Reconstruction of KOMPSAT-l GPS navigation solutions using GPS data generation and preprocessing program

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Acta Astronautica 54 (2004) 571 – 576 www.elsevier.com/locate/actaastro

Reconstruction of KOMPSAT-l GPS navigation solutions using GPS data generation and preprocessing program Byoung-Sun Leea;∗ , Jeong-Sook Leea , Jae-Hoon Kima , Seong-Pal Leea , Jae-Cheol Yoonb , Kyoung-Min Rohc , Eun-Seo Parkc , Kyu-Hong Choic a Communications

Satellite Development Center, Electronics and Telecommunications Research Institute(ETRI), Daejeon, South Korea b KOMPSAT Program O"ce, Korea Aerospace Research Institute (KARI), Daejeon, South Korea c Department of Astronomy and Space Sciences, Yonsei University, Seoul, South Korea Received 15 October 2002; accepted 5 June 2003

Abstract GPS navigation solutions and GPS-related telemetry points are analyzed using one-day playback data from the KOMPSAT-1 Spacecraft. Then, the GPS navigation solutions are reconstructed using simulated GPS raw measurements such as pseudo-ranges between the KOMPSAT-1 and GPS satellites constellation. GPS data generation and preprocessing computer programs are developed and used for the generation of the GPS raw measurement and reconstruction of the GPS navigation solutions. The playback GPS data and the reconstructed data are compared with each other. c 2003 Elsevier Ltd. All rights reserved. 

1. Introduction Korea multi-purpose Satellite-1 (KOMPSAT-1) has been successfully operated since the launch of the spacecraft on the 2l December, 1999 by Taurus launch vehicle. The main mission of the satellite is to perform the cartography of the Korean peninsula using 6:6 m resolution panchromatic camera. The operational orbit altitude of the satellite is 685 km. The primary method of orbit determination for the spacecraft is using GPS navigation solutions. The VICEROY GPS receiver by MOTOROLA is mounted on the KOMPSAT-1 and it uses C/A code on the GPS

∗ Corresponding author. Tel.: +82-42-860-4903; fax: +82-42-860-6949. E-mail address: [email protected] (B.-S. Lee).

L1 frequency for providing Earth Centered Fixed (ECF) position, velocity, and time [1]. The GPS navigation solutions are stored on-board the spacecraft in every 32-s interval and dumped to ground station during the pass time as a playback telemetry data. Batch of the GPS navigation solutions, normally one-day amount, is processed by the orbit determination software in ground control center [2,3]. The accuracy of GPS navigation solutions of the KOMPSAT-1 before and after the termination of the selective availability was analyzed in the previous paper [4]. As a following program of the KOMPSAT-1, the KOMPSAT-2 spacecraft will be launched into the same orbital altitude in 2004. However, the Earth imaging resolution of the KOMPSAT-2 will be upgraded by 1 m in panchromatic and 4 m in multi-spectral mode. To enhance the image product of the KOMPSAT-2, a precise orbit determination

c 2003 Elsevier Ltd. All rights reserved. 0094-5765/$ - see front matter
doi:10.1016/S0094-5765(03)00230-3

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(POD) using diFerential GPS technique will be performed in ground control center. TOPSTAR 3000 GPS receiver by ALCATEL is considered for the spacecraft and it uses GPS L1 frequency for providing the raw GPS data such as pseudo-range and carrier phase [5]. The raw GPS data from the spacecraft and the global reference stations in the international GPS service (IGS) will be used for POD. The target accuracy of the POD is 1 m (1 ) with 1-day raw GPS measurements. The POD software package for the KOMPSAT-2 is under development. The software package is comprised of raw GPS data generation program, data preprocessing program, and precision orbit determination program. In this paper, raw GPS data generation and a part of preprocessing programs are used for reconstruction of the KOMPSAT-1 GPS navigation solutions. At Grst, real GPS data from the spacecraft are processed and analyzed. Then the raw GPS data are simulated using the raw GPS data generation program and the navigation solutions are reconstructed using a part of preprocessing program. Two orbit determinations are performed using GPS navigation solutions from playback data and reconstructed simulation data. 2. GPS data generation and preprocessing The GPS raw data generation program consists of precise dynamic modeling of the low Earth orbit spacecraft and measurement modeling of GPS satellites constellation. The equations of spacecraft orbit are numerically integrated using Adams–Cowell 11th order predictor-corrector method. The force models include Earth’s gravity up to 70 × 70, luni-solar gravity, eight planets of the solar system, solid Earth tides, ocean tides, relativistic eFect, and Earth’s dynamic polar motion. The non-gravitational forces such as atmospheric drag, solar radiation pressure, Earth radiation pressure, and thermal radiation of the spacecraft are also modeled [6]. The GPS measurement models are composed of tropospheric path delay, ionospheric path delay, relativistic eFect, phase center oFset and variation of the GPS receiver antenna, and position variation of the ground stations due to the solid Earth tide, ocean loading, and tectonic plate motion [7]. The GPS raw data generation program simulates Gve GPS measurements such as L1/L2 carrier phases, P1/P2 pseudo ranges, and C/A-code pseudo range. All

Fig. 1. Block diagram of the raw GPS data generation.

of the GPS raw data are generated as RINEX2 format Gles. Initial state vector of the spacecraft and related GPS orbit are required for simulating the raw GPS measurement data. The orbit information of the GPS constellation includes the positions of the GPS satellites and clock errors in 15 min interval. Three kinds of GPS orbit formatted in SP3 such as IGS (IGS Precise), IGR (IGS Rapid), and IGU (IGS Ultra-Rapid) are available in GPS raw data generation program. Fig. 1 shows the block diagram of the raw GPS data generation processing. The GPS raw data pre-processing program consists of correction of the clock errors in the low Earth orbit spacecraft, detection of the cycle slip in carrier phase, and derivation of the navigation solutions. The dilution of precision (DOP) is also derived in the process of estimating GPS navigation solutions of the spacecraft. The GPS navigation solutions such as the position of the spacecraft and receiver clock error are derived using least square method when more than four GPS signals are received. The elevation cut oF angles at the GPS receiver onboard the spacecraft can be used as input parameter. Fig. 2 shows the block diagram of a part of preprocessing used in this paper. 3. Processing of the KOMPSAT-1 GPS data KOMPSAT-1 GPS navigation solutions on the 13 February 2001 are used for orbit determination.

B.-S. Lee et al. / Acta Astronautica 54 (2004) 571 – 576

Fig. 2. Block diagram of the GPS data preprocessing.

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Fig. 3. Position diFerence between GPS navigation solutions and orbit determination results.

Table 1 Orbit determination results and parameter (playback) a (km) e i (deg)

(deg) ! (deg) M (deg)

7058.306 0.0023843 98.15317 305.72997 66.69091 166.79377

S=C area (m2 ) S=C mass (kg) Iterations Drag coeLcient SRP coeLcient

7.5 448 5 2.43 1.32

Atmospheric drag and solar radiation pressure (SRP) coeLcients are estimated in the orbit determination process. Table 1 shows the state vectors and related parameters. The S=C area and mass are Gxed input parameters in orbit determination process. The orbital elements and coeLcients are estimated values. Fig. 3 shows the position diFerences between the GPS navigation solutions and the orbit determination results. The standard deviation of the position diFerences is about 44:1 m. Many bad points in the GPS navigation solutions are found in Fig. 3. KOMPSAT-1 playback telemetry data Gles are received and processed to extract the GPS-related data. The GPS-related telemetry points are GPS navigation solutions such as position and velocity vector, GPS state of health, 3D Gx status, position hold status, 2D Gx status, bad almanac, current DOP, insuLcient visible satellites, storing new almanac, poor geometry, position propagate mode, number of satellites tracked, and number of visible satellites.

Fig. 4. Number of visible GPS satellites (playback).

Fig. 4 shows the number of visible GPS satellites from KOMPSAT-1 spacecraft. The masking angle of the GPS receiver is set to −5◦ elevation. The number of visible GPS is varied with time according to the geometric conGgurations between the KOMPSAT-1 and GPS constellation. Fig. 5 shows the number of tracked GPS satellites. The number of tracked GPS are smaller than those of visible GPS in Fig. 4. The number of tracked GPS is important to deriving the GPS navigation solutions on board the spacecraft. Fig. 6 presents the variation of the GDOP during the 1-day period. Normally good GDOP values are shown

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Fig. 5. Number of tracked GPS satellites (playback).

Fig. 7. GPS receiver status.

Fig. 6. Variation of the GDOP (playback).

Fig. 8. Number of visible GPS satellites (simulation).

except a few sporadic bad points. The bad GDOP values are a cause of the bad GPS navigation solutions in Fig. 3. Fig. 7 shows the status of the KOMPSAT-1 GPS receiver. The points of loss of 3D Gx, position propagate mode, and bad almanac are shown. The points are related with the big GDOP values in Fig. 6.

GPS satellite were generated with adding 5 m (1 ) Gaussian random noises. The data output interval was 32 s same as the KOMPSAT-1 GPS navigation solutions. The GPS antenna was assumed to be located in the zenith directions in local-vertical-local-horizontal and the masking angle of the elevation was set to −5◦ . Twelve channel GPS receiver was assumed in the simulation. Fig. 8 shows the number of visible GPS satellites in the simulation. The plotted number of visible GPS satellites in Fig. 8 is similar to Fig. 4. The actual location of the GPS receiver antenna on board the KOMPSAT-1 is not zenith directions in LVLH coordinates. This causes small diFerences between the Ggures.

4. Reconstruction of the KOMPSAT-1 GPS data GPS raw measurement data at the KOMPSAT-1 spacecraft were generated using orbit determination results in Table 1 and GPS ephemeris in IGS. C/A code pseudo-ranges between the KOMPSAT-1 and visible

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Fig. 9. Visible GPS PRN (simulation).

Fig. 11. Variation of the GDOP (simulation).

Fig. 10. Position diFerence between the reference orbit prediction and derived GPS navigation solutions.

Fig. 12. Variation of the clock error.

Fig. 9 shows PRN of the visible GPS satellites at the KOMPSAT-l orbit during 1-day period. Totals of 28 GPS satellites are shown. GPS navigation solutions were derived from the simulated raw GPS measurements using a part of the preprocessing program. All of the measurements within the visible range of the KOMPSAT-1 were used. Batch least square method was applied to deriving the navigation solutions. Fig. 10 shows the position diFerences between the reference orbit used for generating raw GPS measurements and the derived GPS navigation solutions. The standard deviation of the position diFerence is 11:3 m and the mean value is 28:9 m.

Fig. 11 presents the variation of the GDOP during the 1-day period. All of the GPS measurements in the visible range of the KOMPSAT-1 are used to deriving the GDOP values. So, the best GDOP value in Fig. 11 is worse than that in Fig. 6. There are no sporadic bad points because only Gaussian noises are added in generating the raw GPS measurements. Fig. 12 shows the estimated clock errors in GPS navigation solutions. The clock errors are mainly caused by the random walk noises. Orbit determination was performed using the derived GPS navigation solutions. Table 2 shows the estimated orbit using the reconstructed navigation solutions.

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Table 2 Orbit determination results (simulation) a (km) e i (deg)

7058.306 0.0023845 98.15317

(deg) ! (deg) M (deg)

305.72997 66.69509 166.78954

real space environment. However, the playback GPS data and the reconstructed simulation data showed the similar trends. The raw GPS data generation and preprocessing programs were used for the reconstruction of the KOMPSAT-1 GPS navigation solutions. Two programs will be part of the precision orbit determination package for the KOMPSAT-2 spacecraft that will be launched in 2004. The raw GPS data from the spacecraft and the global reference stations in the international GPS service will be used for POD. References

Fig. 13. Position diFerence between the derived GPS navigation solutions and orbit determination results.

Fig. 13 shows the position diFerences between GPS navigation solutions and orbit determination results. Standard deviation of the position diFerence is 5:9 m and the mean value is 13:0 m. Since the orbit determination process Gts the orbit to the measurement data, the diFerences are smaller than those in Fig. 10. 5. Conclusions The GPS navigation solutions were reconstructed from the simulated raw GPS data of the KOMPSAT-1. The reconstructed GPS data was compared with the real playback data. The real playback GPS data presented a number of sporadic bad points in reMecting the

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