12. 540 Principles of the Global Positioning System Lecture 22 Prof. Thomas Herring 12540Lec22 KinematiC GPS The style of gps data collection and processing suggests that one or more GPs stations is moving(e.g, car, aircraft) To obtain good results for positioning as a function of time requires that the ambiguities be fixed to integer values Track is the MIT implementation of this style of processing
05/07/03 12.540 Lec 22 1 12.540 Principles of the Global Positioning System Lecture 22 05/07/03 12.540 Lec 22 2 Kinematic GPS processing Prof. Thomas Herring • The style of GPS data collection and processing suggests that one or more GPS stations is moving (e.g., car, aircraft) • To obtain good results for positioning as a function of time requires that the ambiguities be fixed to integer values • Track is the MIT implementation of this style of 1
Styles of kinematic GPS Kinematic GPS techniques go by a number of names with features that are often receiver specific Kinematic GPS: Early term which implies that there is no loss of lock while the receiver is moving. In survey mode, if loss of ock occurs the antenna must be returned to a point of know ocation Rapid Static GPS: Technique that uses range and phase to resolve ambiguities. No need to maintain lock while receiver \is ping. Surveying where position during static portion all that RTK Real-time kinematic: Kinematic solution with real-time radio telemetry link. Analysis is done on-the-fly. Very popular now with surveyors because results know instar 12540Lec22 General aspects The success of kinematic processing depends on separation of sites The MIT software allows multiple stations to be used in the positioning(may by kinematic or static For separations 10 km, usually easy(most RTK systems work at these distances) 10>100 km more difficult but often successful Depends on quality of data and ionospheric activity >100 km very mixed results
05/07/03 12.540 Lec 22 3 Styles of kinematic GPS • Kinematic GPS techniques go by a number of names with features that are often receiver specific. of lock while the receiver is moving. In survey mode, if loss of lock occurs the antenna must be returned to a point of known location. resolve ambiguities. No need to maintain lock while receiver moving. Surveying where position during static portion all that is needed. radio telemetry link. Analysis is done on-the-fly. Very popular now with surveyors because results know instantly. – Kinematic GPS: Early term which implies that there is no loss – Rapid Static GPS: Technique that uses range and phase to – RTK Real-time kinematic: Kinematic solution with real-time 05/07/03 12.540 Lec 22 4 General aspects • The success of kinematic processing depends on separation of sites • The MIT software allows multiple stations to be used in the positioning (may by kinematic or static) • For separations 100 km more difficult but often successful. Depends on quality of data and ionospheric activity. • >100 km very mixed results 2
Issues with length As site separation increases, the differential ionospheric delays increases, atmospheric delay differences also increase making modeling of phase data more difficult For short baselines(<10 km), ionospheric delay can be treated as zero and l 1 and l2 resolved separately For longer baselines this is no longer true and ambiguities must be resolved with lc (and often the MW WL L1-L2 number of cycles) Track features Track uses the melbourne -Webena wide Lane to resolve l1-l2 and then a combination of techniques to determine l1 and l2 cycles separately For short baselines uses a search technique and floating point estimation with L1 and L2 separately For long baselines uses floating point estimate with LC and ionospheric delay constraint
05/07/03 12.540 Lec 22 5 Issues with length 05/07/03 12.540 Lec 22 6 Track features separately. separately • As site separation increases, the differential ionospheric delays increases, atmospheric delay differences also increase making modeling of phase data more difficult • For short baselines (<10 km), ionospheric delay can be treated as ~zero and L1 and L2 resolved separately • For longer baselines this is no longer true and ambiguities must be resolved with LC (and often the MW WL L1-L2 number of cycles) • Track uses the Melbourne-Webena Wide Lane to resolve L1-L2 and then a combination of techniques to determine L1 and L2 cycles • For short baselines uses a search technique and floating point estimation with L1 and L2 • For long baselines uses floating point estimate with LC and ionospheric delay constraint 3
Ambiguity resolution Basic problem is determine the integer number of cycles in the carrier phase double differences Two generic classes of approach Searching methods: T wo basic types Search over integer ambiguities checking RMS fit of phase residuals Search over position, minimizing a fit function that does not jer part of an ity(e.g. Cosine of phase Estimation and then resolution using statistical testing Statistical Resolution The most common method now is estimation with statistical assessment of fitting to integer Generic classes of cases: NNNNN.01+0.01 Pretty clearly can be resolved NNNNN. 35+0.40 Highest probability answer is nnnnn but NNNNN+1 also has >10% of being correct. Since 10-100 ambiguities need to be resolved 1-10 of them would be incorrect in the above case NNNNN.01+0.55 clearly close to an integer but+1 value also very likely NNNNN. 35+0.01 should be resolvable to integer but value is ar from integer
05/07/03 12.540 Lec 22 7 Ambiguity resolution differences. – Searching methods: Two basic types • Search over integer ambiguities checking RMS fit of phase residuals • Search over position, minimizing a fit function that does not depend on integer part of ambiguity (e.g.. Cosine of phase residuals) – Estimation and then resolution using statistical testing. • Basic problem is determine the integer number of cycles in the carrier phase double • Two generic classes of approach: 05/07/03 12.540 Lec 22 8 • The most common method now is estimation with statistical assessment of fitting to integer. • Generic classes of cases: NNNNN+1 also has >10% of being correct. Since 10-100 ambiguities need to be resolved 1-10 of them would be incorrect in the above case. very likely far from integer, Statistical Resolution – NNNNN.01±0.01 Pretty clearly can be resolved – NNNNN.35±0.40 Highest probability answer is NNNNN but – NNNNN.01±0.55 clearly close to an integer but +1 value also – NNNNN.35±0.01 should be resolvable to integer but value is 4
Statistical resolution Uncertainties of ambiguities are always uncertain Formal estimates come for inversion but these depend on data noise characteristics(most importantly correlations in data) Many kinematic surveys done with high sampling rates(0. 1-1Hz) so white noise assumptions generate very small error estimates Most testing methods use a"contrast or "ratio"style test (ie, ratio of x2 with best and next best choice of ambiguities and an impact on x2 of setting the value to n integer. Covers last case shown--no integer seems correct implying modeling errors 12540Lec22 LAMBDA Method In addition to individual values Each ambiguity that is resolved effects other estimates and thus there is a cascading effect The LAMBDa Method tries to account for these correlations by projecting the ambiguities into ar orthogonal space. Use of eigenvectors and eigenvalues discussed in earlier classes) Method is from a linear operator that preserves nteger values and transforms ambiguities so that estimates are nearly un-correlated.(Eigenvectors ould make estimates uncorrelated, by integers would be preserved)
05/07/03 12.540 Lec 22 9 Statistical resolution • Formal estimates come for inversion but these depend on data noise characteristics (most importantly correlations in data). • rates (0.1-1Hz) so white noise assumptions generate very small error estimates. • test (ie., ratio of c2 with best and next best choice of ambiguities and an impact on c2 of setting the value to an integer. Covers last case shown--no integer seems correct implying modeling errors.) Uncertainties of ambiguities are always uncertain. Many kinematic surveys done with high sampling Most testing methods use a “contrast” or “ratio” style 05/07/03 12.540 Lec 22 10 LAMBDA Method • In addition to individual values: Each ambiguity that is resolved, effects other estimates and thus there is a cascading effect. • The LAMBDA Method tries to account for these correlations by projecting the ambiguities into an orthogonal space. (Use of eigenvectors and • Method is from a linear operator that preserves estimates are nearly un-correlated. (Eigenvectors would make estimates uncorrelated, by integers would not be preserved). eigenvalues discussed in earlier classes). integer values and transforms ambiguities so that 5
Magnitudes of effects of ambiguities Basic changes in phase with ambiguities △LC f2/f1 4N,-198N MLG=-(2/f1)N1+N2=-0.78N+NM2 Notice that N1=N2=1(not detectable in the Mw Widelane)cause a change of 0.56 cycles in LC and only 0.22 cycles in LG (variations in LG can be several cycles) Combinations such as n1=3. n2=4 and n1=4 and n2=5 can cause small effects in LC (ie, geodetic fit looks good but ionospheric delay in error: if small can be detected but when large can be difficult) 12540Lec22 Cycle slip detection in kinematic processing Detecting cycles slips can be difficult in kinematic processing because of vehicle movement. Normally in static GPS, coordinates are well enough known that changes on phase(LC combination) between epochs of data can be used. If a change is larger an a tolerance level (usually 0.2 to 0.5 cycles) then a cycle slip is detected Cycle slips are resolved to integers to fitting with simply polynomials across the epoch with the jump In kinematic processing this is much more difficult because the position of the moving receiver is not
† 05/07/03 12.540 Lec 22 11 Magnitudes of effects of ambiguities • • a change of 0.56 cycles in LC and only 0.22 cycles in LG (variations in LG can be several cycles) • Combinations such as N1=3, N2=4 and N1=4 and N2=5 can cause small effects in LC (ie., geodetic fit looks good but ionospheric delay in error: if small can be detected but when large can be difficult). DLC = 1 1 - ( f2 / f1) 2 N1 - f2 / f1 1- ( f2 / f1) 2 N2 = 2.54N1 -1.98N2 DLG ( f2 / f1)N1 + N2 0.78N1 + N2 Basic changes in phase with ambiguities Notice that N1=N2=1 (not detectable in the MW Widelane) cause = - = - 05/07/03 12.540 Lec 22 12 processing • Detecting cycles slips can be difficult in kinematic processing because of vehicle movement. Normally in static GPS, coordinates are well enough known that of data can be used. If a change is larger an a tolerance level (usually 0.2 to 0.5 cycles) then a cycle slip is detected. • Cycle slips are resolved to integers to fitting with • known. Cycle slip detection in kinematic changes on phase (LC combination) between epochs simply polynomials across the epoch with the jump. In kinematic processing this is much more difficult because the position of the moving receiver is not 6
Cycle slips Large jumps can be detected by using the pseudorange position estimate. In some cases, Doppler shift is also available and can be used Small slips just a cycle or so can be difficult).More common than expected because the receivers try to fix cycle slips and they often get it wrong be a small amount(slips based in SNR) MW WL and ionospheric delay jumps are common methods but can still leave slips un-detected Basics of MiT track program Track runs using a command file The base inputs needed are Obs_file specifies names of rinex data files Sites can be k kinematic or f fixed Nav file orbit file either broadcast ephemeris file or sp3 file Mode air/short/long--Mode command is not strictly needed but it sets defaults for variety of situations
05/07/03 12.540 Lec 22 13 Cycle slips • Large jumps can be detected by using the pseudorange position estimate. In some • Small slips (just a cycle or so can be difficult). More common than expected because the receivers try to fix cycle slips and they often get it wrong be a small amount (slips based in SNR). • MW WL and ionospheric delay jumps are common methods but can still leave slips un-detected. 05/07/03 12.540 Lec 22 14 situations cases,Doppler shift is also available and can be used. Basics of MIT track program • Track runs using a command file • The base inputs needed are: • Obs_file specifies names of rinex data files. Sites can be K kinematic or F fixed • Nav_file orbit file either broadcast ephemeris file or sp3 file • Mode air/short/long -- Mode command is not strictly needed but it sets defaults for variety of 7
Basic track use Recommended to start with above commands and see how the solution looks Usage: track -f track. cmd >& track out basic quality checks grep RMs of output file Kinematic site rovr appears dynamic Coordinate RMS XYZ2834466253859.17m For 2067 Double differences: Average RMs 17.85mm 12540Lec22 Basic track use and evaluation Check on number of ambiguities fixed grep FINAL output file a 3 in column fixd means fixed. 1 means still If still non-fixed biases or atmospheric delays are estimated then smoothing solution should be made(back_type smooth) output in NEU and/or geodetic coordinates
05/07/03 12.540 Lec 22 15 Basic track use – Kinematic site rovr appears dynamic Coordinate RMS XYZ 283.44 662.53 859.17 m. – For 2067 Double differences: Average RMS 17.85 mm • Recommended to start with above commands and see how the solution looks • Usage: track -f track.cmd >&! track.out • Basic quality checks: • grep RMS of output file 05/07/03 12.540 Lec 22 16 Basic track use and evaluation – grep FINAL output file – A 3 in column Fixd means fixed, 1 means still floating • Check on number of ambiguities fixed • If still non-fixed biases or atmospheric delays are estimated then smoothing solution should be made (back_type smooth) • output in NEU and/or geodetic coordinates 8
More advanced features Track has a large help file which explains strategies for using the program, commands available and an explanation of the output and how to interpret it It is possible to read a set of ambiguities in Works by running track and extracting FINAL lines nto an ambiguity file setting 7 for the Fixd column will force fix the ambiguity ambiguity file is then read into track(a option or ambin _file 12540Lec22 Advanced features Commands allow control of how the biases are fixed and editing criteria for data Editing is tricky because on moving platform jumps in phase could simply be movement lon delay and MW WL used for editing Explicit edit_svs command allows removal of problematic data
05/07/03 12.540 Lec 22 17 More advanced features – Works by running track and extracting FINAL lines into an ambiguity file. Setting 7 for the Fixd column will force fix the ambiguity. ambiguity file is then read into track (-a option or ambin_file) 05/07/03 12.540 Lec 22 18 Advanced features • Track has a large help file which explains strategies for using the program, commands available and an explanation of the output and how to interpret it. • It is possible to read a set of ambiguities in. • Commands allow control of how the biases are fixed and editing criteria for data • Editing is tricky because on moving platform, jumps in phase could simply be movement • Ion delay and MW WL used for editing. • Explicit edit_svs command allows removal of problematic data 9
Some results EXamine results from phase processing of homework #3 Solutions with LC and with L1+L2 (less noise but larger ionospheric delay Output of processing in track.016f. out Solutions in North, East Up differential position from etab d016 NEU. rovr LC and d016fNeU. rovr L1 North evolution dN LC (m) dN L1+L2(m 2000.00 4000.00 16.80 actional Day°s
05/07/03 12.540 Lec 22 19 Some results rovr.LC and d016f.NEU.rovr.L1+L2 • Examine results from phase processing of homework #3. • Solutions with LC and with L1+L2 (less noise but larger ionospheric delay. • Output of processing in track.016f.out • Solutions in North, East Up differential position from etab: d016f.NEU. 05/07/03 12.540 Lec 22 20 North Evolution -6000.00 -4000.00 -2000.00 0.00 2000.00 4000.00 16.80 16.85 16.90 16.95 17.00 17.05 Fractional Day dN LC (m) dN L1+L2 (m) dN LC (m) 10
