Precise Orbit Determination for Alos and the Accuracy Verification by Slr

14 JSTS Vol. 23, NO. 1

PRECISE ORBIT DETERMINATION FOR ALOS AND THE ACCURACY VERIFICATION BY SLR

Ryo NAKAMURA, Shinichi NAKAMURA, and Nobuo KUDO Japan Aerospace Exploration Agency (JAXA) 2-1-1 Sengen, Tsukuba, lbaraki, 305-8505, JAPAN Phone: +81-29-868-2616, FAX: +81-29-868-2990 E-mail: nakamura.ryoh@jaxa.jp Abstract The Advanced Land Observing Satellite (ALOS) has been developed to contribute to the fields of mapping, precise regional land coverage observation, disaster monitoring, and resource surveying. Because the mounted sensors need high geometrical accuracy, precise orbit determination for ALOS is essential. So ALOS mounts the GPS receiver. This paper deals with the precise orbit determination experiments for ALOS using Global and High Accuracy Trajectory determination System (GUTS) and the evaluation of the orbit determination accuracy by Satellite Laser Ranging (SLR) data

by GFZ) has been reported to achieve a few centimeters 1 Introduction accuracy[3][4]. JAXA has developed a precise orbit Japan Aerospace Exploration Agency (JAXA) determination system (GUTS) from 1995. We launched an Earth observation satellite, ALOS, also performed precise orbit determination experiments for called "DAICHI'', from the Tanegashima Space Center ADEOS-II canying a single frequency GPS receiver in Japan on 24 January 2006. ALOS performs Earth and achieved 23 cm accuracy [5]. In the experiments observation at a high resolution, which is expected to using GRACE's double frequency GPS receiver data, contribute to a wide range of fields such as map we achieved 8 cm accuracy compared with the result of compilation, region observation, grasp of disaster DLR [6]. And also in GPS satellites' orbit situation and resource mapping. ALOS has three determination, we achieved 7cm accuracy compared remote-sensing instruments: the Panchromatic Remote- with the ephemeris released by the International GNSS sensing Instrument for Stereo Mapping (PRISM) for Service (IGS). digital elevation mapping, the Advanced Visible and The orbit determination accuracy required for Near Infrared Radiometer type 2 (A VNIR-2) for precise ALOS is within one meter, which is not considered to land coverage observation, and the Phased Array type be difficult to achieve. But the GPS receiver loses lock L-band Synthetic Ape1ture Radar (PALSAR) for day- of GPS signals more frequently than expected. and and-night and all-weather land observation. In order to there are long period during which the receiver locks fully utilize the data obtained by these sensors, ALOS less than four GPS satellites. In order to determine the is designed with two advanced technologies: high speed position and time using GPS signals, it is required that and large capacity mission data handling technology the GPS receiver receive signals from at least four GPS and precision spacecraft pos1tlon and attitude satellites. Because of this, when we deal with ALOS's determination capability. These technologies will be GPS receiver data same as other GPS receivers' data, essential to high-resolution remote sensing satellites in the orbit determination accuracy is less than instances the next decade [1][2]. of orbit determination using double frequency receiver To achieve the high geometrical accuracy required data. So we preformed orbit determination using as for ALOS's sensors, we need to determine ALOS's much data as possible by conducting the preprocessing orbit very accurately. For this purpose, ALOS carries a appropriately. double frequency GPS receiver. Precise orbit Because the ALOS onboard GPS receiver was determination for low orbit satellite using GPS receiver newly developed, we need to evaluate the data received data has been widely studied. The orbit determination from the receiver. For this purpose, ALOS has a Laser for GRACE (Gravity Recovery And Climate Reflector (LR) composed of eight Corner Cube Experiment, managed by National Aeronautics and Reflectors (CCRs ) for SLR operations. We performed Space Administration (NASA) and Deutschcn orbit determination using only SLR data and compared Zcntrums fiir Luft- und Raumfahrt (DLR)) and CHAMP (CHAiienging Minisatellitc Payload, managed 15 JSTS Vol. 23, NO. I

GUTS Gl'S Navigation Data

Data Collection

Onboarcl Gl'SH Gioia) Network GPSR Data GP Navigation Data Fig.I ALOS Preliminary Orbit the SLR-determined orbit with the orbit determined by Point Positioning Determination GPS data. Data Initial Orbit (ALOS, GPS Satellites)

2 Mission Requirement of ALOS For precise mapping using satellite imagery, 1t 1s Evaluation Precise Orbit Propagation necessary to observe the earth with high resolution and determine the geographical positions corresponding to the observation image. Thus, high positioning accuracy Ephemeris (ALOS) and pointing control accuracy are required for ALOS Mission Users (EOC c>td [7], [8], [9], [IO]. The accuracy requirements for satellite attitude and pointing control are shown in Fig.2 System Configuration of GUTS Table I. Taking these errors into account, the orbit determination accuracy is required to be better than l of three functions: preprocessing, preliminaiy orbit meter (peak-to-peak). There are two tools for precise determination, and precise orbit determination. We use orbit determination for ALOS; GPS receiver and laser GPS data (RINEX) as observation data and determine reflector (LR) for Satellite Laser Ranging (SLR). the satellite's orbit by estimating some parameters of ALOS 's G PS receiver was newly developed for its Dynamical Model and Observation Model. We perform mission. Detailed explanation of GPS receiver 1s preliminary orbit determination using GPS navigation described in Toda et al [ 11]. data and ALOS onboard GPS data (point positioning data), and use the orbit as the initial orbit for precise Tab. I Re uired Accurac for Attitude and Pointin orbit determination. Pointing Control Roll, Pitch, Yaw: ±0. I deg Accurac 3.1 Preprocessing Pointing Short Roll, Yaw: 2.0x 10- Before precise orbit determination, GUTS Stability Period Sdeg/0.3 7ms (peak-to-peak) Perfonnsing to collected data as follows. Pitch: I .Ox I 0-5deg/0.3 7ms • correction of clock offset of onboard GPS receiver eak-to- eak data Long No DRC Drive: 2.0x l 0-4 • detection and correction of cycle slip of onboard and Period deg/5s (peak-to-peak) ground GPS receiver data With DRC Drive: 4.0x 10-5 • correction of ionospheric delay de /5s • smoothing and compression Pointing Determination Accuracy 3.2 Preliminary Orbit Determination (after processing on the GUTS estimates the initial orbit from RINEX round navigation data for GPS satellites and from point positioning data for ALOS. 3 System Configuration of GUTS Fig. 2 shows the system configuration of GUTS. 3.3 Precise Orbit Determination GUTS receives onboard GPS receiver data from JAXA In GUTS precise orbit determination, we use GPS Earth Observation Center (EOC) and GPS ground data (carrier phase measurements from RINEX) as stations data from JAXA GPS ground station control observation data and determine the orbit by estimating and the IGS ftp server. The system is mainly composed some parameters of Dynamical Model and Observation 16 JSTS Vol. 23, NO. I

Tab. I Dynamical Model Model GPS D_y_namics ALOS D_y_anmics Geo Potential JGM3(12*12) JGM3(70*70) N Body JPL DE405 (All.£_lanets, Sun and Moon) JPL DE405 (AllJ?lanets, Sun and Moon) Solar Radiation Pressure Model Estimated using Estimated using (including conical shadow model - GSPM.04b · Sphere Model for Earth and Moon) -CODE - Multi-Plane Satellite Model Albedo Albedo Earth Radiation Pressure infrared second-d()gree zonal model infrared second·de_gree zonal model Solid Earth tides Based on IERS2003 Based on IERS2003 Tide Ocean tides Based on IERS2003 Based on IERS2003 Solid Earth _f!_Ole tide Based on IERS2003 Based on IERS2003 Estimated using Atmospheric Drag - Sphere Model - Multi-Plane Satellite Model Relativity Based on IERS200;3 Based on IERS2003 Emprical Acceralation Constant

Tab. 2 Observation Model Model Ground Stations GPS Receiver ALOS Onboard GPS Receiver Geometrical Distance Light Time El1uation Light Time Eguation Satellite Clock/Receiver Clock Of Considered Considered Ionospheric Delay linear combination linear combination Tropospheric Delay Lanyi Model Lanyi Model Phase Center Offset of Different Model for Each 131ocks Different Model for Each Blocks GPS satellites's Antena Phase Center Offset of Different Model for Each Stations ALOS(X,Y,'/,)=( 1.820111, -0.858m, · l.562m) Receiver's Antena Solid Earth tides (Based on IERS2003) Site Diplacement Ocean tides (Based on IEHS2003) Propageted by ALOS Dynamical Model Solid EarthJ_>ole tide (Based on IERS2003) Earth Rotation Parameter Based on IEHS 1998 Based on IEHS1998 Carrier Phase Bias Considered Considered

Model by SRIF (Square Root Information Filter). First, the GPS satellite position is estimated assuming that the the selected GPS satellites' orbits are estimated by positions of the GPS receivers (ground stations) arc using the coordinates of the ground stations published fixed, and in ALOS orbit determination, onboard GPS by the IGS (SINEX). Then, ALOS orbit is estimated receiver's position is estimated assuming that the using the determined GPS satellites' orbits. The detail positions of the GPS satellites are fixed. is omitted here, and the outline is described below. 4 Features of ALOS onboarcl GPS Receiver Data 3.3. l Dynamical Model Considering the performance of GUTS described in Dynamical Model is a set of forces acting on the section 1, it is not so difficult to fulfill the requirement satellite and is used for propagating the satellite' orbits. for orbit determination accuracy, if the ALOS onboard The detail is shown in Tab. I. GPS receiver locks onto four or more GPS satellites 3.3.2 Observation Model signals constantly. However, the ALOS onboard GPS receiver frequently loses G PS satellites signals because The Observation data used for GPS satellites' orbit of electromagnetic interference with other sensors. determination is calculated from ground stations' GPS Because of this problem, the ALOS onboard GPS receiver data and includes various observation errors receiver often can not locks onto four or more GPS other than geometric distances between GPS satellites satellites signals. and ground stations. In order to correct these errors, some factors are modeled. Modeled factors arc 4.1 Interference Problem described in Tab.2. PALSAR, mounted on ALOS, is a radio wave 3.3.3 Estimation Parameters sensor using L band. There was a concern about radio GUTS can estimate not only orbital elements but disturbance due to electromagnetic interference because also various parameters described above. The estimated the transmission frequency (1270MHz) was close to L2 parameters for ALOS orbit determination are shown in frequency (1227.6MHz) of the GPS receiver. To deal Tab 3. Note that in GPS satellites orbit determination, with this problem, an interface was established between 17 JSTS Vol. 2:3, NO. 1

Tab. 3 Estimation Parameters GPS Satellites Orbit Determination ALOS Orbit Determination Orbital Elements Six Elements Six Elements GSPM.04b(Scale Factor, y bias) Solar Radiation Pressure Scale Factor Solar Radiation Pressure CODE(DO, YO, BO, ZO) (10 minutes interval) Atmospheric Density Scale Factor Atmospheric Drag (10 minutes interval) Clock Transmitter/Receiver------Clock Offset Receiver Clock Offset Carrier Phase Bias Estimate Estimate Wet zenith Em_r>rical Acceralation Costant

PALSAR and GPS receiver so that the receiver will As is shown in Figure 5, the number of acquired GPS stop receiving L2 signal when the P ALSAR signal is satellites was four or less during the most of the period transmitted. As shown in Figure 3, GPS receiver stops and the number of GPS satellites useful for orbit receiving GPS signals while PALSAR is active. By this determination further decreased due to erroneous data. method, Electromagnetic interference between GPS and The maximum value of vertical axis is six satellites PALSAR was avoided successfully. However, another because the GPS receiver has six reception channels for unexpected electromagnetic interference was observed both LI and L2. between the ALOS GPS receiver and other ALOS instruments. This electromagnetic interference caused 4.2 Parameter Tuning for Preprocessing the GPS signals to be lost when the operations of As shown in Fig.5, 6, the percentage of the orbit ALOS sensors increased. In satellite positioning using determination using four or more GPS satellites was GPS, at least four GPS signals are required to 35.7%, when we counted the number of GPS satellites determine the position and time. So this interference used for the orbit determination per 60-second period of problem causes a degradation of orbit determination GPS data. The percentage of four or more GPS accuracy. satellites tracked, calculated based on RINEX data, is Figure 4 shows the percentages of unusable GPS 56.6%. We modified the following parameters of data during the orbit determination period. preprocessing to use as much data as possible for orbit Approximately I 0% of LI pseudorangc data, 23% of determination. L2 pscudorange and L2 carrier phase were discarded. • maximum data loss time for pass regarded as Unusable data of LI pscudorangc and L2 pscudorange continuous : A pass is divided if it is longer than this were mainly caused by loss of lock on GPS signals. time. Bad data of L2 carrier phase was mainly due to loss of • minimal time for pass regarded as effective : A pass lock and error of the GPS receiver. We suppose that is rejected if it is shorter than this time. these erroneous data were caused not by the • time for carrier smoothing malfunction of the GPS receiver but by electromagnetic GUTS performs carrier smoothing as follows. interference with surroundings, because the GPS - Calculate the difference between ionospheric free receiver showed a good performance during the ground combination pseudorange data and ionospheric free test. In particular, the percentage of erroneous LI data combination carrier phase data increased after 16 August, from which PALSAR sensor - Take the average of the difference over a certain was frequently operated for observation. Over 20% of period daily observation data was useless for orbit - Make a new pseudorange data by adding the average determination. This is equivalent to that one or two out to the ionospheric free combination carrier phase data of six channels of ALOS GPS receiver are unavailable. 2f>

250-667 micro sec 20

lfi i 10

Fig.3 Interface between PALSAR and GPS Receiver

Fig. 4 Data Rejection Rate 18 JSTS Vol. 23, NO. I

---i 1.2% 2.8%

1: i ··::. .., T..•:; ru=1:,·I

25.0% j ;01 , . . I _:_ _;_ ____J 2oon1812:i 200618123 20061812:1 2oon1812:i 2oon1812a 2oon1s12a 2oon1812al 1:00:00 2:00:00 a:oo:oo 1:00:00 ri:oo:oo 6:00:00 nio:oo

'l'inrn (G l'S'l') J ------Fig. 5 number of locked OPS satellites (dot) and OPS Fig. 6 number ofGPS satellites used for satellites used for orbit determination (solid) orbit determination (time ratio)

-based range to ALOS and orbit-determination (using 5 Experiments GPS data)-based range to ALOS (0-C). In addition, we performed ephemeris comparison between orbit 5.1 Execution Condition determined by only SLR data and orbit determined by We used 40-hour data for orbit determination (30- only GPS receiver data. In SLR-based orbit sec interval data for ground GPS receiver data and 1- determination, we chose a period in which relatively a sec interval data for the ALOS onboard OPS receiver larger amount of data was acquired and estimated the data). The used preprocessing parameters are shown in orbit in short arc. Despite this, SLR-based orbit was Tab.4. less accurate during the period without SLR data. So Tba . 4P p arame ers we evaluated ephemeris comparison during the period Condition Time[ sec] with SLR data. maximum data loss time 2 for regarded as continuous 6 Results minimal time for regarded as effective 120 By tuning the preprocessing parameters, the time for carrier smoothing 60 percentage of period in which the ALOS onboard receiver locked onto four of more GPS satellite signals increase from 35.7% to 49.5%. The result of overlap evaluation is shown in Fig.8. As shown in this figure, 5.2 Accuracy Evaluation orbit determination accuracy is less than about 30- The orbit determination arcs have 16-hour overlap centimeters and better than before tuning. period as shown in Fig.7. We compared the two determined orbit during the overlap period (6-hour T a b5. C 011'!£.aflSOll WI'ti 1 SLR- base d or b't1 overlap period excluding the first and last five hours) Cross- Along- Radial and the maximum absolute difference in the period was Track Track used for orbit determination accuracy. This evaluation -2.98 -4.69 -5.44 is comparative, but can be used for evaluation of Standard 20.54 38.32 28.76 random errors of orbit determination. Deviation( cm) ALOS carries a LR for SLR. We also performed absolute evaluation by taking differences between SLR- data 20 0 12 20 0 12 20 0 12

Over Lap Period Over Lap Period Over Lap Period UT 1:00 --7:00 UT 1:00 --7:00 UT 1:00 --7:00

Fig.7 orbit determination arc and overlap period 19 JSTS Vol. 23, NO. I

[4] Z. Kang, B. Tapley, S. Bcttadpur, J. Ries, P. Nagel and R. Pastor, "Precise orbit determination for the GRACE mission using only GPS data", JOURNAL OF GEODESY 80 (322 - 1··+·· post-tuning I pre .. _l un __ 331) [5] S. Nakamura, S. Katagiri, "Comparison of OD systems of JAXA and DLR GSOC for ADEOS-11", JAXA Tcchinical Note (QNX-050066) [6] S. Nakamura, T. Uchimura, S. Katagiri, "Comparing OD system for GRACE-B between DLR and JAXA-Changing the JAXA gravitational potential model from JGM-3 to GGM-02c - ", JAXA Techinical Note (QNX-050045) [7] T. Iwata, "Precision attitude and position determination for the Advanced Land Observing Satellite (ALOS)'', SPIE 4th 3/27 4/10 4/24 5/8 5/22 6/5 6/19 International Asia-Pacific Environmental Remote Sensing Date Symposium: Remote Sensing of the Atmosphere, Ocean, Environment & Space, Honolulu, U.S.A., November 9,2004 Fig.8 Overlap Comparison Result (8] T. Iwata., K. Toda, T. Hamazaki, "Attitude and Orbit Control System for the Advanced Land Observing Satellite (ALOS)", The result of 0-C evaluation using all 14-day SLR International Workshop on Spacecraft Attitude and Orbit Control data is that the averaged difference is -4. 78-centimeter Systems, Noordwijk, Netherlands, 1997 and the standard deviation is 12.03-centimeter. This [9] T. Iwata, ct al, "Precision Attitude and Orbit Control System means that, other than small errors, there is no for the Advanced Land Observing Satellite (ALOS) ", AIAA Guidance, Navigation and Control Conference, AIAA-2003- inconsistency between the SLR-data-based range to 5783, Austin, U.S.A, 2003 ALOS and orbit-determination (using GPS data)-based [10] T. Iwata, Y. Osawa, T. Kawahara, "Precision Pointing range to ALOS. Management for the Advanced Land Observing Satellite The result of ephemeris comparison is shown in (ALOS)", 23rd International Symposium on Space Technology Tab. 5. The averaged differences are within the values and Science, ISTS 2002-d-56, Matsue, Japan, 2002. of standard deviation in all directions. This result shows [ 11] K. Toda ct al, "GPS Receiver and Precision Position Determination for the advanced Land Observing Satellite that the GPS-based orbit is consistent with the SLR- (ALOS): Flight Result" 50th Joint Conference on Space Science based orbit considering the I sigma error. In other & Technology, Kitakyushu ,2006 (in Japanese) words, the averaged differences can not be regarded as [ 12] h!lp_;j!ju_?.;JJ.,cco. goQ. offsets between the GPS-based orbit and the SLR-based orbit, because the standard deviations are larger than the averaged differences. And considering the I sigma error, orbit determination accuracy of one meter (peak- to-peak) was achieved. 7 Conclusion We performed orbit determination for ALOS, which carries a dual-frequency GPS receiver, and evaluated the accuracy by overlap method and comparison with SLR data. The ALOS onboard GPS receiver has a problem of electromagnetic interference with other onboard sensors and because of the problem, often loses lock onto GPS satellite signals. We performed preprocessing parameter tuning and tried to use as much data as possible. After parameter tuning, period of locking more than four satellites is not enough, but we achieved 1 meter orbit determination accuracy. References [I) T. Hamazaki, "Overview of the Advanced Land Observing Satellite (ALOS): Its Mission Requirements, Sensors, and a Satellite System," presented to ISPRS Joint Workshop "Sensors and Mapping From Space l 999," International Society for Photogrammctry and Remote Sensing (ISPRS), Hannover, Germany, Sept. 27-30, 1999 (2) JAXA Web Site : http://alos.jaxa.jp/indcx-e.html (3) !Jssel, J. vd, P. Visser, CHAMP Precise Orbit Determination Using GPS Data, Adv. Space Res., 31/(8), 2003