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124-27 March 1992 Scientific Paper4. TITLE AND SUBTITLE S. FUNDING NUMBERS k " jr

Azimuth Determination Using GPS Short BaselineCarrier Wave Interferometry

6. AUTHOR(S)

Mahlon C. Hawker

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U.S. Army Topographic Engineering Center REPORT NUMBER

ATTN: CETEC-LOFort Belvoir, VA 22060-5546 R-172

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13. ABSTRACT (Maximum 200 words)

The U.S. Army Topographic Engineering Center (TEC) is involved in

research regarding the determination of azimuth by measurement of the carrierphase of GPS satellite signals between two or more antennas. TEC awarded three

first generation contracts to develop a GPS Azimuth Determining System (GPS ADS).Presented is a discussion of the three approaches, and test data from limited

Government tests of the original systems. DTIC

"he1199Z U14. SUBJECT TERMS 15. NUMBER OF PAGES

Azimuth determination 10GPS satellite signals 16. PRICE CODE

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AZIMUTH DETERMINATION USING GPS SHORT-BASELINE CARRIER WAVE INTERFEROXETRY

Mahlon C. HawkerPhysicist

U.S. Army Topographic Engineering Center* FortBelvoir. Virginia 22060-.5546

"Telephone (703) 355-2799

ABSTRACT.

The U.S. Army Topographic Engineering Center (TEC) is involved inresearch regarding the determination of azimuth by measurement ofthe carrier phase of GPS satellite signals between two or moreantennas. TEC awarded three first generation contracts to developa GPS Azimuth Determining System (GPS ADS). Presented is adiscussion of the three approaches, and test data from limitedGovernment tests of the original systems.

INTRODUCTION

Since the early 1960's, the TEC has been investigating and usingsatellites to determine position. Most recently, TEC has beenstudying the Global Positioning System (GPS) and it's applicationto tactical operations. The use of GPS broadcast signals todetermine azimuth was part of these studies. These studies showedthat azimuths could be accurately determined using the standard GPSbroadcast information.

In May 1989, TEC awarded three contracts for the design anddelivery of technology demonstration models of azimuth determiningsystems utilizing modified commercial GPS positioning receivers.The intent of the program was to demonstrate the feasibility ofdetermining azimuths of an accuracy of 0.5 to 3 mils, in real time,utilizing GPS receivers in a portable field equipmentconfiguration. Contracts were awarded to: Magnavox AdvancedProducts and Systems, Torrance, California; Texas Instruments (TI),Plano, Texas; and Adroit Systems, Alexandria, Virginia. The threetechnology breadboard units were successfully demonstrated to theU.S. Army Field Artillery School on September 27, 1990.

DESCRIPTIONS OF THE SYSTEMS

The TEC approach was to utilize Li carrier phase short baselineinterferometry to determine an angle between the antenna baselineand each of three or more satellites. The phase difference L COS0 is the sum of N integral phase cycles, each cycle determined bythe Li carrier wavelength of 19 cm and a residual phase of lessthan 3600. The integer number N must be known in order to resolveangle ambiguities, the solution to this determination being themajor difference among the three approaches. Solving three or more

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simultaneous equations, one for each satellite used in the azimuthdetermination, gives the orientation of the antenna baseline withrespect to the satellites and thus can be referred to the earth'ssurface. The approach selected by Adroit Systems, Alexandria,Virginia utilized a dual baseline antenna.

The long baseline (0;85 meter) antenna pair resolves the measuredphase into precise multiple solutions.. Each quadrant (9b0) hasmultiple solutions which exactly fit the residual phasemeasurements. Making simultaneous measurements to multiplesatellites increases the total number of solutions. Only onesolution is correct, but the residual differences *are so similarthat selection of one correct solution is very time-consuming anderror-prone involving many iterations and the determination of thelowest residual solution.

Adroit's solution was to use a very short baseline (0.14 meter)antenna pair that resolves an angle to only h to 1 degree. Thiscoarse angle determination is not accurate enough to satisfy theaccuracy requirements of the users, but it is more than adequate todetermine which of the multiple solutions is correct. The shortbaseline has only two ambiguous solutions which are so far removedfrom the connect solution as to be inconsequential.

The Magnavox Marine Systems division approach used a dual frequency(Li and L2) "wide-lane" ambiguity solution method. Subtracting theL2 signal from the Li gives an effective wavelength of 86 cm. Aswith the Adroit approach, the accuracy of the 86 cm. angledetermination does not meet the required specifications, but theambiguous solutions are sufficiently far apart that the correcthigh resolution (Li) solution can be selected.

Texas Instruments utilized a rather straight forward approach,using a two antenna single frequency (Li) 1 meter baseline, anddepending on mathematical computations to resolve the correctsolutions. The TI-420 receiver used incorporated a two stateKalman filter, the only system to do so.

RESULTS OF TESTING

All three systems were demonstrated to the U.S. Army FieldArtillery School, Fort Sill, Oklahoma in September 1990. Theresults (See Figure 1) were obtained during an informaldemonstration by contractor personnel using their own measuringtechniques, which are different for each system. The Adroit datais unique in that it was collected over a period of 1h hours, asopposed to the approximately 15 minute data collection periods of 3r

the other two. It should be noted that all subsequent tests of thesystems at Fort Belvoir, Virginia, by TEC showed results consistent 0with the Fort Sill, Oklahoma data when using the same techniques. 0

Figure 2 shows the results of the three systems located parallel toeach other and separated by three feet. They were not boresighted .to a common azimuth, as the intent was to determine their responses./

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over a period of time. This figure is a graphic example of thelarge differences in the response of the three technologies to what.is essentially the same input. While the ultimate output of eachsystem is a digital azimuth display, the plots of these azimuthvalues bear little resemblance to each other over an extendedperiod of time.

Multipath tests using these systems are. not considered to beeffective. The greatly varying responses to what is assumed to beeffectively the same stimuli negates any real attempt at using onesystem as a standard "reference" unit under benign, no multipathconditions (if this is possible),. and introducing deliberatemultipath conditions to the. second instrument. Only two identicalmatched and calibrated ADS units should be used to try to determinethe effects of multipath at a particular location. We do not havethis capability at this time.

The Adroit antennas were removed from their mount and attached toa roof top fixture which allows the long baseline to be spaced0-842 meter, 1.498 meters, and 2.216 meters. Figures 3, 4, and 5show one half hour of 0.842 meter data, recorded every 10 secondson four different days. Three days are consecutive and all timesare referred to the same sidereal time. The July 31 through August2 data show excellent correlation. It should be noted that themajority of the descenders, momentary large negative excursions inthe data average, occurred on August 2. These momentary excursionsare a result of measurement and computation errors attributed tothe hardware and should be disregarded. The exact mechanismsinducing these errors have not been determined.

Figure 6 data shows the effect of rotating the mounting structure900 CCW. The time corresponds to the August 1 and 2 1.498 metersbaseline data. Some of our data indicates the presence of a one-hour cyclic variation in calculated azimuths with peak-to-peakvariations of nominally 10 mils to as much as 20 mils.

CONCLUSION

TEC has successfully demonstrated the ability to measure azimuthsusing short baseline interferometry with GPS carrier phasemeasurements. The initial proof-of-concept brassboarddemonstration units did not achieve all design and per~ormancegoals but did identify several problem areas that requireaddressing in our follow-on work. The ability of our crude firstgeneration system to achieve an 8-mil probable error accuracyactually exceeded our initial expectations.

TEC has prepared specifications for a second generation ADS whosedesign will specifically address those problem areas presentlyidentified as major accuracy degraders. The U.S. Army FieldArtillery School, the Multiple Launch Rocket System (MLRS) andTrailblazer are actively involved in setting the requirements. Itis anticipated that the second generation ADS will be sufficientlyperfected so as to lead directly into full scale development for

several Army weapon systems.

The Adroit system has been modified by the manufacturer to add acarrier phase logging capability and some software enhancements.An external 386 laptop computer is now used as a display and datalogging facility. New antennas incorporating improvedenvironmental shielding have been added.

A request for proposals has been issued for two second generationADS procurements. Specifications have been tightened to the 0.4 to1.0 mil range to meet published requirements for MLRS and otherartillery systems.

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