SCRIPPS INSTITUTION OF OCEANOGRAPHY › dtic › tr › fulltext › u2 › a220166.pdf · 2011-05-15 · C IV MARINE PHYSICAL LABORATORY SCRIPPS INSTITUTION OF OCEANOGRAPHY San Diego,

C IV MARINE PHYSICALLABORATORY

SCRIPPS INSTITUTION OF OCEANOGRAPHY

San Diego, California 92152

to NAVIGATION SOFTWARE FOR THE(0 MPL VERTICAL LINE ARRAY

NB. J. Sotirin and W. S. Hodgkiss

OTICE LECT E~S APRO 3 1990

MPL TECHNICAL MEMORANDUM 409

MPL-U-12/89March 1989

Approved for public release; distribution unlimited.

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Marine Physical Laboratory (if licable) Office of Naval ResearchI .Department of the Navy

6c. ADDRESS (Cty, State. aiid ZIP Code) 7b. ADDRESS (City; State, and ZIP'Code)University of California, San Diego 800 North Quincy StreetScripps Institution of Oceanography Arlington, VA 22217-5000San Diego, CA 92152

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Office of Naval Research CNR N00014-87-K-0225 and N00014-87_00127

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Arlington, VA 22217-500011. TITLE (Include Security Classification)

NAVIGATICEN SOFTWARE FOR THE MPL VERTICAL LIM ARRAY

12. PERSONAL AUTHOR(S)B. J. Sotirin and W. S. Hodqkiss

t3a. TYPE OF REPORT ft3b. TIME COVERED f4. DATE OF REPORT (Year, Month, Day) .PAGE COUNT,ec : u FROM TO , March 1989 103

16. SUPPLEMENTARY NOTATION

17. COSATI CODES 18. SUBJECT TERMS (Continue on reverse if necessary and identify by block number)FIELD GROUP SUB-GROUP navigation software, vertical line array, travel tirmI nasureients, nonlinear least squares

19. ABSTRACT (Continue on reverse if necessary and identify by block number)

; This report describes the navigation software and demonstrates its operation using

data obtained during an experiment in which a large aperture low frequency acousticarry, designed and built at the Marine PIlysical Laboratory (MPL), was deployed verti-cally from the Research Platform FIP in the NE Pacific. The army was equipped with a12 KHz acoustic navigation subsystem. Travel time measurements from near bottomacoustic transponders of known position were received by specific array elementsthroughout the deployment. These measurements were converted to spatial positions ofthe array elements by a nonlinear least squares technique. The data collection methodsand navigation software programs which locate the transponders, calculate the travel timefrom detected returns and convert travel times to spatial positions are documented.

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DD FORM 1473, 84 MAR 83 APR edition may be used until exhausted. . SECURITY CLASSIFICATION OF THIS PAGEAll other editions are obsolete U.S ,,mnmd Pat~mii Om.ffim 99--.0

Navigation Software for the MPL Vertical Line Array

B. 1. Sotirin and W. S. Hodgkiss

Marine Physical LaboratoryScripps Institution of Oceanography

La Jolla, CA 92093

ABSTRACT

This report describes the navigation software and demonstrates its operation usingdata obtained during an experiment in which a large aperture low frequency acousticarray, designed and built at the Marine Physical Laboratory (MPL), was deployed verti-cally from the Research Platform FLIP in the NE Pacific. The array was equipped with a12 KHz acoustic navigation subsystem. Travel time measurements from near bottomacoustic transponders of known position were received by specific array elementsthroughout the deployment. These measurements were converted to spatial positions ofthe array elements by a nonlinear least squares technique. The data collection methodsand navigation software programs which locate the transponders, calculate the travel timefrom detected returns and convert travel times to spatial positions are documented.

Accession For

NTIS GRA&I f*"

DTIC TAB r-Unannounced 0Justificeation

By--Distribution/

Availability Codes

Dit Special

Navigation Software for the MPL Vertical Line Array

B. J. Sotirin and W. S. Hodgkiss

Marine Physical LaboratoryScripps Institution of Oceanography

La Jolla, CA 92093

Introduction

A high frequency acoustic navigation system is an integral part of the large aperture vertical arraydeployed in 4700 m of water in the NE Pacific during September 1987. Navigation is defined in this con-text as the process of locating individual elements of the array in 3-dimension space at any particular time.The method implemented in the array involves a wansceiver near the ocean surface which sends uniqueinterrogation signals that are detected by bottom moored transponders who reply with a pulse at 12 KHz.This reply is monitored by the array and the time delay between the initiation and reception of each of thepulses is calculated. Knowing the sound speed in the water column, and the location of the transceiver andthe transponders, this travel time defines a slant range between each array receiver and each bottom tran-sponder. A slant range from a known point (a transponder) describes an sphere of possible receiver loca-tions. Intersecting spheres described by the slant ranges from three known points identify a single locationif there are no errors. Because there are always sources of error in real data, a least squares filter is imple-mented to approximate the location by minimizing the squared difference between the calculated andmeasured values, which defines the error in the assumed position. This report documents the data collec-tion methods and the navigation software used to locate 12 array receivers during the September experi-ment. The array itself is documented in [Sotirin et al, 1988] and [Sotirin and Hildebrand, 1988]. The arraynavigation system, least squares filter and navigation data analysis are documented in [Sotirin and Hilde-brand, 19891.

The navigation processing is separated into three programs. The first program locates the bottomtransponders. The second program calculates travel times from a continuous recording of the 12 KHzdetected returns for each navigation receiver. The third program uses the travel times and transponder loca-tions output from the first two programs to estimate the spatial positions of the receivers using a non-linearleast squares filter. The logic flow and input/output files are detailed for each program in the Appendixreferenced in the text followed by a sample run. The software, written in either C or Fortran, is listed inAppendix E.

I. Transponder Localization.

The transponder positions must be surveyed to acquire the location parameters defining the tran-sponder net which are ultimately used to navigate the array. Due to errors in the measured data, an estima-tion technique (least squares) must be implemented. Due to the nonlinear conversion from travel timespace to xyz positional space, the least squares method proceeds iteratively. A data set is obtained contain-ing spatial positions and travel time measurements. If all parameters were known exactly, the travel times

calculated using the spatial positions and the measured value would be identical. This is obviously not thecase, and the difference between the calculated and measured values defines the error in the assumed posi-tion which is minimized during the iteration. Transponder positions with accuracies of less than a meterare achieved by this method. The description below details the data collection, the program inputs, theleast squares implementation, and the resulting transponder positions.

Data Collection. The input data set for the least squares iteration requires initial spatial positions ofthe transponders and transceiver in meters from an arbitrary origin, and slant ranges between the tran-sponders and transceiver. The data collected in the form of travel times and Global Positioning Satellite(GPS) fixes are transformed into the input parameters required by the least squares filter. GPS fixes areconverted into initial xy positions in meters from an arbitrary origin. Travel times are converted to slantranges with knowledge of the local sound speed profile. The travel times required are normally recordedduring an intensive surface ship survey during which the ship criss-crosses the area in which the tran-sponders were deployed recording the travel time data and GPS fixes for its own position. [Spiess, 1985],[Smith et al, 1975] A transceiver is either hull mounted or towed on a short line such that its position rela-tive to the ship is known. The transceiver sends a continuous stream of unique transponder interrogationpulses, and the transponder replies are recorded while the ship criss-crosses the area. Using this method, aseries of travel times are obtained from a wide variety of ship positions. As the ship crosses over the top ofa transponder, an estimate of transponder depth is acquired, and as the transponder baselines are crossed,intertransponder distances are defined.

During the September experiment, although travel times were recorded as described, the 12 KHzreceiver was deployed on a 200 m line due to the noise level of the ship. This introduces theinterrogator/receiver depth as an unknown defining an underdetermined set of equations which cannot besolved for a unique solution. Since the measurements described above were not sufficient for the tran-sponder survey, the travel time data collected by the navigation equipment installed on FLP were utilizedinstead. This is unfortunately not an optimum choice of observation configurations [Spiess, 1985] so theresults of several simulations devised to assess the effect on the estimated positions are also presented. Thehorizontal motion of FLIP was constrained by a three point moor. The FLIP data set provides sufficientrange information but the azimuthal component was not well constrained. The vertical component wasestimated from the echo sounding depth at FLIP and apriori information that the sea floor in the experimentarea was relatively flat. The inputs used to locate the transponders are the vertical sound speed profile inthe test area, the initial xy positions of FLIP from the arbitrary origin, the depth of the interrogation tran-sponder hardwired o the FLIP, the slant ranges from each transponder to FLIP as determined by chartrecorder traces and associated errors, the initial transponder xy positions from the arbitrary origin using thesatellite fixes and the transponder depths.

The initial GPS positions in latitude and longitude are converted to xy distances in meters from anarbitrary origin. For the September experiment, the origin was chosen as 340 47' N, 1260 00' W, with xincreasing positively toward the east and y increasing positively toward the north. A GPS position of theship is recorded during each transponder deployment. The transponders have 45 kg negative buoyancywhen they are deployed from the fantail of the surface ship to insure that the GPS position of the ship is anaccurate initial position of the actual transponder position on the sea floor. Latitude and longitude for eachset of travel time measurements (one measurement from each transponder) are also converted to m to pro-vide the initial estimate of the transceiver position during the survey. The 'survey' from FLIP had such alimited range that the same initial position was used for all measurement sets. During a normal ship sur-vey, travel time data is collected from a large variety of horizontal positions and individual initial positionsare important. If GPS positions are not available, a technique detailed in Appendix D may be used to esti-mate the initial position from the measured travel time data. Positions in latitude and longitude were con-verted to meters by calculating the radius of the earth at the position using the following equations [Sta-cey]:

2

x = r. 8(lat), y = rp cos (lat) 8(long)

r. = a ( 1 -f sin2(lat) ) f = a- c' a

where x is the E-W distance in meters of the position from the origin, y is the N-S distance in meters of theposition from the origin, r. is the radius of the earth at the position latitude, 8(lat) is the difference in lati-tude in radians between the position and the origin, 8(Iong) is similarly the difference in longitude, lat isthe latitude of the position, a is the equitorial radius of the earth in meters and c is similarly the polarradius.

The conversion between travel time and xyz positions requires knowledge of the sound speed profileat the experiment site. The local sound speed profile was calculated from measurements of conductivity,temperature and depth. Travel time deviations due to the variations within the thermocline and to refrac-tion of acoustical energy were shown to be negligible for this experiment therefore a constant sound speed(harmonic mean) was used for the conversion. [Sotirin and Hildebrand, 1989]

The travel time measurements used for transponder localization were collected by transmitting thetransponder interrogation signals from FLIP once an hour for 18 days and recording the returns on a chartrecorder. The transceiver is mounted on the bottom (90 m in depth) of FLIP and the pulse level is adjustedmanually above the ambient noise for consistent transponder replies. Round trip travel times for navigatingFLIP are measured carefully by hand on the chart recorder output with an estimated random gaussian errorof 2-3 ms. The chart recorder trace, set on a one second sweep rate, records the filtered 12 kHz (500 Hzbandwidth) replies received from the transponder being interrogated (Figure 1.1) delayed 6-7 seconds fromthe interrogation pulse. Each transponder is interrogated individually by transmitting its unique signal, trig-gered by the chart recorder, once per second for 45 seconds, and notating its reply on the chart by sweepnumber with color-coded pens (red, green or blue). The figure shows the direct and multipath(surface/bottom bounce) returns for each transponder. Differentiation between the surface and bottombounces is difficult because the FLIP transceiver is the same distance below the surface as the transponderis above the sea floor. The direct return from the red and blue transponders is strong and consistent.Several error modes are evident in the returns from the green transponder however. The sporatic directreturn of the green transponder (Gd) is caused by the transponder detecting either the surface or bottombounce of the transmitted signal. The error was attributed to the transponder because the same transceiversystem detects the returns from all transponders and this type of error was not evident on all transponders.Detection of the multipath arrival of the interrogation pulse delays the arrival of the transponder directreturn such that it arrives at the same time as the multipath return of a direct interrogation pulse (G,, i). Themultipath return of the multipath interrogation arrives 120 ms later (G.2) clearly identifying the first errormode. The second error mode of the green transponder is displayed as the smaller perturbations in the timeof arrival of the Gd return. It is more difficult to identify because the deviation from the true return issmaller (although still significant) and not as consistent as the first type of error. These error modes wereidentified during about 20% of the experiment at random intervals, causing concern during array detectionpost-processing discussed in the next section.

Software Implementation. Once the ingredients for the least squares method have been accumu-lated, all xyz positions are adjusted until the root mean squared (rms) error satisfies the convergence cri-teria. This is accomplished in several parts by first maintaining constant transponder positions and perturb-ing the FLIP positions, then holding the current FLIP positions constant while perturbing the transponderpositions, and finally examining the mean squared error of each FLIP position to determine whether itshould be preserved as a viable contributor. A general outline of the method is presented in Figure 1.2 anddiscussed below. This is followed by a detailed description of the conversion from travel times to slantranges and spatial positions.

The program consists of three concentric stages, the first stage adjusts the xy positions in two innerloops each testing the rms error against specific convergence criteria for the least squares filter, the second

3

.b ..' .. -----

b.iil

- -_ 11-'P

Al11

stage tests the total transponder rms error against a second set of convergence criteria, and the third stagetests yet again. The three stages-are referred to within the software as stage 1: xpfil, stage 2: xploop andstage 3: xpmain. The two first stage loops shown in Figure 1.2 have similar internal operations. The meas-ured travel times, sound speed profile, FLIP depth and transponder depths are considered known, while thexy positions of FLIP and the transponders are considered unknown. The measured travel times are con-verted to slant ranges by multiplying by the harmonic mean of the sound speed profile. This slant range isprojected as a horizontal range and compared to the range calculated from the xy positions. For the firstloop, the squared difference between these two ranges are summed over the number of transponders for aparticular FLIP position and the square root is taken to yield the rms error. If the rms error does not satisfythe convergence criteria, then the FLIP position is adjusted and the loop repeats with the adjusted position.The adjustment is calculated with the search direction as the negative gradient and the step size as a con-stant (1.5 m) unless the rmis error is less than 1 m at which time the step size begins to decrease; as theminimum is approached, the step size is calculated as a function of the percent change in interated rmserror. The convergence criteria for the first stage loops are defined such that an absolute rms error less than0.15 m, a 0.015 percent change in the iterated rms error, or a maximum number of iterations (30) will ter-minate the loop and save the current position. These criteria test each adjusted position individually. Thefirst loop is repeated for each FLIP position. The second loop of the first stage performs the same cadencemaintaining the current FLIP positions constant while adjusting the transponder positions. The rms error iscalculated over the number of FLIP positions, and evaluated using the same criteria. The second loop isrepeated for each transponder position. In the second stage, these two loops are initiated again based onthe percent reduction (< 0.35%) in the transponder rms error summed over all transponders and totalnumber of iterations (> 30). Upon completion of the first two stages, the rms error for all the transpondersis calculated and evaluated according to the following criteria:

1) The rms error * 4number of FLIP positions is < 1.0.2) The percent reduction in rms error is less than a specified value (0.1%).3) The absolute rms error is less than a specified number (0.75).

If the rms error satisfies any of the above criteria, the current positions are written out and the program isterminated. If the rms error does not satisfy any of the above criteria, then the rms errors associated witheach FLIP position are examined and any position with an error greater than the transponder rms errortimes a user specified value (default=2) is deleted from the array and the entire process begins again. Therms error is initialized to 10000 prior to each loop/stage so that unless the absolute error is small, theloop/stage will always be executed more than once.

The conversion from travel time to slant range and the spatial position adjustment is straight forwardand executed within the subroutines xrcor and xpfil. A detailed outline of the program flow is found inAppendix A. The program originates with the Marine Physical Laboratory's DEEPTOW group and hasbeen in existance since the late 1960's. The version documented here is the most recent in a long seriesand, unfortunately, every programmer has left a mark. Consequently, the subroutines each have uniquevariable names for the parameters, increasing the confusion in documentation. The text below attempts tomaintain the variable names within the subroutines specified. The travel time to slant range conversion iscomputed in xxcor. The measured information is input to the program as a slant range assuming a homo-geneous medium with a sound speed of 1500 m/s. This is not normally the case, and for the September testthe sound speed profile was measured and input as a data statement in =rcor into the array vdp as a hor-izontally stratified medium. If sound speed corrections are requested by the user, the input slant ranges areconverted back to the original time measurement (t) and the slant range contribution for each sound speedlayer is summed, returning the corrected slant range (xnew). If the slant range is a depth, for example, thesummation starts at the surface and sums the contribution in each layer until the accumulated time (t.) isequal to the measured travel time. This is an implementation of the following equation in which x is thecorrected slant range:

5

INITIALIZE

- -LOAD NEXT FLIP POSITION

kI,,--FS POSITION

ACCUMULATE ERRORDOES FLIP POSITION RMS ERROR

SATISFY CONVERGENCE CRITERIA?

EN YES

No ARE ALL FLIP POSITIONS ADJUSTED?

N OS

LOAD NEXT TRANSPONDER POSITION

ACCUMULATE ERRORDOES TRANSPONDER POSITION RMS ERROR NO

SATISFY CONVERGENCE CRITERIA?

YES

N1ARE ALL TRANSPONDER POSITIONS ADJUSTED?

PURGE IS FINAL CONVERGENCEFLIP POSITIONS NO CRITERIA SATISFIED?

YES

FTERM INATE

Figure 1.2 Transponder Localization Program Flow. A general outline of 3 stages in the least squares programlogic is presented. During stage 1, the FLIP positions are iterated first, followed by the transponder positions andthe rms errors are calculated for the positions iterated and compared to a set of convergence criteria; stage 2 com-pares transponder rms errors to a second set of convergence criteria; stage 3 compares transponder rms errorsagainst a third set of convergence criteria and determines whether to delete FLIP positions with excessive errors.

6

x = c * t= (zr - zo) * t~dz

where in terms of the subroutine, z0 = xdepth = 0, zT = z is the unknown depth, C(z) = v is the sound speedin the layer, and dz = deltad is the layer depth. When the unknown is a slant range, the implementation isan approximation to the above equation (where dz is the slant range component in the layer assuming thelaunch angle is constant) which is accurate for the configuration of the data set considered for the Sep-tember experiment. Errors increase as the horizontal range or projection (defined below) of the slant rangeincreases however, so for larger scale experiments a more accurate implementation is advised.

Once the slant range has been calculated, the conversion to spatial coordinates is visualized as thegeometric relationship between the transmitter (t subscript) and receiver (r subscript):

slant range = (x, - x,) 2 + (yt - y,) 2 + (Zt - Z,)2 ] *Because the error in the depth parameters are small compared to the horizontal parameters, the slant rangeis projected onto the xy plane prior to the adjustment (Figure 1.3):

1 1

horizontal projection = [(slant range) 2 - (zt - z,)2]Y = [(xt - x,) 2 + (y, - y,) 2]'.

The left half of the equation is calculated in xpread, CRANS(NTR ,NPOS) = (S-D 2)"T where NTR = thenumber of transponders, NPOS = the number of FLIP positions, S = slant range between a position and atransponder, D = transponder depth - FLIP depth, and passed into xpfil as an array IIRAN which isredefined within a loop as a variable HH. The right half of the equation is calculated in xpfil,RNGEC = 4(XDIFF)2 + (YDIFF)2, where XDIFF = XG - XF (NDAT), YDIFF = YG - YF (NDAT),(XG, YG) are the xy positions being interated, (XF, YF) are the fixed positions indexed over NDATA datapoints, and the error ERR, summed over the number of fixed positions, is (RINGEC-HH)2 .

The adjustment is calculated using the steepest descent method to minimize the error by followingthe mean squared error gradient to a minimum. For known transponder positions and the x-direction, theperturbed position is:

XG = tXG + h ERR'(NDAT)

where h is the step size, and ERR' is the negative derivative of the error function with respect to XG. They-direction adjustment is calculated similarly. The error derivative expands to:

ER =d (RNGEC-HH )2 = (H-RGC)d (XDIFF 2 + YDIFF 2 1ERR'- 2(HH - RIVGEC)I Pffi+D12

-H -L RN GEC ) (G -J 2 J TO*XDIFFKNUEE dXG

where RATIO = (HH - RNGEC)IRNGEC, and the constants are absorbed by the step size h.

Simulations. Several simulations of the array navigation system were conducted to examine the sen-

sitivity of the estimated transponder positions to errors in travel time measurements and initial positions.The spatial configuration used closely resembles the experimental set up of the September sea test asshown in Figure 1.4. The transponders and FLIP were initially assigned to known positions with deter-mined slant ranges as shown. Two simulations were conducted to illustrate the transponder positionresponse to errors; two other simulations were conducted to show the effect of transponder and FLIP posi-tional errors on the estimated array positions and are presented in [Sotirin and Hildebrand, 1989]. Theresult of the first simulation was the 3D error surface for various parameters. The second was a MonteCarlo simulation of the initial transponder positions. The simulations provide an understanding of the

7

(2400, 3200, 90)

' IXT, XT2 XT(51 2600) (428)

YYT2

GREEN

4575 in

(4771)- BLUE4575 m

Figure 1.3 Navigation Overview. The horizontal projection is estimated first by using the measured slant rangeand depths (HRAN) and then by using the initial xy positions (mgxy). The initial xy positions of the transponders(Tl=red, T2=green, T3=blue) and FLIP as measured by a GPS fix are notated in meters on the axis, and beneath thetransponder for depth.

- North

BLUE4500 3786 (T3).

RED .

0€F,,

Lo 25000M200

wI-- isee

X (7(T2)

0 500 1000 i500 2000 250 300 3580 400 4500 5000

METERS EAST FROM 1260 W

Figure 1.4 Spatial Configuration for Navigation During the September 1987 Sea Test. The estimated xy posi-tions of the 3 fixed bottom transponders and of FLIP measured hourly over 18 days are shown in plan view. Themooring lines are represented by mrows, and the slant range (s) and horizontal projection of the slant range (h) areindicated in meters from an arbitrary FLIP xy position of (2400,3200) m. Transponder baseline distances are alsoindicated in meters.

a:9

effect of the unconventional survey data.

To examine the error surface for the least squares configuration used in the previous section, specificmodel parameters were perturbed systematically. The error is the rms value of the differences between theslant ranges based on the travel time measurements corrected for sound speed deviations (Eq. 1.1 dividi.gthrough by c) and those calculated from the spatial positions of the transponders and FLIP which were out-put from the least squares filter. Ideally this produces a single well-defined minimum for which the optimi-zation method searches. This is not the case in many real applications however, and the possibility of localminima and/or a broad global minimum should be investigated. The inverted error surface for perturba-tions in the horizontal positions of the blue transponler is shown in Figure 1.5. It exhibits a narrow chan-nel of local minima which appear as a ridge plotted as the negative logarithm of the error against the per-turbation amplitude in x and y position. The ridge is orientated perpendicularly to the FLIP/transponderrange direction indicating that the azimuthal component is not well constrained. Perturbing the horizontalpositions of the other transponders produced similar results.

The system displays a sensitivity to initial positions which is shown to be an artifact of the limitationsin the FLIP 'survey' discussed previously and of the structure of the error surface. The direction in whichthe transponders are moved is constrained to nearly parallel to the FLIP-transponder baseline (Figure 1.6).This occurrs because the distribution of FLIP positions shown in Figure 1.4, provide adequate range infor-mation but minimal azimuthal information regarding the transponder positions as was indicated by theerror surface. There are no sure techniques for locating a global minimum in the company of local minim-ina. Without independent positional data corroborating the results, either Monte Carlo techniques wouldhave to be incorporated in the initial position of each transponder, or knowledge of the error surface wouldhave to be employed as a constraint (search range/azimuth space rather than xy space). Fortunately, theinitial estimate of the transponder positions were acquired from accurate GPS fixes and the resulting errorin GPS positions compared to the estimated FLIP positions at corresponding times had an rms value ofonly 10 m. Thus the estimated transponder positions were declared adequate.

II. Array Travel Time Acquisition.

Travel time measurements were acquired by interrogating three bottom mounted transponders fromFLIP and detecting their replies at the navigation receivers distributed across the 900 m aperture array.There are 24 navigation receivers located at ± 3.75 m from each processor which are separated by 75 mkFigure 2.1). Due to bandwidth constraints, data from 12 of the navigation receivers (one/section) weredecimated and recorded during the September 1987 sea test. The data bit stream from transmit time of theinterrogation pulse to receive time of the reply at the array was reconstructed during post-processing foreach navigation receiver and the travel times calculated.

The navigation timing was based on a 16 bit clock driven at a 1 KHz rate which initiated the naviga-tion sequence. This clock is referred to as the hardware clock to differentiate it from the real-time clock(local time) and the Greenwich Mean Time (GMT) clock. The timing is illustrated in Figure 2.2 anddescribed below. A transceiver located at the bottom of FLIP transmited a series of 65536 ms transpondersequences (Figure 2.2a). A transponder sequence consisted of 4 interrogation pulses 10 ms long at 10 sintervals beginning at a 16 bit clock rollover followed by a 35.536 s silent interval (Figure 2.2b). The firstthree pulses were at the unique transponder interrogate frequencies of the bottom transponders (10, 10.5and 11 KHz). Upon receiving an interrogation pulse the bottom transponders replied with a 3 ms pulse at12 KHz. The turn around time of the transponders is signal to noise dependent, however due to thestrength of the transmitted signal, the delay was assumed to be less than 1 ms. The fourth interrogationpulse transmitted by the FLIP interrogate transceiver was at 12 KHz which simulates a bottom transponder

10

+100

+50

-50

" 0

0-9 +100-0

I-50 +50

-100 x

Figure 1.5 3D Error Surface. The estimated positions of FLIP and the transponders were considered known,selected parameters were perturbed and the resulting error calculated. In this example, the blue transponder x and ypositions were perturbed in 1 meter increments to ±100 m from the original known position which is plotted at thecenter. The z axis is plotted as the negative logarithm of the error to allow visualization of the minima.

II

I I I I ji I t I I I

z IZ 4300,'o

0

" 4200

LL

=I

3900I

400 500 600 700 800 900METERS EAST FROM 12600 W

Figure 1.6 Monte Carlo simulation of initial transponder positions. This simulation illustrates the constraints insearch direction placed on the least squares iteration by the spatial configuration of the survey points. The azimuthalposition of the transponders is not accurately determined by the FLIP data set used. The transponders tend to movealong the transponder to FLIP direction. The initial transonder position is notated by a triangle, the final position bya cross.

12

0 . '-P12

Hi

2

5 0 P1 13P11 NAVIGATION......-H4

P11

75H7

VELO IY meer/s.

~weight,

4

51480 1490 1500 1510 1520 1530 1540

VELOCITY, meters/second

Figure 2.1 Array Navigation Receivers. The array was deployed at three nominal depths during the Septembr1987 experimenL The 12 indentcal array sections are each 75 m in length, with 10 receiving hydrophoe s spacedat equal increments. The location of the navigation receiving hydrophonea is illustrated for one section. The datafrom one navigation receiver per array section (145) was recorded during the experiment The background curvesrepresent the CTD sound speed profile and 3 near surface profiles calculated from XBT data.

13

()= ROLLOVER

a) Series oftranspondersequences

0 65536 128 k 256 k (ms)(64 k)

RED GRN BLU FLPMO kHz 10OOkHz li-Oki 120ktiz

b) Transponder p , , psequence102

0 10 20 30 65.536 (sec)

12 12 12 12kdlz kHz kHzk1z

c) Each array I I I I Ireceiver 10 20 30 65.536 (see)

d) Tapebuffers 1: ms eah

Figure 2.2 Navigation Timing Diagram. The timing associated with the array navigation system is illustrated.(a) The transponder interrogation sequence is initiated every 65536 (64 K) ms by a 16-bit hardware clock rollover(R). (b) Each transponder sequence consists of 4 transmissions at unique frequencies issued from a transceivermounted on the bottom of FLIP (90 m depth) spaced 10 s apart. (c) The array receives the 12 kHz transponderreplies and the 12 kHz pulse from FLIP. The time of arrival of a transponder reply reflects the time it took theacoustic pulse to travel from FLIP to the transponder to the array receiver. The time of arrival of the FLIP 12 kHzpulse corresponds to the distance from the array receiver to FLIP. (d) The hardware clock rollover and 12 kHzarrivals may occur anywhere within a 128 ms tape buffer which complicates the data extraction procedure.

14

reply. The array therefore received four consecutive 12 KHz reply pulses about once a minute (Figure2.2c). The navigation receivers were capable of detecting a 12 KHz signal 6 dB below the noise level in a200 Hz band. The binary output of the detector was sampled at 5 KHz and decimated by 2 to provide acontinuous time series consistent with the 5 bits per processor every 2 ms allowed for navigation datawithin the specified data format. Thus the data from 12 navigation receivers located 3.75 m below eachprocessor (H5 in Figure 2.1) in the array were multiplexed in with the low frequency acoustic data,transmitted to the surface and recorded. The interrogation sequence was synchronized with the timebase inthe array and the initiation time of the sequence (as indicated by the rollover of the hardware clock) wassampled every 128 ms and recorded on the tape. The tape format showing the placement of the navigationdata is illustrated in Figure 2.3. The navigation data time series are reconstructed from this recorded databy the method described below.

The structure of the data on tape was not optimized to facilitate extraction of the navigation bitstream. The header containing the timing information appears in the first 8 words of each tape bufferwhich contains 128 ms of data. Most of the 6280 16-bit words within a buffer are assigned to low fre-quency acoustics rather than navigation as seen in Figure 2.3. To reconstruct the sampled time series out-put from each navigation receiver, the 5 bits/processor must be extracted and stored. The major program-ming effort was in initializing and incrementing pointers and in error checking. A description of the mainprogram and subroutines and a sample run will be found in Appendix B. The extraction of the data beginswith the data buffer containing the hardware clock rollover. The rollover may occur anywhere within thedata buffer. The program examines each buffer header for the hardware clock rollover. This is determinedby testing between consecutive clock times for a negative difference, and linearly interpolating within thebuffer for the correct frame. For example, if the hardware clock in buffer I is hwc I = 65430 and thehardware clock in the next consecutive buffer is hwc 2 = 22, then the difference (hwCv2 - hwcI = -65408) isnegative and the clock rollover occurs within buffer 1 at frame 53; the data stored would begin at frame 0,buffer 1. The time difference from the beginning of the buffer to the rollover is stored in a parametercalled start and passed to the subroutine navloc which calculates the travel time. Once the rollover isidentified, the navigation data associated with each processor is masked off and stored as the 10 mostsignificant bits in a 16-bit word (fillnav).

The travel time calculation is a simple difference once the time of the transponder reply is deter-mined. To locate the reply, the data is treated as a time series of 0.4 ms bits, and a correlation between thetime series data and a replica transponder pulse is initiated as a matched filter detector. Due to inconsisten-cies in receiver detection threshold and noise level, each receiver is assigned an individual replica pulselength. Temperature sensitivity of the capacitors and high failure rate of the inductors in a phase matchingtuned filter caused the mismatch in the detection threshold of the individual receivers. This variation inreceiver error is apparent in the positional error distribution shown in Figure 2.4, in which the onlyhardware or processing difference is in the navigation receiver itself. Additional variation in receiver sig-nal to noise level of the incoming reply could have been caused by the 12 KHz beam pattern of the indivi-dual elements. Each element consisted of two hydrophones wired in series with a spacing between 8 and 9cm such that at 12 KHz, a notch in the beam pattern appears between 460 and 520 from broadside. FromFigure 1.4, the arrival angle of the transponder replies when the array was nominally at 400 m was between580 and 650 from broadside and signal to noise was high enough to be detected; at deeper depths, however,this is potentially a problem. The rms error used in the distribution calculations accompanies each receiverposition estimation described in Section 1H. The transponder reply is a CW pulse, so the correlator isimplemented as a moving adder, with a detection defined as the first occurrance of a normalized correlationamplitude greater than or equal to 1.0 within a valid data window.

The window was installed due to excessive noise levels and the interference seen in the return of asingle navigation receiver from the hardware clock rollover as shown in Figure 2.5. The window must bedetermined prior to program execution. An option in the program harrynav (-p) will print out all bits set bythe navigation receiver, packed into the least significant 10 bits of a 16-bit word (for a maximum of 3FF) toallow the user to determine the window parameter. An example is seen in Figure 2.6: the return for

15

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60 6040 4020 20

60 -60

8 1 2 3 4 5 6 7 8 9 18 0 1 2 3 4 5 6 7 8 9 10

Bins (m) Bins (m)

Figure 2.4 Receiver Error Distribution. The detection threshold mismatch in the array receivers causes a varia-tion in receiver positional error distribution. This rms error is an output of the program flpnav described in Appen-dix C.

17

-35

30

A U-

"0

-203U)

15

10

5

01001

0 025 0.50 075 100seconds

Figure 2.5 Correlated Output from a Single Receiver. A single receiver should ideally display only 4 detectionsduring a transponder interrogation sequence. Shown is a 40 s time series of the correlated output of processor 3.Each line represents one second of data, a signal is detected as a transponder return if the amplitude is above thezero level of the line above it. The noise and interferring signals warrented that only data within a valid data win-dow be considered. The desired detections are indicated by the stipled boxes.

18

processor 2 is clean and the transponder reply is quite clear. The last 4 bits of word i503 are set followedby 8 bits in word 606, for a total of 12 sequential bits set. The words have a maximum of 10 bits or 4 ms.The valid data window length is a minimum of 100 ms and varies with the transponder. Only the leadingedge of the window is specified in the window parameter input file window.dat. An estimate of the leadingedge of the window for a particular receiver p may be obtained by multiplying the first word of the tran-sponder reply W. by 4 and subtracting 184 ms. A window parameter for each processor and each tran-sponder must be specified, 48 total. The algorithm for computing the time of arrival of the leading edge ofthe reply is T= W * 4 + (10 - Bp) * 0.4 - start - c where W. is the first word, and B. is the number ofbits set in the first word, start is the time difference between the beginning of the data buffer and thehardware clock rollover as described earlier (printed in line I of Figure 2.6 as Start: 14), and c is a constant= 134.68 ms; e.g. W. = 1503, Bp = 4, T. = 5865.72 ms which is the result shown in Figure 2.6 for Roll: 1,Xponder 1 Start: 14, Proc# 2 in the Figure. The constant c is a sum of the delays through the array systemin ms determined by laboratory tests to be 7 + (128 - 0.016 * (32 - NPROC)) where NPROC is the numberof processors in the array. The 7 ms is due to data buffering in the array hardware and the remainder is dueto buffering within the driver for the magnetic tape. This buffering delay was discovered later to be inerror by 112 .zs and should have been calculated as (128 ms/frame - [125 (words/frame) - 2 (header words)- NPROC * 8 words/processor] * 0.016 ms/word).

The result of the correlation is seen as a series of returns across the array for each of the transpondersas shown in Figure 2.7. One second of data for each processor is plotted during each transponder reply withthe deepest processor P1 plotted on the bottom. The reply from the bottom transponders (Figure 2.7a, band c) appear at the deepest processor first and arrive sequentially at the shallower receivers as the pulsetravels up through the water column. The pulse from the last transponder (Figure 2.7d) travels down fromFLIP. The squared amplitudes are normalized to 1.0 and plotted such that a value of 1 will be slightlyabove the zero level of the next processor. The noise and interference mentioned earlier is also evident;processor 6 represents a particularly noisy time series and interferring signals are seen travelling up anddown the array, e.g. between 0.6 and 0.7 s on the plot of transponder 3. Each valid data window is markedwith "x's"; the window length for the first and last transponders is 100 ms, but was increased for tran-sponders 2 and 3 to 250 ms to enable detection of the transponder error modes illustrated by the green tran-sponder (transponder 2) in Figure 1.1. The time of arrival of the detected reply may be estimated from theplot by adding the time indicated by the plot to the number of ms notated at the top of the plot and subtract-ing a constant c = 134 ms defined previously. The travel times are converted into slant ranges by assuminga constant sound speed of 1500 m/s and written into ascii files for use in the spatial position calculationcomputed in the next program.

III. Array Spatial Localization.

The navigation algorithm for the array elements is virtually identical to that described for the tran-sponders once the initial array X-Y position is determined, except that the transponders are considered sta-tionary. The element to transponder slant ranges corrected for sound speed, the depth of the transponderinstalled on FLIP and the adjusted transponder locations constitute the data required to navigate the array.The array positions are iterated to achieve the best fit to the data is a least squares sense. A description ofthe main program and subroutines and a sample run will be found in Appendix C. Array element relativelocation accuracies of a few meters are achieved by this method.

The array is not moored but hangs vertically from FLIP under 320 kg of tension; its horizontal rangeof motion is significant during a one minute time interval. Consequently, receiver locations are iteratedusing a single time slice of slant range data. The receiver slant range is the path from FLIP to the tran-sponder to the receiver, FLIP slant range is simply the path to the transponder. The slant ranges are

19

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TRANSPONDER 4: STARTING AT 246 ms AFTER BUFFER

d ) -- D00

-. J-0 0"2 0"4 0.6 VALID 0"8 1.0

seconds DATAWINDOW

Figure 2.7 Transponder Reply Detection. The correlator output for each processor is plotted for 1 second

around the time of each of the transponder replies. The top processor plotted is the top processor in the array. The

valid data window is indicated by the small x's. The bottom mounted transponder replies are evident as upward

traveling pulses, arriving at the bottom of the array first. (a) TI (red) transponder reply arriving between 0.15 and

0.55 for processors 1 and 11 respectively; (b) T2 (green) transponder reply; (c) T3 (blue) transponder reply; (d) the

12 kHz pulse transmitted from FLIP arrives as a downward traveling pulse with the surface bounce 120 ms behind.

21

corrected for deviations due to a sound speed profile which differs from the assumed 1500 m/s, as are thereceiver depths (which are obtained from the travel time measurement of the FLIP transceiver to eachreceiver), using the harmonic mean described earlier. The noise in the receiver slant range measurements(Figure 3.1) is contributed to not only by the receiver noise as shown in the previous section but by thetransponder malfunction described in the first section and by high frequency FLIP motion (recall that FLIPslant ranges are available only once per hour). Constraints on the difference in receiver depths and slantrange measurements from one minute to the next are implemented as user parameters thresd and thres. Ifthese thresholds are exceeded, the noisy data is ignored and another user parameter alter determines whichinterpolation scheme will be implemented, if any.

Localization of the array receivers now proceeds to the least squares filter. A constant initial xyposition is assigned to FLIP based on a GPS position as was done during the transponder iteration for theFLIP initial position; the initial positions for the first receiver iterated is the estimated FLIP position; theinitial positions for subsequent receivers is the estimated position of the previous receiver. The FLIP slantrange is subtracted from the receiver slant ranges leaving the transponder to receiver portion. The horizon-tal slant ranges are calculated and the xy positions are iterated minimizing the squared error in calculatedand 'measured' positions. With the transponder positions fixed, the iteration is confined to FLIP and thearray receiver positions whose x, y and z positions are output with an associated rms error. The resultinganalysis of data processed in this manner is presented in [Sotirin and Hildebrand, 1989].

Acknowledgments

This work was supported by the Office of Naval Research under Contract nos. N00014-87-K-0225 and N00014-87-C-0127. The authors are indebted to the undaunting work of the MPL staff.We wish to thank Vince Pavlicek for engineering expertise, Eric Wolin, Paul Henkart, Dick Cur-rier and the MPL Deeptow Group for software support, Tony Aja, Dave Ensberg and Pam Scottfor their assistance in the data processing effort and Jo Griffith for her illustrative talents. Thecaptain and crew of the research platform FLIP contributed to the sea-going operation. Specialthanks goes to Chris deMoustier for his critical reviews and valuable suggestions.

References

Smith, W., W. M. Marquet and M. M. Hunt, "Navigation Transponder Survey: Design andAnalysis", in I.E.E.E. Ocean 75 Record, San Diego, CA, 563-567, Sept., 1975.

Sotirin, B. J., F. V. Pavlicek and J. A. Hildebrand, "Low Frequency Digital Acoustic Array", inCurrent Practices and New Technology in Ocean Engineering- 1988, ed. G. K. Wolfe and P. Y.Chang, American Society of Mechanical Engineers, NY, OED-Vol. 13, 1988

Sotirin, B. J. and J. A. Hildebrand, "Acoustic Navigation of a Large Aperture Array", in prepara-tion, 1989.

Spiess, F. N., "Analysis of a Possible Sea Floor Strain Measurement System", Marine Geodesy,Vol. 9, No. 4, 385-398, 1985.

22

200-

E 100

z< -100 RED SLANT DATAcc' Median: 9161.9, Mean: 9138-6

-200 - - -- --' I - --- - '

200 .

sE 100 a'

0 0 ~it~U

z a

<-100 GREEN SLANT DATAMedian: 9437.0, Mean: 9434-6

-200 = ==- ' u - ' :== =200 -I . - I

E 100 4

J 0z< -100 BLUE SLANT DATAcc Median: 9713.6, Mean: 9715.9

-200 ... f -. . - 1

260"0 260"5 261 0 261"5 262-0 262-5

TIME (Julian days)

Figure 3.1 Slant Range Data. The slant range data for each of the bottom transponders (as detected by the naviga-tion receiver sampled by processor 8) are plotted relative to the median value for a series of nonconsecutive tapes.The data were clipped at ±200 m from the median value. The errors shown in Figure 1.la for the green transponderare evident as a shadow return 160 m from the direct return. Most of the data appearing at -200 m illustrates a lackof detection due to the navigation of FLIP once an hour for several minutes. FLIP navigation continues to interro-gate the transponders consequently the 12 kHz returns are there, but because the FLIP navigation system is not syn-chronous with the array, the valid data windows filter out most of the unwanted returns.

23

APPENDIX A. Transponder Localization Program Description.

The tables below illustrate the program flow; the text details purpose of each routine. The structure of thetables reflects the main program and subroutine levels. Each subroutine in level I is called by the mainprogram; each subroutine in level 2 is called by the level I subroutine to the left. The structure of the textis similar;, indentations reflect subroutine level. Any file names beginning with xxx may be specified by theuser. The first routine localizes the transponders and is called XPMAIN. It is assisted by an initilizationprogram called XPLOAD. These programs were originally written for navigating the transponders used inthe DEEPTOW program at the Marine Physical Laboratory. A sample run for each program specifing theinput and output file structures and program user inputs follows the description of the program.

XPLOAD

Main Subroutines

Program Level 1 Level 2

XPLOADXPCMLD XPSETBASLIN

XPCMLDEXIT

XPLOAD: initializes the program para-'cters used by the main transponder navigation program XPMAIN.There are three files involved:

XPCOMDAT defines a cor'mon area containing initialized variables. If this file does not exist whenthe program is initiated, it is created.xxx.trs contains the X, Y and depth information for each transponder and provides a convenient wayof inputting the data. The default name is TRANSPONDER.TRS, read in if a carriage return is inputwhen the user is querried for the transponder file.xxx.dst contains the X, Y depth and baseline information for each transponder. The user is querried forthe name, referring to the 'listing' file.

XPCMLD: is a subroutine which reads or writes to a file depending upon the state ofldflg. The file is called XPCOMDAT by default and if it does not exist whenXPCMLD is called, it is created.

XPSET: If xpcom.dat does not exist, the file is created; programfunctions include initializing the deep (1500 m/s) and shallow(1500 m/s) sound velocities, logical unit numbers, plotter inputsand clearing the transponder data buffer areas. Some of this infor-mation is obsolete eg. sound velocities and plotter inputs.

If the transponder data file xxx.trs does not exits, it is created by quering the us--. The inputs for each tran-sponder are 'Label, X, Y, Depth, Comment'. The X and Y values are position '.th respect to an arbitraryorigin. All numeric inputs are in meters.

BASLIN: The baselines between transponders are calculated using the X-Y positionsand are written into the list file along with the X-Y positions.

24

XPMAIN

Main Subroutines


XPMAINXPCMLDXPLNPTXXCOR

LOOP: XPREAD XXCORXPLOOP XPFIL (perturbs FLIP position)

XPFIL (perturbs transponder positions)

XPURGEBASLN2TMDATE

END LOOP:BASLN2BASLN2XPRINTTMDATEXPRIN2EXIT

XPMAIN perturbs the xy positions of FLIP and each transponder until the RMS error is minimized. Thereare four files involved:

XPCOM.DAT: defines a common area which is set up by XPLOAD. The common area contains thetransponder information, nominal sound speed, plotter parameters, and logical device definitions.Sound speed and plotter parameters are obsolete.xxx.dat: contains the time, X and Y position and the depth of the interrogation hydrophone which isdeployed from FLIP and slant range information for each transponder for a series of fixez This file isset up manually by reading the slant range for each transponder off of the chart recorder output (if sur-face ship is used for the survey, by also recording the corresponding LORAN-C or SATNAV positionand converting to meters from an arbitrary origin). The same xy FLIP position is used for all fixesdue to the limited range of the survey.xxx.trsout is an output file containing the adjusted transponder positions.xxx.pos is an output file containing the adjusted FLIP positions.

XPCMLD: is a subroutine which reads the data in the transponder common area from

the file called XPCOMDAT which is written by XPLOAD.

XPINPT: opens the data file xxx .dat, the transponder file xxx.trsout and inquires as tothe minimum number of ranges acceptable (default=3). The option of selecting

specific transponders to be removed from the net is available but will not be used in

25

this demonstration as the minimum number of transponders is 3.XPMAIN requests an operator input for the RMS error factor for fix rejection (default=2) and saves the ini-tial transponder positions for later comparison.LOOP:

XPREAD: opens the data file zr.dat and calculates the horizontal range (XY projec-tion) from the source hydrophone on FLIP to each transponder and for each fix usingthe source depth, transponder depth and slant range.

HH=CRANS (ntrnpos)=4S2 - D2

where ntr indicates a particular transponder, npos indicates a particular fix or position,S is the slant range from the source to that transponder and D is the transponder depth -source depth.

XPLOOP. initializes the minimization of the RMS error in the horizontal range. This isaccomplished in a loop in which XPLOOP calls XPFIL transfering the appropriateparameters to first adjust the FLIP positions (npos fixes) for each transponder and thenadjust the transponder positions for each FLP position (fix). The RMS error is theerror after perturbing the transponder positions. If the reduction in normalized RMSerror from one loop to the next is less than 0.015 or is the number of loops exceeds 30,XPLOOP is terminated.

XPFIL: adjusts the xy positions. The first sequence during which itis called within the XPLOOP loop, it sums the squared error in thexy projections, 1,(rngec - HH)2 where mgec is the xy projectioncalculated from the xy positions of the FLIP and the transponderand HH is calculated as described in XPREAD, for each tran-sponder. The FLIP position is then adjusted:X = X + DX * RATIO * GAIN / N where X is the original Xposition of the FLIP, DX = the difference in X-positions of theFLIP and the transponder, RATIO = (HH-RNGEC)/RNGEC,GAIN = 1.5, N = number of transponders. If the root-mean of thesquared error is less than 0.25 or if the normalized reduction in rmserror is less than 0.035 it returns to XPLOOP. Otherwise thesquared error is cleared and calculated again. This continues untilthe rms error satisfies one of the above criteria or the number ofadjustments exceeds 30. Although the final error calculated fromthe FLIP position adjustment .s passed back to XPLOOP it is notused. The parameters associated with the next FLIP position areamassed and XPFIL is called again. This continues for all FLIPpositions or fixes in xxx.dat. The second sequence during whichXPFIL is called the proceedure is essentially identical except thethe squared error is summed for each FLIP position, the positionadjusted is the transponder position, and N is the number of FLIPpositions.

XPMAIN then evaluates the rms error passed back by XPLOOP for the following criteria:If the RMS error * 4 number of fixes in xxx.dat is < 1.0.If the normalized reduction in RMS error per loop [(OLD error - current error) / OLD error] < 0.035.This terminates the loop if the reduction in error is less than a specified value or if the error increases.If the RMS error is less than a specified number (0.75).

If any of the above criteria are met, XPMAIN jumps out of the LOOP and begins the print sequence. Theerror must converge to one of these criteria, there is no limit as to the number of times the LOOP may be

26

executed.XPURGE removes any FLIP fixes which have an rms error greater than the fix rejec-tion parameter input from the keyboard earlier (default=2) times the total transponderrms error.

BASLN2 calculates baselines from transponder xy positions and writes to the screen.XPMAIN stores the new transponder positions.

TMDATE calculate real time and date from a system call.XPMAIN writes the date, time and transponder positions to an intermediate file which is defined currentlyas the screen.GOTO LOOP

BASLN2 calculates baselines from initial transponder xy positions and writes to thescreen.

BASLN2 calculates baselines from final transponder xy positions and writes to thescreen.

XPMAIN calculates the difference between the initial transponder baselines and the final transponder base-lines and prints to the screen.

XPRINT writes to the screen the adjusted transponder ranges and the time, X, Y, errorand depth for the FLIP transceiver.

XPRIN2 writes to the output file xx.x.pos the time, X, Y, error and depth for the FLIPsource, a quality factor, and the original slant ranges to the transponders in the sameformat as the input file xxx.dat.

27

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38

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APPENDIX B. Array Travel Time Acquisition Program Description.

HARRYNAV

Main Subroutines

Program Level 1 Level 2 Level 3

HARRYNAVgetnav whichtp printnavp

printnavtskipread whichtpgettimeskipreaddtfposskipreadfillnav navlocdtfposskipread

HARRYNAV: Enters user parameters from the program line. Parameters are the option flags, number ofrollovers to skip between processing, total number of rollovers to process, rollover to start processing with,processor ID file, FLIP slant range data file name, and output file name. System subroutines are called toread the input program line and the processor ID file.

gemav: System subroutines are called to read the FUP slant range data, parametersare initialized. Subroutines whichtp and skipread are called to readin the buffer of data containing the rollover requested. If the time flag is set,the first buffer on the tape is read, the header time parameter is converted to aninteger and compared to the user input time parameter also converted to aninteger (by gettime). Due to a lack of space in the tape header, only theleast significant digit of the GMT hour was recorded so the local hour is con-veted to GMT for the correct GMT time. The time in each buffer header iscompared until the header time in the rollover buffer is greater than the timerequested. If the time flag is not set, buffers are read in until the rollovernumber requested is equal to the number of rollovers read in. A loop indexed byi over the number of rollovers requested is initialized. The file window.datcontaining the valid window parameters is read in. Difpos is called to writethe date and time into the output file and FLIP slant range into the outputfile. A loop for extracting the data for each transponder is initialized indexedby j. Skipread is called for the remaining three tansponders. Navtst)is called to locate the return. Files are output containing the valid data markers(pltwin.sio) and the received data (navplt.sio) for plotting. Current valid datamarkers are written into window.dat and the date and time are appended to thebottom of the file by dtfpos. A system call to pltsav.scr plots the data tothe screen. If pltsav.scr does not exist when called, a message is displayed andthe program continues.

whichtp: Requests the user to load the magnetic tape and enter the tape

40

drive number. If the verbose flag is set, printnavp andprintnavt are called.

printnavp: prints the array slant ranges by processor. printnavt:prints the array slant ranges by transponder.

skipread: Reads the first buffer requested. Buffers may be requested byrollover, by GMT time or by hardware clock time. If end of tapeindicators are true then whichtp is called and the programcontinues extracting the navigation time series.

gettime: Translates the ascii string indicating GMT time into an integer.

dtfpos: is called to write the date and time into the output file andlinearly interpolate the FLIP slant range data to obtainthe slant range corresponding to the time of the rollover. Thelocal date is converted to Julian day, and the time is GMT. Ifthe passed parameter 'pall' is negative (0) then only the data andtime are printed.

fillnav: For each transponder interrogation, the navigation bits areextracted from the data stream and stored as a time series foreach processor. Each time series begins with the buffer followingthe interrogation transmission and ends 74 buffers later (9.472 s).Various ID's and counters are verified. Errors in the buffer

counter, frame sync word or frame counter are fatal. If an error inprocessor ID is detected, the data for the processor in question iszero filled.

navloc: The data are received by navloc as the most significant 10 bitsin a 16-bit word and repacked into the 30 least significant bitsin a 32-bit word. The correlation is done as a moving adder withleading pointers iI (word) and jI (bit) and trailing pointers k(word) and I (bit) starting from the beginning of the plotwindow (bwinw[xpndr] + bwinb[xpndr] the window word and bit

pointers) for SAVSZ bits (1 second of data). The plot window isset up as WINOFF words before the valid data window when theprogram is initiated and allowed to move as the reply moves byevaluating the average movement of the replies across the array.The detection is valid only within the valid data window, withparameters pstart, pend, LENGTH; and is defined as a normalizedcorrelation amplitude of detec=1 (normalized by PINGSIZE). Thetime of the detection is the start of the plot window plus thenumber of bits, p, as the adder was initilized with the firstPINGSIZE outside the correlation loop.

41

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4143

APPENDIX C. Array and FLIP Localization Program Description.

FLPNAV

Most of the code for this program is involved in reading the inputs. There are some minor system subrou-tines called which interpret ascii input files. Only subroutines which pertain to the array navigation prob-lem will be described here.

Main Subroutines


FLPNAVlinintxcorfix3

FLPNAV: Locates the array receiver positions in a least squares sense using parameters entered by the userin a command file. User parameters define the transponder locations, slant range input data file name, out-put file name, sound speed profile, interpolation method for the FLIP slant ranges and a method for con-straining deviations in slant range with time. The set of measurements taken at any one particular time arereferred to as a "fix". Some parameters may be input from the command file or from separate existing filese.g. dvfilefilename inputs depth vs. sound speed values from the named file. Parameters which are notspecifically declared in the command file are preset to the value indicated below. Some of the parametersdefined below were used for test purposes only.

The information required by this program is:1) Fixed x, y, and z positions for each of 3 transponders.2) Initial x y positions for each receiver and FLIP.3) Fixed depth of each receiver and FLIP.4) The slant range of each receiver and FLIP to each transponder.

The array receiver slant ranges must be the distancefrom FLIP to transponder to array receiver.

5) Sound speed profile.

The navigation procedure is:1) Discard noisy slant ranges using the parameters THRESD and THRES.

An entire fix is thrown away if more than 7 of the slant ranges forany one transponder are zero (this is to find the fixes that occurwhen the navigation is turned off - usually around the hour whenFLIP is navigated).

2) Adjust the receiver depths and slant range measurements using theharmonic mean based on the sound speed profile measured by a CTD.

3) Interpolate for a new FLIP slant range set if files are available;subtract the FLIP slant range from the receiver slant range leavingthe range from transponder to receiver.

4) Calculate a horizontal range for FLIP and each array receiver5) Iterate the FLIP and receiver positions minimizing the rms error

in a least squares sense.

44

Input Parameter Definitions:ifile - The file name of the file containing the slant ranges and depths of FLIP

and the array receivers whose positions are to be iterated. A FLIP slant rangeis the path from FLIP to the transponder, a receiver slant range is the pathfrom FLIP to the transponder to the receiver.Preset = rnglog.dat e.g. ifile /localdata/ranges/data

ofile - The output file name. When given, the output xyz information will be written tofile ofile instead of standard OuLPreset =' ' e.g. ofile output

trlocs - A list of the transponder locations, given as x, y, z, and variance for eachtransponder.Preset = none e.g. trlocs 1000 1000 2000 1.0 3000 1000 2100 2.0 2000 2000 1500 1.0

depths -A list of depths of all the receivers, replacing the receiver depths in ifile.All depths including FLIP's must be given if any are given. DEPTHS is ignoredunless given.Preset = 0 e.g. 90 300 375 450 525 600 675 750 825 900 975 ....

nsta - The number of fixes in ifile.Preset = 1

stainc -The increment between fixes in ifile to interate. The first fix in ifileis always the first. e.g. stainc 2 skips every other fix.Preset = 1. e.g stainc 10

n2ave - The number of consecutive fixes to average before trying to calculate eachlocation. The average is performed by doing a running average of one-way slant ranges(transponder to receiver) of n2ave fixes. Noisy fixes (where more than 1/2 ofthe ranges are "noisy") are nc,* included in the average. n2ave may not exceed 10.Preset = 1 e.g. n2ave 10

alter -The alternatives for noisy slant range data. Slant ranges more than thresdifferent from the slant range for the same receiver/transponder of the previousday/time will be "noisy".

=0, No data are discarded as being "noisy".-1, A single noisy slant range out of the three slant ranges for a single receiver

will cause the data for that receiver to be discarded. The output file will notcontain a position for the receiver for that day/time. The file will still containthe total number of receivers, but some receiver positions will be duplicated (thisallows plot programs to just plot the same receiver on top of itself).

=2, The noisy slant range will be replaced by the slant range for the same receiverand transponder from the previous day/time in the input file.

=3, The "noisy" slant ranges will be replaced by interpolating the slant range to thesame transponder from adjacent receivers. A noisy receiver on either end of thearray will receive the slant range from the previous fix. Likewise, if two adjacentreceivers have noisy slant ranges, the first one will receive the slant range forthat receiver from the previous day/time.

-4, Noisy slant ranges are calculated by extrapolation or interpolation of adjacentnon-zero slant ranges.

Preset = 3thres - The threshold slant range in meters used in determining noisy slant ranges.

Preset = 100. e.g. thres 50thresd -The threshold depth difference in meters used in determining "noisy" depths.

A depth is considered noisy if it is more than THRESD away from the depth ofthe same receiver on the previous day/time.

45

Preset = 5trfile - A file containing the transponder locations ( x, y, z ) in the format written by

program XPMAIN with an additional column specifing the variance.Preset = none e.g. trfile trs

dvfile - A file containing the depth-sound speed pairs as described below.vdp - A list of sound speed depth-pairs to use in making sound speed corrections. The

sound speed correction is made by first converting every depth and slant range to timeusing a constant sound speed of oldcv. The speeds in vdp are assumed to bethe speed of sound at the depth given. The speed used between specified depths is the"interval" speed which is the average speed of the two adjacent depths.

dvp - A list of depth-speed pairs. This is identical to vdp, except that theorder of the pairs is depth followed by speed.Preset = none

oldcv - The constant speed used in obtaining the slant ranges.Preset = 1500. e.g. oldcv 1450

calctd - Assuming that the array is vertical and directly below FLIP, data from twoarray receivers may be used to estimate the transponder depths. Enter the two arrayreceiver numbers for the calculation. Test mode only.Preset = 0 e.g. calctd 3 4

lpfiie - The pathname of an output file containing the slant ranges in the format expectedby the looping program XPMAIN. The file contains the slant ranges from the transponderto each receiver.Preset = none e.g. lpfile slr.loop

eqle- The pathname of a file in which data error messages will be written. No data errormessage will be written unless erfile is given.Preset = none e.g. erfile oops

fudge - A factor to test FLIP's slant ranges. Test mode only.Preset = 0. e.g. fudge 10.

Subroutines:linint: Interopolates the FLIP slant ranges linearly.xcor: The harmonic mean of the sound speed profile is used in correcting the slant

ranges and depths which were calculated assuming a constant sound speed of 1500 m/s.trgss: Not used for the September 87 data. Calls Fix2 to calculate twopossible 'fix' positions. The position closest to the third transponderposition is selected.fix2: Nor used for the September 87 data. Calculates the two positionsdefined by the intersection of the slant ranges from the firsttwo transponders. See Appendix C.fix3: Adjusts the of first the FLIP position and then each arrayposition using a single fix to obtain a least squares convergence.Outputs the adjusted xy position and the rms error for each call.

46

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APPENDIX D. Initial Position Calculation.

To calculate an array X-Y position (Figure D 1), the horizontal range from FLIP or the array to any tran-sponder is given by

Hproj I = S2 - (DTI-DFUp) 2

Hproj 1 is the horizontal range from transponder 1 to FLIP,S is the one way slant range from FLIP to transponder 1,DTI is the depth of transponder 1 andDFUp is the depth of the transmitter mounted on the bottom of FLIP.

Two X-Y positions are calculated from the intersection of the arcs defined by the horizontal projections:XT1 YTI = the X-Y position of transponder 1;XT2,YT2 - the X-Y position of transponder 2;X 12 and Y12 are the differences between the X-Y transponder positions (X 12 = XTI-XT2);

Bline = the transponder baseline range = (X?2 + Y?)4;Hproj 1 = transponder 1 to fix range;Hproj2 = transponder 2 to fix range;P1 = projection of fix onto baseline point to transponder 1P2 = projection of fix onto baseline point to transponder 2C = perpendicular distance from fix to baselineXpj = X-coordinate of the projection of the fix onto the baselineYpj = Y-coordinate of the projection of the fix onto the baselineXI,Y1 = X-Y position of the first fixX2,Y2 = X-Y position of the second fix

Bline =P 1 +P2

C 2 =Hproj 12 - P 12 = Hproj 2 2 - P 2 2 = Hproj 2 2 - (Bline - P 1)2

Hproj 12 - P 12 = Hproj22 - Bline 2 + 2P I Bline - P 12

P 1 = Hproi 12 - Hproi2 2 + Bline2

MBine

XI2 ; YI2cos0 = X12 ; sine =

Xpj =XT, + P cosO; Yp= YTI + P IsnO

XlfXp-CinO; YlfYpJ+CcosO

X2=Xpj+CsinO; Y2=Yp-CcosO

48

Y

YTI

Ple I 2

~ccos! /1Hproj2

Y2 -2 o

, \_ \

Y7 I / I\

- - -ix'j- 1 -J

I xXT, X2 XpJ Xl X

Figure D.1 Array Position Calculation. The parameters required to calculate the lateral positionof an array element. The points at FIX 1 and FIX2 give the intersection of the horizontal projec-

tion arcs for two transponders. A third transponder range is used to choose between these two po-sitions.

49

APPENDIX E. Software Listings.

The software discussed in this report is listed below. The listings contain the main program,subroutines, makefile and include files for each program discussed.

50

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SCRIPPS INSTITUTION OF OCEANOGRAPHY › dtic › tr › fulltext › u2 › a220166.pdf · 2011-05-15 · C IV MARINE PHYSICAL LABORATORY SCRIPPS INSTITUTION OF OCEANOGRAPHY San Diego,