THEOS MISSION: TRANSFER AND FIRST YEAR OF OPERATIONS

Anne-Hélène Gicquel(1), Philippe Cayeux(1), David Joalland(1) Pornthep Navakitkanok(2)

(1) EADS 31, Avenue des Cosmonautes, F-31402 TOULOUSE CEDEX 4 (Tel. (+33) (0)5 62 19 66 99, E-mail [email protected])

(2) Geo-Informatics and Space Technology Agency (GISTDA) 196 Phahonyothin Road, Chatuchak, Lad Yao Bangkok 10900 (Tel (+66) (0)2 940 6421, E-mail [email protected])

Abstract In addition, the paper presents the first year of operations as seen from GISTDA operator. It THEOS is the first Thai Earth Observation describes the daily Flight Dynamics activities (Fig.1.). It has been launched 1st and also puts forward flight data dealing with October 2008 and is currently operated by operational parameters station-keeping GISTDA. The transfer phase has been (ground track error at equator and local solar performed by Astrium Flight Dynamics team time error evolution). It finally describes the leading to a hand over to GISTDA team mid- activities planned for the first OCM October 2008. This was the starting point of preparation. GISTDA Flight Dynamics operational experience following a 21 months training at Astrium premises before the launch. 1. TRANSFER PHASE OR HOW TO REACH MISSION REFERENCE ORBIT

1.1 Phasing with SPOT: The reference orbit definition The 26 positions shown on Fig. 2. correspond to the possible positions in argument of latitude to match the SPOT ground track reference grid.

Fig. 1. Artist view of THEOS spacecraft THEOS The THEOS spacecraft mission orbit has the same repetitivity (14 + 5/26) as the SPOT spacecraft, i.e. the same altitude of 822 km, but a different mean local solar time. THEOS and SPOT satellites follow the same grid on Earth. 97° The paper presents how the reference orbit has been chosen to phase THEOS with the SPOT SPOT 5 spacecraft. Then, the transfer strategy design Fig. 2 : Argument of latitude phasing to reach this mission orbit as well as its operational realisation are detailed.

- 1 - In order to avoid simultaneous visibility of According to the SPOT 5 station keeping SPOT 5 and THEOS spacecraft from the Thai strategy, the local solar time of SPOT 5 is ground station, THEOS argument of latitude assumed to drift towards 10h15 (at descending shall be selected in the interval shown on Fig. node) in 2009. This LST shift is equivalent to 2. This choice takes into account the end of a shift of 53.2 deg in argument of latitude with life drift in local solar time of SPOT 5 and respect to its nominal position towards ensures a minimum separation with SPOT 2. THEOS position. The THEOS spacecraft shall follow the SPOT In addition, though the mission requirement ground track grid but with a shift in local solar for THEOS in terms of LST window is 10h00 time. at descending node +/- 15 minutes, the LST The local solar time (LST) at descending node control window is reduced in order to cope is 10h00 for the THEOS mission (10h30 for with SPOT2 and SPOT5. At the beginning of SPOT). The shift in local solar time with life (after the transfer phase) the THEOS LST respect to SPOT is then half an hour, which initial shift is limited to -5 minutes in order to corresponds to a shift of 7.5 deg in longitude ensure the separation with SPOT2. Then after (towards West) at equator crossing for a given the first LST control manoeuvre, the LST argument of latitude. window will be reduced to +/- 2 minutes in The longitude interval between two order to limit the libration of THEOS in consecutive nodes of the grid is ≈0.98 deg. argument of latitude (Fig. 3). Thus, the solar local time shift is equivalent to a shift of ≈ 7.7 positions in argument of latitude. In terms of argument of latitude, the 1.2 Transfer Strategy Design total shift is equivalent to 106.44 deg. That means that the equator crossings of a Due to launcher limit of capability, spacecraft at 10h00 LST are at the same the altitude of injection orbit was 680 km, i;e. longitudes as for a spacecraft that would be at 142 km below the mission orbit. 10h30 LST with a shift of 106.44 deg in argument of latitude. In order to cross the The transfer phase consisted therefore in equator at the same longitude, the spacecraft at increasing the semi-major axis and correcting 10h00 LST should be ahead of the spacecraft the inclination while phasing with the at 10h30 LST in terms of argument of latitude. reference mission orbit.

Within the argument of latitude interval An injection this far from the mission orbit chosen for THEOS, only one position matches enables to have a constant transfer duration of the equator crossings of the SPOT grid (Fig. 14 days whatever the initial phasing 3). conditions. ∆H=-5 min ∆H=-2 min

nominal reference position

∆H=+2 min

THEOS

97°

SPOT 5

Fig. 3 : THEOS argument of latitude window Fig. 4 : THEOS transfer and phasing overview 3

Indeed, depending on the launch date, 1 to 3 Fig. 5. Transfer Plan overview days of free drift are inserted between the Day Manoeuvres groups of manoeuvres in order to achieve the Day 3 2 in-plane calibration manoeuvres of orbit phasing with the reference grid 2 m/s

Day 4 Drift Day Day 5 4 in-plane manoeuvres of 3.4 m/s The transfer strategy was designed taking into account platform constraints (manoeuvre Day 6 4 combined manoeuvres of 3.4 m/s maximum duration, AOCS and power Day 7 4 in-plane manoeuvres of 3.4 m/s constraints) and operational constraints: Day 8 4 in-plane manoeuvres of 3.3 m/s • First calibration manoeuvre is Day 9 Drift Day performed in visibility from polar station Day 10 4 in-plane manoeuvres of 2 m/s Kiruna Day 11 2 in-plane manoeuvres of 2 m/s • Manoeuvres scheduled according Kiruna passes sequence in order to ensure the Day 12 2 in-plane manoeuvres of 1.05 m/s ground station acquisition after a non-nominal Day 13 Drift Day manoeuvre (Kiruna has autotrack capability Day 14 2 in-plane manoeuvres of 1 m/s while Siracha has only an on-ephemeris mode) • If inclination correction is needed, combined manoeuvres (inclination/semi-major axis correction) are proposed to save Manoeuvres started on Day 3 as Day 1 & 2 propellant and are thus performed at nodes were dedicated to spacecraft initialisation. The • The in-plane manoeuvre argument of transfer phase was completed on 15th October latitude is chosen to target the frozen 2008. eccentricity • The strategy is robust to an ARO The 28 Orbit Control Manoeuvres (OCM) postponing a group of manoeuvres for 24h: a corrected 142 km of semi-major axis (Fig. 6.) simple criterion is to target roughly +5 km on and -0.015 deg of inclination the ground track error at equator and to plan daily DV always approximately smaller than the total remaining DV. The ground track error target allows remaining in the control window even with a delay of 24h on the last group of manoeuvres.

• The strategy is robust to contingency cases (loss of 2 thrusters) The transfer strategy was thus made of drift days and manoeuvres days. During a manoeuvre day, 2 to 4 manoeuvres were planned in the afternoon. The number of manoeuvres per day could have been extended to 8 in case of thrusters’ failure.

1.3 Transfer realisation Fig. 6. Semi-major axis raising sequence st The launch date of 1 October and very small The manoeuvres have been calibrated during dispersions on injection orbit induced a the transfer and the accuracy was about 1% transfer sequence composed of 9 manoeuvres days (including only 1 day with combined The accuracy of the last pair of manoeuvres manoeuvres) and 3 drift days on days 4, 9 and enabled to initialize properly the station- 13, as described in Fig. 5. keeping. No manoeuvre was foreseen during the first 6 months of operations (Fig. 7.). 4

• Image evaluation phase to confirm and guarantee the quality of the images by GISTDA and specialists in Thailand. • Operational Readiness Review phase I: to evaluate product ordering system, image acquisition and production altogether with contacts to users. All scenarios assumed to be fulfilled by THEOS product were tested. After this test campaign, GISTDA declared THEOS as fully operational for national users on 1st June 2009. • Operational Readiness Review phase II: to extend capability of THEOS , GISTDA is currently preparing THEOS polar station [2] and selecting THEOS distributor to serve all the international users. Fig. 7. Ground track error evolution prediction

at the end of the transfer

2. FIRST YEAR OF OPERATIONS AS SEEN BY GISTDA

2.1 Overview of THEOS operations

The initial THEOS operations can be divided Fig. 8. Timeline of THEOS operations in 5 phases that are presented in Fig. 8. • Launch and Early Operations Phase The operations are performed from THEOS (LEOP) supervised from Astrium Satellites: ground segment. GISTDA operator is now After THEOS separation from the launcher at fully used to operate nominally THEOS an altitude of 680 km, the communication Control Ground Segment and is able to handle system was activated and switched to normal anomaly situation and analyze telemetry. The mode. THEOS took its first set of images over Control Ground Segment is composed of three Thailand. Series of OCM were commanded in main elements: The Flight Dynamics Centre, order to raise THEOS to its operational orbit the Satellite Control Centre and the Mission within 14 days. The remaining propellant, Planning Centre. The Image Ground Segment more than 54 kg, guarantees large margin for is dedicated to image data acquisition and station-keeping (SK). The initial conditions for production [3]. Currently over 48,000 of PAN SK were ground track (GT) error = +5 km and and 18,000 of MS scenes are available in Local Solar Time (LST) = 21.55 PM. THEOS archive. • In Orbit Test phase: the platform and payload commissioning began in mid of 2.2 Flight Dynamics routine activities October with 2 cycles of 26 days each. The The daily routine activities for Flight whole system and image performances were Dynamics team start when the GPS and evaluated /calibrated [1]. All equipments Doppler data are available after the first performed as expected. Payloads demonstrate satellite contact in the morning. All the excellent performance with good geometric activities are performed with the operational and radiometric accuracy. At the end of this software package QUARTZ developed by phase, the In Orbit Acceptance Review Astrium Satellites. The routine activities supported by Centre National d'Etudes consist in: Spatiales (CNES) was declared successful. • Orbit determination (OD): The operational orbit determination is based on a batch weighted least square (WLS) using GPS

5 based navigation solutions from dumped 2.3 Orbit Maintenance Strategy telemetry. The result of operational orbit The orbit maintenance strategy shall not determination is the estimation of spacecraft disturb the payload operation. Therefore, the orbit for a specific epoch. Solar flux and drag SK shall be optimised to plan only necessary error can be estimated when needed. manoeuvres. The two parameters to be QUARTZ determines the orbit precisely. The considered for SK are Ground Track error and “global delta” is the difference between Local Solar Time error. predicted position over 24 hours and actual Ground Track of the orbit is defined as the position of the satellite obtained with OD. This locus of points projected on the Earth's surface difference is 100% less than 3 meters and 75% directly "beneath" the spacecraft orbit. less than 1 meter as shown in Fig. 9. Due to the time varying nature of the perturbations on the orbit, deviations from the reference orbit lead to ground track drift. To understand this, consider the orbit as it crosses the equator (called ground track error), as the earth rotates from one node crossing to the next the ground track moves westward. If the is exactly right, successive node crossings match successive reference nodes. If the period is too short, the Earth does not rotate quite far enough, and the true node falls eastwards of the reference node. If the period is too long, the earth rotates too far, and the true node falls westwards. After several orbits, the ground track moves further and further to Fig. 9. Delta of semi-major axis between one direction or another and a ground track actual and predicted over 24 hours drift develops. Ground track maintenance manoeuvres must be performed to maintain the ground track within a predefined control band • Events prediction, Station Keeping around the reference ground track. For parameters evolution monitoring and UTC THEOS, this band is ±40 km. as shown in time reference update. The orbit propagation is “Fig. 10.” carried out thanks to an Adams-Moulton integrator which is initialised by Runge-Kutta integrator. The main perturbations taken into account are: geo-potential, solar radiation pressure, Sun/Moon attraction and drag. • Generating files for Mission Planning Center as Orbit Ephemeris for mission planning purpose and Transponder activation plan to generate a TC file to activate spacecraft transponder when the spacecraft is in geometrical visibility of one or more ground Fig. 10. Ground track drift, measured at the stations. equator • Generating Antenna pointing file for S band station and an orbit in TLE format for X The Local Solar Time of an orbit is defined as the angle between the orbit’s ascending node band station and the mean Sun as shown in Fig. 11. A Sun- synchronous orbit, such as THEOS one, is

6 designed to maintain a constant LST by component is likely to be large with respect to matching the J2 nodal rate of the satellite with the in-plane component, and the attitude error the nodal rate of the mean Sun. The LST is may induce a large effect on the ground track often presented in units of time with 10:00 PM error. Therefore, an additional ground track – or noon describing a Sun-synchronous orbit control manoeuvre should be planned few days that places the Sun directly at zenith when the after a combined manoeuvre in order to spacecraft is at the ascending node. Orbital compensate for the effect on the ground track perturbation caused by the Sun and the Moon error. are responsible for the deviation of the actual LST of a spacecraft from a fixed value [4]. 2.4 Pre-OCM activities The THEOS spacecraft is required to maintain Once the need of a manoeuvre has been a LST between 22:00±2 mins to provide a established, the manoeuvre shall be computed nearly constant geometry despite these in terms of size, start date, duration and all the deviations. related constraints verification. Then the The decision to perform an orbit control corresponding satellite Tele-command plan maneuver should be taken several days in shall be generated and uploaded. The work advance. In-plane manoeuvres are used for flow to prepare the OCM is shown in Fig. 12. altitude adjustment to compensate for the effects of air-drag. This altitude decrease Pre OCM activities Post OCM activities affects the ground-track repeatability, mainly Check station Update new configuration Checkkeeping station Update new configuration in the equatorial regions. The frequency of (groundkeeping track and (groundlocal solar track time) and local solar time) these manoeuvres is determined by the rate of Thruster calibration the semi-major decrease. Out-of-plane Thruster calibration corrections are used to correct the steady drift Orbit planning Orbit planning• Propellant accounting • Compute delta semi-major Propellant accounting of inclination mainly caused by solar and lunar Computeaxis or delta delta inclination semi-major axis or delta inclination gravity perturbations. •Delta v computation •Delta v computation •Maneuver time and duration Telemetry •Maneuver time and duration • • Predicting station evolution take in to Predictingaccount of station maneuver evolution take in to account of maneuver Upload plan to satellite OCM planning OCM planning generated generated

Fig. 12.Orbit Control Maneuver work flow

Mean Sun 2.5 Post-OCM activities Mean Local solar time Post manoeuvre activities consist in estimating Ascending the efficiency of the manoeuvres based on the node localization measurements. The calibration coefficient is then used for the computation of the next OCM. It is also necessary to estimate Fig. 11. Definition of Local Time of the remaining propellant mass after thrust the Ascending Node using the propulsion tank telemetry parameters (temperatures and pressure). The remaining propellant is currently around 54 kg which is When a manoeuvre is required to control sufficient for orbit maintenance over 10 years. Local Solar Time (out of plane manoeuvre), a st combined manoeuvre to correct both LST and 2.6 1 OCM preparation ground track (in plane component) is planned. The operators perform SK long-term In terms of delta velocity magnitude and prediction based on weekly basis to check propellant consumption, the out of plane evolution of ground track and local solar time. 7

The ground track and local solar time real Conclusion evolution since the LEOP and prediction of ground track and local solar time up to the end THEOS satellite and ground segment of 2010 are shown in Fig. 13. and Fig. 14. developed by Astrium Satellites for GISTDA are functioning as specified and are now fully operated by GISTDA. The entire THEOS system has been tested, monitored, calibrated and validated and is now ready to operationally deliver imaging products to world wide customers. The daily routine Flight Dynamics activities are now handled by GISTDA FD operators. They will perform the first OCM activity around end of 2010 (depending on solar activity) with Astrium Satellites support.

Fig. 13. Evolution of ground track error

References:

[1] R. Nutpramoon, K.Weerawong and P. Apaphant, In-Orbit MTF measurement for THEOS imaging system, Asian Conference on Remote Sensing (ACRS), 2007. [2] D.Niummuad, P. Navakitkanok and A. Öskog, THEOS and a first step to international user, European Ground System Architecture Workshop (ESAW), 2009. [3] D. Niammuad, R. Nutpramoon and R. Fraisse, THEOS data processing and its image quality, Asian Conference on Remote Sensing Fig. 14. Evolution of local solar time (ACRS), 2006. [4] David P. McKinley, Long term mean local Currently the ground track error at equator and solar time of the ascending node prediction, local solar time are respectively 1.1 km a.i. solutions. (Ground track error window is +/-40 km) and 21:59:30 (LST window is 22:00±2 mins). Because of the accuracy of achieved injection local solar time and the achieved ground track error after LEOP, no manoeuvre has been necessary so far. The first foreseen OCM should occur in December 2010 and is an out of plane manoeuvre. This date is likely to change according to the real solar activity. The 1st OCM preparation will be supported by Astrium Satellites.

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