SARAL (Satellite with ARgos and ALtiKa)

SARAL is a cooperative altimetry technology mission of ISRO (Indian Space Research Organization) and CNES (Space Agency of France). In this setup, CNES is providing the payload module consisting of the AltiKa altimeter, DORIS, LRA, and Argos-3 DCS (Data Collection System), and the payload data reception and processing functions, while ISRO is responsible for the platform, launch, and operations of the spacecraft. A CNES/ISRO MOU (Memorandum of Understanding) on the SARAL mission was signed on Feb. 23, 2007.

The overall objectives are to realize precise, repetitive global measurements of sea surface height, significant wave heights and wind speed for: ¹⁾

• The development of operational oceanography (study of mesoscale ocean viability, coastal region observations, inland waters, marine ecosystems, etc. )

• Understanding of climate and developing forecasting capabilities

• Operational meteorology.

The SARAL mission is considered to be complementary to the Jason-2 mission of NASA/NOAA and CNES/EUMETSAT (it is also regarded a gap filler mission between Envisat and the Sentinel-3 mission of the European GMES program). The combination of two altimetry missions in orbit has a considerable impact on the reconstruction of SSH (Sea Surface Height), reducing the mean mapping error by a factor of 4. ^{2) 3) 4)}

AltiKa, the altimeter and prime payload of the SARAL mission, will be the first spaceborne altimeter to operate at Ka-band.

Background: The AltiKa concept, based on a wideband Ka-band altimeter (35.75 GHz, ~500 MHz) concept, was initially proposed in 2002 as a CNES altimetry minisatellite mission (150 kg) on the Myriade platform. Feasibility studies were also made to accommodate AltiKa on the TopSat platform of SSTL (Surrey Satellite Technology Ltd.). In Dec. 2005, CNES approved the development of the AltiKa payload since an opportunity arose to embark the instrument on a cooperative mission of ISRO and CNES, namely OceanSat-3. However, early in 2006, the OceanSat-3 launch was postponed to the period 2011/12, beyond the schedule objective of AltiKa. As a consequence, CNES and ISRO established an alternative option, based on a dedicated minisatellite using a new SSB (Small Satellite Bus) platform to be developed as ISRO. ^{5) 6) 7)}

Figure 1: Overview of current and future altimetry missions (image credit: CNES)

Spacecraft bus and payload module development at ISRO:

The minisatellite, as provided by ISRO, involves nothing less but the development of a new spacecraft bus. The general spacecraft architecture consists of two modules: namely the introduction of a new modular bus (under development at ISRO as of 2007), referred to as SSB (Small Satellite Bus), and PIM (Payload Instrument Module). Two SSB designs are under development: ^{8) 9)}

1) The first one is being developed for a minisatellite series with a total launch mass of about 450 kg, including a payload mass of ~ 200 kg.

2) The second one is considered for a microsatellite series with a total launch mass of around 100 kg, including a payload mass of 20-30 kg

The design layout is such that the bus module and the payload module of each series may be integrated and tested separately (thus reducing the interdependency during the realization between both modules). SSB is also designed to accommodate different types of payloads with minor modification from mission to mission. SSB is also developed with the intention to permit a multiple payload launch by PSLV using the DLA (Dual Launch Adaptor), which caters to minisatellites in the class of 450-600 kg.

The structure of the bus is built with aluminum honeycomb panels. It consists of 3 horizontal decks, namely the bottom deck, top deck and payload deck, and four vertical equipment panels. The bottom deck has the interface ring (with Merman band interface 937VB) bolted to it. The design employs a number of stiffness measures to avoid any resonant frequencies of the launcher in both lateral and longitudinal directions.

The SSB system design includes all general platform services functions such as the EPS (Electric Power Subsystem) with battery and solar arrays, the AOCS (Attitude and Orbit Control Subsystem), TT&C (Telemetry, Tracking & Command) subsystem, RCS (Reaction Control Subsystem), the thermal subsystem for bus (a passive system is used), and mechanisms (for solar panel deployment), etc. Most of the systems have redundancy/large margins or space capacity.

Figure 2: Overview of the SARAL spacecraft configuration (image credit: ISRO/CNES)

Table 1: Overview of preliminary SSB minisatellite bus series specifications

The minisatellite bus module has a standard simple interface for the payload module. The new bus derives its shape from previous IRS configurations with cuboid structure. The bottom deck of the minisatellite bus is measuring 900 mm x 900 mm providing an interface with launch vehicle. The launcher interface uses a Merman clamp band, 937VB. The top deck is of the same size as the bottom deck featuring four corner points extending as pillars to provide a four-point interface for the payload module.

The PIM (Payload Instrument Module) consists of a set of CFRP (Carbon Fiber Reinforced Plastic) based panels with appropriate interfaces for mounting onto the main platform and for mounting of the payloads and associated elements. The payload-related systems like data handling, SSR (Solid State Recorder), etc., are mission specific functions.

Figure 3: General configuration of the PIM (image credit: ISRO)

The PIM is also of square shape and of the same cross-section size as that of the bus with four corners (payload interface pods) interfacing with main bus. The PIM, unlike the bus module, has the freedom to grow vertically; the only limitation is the available volume within the heat shield enclosure of the launch vehicle and allowable frequency constraints.

Figure 4: General block diagram of the SSB (Small Satellite Bus), image credit: ISRO ¹⁰⁾

SARAL spacecraft:

The spacecraft is 3-axis stabilized - using the bus SSB1. Attitude is sensed by a double-head miniature star sensor providing an attitude knowledge of < 30 µarcsec about all axes. This attitude information is also used to update the inertial angle information of the gyros. The gyros are also of a miniaturized version. Further attitude sensors are sun sensors (4π FOV) and a magnetometer. Actuation is provided by reaction wheels (5 Nms) and magnetorquers. The latter one are used for the momentum dumping of the wheels. The monopropellant RCS uses a single tank (21 kg of propellant) with two blocks of 1N thrusters. It is mainly used for orbit maintenance.

The EPS employs a single bus with a bus voltage of 28-33 V. An average power of ~500 W is provided with articulated solar panels. A Li-ion battery with 18 Ahr capacity is used for eclipse operations. The power conditioning and the power distributing converters have full redundancy.

The BMU (Bus Management Unit) with the OBC takes care of all data handling functions using the MIL-STD-1553B standard interface with all onboard subsystems. The payload data handling subsystem employs serial interfaces to the baseband data handling formatter. The formatter receives the payload data packets, annotates these with the housekeeping data, and deposits them onto the SSR.

The SARAL spacecraft has a mass budget of about 346 kg. The SSB has a total mass of 199 kg, while the total payload mass is 147 kg (with 5% margin). The design life is 5 years.

RF communications: There is no real-time data transmission requirement for SARAL. The SSR has a capacity of 32 Gbit. The TT&C data are transmitted via S-band. The payload data are transmitted in X-band at data rates of up to 20 Mbit/s. The formatted and encoded data from SSR is PCM/QPSK modulated. The transmitter uses a SSPA (Solid State Power Amplifier) with an EIRP output of ~5 W. In addition, the UHF link is used to collect the data from the ground segment with the Argos-3 onboard instrument while the L-band is used to transmit the collected data to the data centers.

Figure 5: Artist's rendition of the SARAL spacecraft (image credit: CNES)

Launch: A launch of the SARAL minisatellite is planned for late 2009/early 2010 from SDSC-SHAR (Sriharikota, India) on a PSLV launcher of ISRO.

Orbit: Sun-synchronous near-circular dawn-dusk orbit, altitude of ~800 km, inclination of 98.55º, orbital period of 100.6 minutes, LTAN (Local Time on Ascending Node) = 6:00 hours, repeat cycle = 35 days.

Sensor complement: (AltiKa, DORIS, LRA, Argos-3)

AltiKa (Altimeter in Ka-band). The AltiKa project of CNES is based on a wideband Ka-band altimeter (35.75 GHz, 500 MHz), which will be the first oceanography altimeter to operate at such a high frequency. The high-resolution AltiKa altimeter has a dual-frequency radiometric function which allows the altimetry measurements to be corrected for the effects due to the signal passing through the wet troposphere. This is coupled with the DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) tracking system and an LRA (Laser Retroreflector Array) for the measurement of precision orbits. ^{11) 12)}

The key feature of the altimeter payload has been the selection of Ka-band (35.75 GHz) for the altimeter avoids the need for a second frequency (necessary when using the Ku-band) to correct the ionospheric delay; the configuration allows the same antenna to be shared by the altimeter and the radiometer. This single antenna solves the accommodation problem of a conventional altimetry payload on a minisatellite (150 kg class). The Ka-band concept allows also the improvement of the range measurement accuracy by a factor of 3:1 due to the use of a wider bandwidth and a better pulse-to-pulse echo decorrelation.

The AltiKa design and development is a partnership of CNES, scientific laboratories (LEGI/CNRS, IFREMER, CLS, etc.) and industry, with TAS-F (Thales Alenia Space-France) as prime instrument contractor. The AltiKa instrumentation consists of an integrated altimeter/radiometer instrument. It is composed of the following elements:

• ORA (Offset Reflector Antenna) shared by the altimeter and the radiometer

• AMU (Altimeter Microwave Unit). The objective is to gather all analog support tasks of the altimeter operations (bandwidth expansion, frequency translations, local oscillator generation, high power amplification, transmitter/receiver/antenna duplexing, low noise amplification, deramp processing, bandwidth filtering, IF processing).

• RMU (Radiometer Microwave Unit). The objective is to gather all analog support tasks of the radiometer operations (low noise amplification, bandwidth filtering, high gain amplification, power detection, low pass filtering).

• DPU (Digital Processing Unit). The objective is to gather all the (hardware and software) processing functions of the instrument (digital chirp generator, time FFT, altimeter echo processing, radiometer signal processing, instrument interface handling with the platform computer).

• RCU (Radiometer Calibration Unit). The objective is to assure the interconnection and switching between the antenna, the radiometer receivers, and the radiometer calibration loads or horn.

Figure 6: Functional block diagram of the integrated AltiKa instrument (image credit: CNES)

The altimeter design principle is of Poseidon heritage (Ku-band instrument flown on the Jason missions) and is based on the classical deramp technique for pulse compression Four key evolutions on AltiKa have improved significantly the radar performance compared to conventional Ku-band altimeters:

1) The larger bandwidth from 320 MHz (of Ku-band instruments) to 480 MHz (Ka-band) provides a vertical resolution improvement from 0.5 m to 0.3 m, providing also the same magnitude of improvement in the range accuracy.

2) The shorter decorrelation time of the sea echoes at Ka-band permit an increase in the PRF (Pulse Reception Frequency) for averaging efficiently more echoes in the same integration time. Hence, the PRF has been doubled with respect to the Ku-band (Poseidon) instruments. AltiKa provides a PRF of about 4000 Hz adjustable along the orbit to cope with altitude variations in the surface profile.

3) The antenna has a smaller beamwidth due to the increase in signal frequency, thus providing a smaller footprint in the target area. At an orbital altitude of 800 km, the 6 dB footprint is around 8 km for AltiKa, compared to 30 km for Poseidon. Hence, more accurate measurements in the coastal regions can be expected (better discrimination in transition zones).

4) An innovative echo tracking concept is being employed based on an internally stored DEM (Digital Elevation Model) of the sub-satellite track (ocean and land surfaces) and on the use of the real-time satellite altitude information provided by the DORIS navigator software DIODE. These features help in providing altimetric measurements on surfaces where conventional closed-loop tracking solutions have difficulties to keep the echoes within the narrow range window.

The range measurement accuracy versus wave height is illustrated in Figure 7. The improvement for AltiKa compared to Poseidon-2 is in the order of 3.

Figure 7: Comparison in range measurement accuracies in Ka- and Ku-band (image credit: CNES)

Table 2: Key parameters of the AltiKa instrument

Radiometer design: The instrument is a total power radiometer with a direct detection capability. It consists of two RF receivers, centered on 37 and 23.8 GHz, and a calibration unit enabling the connection of the receivers either to a sky horn pointing to provides a cold space reference, or to a load at ambient temperature (hot reference).

In the nominal mode, the radiometer receivers measure the antenna temperatures (RM1 to RM5). In the internal mode, the receivers are either connected to a sky horn pointing to deep space or to a load at ambient temperature. This internal calibration can be performed every few seconds.

Table 3: Key radiometer parameters and performances

Figure 8: Illustration of the AltiKa antenna (CAD model), image credit: CNES

The critical technologies for AltiKa development concern the Ka-band functions except the radiometer receivers that are taken from the receivers of the Megha-Tropiques mission (ISRO/CNES). This includes:

• The multi-frequency antenna

• The Ka-band SSPA (Solid State Power Amplifier)

• The Ka-band LNA (Low Noise Amplifier)

• The Ka-band radiometer calibration unit

• The Ka-band altimeter duplexer equipment (ADE)

The AltiKa antenna (Figure 8) is a fixed offset reflector (1 m aperture diameter, 0.7 m focal length, 0.1 m offset) with a tri-band feed (35.5 - 36 GHz, 23.6 - 24 GHz, 36.5 - 37.5 GHz). The feed includes an OMT (Ortho-Mode Transducer) device for the separation of the altimeter and radiometer channels which use perpendicular polarizations, and a diplexer for the separation of the radiometer (K-band, Ka-band) channels.

The characteristics of the SSPA are the following: gain of 35 dB, useful bandwidth of 500 MHz, output power of 2 W (33 dBm). The power dies are being built in D01PH technology of OMMIC, France. The output combiner is in waveguide technology to minimize losses. All RF components of the SSPA are provided with a pulsed power converter to minimize power consumption and dissipation, and to decrease the junction temperatures of the components.

Figure 9: RF synoptic of the pulsed SSPA (image credit: CNES)

The real-time telemetry data rate of AltiKa (altimeter+ radiometer) is about 43 kbit/s.

RCU (Radiometer Calibration Unit). The RCU consists of two separate channels operating in the K- and Ka-bands. Two ferrite switches are provided in each channel to allow selection of one of the three different operational paths, corresponding to each of the operational modes. Each channel uses a very low VSWR waveguide load, which acts as the "hot" calibration source. Additionally, the Ka-band channel contains a bandpass filter to improve the isolation between the RCU channels and the altimeter.

The ADE (Altimeter Duplexer Equipment) consists of a single ferrite switch on the transmit path and four ferrite switches in the receive path. The transmit and receive paths are connected via a ferrite circulator which also provides a directional path to and from the antenna port. Two cross-guide couplers provide the calibration path. The RCU and the ADE are developed at COM DEV, UK.

Figure 10: View of the RFU instrumentation (image credit: COM DEV)

Figure 11: CAD view of the integrated AMU (Altimeter Microwave Unit), image credit: CNES

Table 4: AltiKa instrument budgets

DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite). DORIS is a dual-frequency tracking system (400 MHz and 2 GHz) based on network of emitting ground beacons spread all over the world. The DORIS on-board package is composed of:

• A dual-frequency antenna (omnidirectional antenna located on the nadir face of the satellite). The antenna has a mass of 2 kg and a size of 420 mm (length) and 160 mm in diameter.

• A BDR (DORIS Redundant Box) which is composed of two DORIS chains in cold redundancy accommodated inside the same electronic box. Each DORIS chain includes a MVR (Mesure de Vitesse radiale) unit achieving beacon' signal acquisition and processing, navigation, Doppler measurements storage and formatting, electrical satellite interfaces management functions, and a USO (Ultra-Stable Oscillator) delivering a very stable 10 MHz reference which is also used by the altimeter. BDR has a mass of 18 kg and a size of 390 mm x 370 mm x 165 mm. ¹³⁾

Figure 12: View of the DORIS BDR and antenna (image credit: TAS-F)

LRA (Laser Retroreflector Array). The objective is to calibrate the precise orbit determination system and the altimeter system several times throughout the mission. The LRA is a passive system used to locate the satellite with laser shots from ground stations with an accuracy of a few centimeters.

Argos-3 (Data Collection System) of CNES, manufactured by TAS (Thales Alenia Space). The objective is to collect data from remote terminals in the ground segment referred to as PPTs (Platform Transmitter Terminals).

The Argos-3 onboard package represents the newest generation of the Argos system. The major improvement of the new Argos-3 system is that it will now be able to send orders to its terminals whereas before the onboard instruments were only capable of receiving data (up to Argos-2 inclusive). The MetOp-A spacecraft of EUMETSAT (launch Oct. 19, 2006) is carrying the first Argos-3 instrument demonstrator package. In comparison to previous generations, system performance is enhanced via a unique downlink and a high data uplink (4800 bit/s versus 400 bit/s), while insuring complete compatibility with existing systems in the ground segment. Thanks to digital processing, the new instrument is lighter and more compact than its predecessors on analog basis. Argos-3 is capable of receiving messages from over 1000 PTTs (Platform Transmitter Terminal) simultaneously within the satellite's FOV (Field of View).

The Argos-3 onboard instrument is composed of the following components:

• The RPU (Receiver Processor Unit) providing the following functions:

- Processing of the received uplink signals

- Downlink management

- Interfaces with the receiver, the TxU and the satellite

• The TxU (Transmitter Unit) is sending the emissions (messages) to the PTTs in the ground segment including error-free message acknowledgement signals.

• The harness for the RPU to TxU connection

The RPU (16 kg, 36 W) and TxU (8kg, 26 W) boxes have a cold internal redundancy that can be activated by TC level. In the same way, the USO (Ultra Stable Oscillator) has a cold redundancy. RPU has dimensions of 365 mm x 280 mm x 365 mm, TxU has dimensions of 100 mm x 280 mm x 310 mm.

RPU (Receiver Processor Unit). The RPU onboard a spacecraft processes received uplink signals @ 401.6 MHz, measures the incoming frequency, time-tags the message, creates and buffers mission telemetry, manages the downlink and acts as interface between the receiver, the TxU (Transmitter Unit) and the satellite. Featuring fully digital processing, the RPU stores messages and either relays them in real-time to the nearest regional antenna - or in deferred time to a global center (maintained by NOAA, Eumetsat). A backup RPU is included as part of the device.

Figure 13: Illustration of the Argos-3 onboard instrument package (image credit: TAS-F)

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