The Global Positioning System (GPS) is becoming more attractive navigation means for LEO spacecraft due to the data accuracy and convenience for utilization. The anomalies such as serious variations of Dilution-Of-Precision (DOP), loss of infrequent 3-dimensional position fix, and deterioration of instantaneous accuracy of position and velocity data could be observed, which have not been appeared during the ground testing. It may cause lots of difficulty for the processing of the orbit control algorithm using the GPS data. In this paper, the characteristics of the GPS data were analyzed according to the configuration of GPS receiver such as position fix algorithm and mask angle using GPS navigation data obtained from the first Korea Multi-Purpose Satellite (KOMPSAT). The problem in orbit tracking using GPS data, including the infrequent deterioration of the accuracy, and an efficient algorithm for its countermeasures has also been introduced. The reliability and efficiency of the modified algorithm were verified by analyzing the effect of the results between algorithm simulation using KOMPSAT flight data and ground simulator

Chang, Young

Lee, Jin

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Reference No: SSC00-V-3Session: V (Lessons Learned in Success & Failure)1Young K. Chang            14th Annual/USU Conference on Small SatellitesIntroductionThe orbit control of KOMPSAT is mainly performedbased on the data that the GPS receiver provides. TheGPS receiver is capable to track the signal from GPSsatellites and achieve 3-dimensional position fix withinless than 30minutes if it was properly initialized withall the input data that are necessary1. KOMPSAT cancontinuously read out the right position of itself fromthe receiver unless any anomaly happens. Althoughranging by ground station would inevitably be involvedwhen KOMPSAT is in LEOP or emergency, there is noneed to upload additional commands for orbit controlonce if the GPS receiver acquires the position fix. Thisis a great advantage that can effectively reduce theworkload on ground and performs more precise orbitcontrol than ranging does. The GPS information showsless than 120m position errors (2s) in any missionphase and can be an effective means for a LEO earthobservation satellite like KOMPSAT2.Despite the accurate information and convenience GPSprovides, infrequent deterioration and loss of positionfix has been a burden of the ground station operationthroughout the KOMPSAT LEOP. This symptom couldimpact on the on-board ephemeris propagation ofKOMPSAT and prevent it from using GPS informationany more eventually. If the spacecraft ignores severalframes of GPS data because of their accuracy, it willlead permanent loss of GPS lock due to the nature oftracking algorithm. The intervention of ground stationwould be inevitable to avoid the accumulation ofposition errors when the loss of GPS lock happens3.The upgrade or modification of the tracking algorithmA Study on GPS Data Tracking Methodfor KOMPSAT Spacecraft Orbit ControlJin H. Lee*, Young K. Chang§* Senior Researcher, Korea Aerospace Research Institute, P.O.Box 113, Yusung, Taejon, Korea§ Principal Researcher, Korea Aerospace Research Institute & Professor, Hankuk Aviation University,P.O.Box 113, Yusung, Taejon, KoreaAbstractThe Global Positioning System (GPS) is becoming more attractive navigation means for LEO spacecraft due to thedata accuracy and convenience for utilization. The anomalies such as serious variations of Dilution-Of-Precision(DOP), loss of infrequent 3-dimensional position fix, and deterioration of instantaneous accuracy of position andvelocity data could be observed, which have not been appeared during the ground testing. It may cause lots ofdifficulty for the processing of the orbit control algorithm using the GPS data. In this paper, the characteristics of theGPS data were analyzed according to the configuration of GPS receiver such as position fix algorithm and maskangle using GPS navigation data obtained from the first Korea Multi-Purpose Satellite (KOMPSAT). The problem inorbit tracking using GPS data, including the infrequent deterioration of the accuracy, and an efficient algorithm for itscountermeasures has also been introduced. The reliability and efficiency of the modified algorithm were verified byanalyzing the effect of the results between algorithm simulation using KOMPSAT flight data and ground simulator.Young K. Chang             14th Annual/USU Conference on Small Satellites2was necessary to overcome the infrequent deteriorationof GPS information and to assure the spacecraft safety.In this paper, the characteristics of KOMPSAT GPSinformation and the problem of data tracking arediscussed in chapter 2 and chapter 3, respectively. Inchapter 4, a new tracking algorithm that effectivelyimproves the performances is presented with thesimulation results. It is followed by chapter 5-theconclusions.Characteristics of KOMPSAT GPS InformationThe KOMPSAT GPS receiver provides very accurateinformation for the time, velocity and position of thespacecraft, which are essential for the operation. Theinfrequent deterioration of the GPS information,however, had restricted its utilization so far. It may becaused by the rapid turnover of GPS satellites, whichoccurs when the spacecraft navigates more than7km/sec speed in space. The receiver sometimes couldnot secure 4 GPS satellites, which are minimumnumber of satellites for 3-dimensional position fix sothat the receiver loses its accuracy as a result. Evenwhen the receiver contacts more than 4 GPS satellites,the accuracy drops could be observed, but it depends onthe geometrical distribution of GPS satellites3. TheDOP (Dilution Of Precision) is quite useful whenevaluating the accuracy of GPS data. Figure 1 showsthe DOP distribution of KOMPSAT GPS data.Figure 1. DOP DistributionKOMPSAT downlinks the DOP status once in every 16seconds3. Although every variation of DOP numberc uld ot be tracked, the pattern is obviously worsethan that of ground test. In the KOMPSAT test, thereceiver showed quite stable DOP status that mostlystayed below 2.0. The “0” DOP means that the receiverlost 3-dimensional position fix in the Figure 11.Generally, the number of GPS satellites that can betracked is reduced near earth pole in the KOMPSATtelemetry. The accuracy drops, however, could occurregardless of the spacecraft position3. F gure 2 displaysthe number of GPS satellites tracked by the receiver incomparison with the number of visible satellites. Thecomposition of GPS satellites tracked is changed tooof n as shown in Figure 2, which impacts the stability receiver output.Figure 2. Visible & Tracked GPS satellitesIt is important to analyze the GPS data trend for thespacecraft operation when the deterioration happens.When the GPS receiver losses 3-dimensional fix, theGPS output, especially velocity portion, drifts tosomeplace else gradually. It, then, suddenly performs ajump to where it is supposed to be, when position fix isreacquired. This usually lasts for a short time – lessthan 10 seconds or little more, but the jump issometimes relatively long, with which any trackingalgorithm can hardly bear. It happens throughout theentire orbit and the occurrence is highly unpredictable.Figure 3 shows a good example of the GPS data trend,when position fix is lost.Young K. Chang             14th Annual/USU Conference on Small Satellites3Figure 3. GPS velocity output when position fix is lostThe status of the GPS receiver is one of the majorfactors that determine the accuracy of its data. It isrequired to set the receiver to minimize the frequencyof GPS satellites turnover. The items that the user canset for the GPS operation are mask angle, the type ofposition fix algorithm, DOP hysteresis and 2D DOPthreshold1. Table 1 lists the values for each item, whichwere used in the on-orbit test.Table 1 Configuration of GPS receiverItems Values usedMask Angle -5°, 0°, 10°DOP Hysteresis 1.0, 6.02D Threshold 20.0Position Fix Algorithm n-in-view, best-of-4KOMPSAT Orbit ControlThe KOMPSAT uses GPS position and velocity datafor its orbit control only when the GPS data areavailable and reliable. The simple ephemerispropagator, located in one of the KOMPSATprocessors, was involved to justify the incoming GPSinformation including XDOP, the status of 3-dimensional fix and position data. The trackingalgorithm compares the GPS position and velocity tothe values which the ephemeris propagator calculatesbased on the position and velocity data just a secondago and then uses them for orbit control if thedifferences are within the range fixed by ground station.It has been frequently observed that the output data ofthe GPS receiver were sometimes getting relativelypoor and they became less reliable as stated above.Although the period was usually short, some excessiveerrors of velocity data could cause the loss of GPS lockin the propagator during that time. Once it rejects GPSdata because of their accuracy, it is not guaranteed toregain the GPS lock, even though the GPS data fullyrecover the required accuracy. Figure 4 shows theposition and velocity differences between GPS data andthe propagator estimations, and well describes how thepropagator is losing the GPS lock when the allowableerror values are 1km for position and 2m/sec forvelocity, respectively.(a)(b)Figure 4. RSS Error of (a) Position & (b) Velocity(limit: position 1km, velocity 2m/sec)Figure 5 also displays the position and velocitydifferences when the position limit is 6km and thevelocity limit is 6m/sec, respectively. The ephemerisYoung K. Chang             14th Annual/USU Conference on Small Satellites422)(5.0)(5.0bDvbDPvpePeP--==propagator only can buy several minutes more byexpanding the acceptable range, consequently. There isnot a significant improvement even when the velocitylimit is exceptionally increased to 20m/sec or higher. Ahigher limit for velocity data can cause moredifferences in a short period due to the huge initialerrors of velocity components that have been acceptedin the moment of break off, and it will surely make thesituation worse. The analysis shows that the reliableephemeris propagation based on GPS data canno beachieved only by controlling the acceptable error limits.(a) (b)Figure 5. RSS Error of (a) Position & (b) Velocity(limit: position 6km, velocity 6m/sec)New Tracking AlgorithmIt is not possible to replace the whole flight software ofKOMPSAT since it is in operation on-orbit. The partialpatch is the only method to be considered. The patch offlight software is quite limited by the vacant memoryspace and processor throughput. Any sophisticatedtracking algorithm that has been proven on groundcannot be applied to KOMPSAT directly because of itsize and computational load. The new algorithm shouldrequire minimum modification of the flight softwareand throughput to meet the requirement in patchprocess.In this paper, a simple algorithm that associates theprobability of measurement accuracy based on aGaussian fit is proposed. The new tracking algorithmassigns the suitable weight calculated from theGaussian covariance of estimated point to eachmeasurement and updates the spacecraft position andvelocity by the measurement distances from theestimated points, which were already multiplied bytheir weights. Figure 6 shows the Gaussian fit curvethat has been used for the evaluation of the algorithm.The equations used for the Gaussian curve are listed inequation (1).ProbabilityDistanceFigure 6. Gaussian Fitting Curves used      (1)Young K. Chang             14th Annual/USU Conference on Small Satellites5where: Pp = Probability of Position DataPv = Probability of Velocity DataDp = Position Distance RSSDv = Velocity Distance RSSThe new algorithm uses the ephemeris propagator as itwas and requires minimum change of the flightsoftware – only 6 more lines of codes would be addedfor this improvement. For the purpose of evaluation, theflight software was patched to provide the GPS positionand velocity data every second and the related softwaremodules were rarely modified for the adoption of thealgorithm and ground use.(a)(b)Figure 7. RSS Error of New Algorithm Tracking(a) Position & (b) VelocityThe simulation results are pretty encouraging with theexpected performances. It has never lost the GPS lockduring the two weeks of evaluation period despite theaccuracy drops. Figure 7 displays the tracking results ofthe proposed algorithm when the GPS data are notreliable.Another simulation using the KOMPSAT simulator hasalso been done which presented no significantdisturbance in spacecraft attitude with the goodperformance of the modified propagator. There was noinflue ce to the spacecraft attitude in roll and pitchdirection when the GPS data are used. In yaw direction,high frequency jitter was induced due to the error of theGPS data as shown in Figure 8. Although the amplitudeof jitter is quite small and it is within the KOMPSATrequirement, it should be resolved when fine attitudecontrol is required.Figure 8 KOMPSAT Yaw Error when GPS is usedConclusionGPS is getting widely applied in space since it canprovide very accurate navigation solution. It, however,is quite limited in use due to infrequent deteriorationand loss of position fix. This imperfect operation couldimpact spacecraft normal operation as seen inKOMPSAT case. The possibility of anomaly should bethoroughly reviewed from the design phase for anyspace-born GPS applications.T e proposed algorithm shows good performance and it0.5 1 1.5 2 2.5 3 3.5x 104-0.08-0.06-0.04-0.0200.020.040.060.080.1time(sec)Yaw Error(swerr[2])(Deg)SW Estimated S/C Yaw Error in KOMPSIM-kpb00609Young K. Chang             14th Annual/USU Conference on Small Satellites6can be a solution when the deterioration of GPS datahappens. The simulation was done with using real flightdata, and proved the capability and reliability of thenew algorithm. The use of the GPS noise matrix,instead of a simple Gaussian curves, can be consideredfor better results.References1. Motorola, VICEROY GPS Receiver S/W ICD, 99-P30205Y-XB, 1994.2. J.H.Lee, “GPS Data Tracking Method forKOMPSAT orbit control,” KSAS proceeding, 20003. J.H.Lee, "GPS Data Use,"KOMPSAT.00.BUS.001, 20004. Navstar GPS Space Segment / Navigation UserInterfaces - Revision C, ICD-GPS-200, 19975. T.Logsdon, The Navstar Global PositioningSystem, VNR, 1992

A Study on GPS Data Tracking Method for KOMPSAT Spacecraft Orbit Control

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A Study on GPS Data Tracking Method for KOMPSAT Spacecraft Orbit Control

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