SENTINEL-1A FLIGHT DYNAMICS LEOP OPERATIONAL EXPERIENCE M. A. Martín Serrano(1), M. Catania(2), J. Sánchez(3), A. Vasconcelos(4), D. Kuijper(5) and X. Marc(6) (1)(4)SCISYS Deutschland GmbH at ESA/ESOC, Robert-Bosch-Straße 5, 64293 Darmstadt Germany, telephone: +496151902273, E-mail: [email protected], telephone: +496151902235, E-mail: [email protected], (2)(5)CGI at ESA/ESOC, Robert-Bosch-Straße 5, 64293 Darmstadt Germany, telephone: +496151903215, E-mail: [email protected], +496151902376, E-mail: [email protected], (3)GMV at ESA-ESOC, Robert-Bosch-Straße 5, 64293 Darmstadt Germany, telephone: +496151902416, E-mail: [email protected], (6) ESA/ESOC, Robert-Bosch-Straße 5, 64293 Darmstadt Germany, telephone: +496151902210, E-mail: [email protected], Abstract: Sentinel-1 is a two-satellite mission with each satellite carrying a C-Band Synthetic Aperture Radar. The Sentinel-1 satellites are part of the Sentinels fleet, which has been developed for the European Earth Observation Copernicus Programme. Sentinel-1A was launched on April 03 2014 by a Soyuz from Europe’s Space port in French Guiana into a 700 km altitude, frozen eccentricity, dusk-dawn sun-synchronous orbit with a ground-track repeat cycle of 12 days. This paper presents a report of the most relevant Flight Dynamics operations that were conducted at the European Space Control Centre (ESOC) in Darmstadt, Germany during the Sentinel-1A three day LEOP. These activities included the assessment of the injection orbit achieved by the launcher using S-band radiometric measurements during the first hours of LEOP, the monitoring of the Spacecraft appendages deployment sequence, monitoring and supporting the Spacecraft mode transitions and the implementation and execution of the first collision avoidance manoeuvre for this mission during the second day of LEOP. Keywords: Copernicus, Earth Observation, LEOP operations, LEO, Sentinels. 1. Introduction Sentinel-1A was launched by a Russian Soyuz-ST launcher equipped with a Fregat-M upper stage on April 03 2014 at 21:02:26 UTC from Europe’s Space port in French Guiana. Sentinel-1A is the first in-orbit spacecraft (S/C) from the new ESA Sentinels fleet developed for the European Earth observation Copernicus Programme, previously known as GMES (Global Monitoring for Environment and Security). It is also the first of a two-satellite System (Sentinel-1B currently planned for launch in April 2016), each carrying a C-band Synthetic Aperture Radar (SAR) as well as a laser communication payload to transmit data to the geostationary European Data Relay System for continual data delivery. The Sentinel-1 mission provides continuity of crucial data for user services initiated with the ERS and Envisat missions. This data is already benefiting numerous services. For example, services that relate to the monitoring of Arctic sea-ice extent, routine sea-ice mapping, surveillance of the marine environment, including oil-spill monitoring and ship detection for maritime security, monitoring 1 land-surface for motion risks, mapping for forest, water and soil management and mapping to support humanitarian aid and crisis situations. Sentinel-1A is controlled around a sun-synchronous reference orbit with a ground-track repeat pattern of 175 orbital revolutions in 12 days and a Mean Solar Local Time of the Ascending Node (MSLTAN) of 18:00 h. Figure 1. Sentinel-1A stowed representation (in RDM and SHM). +X S/C axis points towards the flight direction. S/C Y axis is aligned with the Sun direction. Solar Array –Y illuminated when stowed. The S/C Attitude and Orbit Control System (AOCS) consists of the following sensors and actuators: fine sun sensors, magnetometers, gyroscopes, star trackers, GPS receivers, magnetic torquers, a reaction wheels assembly and a monopropellant (hydrazine) propulsion system. The propulsion system has 3 pairs of 1 N Orbit Control Thrusters and 4 pairs of Reaction Control Thrusters for attitude correction. Every pair is made up of a prime and a redundant component. The attitude control thrusters are fired when the S/C enters Rate Damping Mode (RDM) after separation, damping any residual rotation left by the launcher upper stage and achieving a S/C pitch rotation of -8 times the orbital period. In the subsequent AOCS mode called Safe Hold Mode (SHM) magnetotorquers and reaction wheels maintain the attitude and reduce the pitch rotation rate to twice the orbital period. The periodic behavior of the Earth’s magnetic field in a polar orbit and the polarization of the angular momentum with the loading of the reaction wheels allow the magnetotorquers to maintain this pitch rate while aligning the S/C –Y axis with the orbit normal, which in a dusk-dawn orbit coincides with the direction to the Sun (see Figure 1). When the appendages deployment commences, the effect of the gravity gradient torque dominates over the magnetic torque, resulting in the alignment of the S/C X axis (appendages axis) with the nadir direction, maintaining thus a pitch rate equal to the orbital period. Upon ground telecommand a transition into the Normal Pointing Mode (NPM) occurs, where the S/C performs a fine attitude control based on the use of reaction wheels in close loop with star trackers, gyroscopes and GPS, and magnetotorquers for wheel unloading. 2 Sentinel-1A operations are conducted at the European Space Operations Centre (ESOC) in Darmstadt, Germany. During the three-day LEOP the main activities of the Flight Dynamics (FD) team as part of the ESOC Mission Control Team were: • Determine the injection orbit achieved by Soyuz/Fregat and support the Ground Station Network in acquiring the S/C signal at every scheduled visibility. Antennas were located in Svalbard (Norway), Alaska, Kiruna (Sweden) and Troll (Antarctica). • Monitor the AOCS telemetry during the deployment of the SAR wings and the Solar arrays as well as the S/C mode transitions ranging from Rate Damping Mode to its final Normal Pointing Mode required for operating the S/C during the Commissioning and Routine Phases. • Generate the AOCS commands required to re-initialise the on-board orbit propagation throughout its different accuracy modes to allow the S/C mode transitions. • Start the preparation of a manoeuvre sequence to acquire the Mission Reference Orbit. The manoeuvre sequence selection was driven by the overall duration of the acquisition period and the fuel consumption. This paper presents the FD operational activities conducted during the three-day LEOP. 2. Separation and first acquisition Figure 2. Sentinel-1A LEOP Ground Stations Network and S/C ground-track evolution during the first 4 hours after separation. 3 The Russian Soyuz launch vehicle was released from its launch pad at the Europe’s Space Port in French Guyana on April 03 2014 at 21:02:26 UTC. Sentinel-1A separation from the Fregat-M upper stage took place 1404 seconds after lift-off, following a nominal ascending flight sequence which included separation of the Soyuz two main stages, payload fairing jettisoning, third stage boost, nose module separation and Fregat-M main burn. Separation occurred within visibility of KSAT ground station at Svalbard (see Figure 2) and three minutes before entering visibility from the SSC/USN antenna in Alaska. Approximately one minute after physical separation, the on-board automatic sequence started, switching on the S-band transponder and setting the S/C mode to RDM. The first TM frames were successfully received at Svalbard station and forwarded to ESOC at 21:27:15 UTC. At the end of the first Svalbard-Alaska combined ground pass the S/C rates had been damped as expected (see Figure 3) and auto-convergence to the next AOCS mode SHM had occurred. The received TM indicated a fully nominal S/C behavior and all commanding activities could be completed as planned. Figure 3. First pass (shaded region) S/C rates measured by the gyroscopes. The S/C follows the planned damping in X and Z axes and achieves a rate equal to -8 times the orbital period in Y axis. The S-band transponder was set to incoherent mode to avoid noisy signal at initial acquisition. This implies that no 2-way Doppler measurements were performed during the first pass. Both ground stations acquired the S/C signal and remained in auto-track during the whole pass duration; the carrier uplink was performed first at Svalbard and it was passed to Alaska before the end of the combined pass. Svalbard station reported time offset values (TOV) of 0.3 seconds and 1 second S/C late at the start and end of the pass respectively. Alaska reported 1.5 seconds S/C early at the beginning of the pass. The FD Orbit Determination Team performed an injection orbit assessment at the end of the first ground pass using angular and ranging data retrieved from both stations. 4 A coarse orbit determination employing this set of tracking data did not lead to a conclusive state vector determination. Solutions with a semi-major axis difference with respect to the nominal injection varying from -4.0 to -8.0 km were obtained with similar goodness-of-fit. The reported TOVs by the two stations were contradicting and therefore could not be trusted to decide which orbit solution could be used to generate TOV predictions for the upcoming pass. Consequently FD did not provide a TOV for the subsequent Troll pass but only an indication to expect AOS earlier than predicted. Troll station acquired the Sentinel-1A signal without difficulties, reporting a 1 second early TOV at the beginning of the pass, evolving to 2 seconds early before LOS. At the end of the pass a new assessment of the orbit injection was performed using the ranging and angular tracking data retrieved after the Troll pass.