SSC15-I-2 Rosetta-Philae RF link, from separation to hibernation Clément Dudal, Céline Loisel, Emmanuel Robert CNES 18 avenue Edouard Belin 31401 Toulouse Cedex 9 France; +33561283070 [email protected], [email protected], [email protected] Miguel Fernandez, Yves Richard, Gwenaël Guillois Syrlinks rue des Courtillons, ZAC de Cicé-Blossac, 35170 Bruz, France; [email protected], [email protected], [email protected] ABSTRACT The Rosetta spacecraft reached the vicinity of the comet 67P/Churyumov-Gerasimenko in 2014 and released the lander Philae for an in-situ analysis through ten scientific instruments. The analysis of the lander RF link telemetry reveals major information on the lander behavior and environment during the 50-hour mission on the comet. INTRODUCTION Table 1: Transceiver technical details The ESA/CNES/DLR Rosetta spacecraft was launched Mass 950 g in March 2004 with the objective to reach the comet 67P/Churyumov-Gerasimenko 10 years later. One of its Volume 160 mm x 120 mm x 40 mm 1.7 W Rx only main assignments was to carry out in-situ analysis using Power consumption 6.5 W Rx/Tx at 20°C Philae, a small lander of about 100 kg equipped with (28 V power bus) scientific instruments. The S-Band RF link between (1 W RF output power) Rosetta and Philae was, after separation, the only mean Temperature Operational: -40°C to +50°C of communication with the lander. This paper proposes Radiation 10 krad (cumulated doses) an analysis of the RF link telemetry during the Telecommand link: 2208 MHz Frequency Separation, Descent and Landing phase (SDL) and Telemetry link: 2033.2 MHz during the First Science Sequence after landing (FSS). Modulation QPSK As the comet landing was epic, the descent and landing Differential coding are studied in two different parts. A cross-comparison Data filtering Nyquist half raised cosine filtering of our analysis is made with the one of other scientific (roll-off is 0.35 in Rx, 1 in Tx) teams to strengthen the raised conclusions. Telecommand link 16000 bps Data rate Telemetry link 16384 bps ROSETTA-PHILAE RF LINK OVERVIEW Rx sensitivity range -50/-120 dBm The transceiver is a full duplex S-band transmission set Tx: convolutional coding for digital data developed specifically for space Channel coding (L=7, R=1/2) applications. The conception made by Syrlinks was Rx: Viterbi soft decision decoding done with drastic objectives for mass and power Electrical interfaces RS 485 and CMOS consumption. For this, the use of commercial parts was decided leading to a low cost product widely used There are two transceivers on both sides of the RF link. afterwards on the Myriade platform family. The The redundancy is activated with RF switches on transceiver is composed of a transmitter, a receiver and orbiter side (1 Tx/1 Rx active) and with diplexer on a reception filter for dual antenna use (Figure 1). The lander side (1 Tx/2 Rx active). filter protects the receiver from out-of-band signals, particularly from the transmitter. The two functions The choice of implementing identical RF chains for (receiver and transmitter) are fully independent and can transmission and reception on the orbiter and the lander be activated separately. Technical details are given in has given great advantages, such as cutting procurement the Table 1 and an illustration in Figure 2. costs and simplifying qualification, integration and testing. Dudal 1 29th Annual AIAA/USU Conference on Small Satellites and Visible Analyzer) and ROLIS (Rosetta Lander Imaging System) photographs taken before touchdown. As expected with increasing distance between the lander and the orbiter, the RSSI decreases over time (Figure 3). Figure 1: Rosetta-Philae bidirectionnal RF link With 1W RF output power and 1 dBi gain (@ 60°) patch antennas, link establishment is possible for distances up to 150 km. The lander telecommunication system answers to a request-to-send protocol from the orbiter at any time. This handshake protocol, which implies full duplex Figure 3: RSSI level during the descent toward the equipment and which was specifically designed for comet Rosetta mission ensures a desired quality of transmission even when the relative geometry and Independently of the global decreasing level, low visibility between the orbiter and the lander is not frequency oscillations are noticeable on the RSSI, of an favourable. average duration of 59 min (orange lines on the Figure 3). It could be explained with a multipath effect on the orbiter structure. LANDING The touchdown on the chosen landing site Agilkia was expected at 15h34m10 UTC and occurred with a precision of a few seconds. After touchdown, despite the announced activation success of the anchoring system, the received RF link telemetry indicated multiple and regular interruptions during two hours. The investigation carried out helped to establish the failure of the anchoring system, causing Philae to Figure 2: Rosetta ISL transceiver rebound on the comet surface. There were two In the housekeeping telemetry available at orbiter side, rebounds before stabilization on the ground; they are one parameter is particularly interesting to get studied from a RF point of view below. information beyond its intrinsic value: the Received Signal Strength Indicator (RSSI). From this raw First rebound telemetry value, it is possible to extract the received After first touchdown (TD1), the lander rebounded and power level on orbiter side, which can be then moved away from the landing site. According to other processed as shown in the following parts. instrument teams, it tumbled around the 3 axis keeping a relatively stable position around Z-axis. The rebound SEPARATION AND DESCENT lasted roughly 2 hours. During this period, the RF link Before separation, several milestones had to be suffered multiple and periodic interruptions. The respected in order to deliver the lander in optimal estimated RSSI on telemetry link presents high and fast conditions. Despite some late complications on the variations in the range -80 dBm/-120 dBm reaching the battery heating, the separation of the lander occurred limit of the Rx sensitivity range and leading to those nominally at 8h35 UTC. Before being able to establish link interruptions. the RF link, the Rosetta spacecraft had to maneuver to point its Inter Satellite Link (ISL) antennas toward the After first touchdown (TD1), the lander rebounded and lander, leading to an AOS roughly 2 hours after moved away from the landing site. According to other separation. instrument teams, it tumbled around the 3 axis keeping a relatively stable position around Z-axis. The rebound The link was established within 5 min, as expected, and lasted roughly 2 hours. During this period, the RF link lasted during the 5 remaining hours of descent. It has suffered multiple and periodic interruptions. The allowed the transmission of the CIVA (Comet Infrared estimated RSSI on telemetry link presents high and fast Dudal 2 29th Annual AIAA/USU Conference on Small Satellites variations in the range -80 dBm/-120 dBm reaching the limit of the Rx sensitivity range and leading to those Periodical link interruptions. phenomenon Periodical Freq = 0.0194 Hz phenomenon Freq = 0.044 Hz 16:20 TD2 TD1 17:25 15:34 Figure 4: RSSI level during first rebound Figure 5: RSSI spectrum during first rebound, At 16h20 UTC being 45 min after touchdown, a change before and after 16h20 is observed on the RSSI profile (Figure 4). The maximum measured value is roughly twice higher. The In order to solve the ambiguity on the aliasing effect, a lander motion is directly responsible for this change and cross-comparison is made with ROMAP (Rosetta particularly the spin around the Z axis created during Magnetometer and Plasma-monitor) team analysis. the separation. An analysis in the frequency domain According to them, at 16h20, the lander may have provides more information on any periodical collided with a surface feature which had for effect to phenomenon like a spin motion and its value. slow the lander spin around its Z-axis, changing from 13 s per rotation to 24 s per rotation. It probably also The orbiter telemetry is sampled at Fs = 0.1 Hz (1 reduced the lander tumbling, decreasing the dispersion sample every 10 s). According to Shannon theorem, the around the spin axis. The lander antennas were then maximum frequency for a possible well-sampled better pointed leading to a better signal reception on phenomenon is 0.05 Hz (Period of 20 s). For faster orbiter side. phenomena, aliasing occurs and the frequency of the phenomenon must be interpreted according to the Table 2: Comparison of ROMAP and RF analysis sampling frequency Fs. For a periodic phenomenon at a frequency F higher than Fs/2, the observed frequency Timeline ROMAP RF analysis will be Fo = Fs - F. Before Lander spin at Phenomenon of period The Fourier Transform of the RSSI (averaged 16h20 13 s/rot 12.41 s detected periodogram with Nfft = 128) is computed and After Lander spin at Phenomenon of period analyzed over the period of the first rebound (Figure 16:20 24 s/rot of 22.7 s detected 5). This allows concluding that the periodical phenomenon • Before 16h20, a periodic phenomenon is identified observed before 16h20 is actually biased with aliasing at a frequency of 0.0194 Hz (period = 51.5 s). If and represents the lander spin at 13 s per rotation. The this measure corresponds actually to a faster spin decrease at 16h20 is perfectly visible in the phenomenon and is biased with aliasing, taking analysis. into account the sampling frequency, the real frequency would be 0.0806 Hz (period = 12.41 s). This template has been set up with several features to make it easier for you to format your paper properly.