Correcting Scale, Roll and Heading in QT Modeler

8/3/2019 Correcting Scale, Roll and Heading in QT Modeler

1/27

ALTM

AIRBORNE LASER TERRAIN MAPPER

ACalibPro Analysis Tool


2/27


3/27

ALTM

AIRBORNE LASER TERRAIN MAPPER

ACalibPro Analysis Tool

Optech100 Wildcat Road

Toronto, Ontario M3J 2Z9Canada

Telephone: (01) 416-661-5904Fax: (01) 416-661-4168

Email: [email protected]: www.optech.on.ca

Document No. 0005830 Release A.0 12 Dec 02


4/27

COPYRIGHT

2002 by Optech Incorporated. All rights reserved. This item and the information contained herein are the propertyof Optech Incorporated. No part of this document may be reproduced, transmitted, transcribed, stored in a retrievalsystem, or translated into any language or any computer language in any form or by any means otherwise, without theexpress written permission of Optech Incorporated, 100 Wildcat Road, Toronto, Ontario, Canada M3J 2Z9.


5/27

Table of Contents

1 Introduction............................................................................................................................. 12 Pitch Misalignment ................................................................................................................. 1

2.1 Data Required.....................................................................................................................12.2 Preparation of the Target Data............................................................................................1

2.2.1 Editing the Target File Manually............................................................................... 2

2.2.2 Editing the Target File in ACalibPro ......................................................................... 32.3 Pitch Analysis.....................................................................................................................42.4 P/R Misalignment Parameters ............................................................................................4

2.5 Determining Pitch Correction............................................................................................. 52.6 Understanding the Log File ................................................................................................ 6

2.7 The Data View Window..................................................................................................... 82.7.1 Colored Marks ........................................................................................................... 82.7.2 Operations .................................................................................................................. 9

3 Roll Misalignment................................................................................................................. 10

3.1 Data Required................................................................................................................... 103.2 Direction of Offset Distance............................................................................................. 11

4 Elevation Analysis ................................................................................................................124.1 Data Required................................................................................................................... 12

4.2 Preparing Control Points................................................................................................... 134.3 Parameters Setting ............................................................................................................13

4.4 Understanding the Log ..................................................................................................... 135 Scale Analysis ....................................................................................................................... 14

5.1 Data Required................................................................................................................... 15

5.2 Parameter Settings ............................................................................................................155.3 Charts in Data View..........................................................................................................16

5.4 Understanding the Log ..................................................................................................... 17

6 Scan Lag Analysis................................................................................................................. 186.1 Data Needed for Scan Lag Analysis.................................................................................196.2 Understanding Charts ....................................................................................................... 196.3 Understanding the Log ..................................................................................................... 20

List of Figures

Figure 1:ALTM Profile Data...........................................................................................................2Figure 2: Target Editor ....................................................................................................................3Figure 3: P/R Misalignment Parameters Setting.............................................................................. 4

Figure 4:Profile Calibration.............................................................................................................5

Figure 5: Data View Window Showing Profile Data ...................................................................... 8Figure 6: Scans in Roll Misallignment ..........................................................................................10Figure 7: Calibrating Scan Lines ...................................................................................................11Figure 8: Open Elevation Analysis Data and Parameter Settings.................................................. 12

Figure 9: Scale Calibration Data.................................................................................................... 15Figure 10: Scale Calibration Parameters ....................................................................................... 16

Figure 11: Scale Analysis Results ................................................................................................. 16Figure 12: Scan Lag in Elevation and Range Data ........................................................................18


6/27


7/27

ACalibPro Analysis Tool Optech Incorporated

Doc. No. 0005830 Release A.0 12 Dec 02 1

1 Introduction

ACalibPro is an analysis tool used to calibrate ALTM survey data. The software can adjust lasershots by centimeters in roll, pitch, elevation, scale, and lag errors. The optimized analysis engine

provides effective and fully automatic two-pulse analysis. Using this software can significantlyimprove the accuracy of the ALTMs data end product.

The software includes analysis of the following: Pitch misalignment Roll misalignment Elevation analysis Scale calibration Lag Analysis

2 Pitch MisalignmentPitch misalignment analysis consists of survey data acquired over a pre-measured target, usually a

large building. Each time the aircraft passes over the target building, laser shots are fired in profile(i.e., non-scanning) mode. ACalibPro helps detect with greater precision the edges of the target

building, which are reported by laser shots from every pass. The software detects first- and last-

pulse laser shots respectively. It can also calculate the offset distances between the pulses and thetrue target edge in the flight direction.

2.1Data Required

Pitch misalignment analysis requires the following data:

Profiles data output by REALM in ENH (*.ASC) Target data (building coordinates)

2.2Preparing the Target Data

Optech recommends using a cubic building with a flat roof as a target. The target building must be

greater than 5 meters in height, and have at least two parallel long edges. The distance between the

two edges must be wide enough so that more than 100 laser shots can register on the rooftop (e.g., >40 m) (Figure 1).


8/27

Optech Incorporated ACalibPro Analysis Tool

2 Doc. No. 0005830 Release A.0 12 Dec 02

Figure 1:ALTM Profile Data

The corners of the target building must be measured with corresponding points in between. Theedge between two consecutive points must be a straight line. The points must be in an ASCII file,

and in a clockwise or counter-clockwise position in Easting, Northing, Height. The columns alsomust be right aligned.

2.2.1 Editing the Target File Manually

ACalibPro generates the lines used to calculate the offset distance calculation according to thepoints (rows) defined in the target file. Not every line is included in the calculation; therefore, youmust indicate which line(s) are to be used.

Any text editor may be used to open the target file. Insert the characters, "_*" (one space and one

asterisk) and a hard return after the elevation, H. ACalibPro automatically chooses the marked row(point) together with the next row to generate a line for the calculation (black color). If the nextline is unwanted, insert "_ _" (two spaces) and a hard return after the elevation, H. The program

will ignore the lines marked with the two spaces (blue color) and choose the next line. If the lastrow in the file is chosen, the program will choose the first row as the second point of the line.


9/27



2.2.2 Editing the Target File in ACalibPro

Another method for editing the target points is to use AcalibPros Edit Target function. From themain menu select File > Edit Target. The EdiTar window will appear (Figure 2).

Figure 2: Target Editor

1. Load the target file by clicking on the Import button. The left side of the interface showsthe point data of the target (X Y Z). The right side of the interface shows the shape of thetarget and the selected lines (black color). The points have to be in order in a clockwise orcounter clockwise direction.

2. Highlighting a value from the Easting (X) column in the data window will produce an X

at this point in the Target frame on the right side of the window (Figure 2). If required, thevalue may be edited in the three X Y Z windows above the data point columns.

3. Check the Select box to mark the point. Selecting the point indicates that this line(consisting of this point and the next point) will be used in the calibration as a reference

line.

Note: If the point was previously selected, then it will be deselected.

4. Click on the Set button to select that point in the window. An asterisk ( * ) is appended tothe end, and the line color changes to black.

5. If the points are out of order (check the Target frame in the right side of the data window),

highlight the misplaced point and use the Move Up and Move Down buttons to positionthe point correctly.

Use the keyboards arrow keys () to move through the points to adjust the sequence. Click onthe Insert button to add points. The sequence of points is very important because the program

generates lines by every two continuous points. Do not use the zone number with the Easting (X)value even if it appears in the elevation data.


10/27



2.3Pitch Analysis

There are three ways to begin the pitch analysis.

1. From the main menu select File > Open > P/R Misalignment (Pitch/Roll Misalignment).2. From the main menu select P/R Misalignment > Open Files.

3. Click on the P/R Misalignment icon in the tool bar.

The Open Pitch/Roll Misalignment Data window appears. First, browse to the data file to be

analyzed. Next, browse to the target file (building points). A log file will be created and written toa predefined location (i.e., a default path that can be modifed by the user). See Section 2.4 for

information on the P/R Misalignment Parameter settings.

2.4P/R Misalignment Parameters

All the parameters needed for P/R Misalignment are set in the P/R Misalignment Parameters

window (Figure 3). The parameters help the program determine which shots are the start and endof a pass over the target buildings roof.

Figure 3: P/R Misalignment Parameters Setting


11/27



Parameters

Control Distance The maximum distance the shots can be from the target before theprogram starts to analyze the data to determine the elevation break point.

Minimum shots per pass The minimum number of shots required per pass for inclusion inthe analysis.

Minimum Building Height The minimum height required for the building. Flight Height The altitude (in meters) at which the aircraft surveys the building.

Threshold on Edge The program compares every laser shot with buildings pre-measuredheight until it finds a shot within the threshold values plus or minus range. The program tags

this shot as the start or end point of a pass. The larger the threshold, the faster the programwill find a shot. The smaller the threshold, the longer the program will take to find a shot.

Threshold on Roof This value specifies which laser shot is still on the roof. The parameteris designed for a non-flat roof. Generally, it will not affect the results, but if it finishes a passin the middle of the building the values is too small; if it finishes the pass beyond the edge ofthe building the value is too large.

Constant Corrections Used These help keep track of the different system parameters usedduring calibrations. These values are not used in the softwares calculations.

Once the P/R Misalignment Parameters are entered, clickOKto begin the pitch analysis.

2.5Determining Pitch Correction

What is measured is the distance from the laser point to the edge of the building. Depending on thedirection of the trajectory ,the shots will either need to be corrected positive (i.e., with the direction

of the flight path), or negative (i.e., againstthe direction of the flight path). For example, in Figure4, the values are negative because the laser shots need to be corrected back to the edge of the

building. The arrows indicate the direction of the flight paths.

Figure 4: Profile Calibration


12/27



2.6Understanding the Log File

ACalibPro outputs the processing history and results in the console window and log file. The

location of this log file is determined at the beginning of the processing.

>Target file E:\DATA\CALIB\NEWTARGETR.TXT loaded! 27 control points include, 4 line(s) were selected.

>Data file R:\Output\02d132\Calib_TO\03402\Pitch\ND\TimeLp_0.all is loading, please wait...>Two pulse profile, 657011 shots loaded, 703015 total in the file.

>6 strips found!

The above information indicates that the target and the data files are loading. ACalibPro readsthe number of points in the file, whether it is a profile or scan and the number of strips.

>Average elv. of shots on edge T1:237.541 T2:237.260. In target file, it is 237.550. T1 was used!

The average elevation of the edge is reported. The difference between the first- and last-pulse lasershots and the target indicates the quality of the data. The recommended default value of theThreshold on Edge is 0.30 m. Increasing or decreasing this value will affect the results by

including or excluding data in the final totals.

>6 pass(es) of laser shots found on top of target building:

Pass# StepOn StepDown

__________________________________________________________________________________________________________

Strip 1: 60557.51123 - 60561.43035 Ave. elv. of shots on edges: 237.360

1. 0: 237.49 231.76 122 1: 237.26 237.30 65038 2: 237.53 237.57 103 3: 237.47 227.33 1 60558.4250 60560.4462Strip 2: 60804.73344 - 60807.42717 Ave. elv. of shots on edges: 237.290

2. 0: 237.48 227.46 76 1: 237.30 237.29 43220 2: 237.24 237.27 87 3: 237.57 231.70 1 60805.4551 60806.7743

Strip 3: 61073.94520 - 61077.84806 Ave. elv. of shots on edges: 237.3433. 0: 237.54 227.35 133 1: 237.30 237.35 65111 2: 237.57 237.60 0 3: 237.57 237.60 0 61074.9167 61076.8798

Strip 4: 61344.93046 - 61347.67588 Ave. elv. of shots on edges: 237.280



6. 0: 237.55 227.53 406 1: 237.19 237.18 46528 2: 236.97 236.96 332 3: 237.53 226.24 1 61904.5366 61905.9551

___________________________________________________________________________________________________________* 0: T1 step on 1: T2 step on 2: T2 step down 3: T1 step down pass start pass end

The software finds the first and the last laser shots of every pass over the target roof. The programcalculates the distances between the points and the edges by first- and last-pulse respectively.

The four points are determined sequentially, and the program marks the stages numerically:

0 The first column indicates when the first-pulse has stepped onto the building top. The last-

pulse may or may not have stepped onto the building. The next number indicates how manyshots it takes until both the first- and last-pulse points step onto the top of the building.1 Both the first-pulse and the last-pulse have stepped onto the top of the building. The next

number indicates how many shots hit the top of the building from the beginning to the end of

that particular pass.


13/27



2 The first-pulse is still on the top of the building and the last-pulse has stepped off. The nextnumber indicates how many shots until both shots have stepped off the building.

3 Both the first and last-pulse points have stepped off the building.

The last two values are the start and end times of the passes. The times are displayed as GPS time.

>Calibration distances calculated by every pass (m):

First-pulse Value Last-pulse Value Middle Value

Pass # StepOn StepDwn StepOn StepDwn StepOn StepDwn

______ _____________ _____________ _____________

1. 0.355 0.060 0.092 0.274 0.224 0.166

2. 0.121 -0.122 -0.097 0.131 0.012 0.004

3. 0.329 0.030 0.056 0.030 0.193 0.0304. 0.150 -0.126 -0.087 0.125 0.031 -0.000

5. 1.062# -0.390 -0.089 0.577 0.487 0.0936. 0.760 -0.435 -0.238 0.514 0.261 0.039

______ _____________ _____________ _____________

Min: -0.435 -0.238 -0.000Max: 0.760 0.577 0.487

StDev: 0.343 0.245 0.146

RMS: 0.350 0.268 0.194Count: 11 12 12

Ave: 0.067 0.107 0.128

Accur 0.329 63.6% 0.131 66.7% 0.166 66.7%

______ __________________________________ _____________

Ave T1 & T2: 0.088 Ave: 0.128

Count: 23 Count: 12_____________ _____________ _____________

First-pulse Value Last-pulse Value Middle Value

This section gives the object points found in the passes, and calculates distances by points, thenprovides the corresponding statistics. Averaging T1 and T2 will indicate an accurate distance.Values marked by the character # will be discarded by statistics because (a) the values are greaterthan the specified control distance; (b) the laser shot has connected with an edge, which has not

been selected as a reference line.

-- Parameters Set --Control Distance: 1.000 Min shots per pass: 100

Min Building Height: 3.000 Threshold on Edge: 0.800 Threshold on Roof: 3.000

Parameter settings set at the beginning of ACalibPro used for the processing.

-- Constant Corrections Used --Pitch: Roll: Heading: Scale:

Offset1: Offset2: Offset:

End time: 15:33:25, 06/25/2002

Constant corrections are used for tracking purposes only.


14/27



2.7The Data View Window

The processed data and calibration target are depicted in the Data View window (Figure 5).

Figure 5: Data View Window Showing Profile Data

2.7.1Colored Marks

Target building: Black lines are the edges selected for use in the calculations. Light bluelines are edges not used.

Red points:Laser shots detected on the building roof. Blue points: Laser shots detected on the ground. Black points: First-pulse laser shots on the building roof only. Red rectangle: Start or end laser shots of a pass over the roof detected by first-pulse. Red line: Distance, direction and crossing point from the red rectangle point to the edge. Green rectangle: The start or end shot of a pass over the roof detected by last-pulse.


15/27



Green line: The distance, direction and crossing point from the green rectangle point to theedge.

Blue circle: The middle point, a point created by the program between the first and last-pulseshots.

Blue Line: The distance, direction and crossing point from the middle point to the edge. Shallow blue rectangle and line: A shot found touching the unselected edge; shot ignored. Gray rectangle and number: The start point of a strip and the strip number. Gray circle: The end of a strip.

2.7.2 Operations

Zoom In:Left click in the Data View window. Zoom Out:Ctl + left click. Zoom to Default Size:Shift + click. Now the default size is 2000:1. Hide/Show Strips:Click the start or end mark of a strip.


16/27



3 Roll Misalignment

Like pitch analysis, there are three ways to start roll misalignment analysis:1. From the main menu select File > Open > P/R Misalignment.

2. From the main menu select P/R Misalignment > Open Files.

3. Click on the P/R Misalignment icon in the tool bar.

The same parameters are also used. ACalibPro will output the result of scans in the console and thedata view window (Figure 6).

Figure 6: Scans in Roll Misalignment

3.1Data Required

Roll misalignment analysis requires the following data:

Scans data output by REALM in ENH (*.ASC). Target data (building coordinates)


17/27



3.2Direction of Offset Distance

In the scans analysis the data are made up of scan lines that move from right to left andleft to right.

If the scan is from right to left, the correction is negative. If the scan is from left to right, the

correction is positive. Figure 7 depicts scan lines over a building rooftop with the flight directionindicated. Scan line A is moving right to left. Therefore, the correction needed is negative. Scanline B is moving left to right so the corresponding correction would be positive. Scan line C is thenmoving right to left, and would require a negative correction. These corrections would continue

this pattern for all scan lines on the building top.

Figure 7: Calibrating Scan Lines


18/27



4 Elevation Analysis

Like roll misalignment analysis, there are three ways to start the elevation analysis:1. From the main menu select File > Open > Elevation Analysis.

2. From the main menu select Elevation Analysis > Open Files.

3. Click on the Elevation Analysis icon in the tool bar.

After entering the parameters click on OK(Figure 8). The analysis compares the laser shots withsome pre-measured points from a large, flat, hard surface (usually a runway). The differences

found between the laser shots and the control points are used to calibrate and correct parameterssuch as TIM corrections, roll, scale and lag.

Figure 8: Open Elevation Analysis Data and Parameter Settings

4.1Data Required

Elevation analysis requires the following data:

Elevation data (*.ASC) in ENH from REALM. Control points (runway data)


19/27



4.2Preparing the Control Points

All the control points must be in the format of Easting, Northing, Height (without zone number).Every column must be right-aligned.

4.3Parameters Setting

Browse to the appropriate data and control files (Figure 8).

Search Settings

Radius The radius of laser shots surrounding a control point that is compared with that

control point. Vertical Control Distance If a laser shot is within the radius of a search area but the

vertical distance between it and the control point is greater than the size of the radius, it willnot be compared.

Max Radius in Pre-search A pre-search function for reducing the size of data sets andnumber of comparisons.

Constant Corrections Used

These parameters are not involved in the calculations.

4.4Understanding the Log File

The log file begins with stating what files have been loaded, the total number of shots and thenumber of strips. In addition, the search radius used in the statistical analysis is given.

>Control file E:\SCALETEST\OUTPUT\RUNCONTROLENZ.PTS, 2087 control points loaded!

>Data file E:\Scaletest\output\South 3.asc is loading, please wait... (REALM3.03 format)

>Two pulse (REALM 3.03 #011025)scan, 20174 shots loaded, 35893 total in the file.

>1 strip found!

>Statistic Results: Search Radius=0.500

The next section shows the processed laser points and the statistical results that the software hasdetermined. The results indicate that TIM 1 is .076m away from the ground control aftercomparison. TIM 2 is .133m away from the ground control. Therefore, the corrections to the TIMvalues would be the original values used in the processing plus these respective values. Makingthis correction in the REALM software and re-testing should produce a lower average for T1-Ctrl

and T2-Ctrl.


20/27



-- Elevation (m) --

T1 T1-Ctrl T2 T2-Ctrl T1-T2

98.740 0.041 98.790 0.091 -0.05099.140 0.090 99.200 0.150 -0.060

.

.

.

98.990 0.095 99.040 0.145 -0.050

____________________________________________________________________________Min 97.370 -0.246 97.430 -0.186 -0.100

Max 100.560 0.322 100.620 0.372 -0.040

Stdev 0.993 0.127 0.995 0.128 0.012RMS 99.028 0.148 99.086 0.185 0.058

Count 47 47 47 47 47

Ave 99.023 0.076 99.081 0.133 -0.057

Accur 1.183 66.0% 0.158 68.1% 1.189 68.1% 0.187 68.1% 0.060 76.6%

0.150 59.6% 0.150 48.9%

The columns processed include:

T1:

T1-Ctr

l:

T2:

T2-Ctr

l:

T1-T2:

Elevationfrom

TIM1.

Elevationdifference

betweenTIM1and

control.

Elevationfrom

TIM2.

Elevationdifference

betweenTIM2and

control.

Elevationdifference

betweenTIM1and

TIM2.

5 Scale AnalysisScale Analysis is based on the results of the runway analysis. It calculates all the differences

between the nadir and the laser shots. The program evaluates how much scale can be eliminatedfrom these differences. The software determines the difference in the exact scan angle of TIM1 and

TIM2 respectively. The output shows an approximate calibration result chart by the average scaleof TIM1 (Figure 11).


21/27



Figure 9: Scale Calibration Data

5.1Data Required

Scale analysis requires the following data (Figure 9):

Elevation Data (laser data) Control Points (surveyed data points) Laser Data (*.LSR) Time Tag (*.TAG)

The flight must be perpendicular to the control area (runway). One strip is recommended for each

processing session.

5.2Parameter Settings

Like elevation analysis, there are three ways to start the scale analysis:1. From the main menu select File > Open > Scale Analysis.2. From the main menu select Elevation Analysis > Open Files.

3. Click on the Scale Analysis icon in the tool bar.

At the Open Scale Calibration Data window (Figure 9), click on the Parameters button to open

the Elevation Analysis Parameters window (Figure 10). The Search Settings field remains thesame as for the Elevation Analysis. In the Constant Corrections Used field the scale value thatwas used in processing must be typed into the Scale box. If it is not, then the program assumes adefault scale value of 1.000. If the processing occurs with the default scale set to 1.00 then theresulting average scale must be corrected. This value is based on 1.000, and must be multiplied by

the scale value used in processing. This product will be the new scale value.


22/27



Figure 10: Scale Calibration Parameters

5.3Charts in Data View

The charts, arranged by scan angle, show the visible results of how the scales affect the data (Figure11). The first chart, T1-Ctr, Scale=1.00000, shows the elevation differences between TIM1 andcontrol points by every scan angle. The second chart, T1-Ctr Calibrated, Scale=0.999146, shows

the new elevation difference after calibration by the new scale.

Figure 11: Scale Analysis Results


23/27



5.4Understanding the Log

Once ACalibPro has finished processing the data, an average scale is determined. This value is

used to reprocess the data in the REALM processing software. However, if the processing occurswith the default scale set to 1.00, then the resulting average scale must be corrected. This value is

based on 1.000 and must be multiplied by the scale value used for processing. In this example, theoriginal scale was 1.0096, and the T1-Ctr Calibrated Scale = 0.999146. The resulting product is1.008737, which is the new scale.

Scan# Angle T1-Ctr. T2-Ctr. Scale1 Changed Height Range Scale2 Changed Height Range

94 -19.0818 -0.130 -0.080 1.01122 0.21566 1150.307 1217.190 1.01137 0.23567 1150.307 1217.690120 -18.8278 -0.103 -0.053 1.01107 0.18867 1149.220 1214.190 1.01122 0.20867 1149.220 1214.690

151 -18.5250 -0.135 -0.095 1.01132 0.22066 1150.601 1213.480 1.01155 0.25066 1150.601 1213.980

254 -17.5189 0.001 0.051 1.01038 0.08466 1149.401 1205.310 1.01046 0.10467 1149.401 1205.800

279 -17.2747 -0.047 0.013 1.01084 0.13266 1149.428 1203.730 1.01093 0.14267 1149.428 1204.220

329 -16.7863 0.031 0.071 1.01021 0.05466 1148.220 1199.330 1.01049 0.08467 1148.220 1199.830

.

.

.

4092 19.9707 0.135 0.095 1.0096# -0.17666 1075.858 1144.620 1.0096# -0.12678 1075.858 1144.160---------------------------------------------

Ave Scale: 1.008737

The listed columns are:

Sca

n#

Angle

T1-

Ctr

T2-

Ctr

Sca

le1

Chan

ged

Height

Ran

ge

Sca

le2

Chan

ged

Height

Ran

ge

Reportedscansteps

Reportedangleforthelasershot

TIM1elevationminustheControl

point

TIM2elevationminustheControl

point

TIM1scalevaluecalculatedforthe

lasershotatthereportedangle

TheTIM1adjustmentvalue,value

nee

dedtobringthelasershottothe

elevationreportedatnadir

Theapproximatedistancebetweenthe

airplaneandgroundforTIM1

TIM1rangereported

TIM2scalevaluecalculatedforthe

lasershotatthereportedangle

TheT

IM2adjustmentvalue;thevalue

nee

dedtobringthelasershottothe

elevationreportedatnadir

Theapproximatedistancebetweenthe

airplaneandgroundforTIM2

TIM2rangereported


24/27



6 Scan Lag Analysis

ALTM models operating at 50 kHz can exceed the electronic circuitrys capacity to accurately readthe associated scan angle of a particular laser shot before the next laser shot is returned.

The software analyzes the laser shots on opposing forward and backward scans, and determines ifany scan lag exists in the elevation and range data (Figure 12). The product of the analysis is a timecorrection used to determine the angle of the laser shot. The timing correction is in microseconds.

This correction will allow the correct angle to be associated with the appropriate laser shot.This correction is applied to the ALTM processed data. Although scan lag is apparent in ALTMmodels operating at 50 kHz, it can occur in any ALTM.

Figure 12: Scan Lag in Elevation and Range Data

After the scale calibration, go to the main menu and select Scale Analysis > Scan to Scan

Analysis. (Or click on the Scan Analysis icon from the tool bar). Two options follow:

1. By All Data

2. By Data Within Polygon

By All Data is the square polygon around the area containing the runway. By Data Within

Polygon refers only to those points that fall over the runway. The program will present the datalist, elevation charts, and range and intensity differences by opposing forward and backward scans.The information is presented in both the console and the data view window.


25/27



6.1Data Required for Scan Lag Analysis

Scan lag analysis requires the following data:

Elevation Data (output from REALM Software) Control points (control points of the runway) Range Data (*.LSR) (file location determined from the tape decode report) Time Tag (*.TAG) (file location determined from the tape decode report)

The flight perpendicular to the control area (runway) is recommended.

6.2Understanding Charts

The program classifies the analysis results by scan angle, and shows the differences by angle in thecharts. The differences include elevation, range, intensity, TIM1 and TIM2 respectively; however,

it only shows the TIM1 results (Figure 12).

T1 Elv. Dif. Between Forward and Backward Scan The dots represent the averageelevation difference between two scans in a certain scan angle between -20 and +20 degrees.All the dots should be around zero. If they are distributed along a sloped line crossing zero

area, then the scan lag was present in the elevation data. T1 Range Dif. Between Forward and Backward Scan The dots represent the average

range difference between two scans in a certain scan angle between -20 and +20 degrees. All

the dots should be around zero. If they are distributed along a sloped line crossing the zeroarea, then the scan lag was present in the system hardware.

Intensity1 Dif. Between Forward and Backward Scan All the dots should be around the

zero area.


26/27



6.3Understanding the Log

>Difference calculated by forward scan and backward scan

Scan# Elv1-Elv1 TIM1-TIM1 Elv2-Elv2 TIM2-TIM2 Int1-Int1 Int2-Int24 0.02 -0.00 0.02 -0.00 -0.29 -0.29

5 0.02 -0.01 0.02 -0.01 -1.12 -1.126 0.04 -0.04 0.04 -0.04 0.03 0.03

7 0.03 -0.02 0.03 -0.02 -1.02 -1.02

8 0.06 -0.05 0.06 -0.05 0.27 0.279 0.05 -0.03 0.06 -0.04 -0.28 -0.28

10 0.08 -0.06 0.08 -0.06 -1.89 -1.89

11 0.04 -0.02 0.05 -0.03 -0.05 -0.05.

.

.4089 -0.03 0.03 -0.03 0.03 -1.41 -1.41

4090 -0.03 0.02 -0.02 0.01 -0.72 -0.72

The listed columns are:

Scan#

Elv1-Elv1

TIM1-TIM1

Elv2-Elv2

TIM2-TIM2

Int1-Int1

Int2-Int2

Reportedscansteps

Elevation1difference

be

tweenforwardand

ba

ckwardscan

Range1difference

be

tweenforwardand

ba

ckwardscan

Elevation2difference

be

tweenforwardand

ba

ckwardscan

Range2difference

be

tweenforwardand

ba

ckwardscan

In

tensity1difference

be

tweenforwardand

ba

ckwardscan

In

tensity2difference

be

tweenforwardand

ba

ckwardscan

- Scan Lag Stats -

By Elevation 1 Ave Min Max StDev RMS CountAngle (deg): -0.00078 -0.01998 0.01998 0.00842 0.00846 5639Time (s): 0.2983 -7.4903 7.6658 2.8715 2.8870 5637

By Range 1 Ave Min Max StDev RMS CountAngle (deg): -0.00038 -0.02000 0.02000 0.01010 0.01011 4607Time (s): 0.1163 -7.6273 7.4966 3.4302 3.4322 4605

Two types of scan lag are analyzed in the program:

1. Angle in degree2. Time in microseconds for both Elevation1 and Range1.

Usually, the average values are applied to re-adjust the laser shots in REALM; but in scan lagcalibration, because the adjustment is very small and is greatly affected by noise, the average values


27/27


calculated cannot be used. To decide the accurate value of the scan lag, the standard deviation andRMS are considered. Because the standard deviation and RMS are always positive, they arecompared with the average to determine the sign for the correction value. This correction value isthen applied in REALM, and re-tested in ACalibPro. The correction in this case is 3 s.

Documents

Correcting Scale, Roll and Heading in QT Modeler