Figure 1. System overview
The Polar Platform carries a wideband communications
system - the DRS Terminal - which will transmit the scientific
data generated by Envisat's instruments to the ground via the
Artemis data-relay satellite. The characteristics of this
Ka-band link impose stringent requirements on both the
Terminal's mechanical and electrical elements in order to
preserve end-to-end data integrity during the communication
sessions. A complex control system is needed to ensure that
correct Artemis pointing and tracking is maintained by the DRS
Terminal on the orbiting Polar Platform.
Envisat will be the first ESA mission to use Artemis as an
integral part of its nominal operational service.
For the Envisat mission, the Polar Platform (PPF) will carry a number of Earth-observation instruments that will generate extremely large volumes of scientific data. There will be two different systems available to communicate that data to the ground, one operating in X-band and the other in Ka-band. Like the X-band systems used on ERS-1 and 2, the Envisat X-band system will provide a direct ground link, but data can be downloaded only when the ground station is visible from the Polar Platform. Real-time transmission and a higher information transmission capacity are also needed to cope with the much greater volume of observational data that will be produced by the new generation of Envisat instruments, leading to the addition of the Ka-band system - known as the Data-Relay Satellite Terminal (DRST) - to operate in combination with Artemis.
This will be the first time that an ESA mission has used a DRS service during the routine operational phase. Some four years ago, an Inter-Orbit Communication experiment was successfully conducted using ESA's Olympus satellite as the relay between the Eureca retrievable carrier in orbit and the ground (from July 1992 to June 1993).
Three main components - the PPF, Artemis and the User Earth Terminal - make up the end-to-end system. The Polar Platform's Envisat mission calls for a low-altitude polar orbit with a mean height of 800 km (orbital period approx. 100 min) and a 35-day repetition cycle, providing global Earth coverage for most of the onboard instruments within three days. The Polar Platform is designed to support a four-year Envisat mission lifetime.
Artemis will be located at the 16.4°E geostationary slot, with east-west and north-south accuracies of 0.14°. The greatest range to Envisat will be about 45 500 km, and this worst case has been used as the reference for link-budget computations.
The User Earth Terminal (UET) is the receiving station in the Envisat Payload Data Segment (PDS) responsible for the reception, demodulation and processing of the instrument data. It is a new, dedicated facility to be built at ESRIN, in Frascati near Rome (I). Figure 1 is a schematic of the complete system. The connections between the Envisat and Artemis Operation Control Facilities are also shown.
Figure 1. System overview
Ka-band transmission of the scientific data will occur in two stages. The signal will first be sent from the PPF DRS Terminal to Artemis via the Inter-Orbit Link (IOL), which has three 250 MHz channels centred at 26.850, 27.100 and 27.350 GHz. Two channels can be used simultaneously, and each active channel can transmit at 50 or 100 Mbit/s (rate selectable from the ground). Next, this signal is frequency-downconverted to 20 GHz, power-levelled by the Artemis SKDR payload (no further data processing is performed at this point)and relayed to the UET via the Artemis feeder link.
The Envisat mission planning envisages communication sessions through the DRS lasting an average of 30 min per orbit. The Earth coverage achieved with this baseline is depicted in Figure 2, where the shadowed zones represent areas for which the Ka-band link is not available. This coverage pattern has been computed taking into account the DRS Terminal's exact location on the Polar Platform, some occultation due to the latter's appendages (e.g. the solar array), and finally the Earth shadowing. Occultations observed in Europe, Central Asia or South America can occur in either an ascending or a descending orbit.
Figure 2. Earth coverage with Artemis at 16.4°E
It is estimated that a total of 21 000 such communication sessions between the DRS Terminal and Artemis will take place during the four years of the Envisat mission.
Figure 3 shows the structural model of the Polar Platform in the ESTEC test facilities. The DRS Terminal's antenna is visible on the upper left-hand face of the spacecraft.
Figure 3. Structural model of the Polar Platform in the
ESTEC test facilities (LEAF)
The Terminal consists of two parts, the In-Board Assembly (IBA) mounted inside the Payload Equipment Bay (PEB), and the Out-Board Assembly (OBA) mounted externally on the zenith-pointing face of the Platform. The IBA contains all the electronic equipment responsible for RF carrier generation, modulation, signal amplification and filtering. There are three identical RF chains centred at the three transmission frequencies assigned to the Inter-Orbit Link. Following the mission requirements, up to two RF chains can be active simultaneously. The third one is a spare. Figure 4 shows the engineering model of the Ka-band Transmit Panel during the test phase.
Figure 4. Engineering model of the Polar Platform's Ka-band
Transmit Panel. (photo courtesy of Alcatel Telecom)
The OBA contains the mechanical elements that sustain the Ka-Band Antenna and allow the tracking of Artemis during the DRS communication session. The OBA consists of a 2 m-long mast, the Antenna Pointing Mechanism (APM) and the Ka-Band Antenna. In launch configuration, the OBA is stowed as shown in Figure 5.
Figure 5. The OBA in launch configuration (up) and its
stowage within the Ariane-5 fairing (down).
To support this configuration and the subsequent deployment, the OBA includes a hold-down/release and a deployment mechanism. There are four hold-down points located below the Antenna and APM, and the deployment mechanism is at the base of the mast. Once in orbit, the OBA deployment begins after the sequential activation of four pyrotechnic devices housed in each hold-down point. The deployment manoeuvre, controlled by a damping device, lasts approximately 1 min, ending with the mast locked in deployed configuration.
The APM (Fig. 6) has two motor gears mounted on an L-shaped bracket. The motors provide independent rotation axes in azimuth (±165°) and elevation (range 90° to -30°). Both motor gears have optical encoders to record the actual antenna position.
Fig. 6a.
Fig. 6b.
Figure 6a,b. Schematic of the APM (a), and its qualification model (b). (photo courtesy of Alcatel Espacio/Inta)
The Ka-band Antenna is a high-gain (44 dB) Cassegrain design with a main reflector diameter of 0.9 m, radiating in right-hand circular polarisation. The RF connection between the IBA and the Antenna is via a waveguide and rotary joints. Thermal control is provided by heaters commanded by the Platform's thermal-control subsystem. The overall OBA is thermally protected by multi-layer insulation.
Functionally associated with the OBA, although physically located within the PEB, are the Antenna Pointing Controller (APC) and the Pointing Mechanism Drive (PMD). The APC controls the Antenna's movement during communication with Artemis. The APC provides two signals, for azimuth and elevation movements, to the PMD, which converts them into the required current levels for driving the APM's motor gears.
Envisat's PEB (Fig. 7) accommodates the data-handling units that interface the scientific instruments with the communications systems. These units collect, format, record and code the instrument data before X- or Ka-band transmission.
Figure 7. Schematic of the Envisat PEB, showing the Ka-band
data-transmission path
Units like the High-Speed Multiplexer (HSM), Tape Recorders (TRs) and the Encoding and Switching Unit (ESU) are part of the PEB's Payload Data Handling (PDH) system. Others such as the Remote Terminal Units (RTUs), Power Distribution Units (PPDUs) and Heaters Switching Unit (HSU) are peripheral elements to support the data handling and other functions.
The scientific instruments send their data in source-packet formats to the HSM. This unit can handle both Low Bite Rate (LBR) data, for instruments with transmission rates below 10 Mbit/s, and Medium Bit Rate (MBR) data, for instruments with rates below 32 Mbit/s. In both cases the interfaces are asynchronous and each instrument supplies its own clock signal. The HSM collects data from nine LBR instruments - ASAR and MERIS in LBR mode, GOMOS, MIPAS, RA-2, AATSR, DORIS, MWR and SCIAMACHY - and from MERIS in MBR mode. It formats these instrument data into Virtual Channel Data Units (VCDUs) with a Reed-Solomon (RS) protection code.
The HSM sends the LBR information to the four onboard 16-track Tape Recorders (at 4.6 Mbit/s) and an LBR and MBR composite to the Encoding and Switching Unit (at 50 Mbit/s). Each Tape Recorder has a capacity of 30 Gbit and allows data input at 4.6 Mbit/s and playback at 50 Mbit/s. This one order-of-magnitude ratio between the recording and playback speeds is the most challenging tape-recorder requirement.
Then, the Encoding and Switching Unit (ESU) receives data from three different sources: the Tape Recorders and the HSM both at 50 Mbit/s, and the ASAR in High Bit Rate (HBR) mode at 100 Mbit/s. The ESU's architecture provides the flexibility to route the input data from any of these sources to any of the three Ka-band modulator interfaces. Data are differentially encoded and, to increase immunity to noise, half-rate convolutionally encoded. Finally, the data are delivered in BPSK or QPSK format for RF modulation. This unit's high-performance capability must ensure data-processing integrity with less than one bit error for each 50 000 million transmitted bits.
The communication sessions via DRS will be conducted according to a predefined protocol. Artemis will point its IOL antenna towards the PPF. The Platform's Ka-band antenna boresight must point towards and track Artemis to cope with the relative motion of the two spacecraft during each session. The open-loop pointing approach adopted for the DRS Terminal requires complex algorithms, shared between the ground and space segments, which determine the correct orientation for the antenna and control its movement.
The Envisat mission-control scenario will define the start and end times of the DRS communication sessions during each orbit. With this information, orbitography computations will be performed to define the Ka-band Antenna trajectories needed to locate and track Artemis. From these trajectories, pointing tables containing the required antenna azimuth and elevation angles at 2 min intervals will be derived and uploaded to the PPF on a daily basis and stored by the Payload Module Computer (PMC). The PMC will then perform an interpolation to compute the pointing directions at 1 s intervals, sending two types of commands to the APC: a prepointing command at the beginning of the session to move the antenna from its current position to point towards Artemis and, once in tracking mode, a series of acquisition commands every second conveying the azimuth and elevation angles that the antenna must satisfy 2 s later. The APC will use this information and the actual antenna position supplied by the APM's optical encoders to further interpolate the antenna trajectory at approximately 23 ms intervals. Pulse trains modulated by elevation and azimuth velocities will be sent to the PMD with the same frequency. The PMD will then generate the driving currents for the APM motors to ensure that the antenna describes the desired trajectory (Fig. 8). The pointing accuracy achieved, thanks to the sophistication of the APM's design features, will better than one thousandth of one degree (0.36 arcmin).
Figure 8. The pointing and tracking functions
The Antenna Pointing Mechanism is particularly critical element in the DRS Terminal's performance, not only because the stringent pointing-accuracy requirements, but also because of the number of operating cycles that it will have to endure during Envisat's four-year mission lifetime. For reason, the mechanism has been submitted a 90 000 cycle life test based on a 211° angular excursion profile, with a velocity 4.2°/s and an acceleration of 0.5°/s² (considerably higher that the maximum velocities and accelerations expected during flight). To verify the complete the rotational range, the origin of the angular movement was shifted by a few degrees every 100 cycles. The test showed that the mechanism's functional performance remained within specification and also allowed important design features such as the liquid lubrication to be tested (Fig. 9).
Figure 9. Qualification model of the APM's azimuth motor
gear (photo courtesy of Alcatel Espacio/Inta)
Because of the open-loop pointing approach, it is important to consider the factors that can induce pointing instabilities. These factors, such as residual misalignment between the OBA elements, depointing due to thermal distortion, or random errors due to the APM, have been identified, analysed and budgeted for. The appropriate power level and radiation pattern (Fig. 10) from the DRS Terminal must ensure stability of the signal received by Artemis.
Figure 10. Flight model antenna's co- and cross-polar
radiation patterns at 27.1 GHz (courtesy of Saab Ericsson)
The Ka-band link is designed to comply with the mission requirement of providing a bit error rate for payload data of better than 10(exp. -6) , with a link availability of 99%. Table 1 presents the link-budget summary for QPSK modulation, showing that the requisite BER figure is met.
Inter Orbit Link (IOL) Artemis Feeder Link
Link Characteristics Link Characteristics
Availability 99 % Availability 99 %
Bit Error Rate 1E-06 Bit Error Rate 1E-06
Modulation Type QPSK Modulation Type QPSK
Information Bit Rate 100 Mbit/s Information Bit Rate 100 Mbit/s
Transmission Bit Rat 200 Mbit/s Transmission Bit Rate 200 Mbit/s
Required E(b)/N(0) 10.5 dB Required E(b)/N(0) 10.5 dB
Coding Diff-Conv 0.5 Coding Diff-Conv 0.5
Coding Gain 5.5 dB Coding Gain 5.5 dB
Polarisation RHCP Polarisation Linear Vertical
Frequency 27.1 GHz Frequency 20 GHz
SKDR Noise BW 234 MHz SKDR Noise BW 234 MHz
PPF Spacecraft Artemis Spacecraft
Transmit Power 58.50 W Transmit Power 30.00 W
Transmit Power (dB) 17.67 dBW Transmit Power (dB) 14.77 dBW
IBA +OBA RF loss - 4.34 dB Total SKDR RF loss 4.86 dB
Antenna Gain (EOC) 43.93 dBi Antenna Gain (EOC) 36.8 dBi
EIRP 57.26 dBW EIRP 46.71 dBW
Required EIRP 55.20 dBW
Margin 2.06 dB
Path Path
Distance 45 400 km Distance to UET 40 500 km
Space Loss - 214.24 dB Space Loss 10.61 dB
PFD at Artemis - 106.87 dbW/m² PFD at UET 116.43 dBW/m²
Ground
UET Receive G/T 37.20 dB/K
C/N(o) 90.68 dB
E(b)/N(0) 10.68 dB
Channel Degradation 2.90 dB
Required E(b)/N(0) 5.00 dB
(with cding gain)
Margin 2.78 dB
Needed E(b)/N(0) 7.75 dB
for VCDU10(exp.-11)
Needed Margin 0.03 dB
On the ground, the signal, once demodulated, is sent to the Viberti and Reed-Solomon decoder. The impact of the coding and decoding sequence used in the Ka-band link has been the subject of a specific study under an ESA contract to analyse and simulate the end-to-end link in terms of the required bit-error and VCDU deletion rate figures. The results show that the required VCDU deletion rate of 10(exp. -11) is met with a signal-to-noise ratio of 7.5 dB in BPSK modulation, and 7.75 dB in QPSK. Given the current Ka-band link budget, and considering that the Envisat onboard noise does not exceed 10(exp. -6), this requirement is marginally satisfied for the case of QPSK modulation, as shown in Table 1. Potential improvements in the data processing between the Viterbi and RS decoders have been investigated which, if required, can improve the current margin by 1 dB.
Two DRS Terminal models are envisaged in PPF programme, namely an engineering and a flight model.
The engineering model was completed before the end of 1995 and the Transmit Panel part has been integrated at higher level (PEB and PPF) for testing and validation at module and system level, respectively. The OBA part has been submitted to alignment checks and thermal-balance/thermal-vacuum and modal-survey tests. Later it was integrated into the PPF structural model for the modal-survey, acoustic and pyro-shock release tests in the LEAF and vibration testing in the new HYDRA facilities at ESTEC (NL).
Delivery of the various units for the flight-model DRS Terminal was completed in September 1996. Tests at overall Terminal level are scheduled until November 1996, followed by subsequent delivery and assembly, integration and test tasks at Payload Equipment Bay and Polar Platform level.
Matra Marconi Space (UK) is the Polar Platform Prime Contractor. Dornier GmbH (D) is the Payload Equipment Bay contractor, and within this module, responsible for the DRS Terminal. The industrial organisation charged with building the DRS Terminal is summarised in Figure 11. Alcatel Telecom (B) is responsible for the IBA and most of the OBA units, whilst Sener (E) has supplied the reminder of the OBA elements.
Figure 11. The industrial organisations building the DRS
Terminal
ESA Bulletin Nr. 88.