1 Geodetic and Geoinformation Science Geometric Approach to Position Determination in Space: Advantages and Limitations Dorota Grejner-Brzezinska and Tae-Suk Bae Department of Civil and Environmental Engineering and Geodetic Science The Ohio State University and Jay Kwon Department of Earth Sciences Sejong University Korea OSU ION Annual Meeting 2002 Albuquerque, June 24-26 2002
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Geodetic and Geoinformation Science
Geometric Approach to Position Determination in Space: Advantages
and LimitationsDorota Grejner-Brzezinska and Tae-Suk Bae
Department of Civil and Environmental Engineering
and Geodetic Science
The Ohio State University
and
Jay Kwon
Department of Earth Sciences
Sejong UniversityKorea
OSU
ION Annual Meeting 2002
Albuquerque, June 24-26 2002
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Geodetic and Geoinformation Science
OutlineOutline
Kinematic POD with triple differencesKinematic POD with triple differences
Data screening (CS detection) Data screening (CS detection)
Orbit smoothing Orbit smoothing
Achievable accuracyAchievable accuracy
SummarySummary
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Geodetic and Geoinformation Science
Kinematic PODKinematic POD
AdvantagesAdvantagesNo force model error affects the solutionNo force model error affects the solution
Fast (potential for nearFast (potential for near--real time)real time)
Quality solution for good PDOPQuality solution for good PDOP
DisadvantagesDisadvantagesNo dynamics to compensate for weak geometryNo dynamics to compensate for weak geometry
No solution or weak solution for weak geometryNo solution or weak solution for weak geometry
Requires correct coordinates for a starting epoch Requires correct coordinates for a starting epoch (forward solution only)(forward solution only)
OSU software GODIVA (1995): triple difference OSU software GODIVA (1995): triple difference approach to GPS POD approach to GPS POD
OSU software OSU software PP--KODKOD ((PPrecision recision KKinematic inematic OOrbit rbit DDetermination)etermination)
Extension of GODIVA to handle LEO (Low Earth Extension of GODIVA to handle LEO (Low Earth Orbiter) POD in kinematic mode (2001)Orbiter) POD in kinematic mode (2001)
UTX (Byun, S. H., 1998) LEO kinematic PODUTX (Byun, S. H., 1998) LEO kinematic POD
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Triple Difference PODTriple Difference POD
Primary advantage: fast, no ambiguity fixing Primary advantage: fast, no ambiguity fixing
diagonal variancediagonal variance--covariance matrix)covariance matrix)Cholesky decomposition and decorrelation schemeCholesky decomposition and decorrelation scheme
Requires good approximated orbit to detect CS Requires good approximated orbit to detect CS
(large residuals)(large residuals)
Equivalent to double difference with float Equivalent to double difference with float
ambiguitiesambiguities
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Geodetic and Geoinformation Science
PP--KOD Data Processing: CHAMP KOD Data Processing: CHAMP
24 hour data sets processed24 hour data sets processed65 IGS tracking stations65 IGS tracking stations
3030--s data sampling rates data sampling rate
Elevation cut off Elevation cut off angle angle 00ºº (CHAMP) and 10(CHAMP) and 10ºº ((stationsstations))
CS detection based on initial SNR prescreening, and CS detection based on initial SNR prescreening, and triple difference residual analysistriple difference residual analysis
NNormal matrix is accumulated until a singularity ormal matrix is accumulated until a singularity point is reached (too few observations or bad point is reached (too few observations or bad geometry)geometry)
Initial epoch released (forward/backward filter)Initial epoch released (forward/backward filter)
LEO orbit interpolation between epochs of observation
IGS reference stations and GPS orbit data or OSU
GODIVA
Station clock error estimation
LEO observation data
Construct triple phase differences
LEO POD Main Procedure
A priori values for LEO and station coordinates
Normal matrix
Reduce normal matrix
Solution and update of the a priori values
Data prescreeningSNR analysis
Binary local data base
Cycle slips detection
Forward/Backward Solution
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Geodetic and Geoinformation Science
Example ResultsExample Results
Good orbit approximation available to clean Good orbit approximation available to clean (remove) CS as large triple difference residuals(remove) CS as large triple difference residuals
One iteration allows for convergenceOne iteration allows for convergence
Forward filtering (batch least squares)Forward filtering (batch least squares)
Backward filteringBackward filtering
Average percentage of CS in the dataAverage percentage of CS in the dataCHAMP: 5CHAMP: 5--66
Tracking stations: <0.5Tracking stations: <0.5
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Geodetic and Geoinformation Science
873873# of Epochs with C/S# of Epochs with C/S
28802880Total # of EpochsTotal # of Epochs
20072007# of Epochs with no C/S# of Epochs with no C/S
Distribution of Cycle Slips: 24 h Data Set, June 15, 2001DistributionDistribution of Cycle Slips: of Cycle Slips:
24 h Data Set, 24 h Data Set, June 15, 2001June 15, 2001
269 (0.2%)269 (0.2%)9226 (5.5%)9226 (5.5%)
Total = 9495 (5.7%)Total = 9495 (5.7%)
Number of Number of C/SC/S
3% of all CS3% of all CS97% of all CS97% of all CS
Stations (65)Stations (65)CHAMPCHAMP
166495166495Total No. of Total No. of ObservationsObservations
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Geodetic and Geoinformation Science
Number of Satellites and
GDOP per Epoch
Number of Number of Satellites and Satellites and
GDOP per EpochGDOP per Epoch
Observations and Baselines per
Epoch
Observations and Observations and Baselines per Baselines per
EpochEpoch
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Geodetic and Geoinformation Science
Examples of Weak GeometryExamples of Weak GeometryExamples of Weak Geometry
CS caused by low SNR due to bad ionospheric CS caused by low SNR due to bad ionospheric conditions, multipath, high receiver dynamics or conditions, multipath, high receiver dynamics or low elevation anglelow elevation angle
Raw signal strengthRaw signal strength
3 types of SNR3 types of SNR-- S1, S2 : L1, L2 phase observationsS1, S2 : L1, L2 phase observations
-- SA : SNR for C/A channel (CHAMP ext.)SA : SNR for C/A channel (CHAMP ext.)
Related to the elevation angleRelated to the elevation angle
Cycle Slip Detection Using SNR: CHAMPCycle Slip Detection Using SNR: CHAMP
S A S 2 T D S N R 0 – 5 d e g
5 – 1 0 d e g
1 0 – 1 5 d e g
1 5 – 2 0 d e g
O O 8 0 2 9 0 0 O X 1 0 1 8 1 1 1 2 0 2 2 X O 1 7 2 1 0 0 O O 8 3 3 0 0 0 O X 7 1 7 1 1 1 2 1 2 2 X O 1 9 2 5 0 0 O O 8 4 3 1 0 0 O X 6 1 6 1 1 1 2 2 2 2 X O 2 0 2 7 0 0 O O 8 6 3 1 0 0 O X 4 1 6 1 1 1 2 3 2 2 X O 2 0 2 8 0 0 O O 8 7 3 2 0 0 O X 3 1 5 1 1 1 2 4 2 2 X O 2 3 2 8 0 0 O O 8 7 3 3 0 0 O X 3 1 4 1 1 1 2 5 2 2 X O 2 8 2 9 0 0
X X –– no C/S no C/S
0 0 –– C/SC/S
Total of 29 satellites tested over 500 epochsTotal of 29 satellites tested over 500 epochs
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Geodetic and Geoinformation Science
Cycle Slip Detection Using SNR: CHAMPCycle Slip Detection Using SNR: CHAMP
Number of C/S in TD residuals: 1657Number of C/S in TD residuals: 1657
Total number of TD: 34374Total number of TD: 34374
500 epochs tested, all PRNs included500 epochs tested, all PRNs included
Direct Form II TransposedRMSx = 0.092 mRMSy = 0.220 mRMSz = 0.154 mRMS3D = 0.283 m
Orbit SmoothingOrbit Smoothing
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Geodetic and Geoinformation Science
SummarySummary
Kinematic triple difference POD works well for good Kinematic triple difference POD works well for good geometrygeometryShort processing time (less than Short processing time (less than 2 h, forward and 2 h, forward and backward, on 1.8 GHz Pentium processorbackward, on 1.8 GHz Pentium processor ))Problems with weak geometry Problems with weak geometry CS cleaning is not easy (high dynamics, LEO in the CS cleaning is not easy (high dynamics, LEO in the middle of the ionospheric layer)middle of the ionospheric layer)
SNR plus orbit smoothing give promising resultsSNR plus orbit smoothing give promising resultsMore work needs to be done on SNR threshold selectionMore work needs to be done on SNR threshold selection
Gaps in the solution Gaps in the solution –– reduced dynamics needed for reduced dynamics needed for orbit continuity and balance between geometry and orbit continuity and balance between geometry and force model force model
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Geodetic and Geoinformation Science
ACKNOWLEDGEMENTSACKNOWLEDGEMENTS
This project is supported by a NASA This project is supported by a NASA Goddard Space Flight Center NIP Award Goddard Space Flight Center NIP Award OSU project number OSU project number 740809.740809.