International Technical Symposium on Navigation and Timing November 16th, 2015 Development in GNSS Space Receivers Lionel RIES - CNES 1
C O GNSS in Space : Use-cases and Challenges Receivers State-of-the-Art Examples of Challenging Applications Concluding Slide : what s next? N T E N T S 2
GNSS Use in Space GPS originally designed for use on Earth. It drew interest since the early 80s for orbit determination. Location of space vehicles by means of ground-based equipment Location of space vehicles by means of GNSS space-based equipment Topstar3000 L1 (TAS-F) Use in space spread to cover most known applications of GNSS, adapted to space missions and environment 3
GNSS Use in Space Challenges facing GNSS receivers Mass, power consumption and volume (MCV) Accommodation to space environment Radiation may significantly impair electronics Total doses exceed 10 krad, single event (ions) Thermal control (vacuum : no convection), vibrations (launch period) Limited access, no maintenance, autonomy requirements Small series Dynamic range of signal parameters Parameters Max. value considered Comments Received power dynamic 15 dbhz - 45 dbhz 35 45 dbhz in LEO 15-45 dbhz for GEO Range Up to 80 000 km GEO or elliptical orbits (GTO, HEO) Doppler (Velocity) 50 khz Doppler rate (Acceleration) 100 Hz/s Up to 250 Hz/s for launcher Doppler rate changes (Jerk) is limited, except for launchers and re entry vehicle : transient may exceed 100 Hz/s² Challenge : trade-off State-of-the-Art algorithms and receiver design (often existing for terrestrial applications) to accommodate constraints imposed by space environment 4
State-of-the-Art Architecture is similar to the one of terrestrial receiver RFFE Baseband processor + CPU (eg. : within System-on-Chip) DC/DC Adapted to stand space environment Complemented with a specific space feature : Orbital Filter A tight hybridization with orbit model (keplerian parameters, orbital mechanics, etc. depending on targeted accuracy) It smoothes measurements (accuracy down to ~1 m with C/A), fills measurements gap, aids signal processing (acquisition, tracking) «Advantageous IMU+Map-mathching» : SW only, no drift, no map update required 3 main classes of receiver technologies HI-REL : specific Radiation-Hardened design, Lifetime up to 15 years, used by most missions (except nanosatellites) COTS receivers : ruggedized COTS (automotive, professional, industry grade) SW update to cope with higher dynamic (and ITAR restrictions). Lifetime ~2 3 years, used for experiments or nanosatellites COTS SDR : SDR using COTS components and implementing Radiation-Tolerant design (triplication, etc.) thanks to large FPGA matrix. Lifetime ~5 years (targets up to 7 years) Old concept, but emerging operational use, for all types of satellites 5
State-of-the-Art High-Reliability Receivers Stand-alone equipment : multi GNSS, single or multiple frequency Parameters Typical values Mass-Volume 2 5 kg, a few dm 3 Power consumption Main functions Lifetime 10-20 W 24 36 channels Multi-GNSS, S-F/M-F TTFF (cold start) < 150 s Up to 15 years RG2M (TAS-F) GNSS processor core : State-of-the-Art Rad-Hard System-on-Chip (baseband + navigation processing) Former generation had separate baseband and CPU : Agga-2, PEGASE (TAS-F) Example of SoA GNSS SoC : Agga-4 (ADS), TIGRE (TAS-F) GNSS SoC (space) typ. value 0,18 µm ; 3M-6M Gates ~ 150 mm² die CPU < 100 MHz 36 Channels In comparison, terrestrial GNSS SoC Smaller More channels (32 several 100s) Acquisition engine May include single frequency FE May include other communications functions Trends by 2020-2025 : introduction of 65 nm technology and ARM CPU could be expected 6
State-of-the-Art Software-Defined-Radio Receivers Stand-alone equipment : multi GNSS, single or multiple frequency Skyloc (Syrlinks) Parameters Typical values Mass-Volume ~1 kg, ~ 1 dm 3 Power consumption Main functions Lifetime ~ 5 W 12 36 channels or more Multi-GNSS, S-F/M-F TTFF (cold start) < 100 s Variable - 5 years, 7 years targeted SGR-ReSI (SSTL) TechDemoSat Eg. Skyloc (Syrlinks) equipment to fly on CNES MICROPSCOPE mission as operational equipment Completed VT / AIV beginning of November. Launch in April 2016 Expected experiments with GPS/Galileo signals in 2016-2017 MICROSCOPE GNSS processor core : commercial FPGA Matrix size allows to implement redundancy and Rad-Tolerant design Easy accommodation and reconfiguration to any kind of missions Trends by 2020-2025 : significant growth of such receiver design is expected in space market Thanks to Moore s law : FPGA SoC with more channels, lower size and power consumption, and better tolerance to radiation SDR technology is expected to grow in the space sector (larger series communalized on different missions) 7
Example of challenging applications GNSS in GTO/GEO (1/2) Context : Autonomous navigation in GEO transfer, based on GNSS, could reduce cost of operations Localization in GEO is currently performed on ground GNSS GEO station keeping : few satellites, low SNR, alternative exist GEO Transfer Orbit (GTO) : Interest has recently raised with electrical propulsion. GTO phase may last month to save propellant, requires more autonomy to save operation costs Challenge : ensure GNSS availability despite degraded conditions, at lowest cost to compete with legacy ground segments During GTO : GNSS signals are received in very different configurations (below and above GNSS MEO orbits) High dynamic, low SNR (GNSS SV secondary lobes) Poor geometry, and situation with less than 4 SV visible is common Satellite accommodation constraints the receiving antennas Autonomy requires minimal operations and external aids Technical solution : Two antennas to manage different configuration (LEO/GEO) and spacecraft attitudes during GTO phase Low SNR tracking combined to orbital filter to fill measurements gaps and filter noise and poor geometry High quality reference clock (space qualified OCXO) 8
Example of challenging applications GNSS in GTO/GEO (2/2) 9
Example of challenging applications Critical applications in space Launchers : GNSS may be used for trajectography and may contribute to safety management Issue : real time availability, mission loss, Safety-of-Life issues in case of incident Challenges : GNSS integrity, high dynamic and spin, atmospheric effects, vibrations Technical solutions envisaged : Multiple antennas Multi GNSS, multi frequency GNSS hybridized with IMU (tight or ultra tight) Rendezvous (docking eg. ISS service modules) : GNSS may serve approach phase and contribute to close RdV. Issues (in case of collision) : mission loss, debris, SoL Challenges : GNSS integrity, high accuracy, but usually in LEO with good reception conditions Technical solutions : vision-based GNC, coupled to GNSS for approach Re Entry Phases : GNSS may contribute to trajectography, and maybe to GNC Issues : similar to launcher Challenges : black-out phase, high dynamic and spin, extreme T Technical solutions : TBD, requirements and mission needs are to be consolidated 10
Example of challenging applications GNSS Radio-Occultations Objective : provide atmospheric sounding for meteorology using GNSS signals received in LEO Principle : GNSS signal path are bended between MEO and LEO receivers, according to atmosphere parameters to be estimated (T, pressure, etc.) Phase Delay Bending Angle Atm. Refractivity Atm. parameters T, P, P w Challenge : to achieve high accuracy phase measurements, at low SNR and with potentially high phase noise (scintillations, multipath) Complemented with satellite precise orbit (POD) Preferred implementation Integrated RO and POD GNSS receiver Directive antennas with Beam Forming Network to RO Open loop processing, driven around expected PRN bin (Time of Arrival) by model operated in receiver (MEO and LEO SV position, atmospheric model, clock drift, etc.) Configuration of GRAS Instrument for METOP and METOP-SG (RUAG-ESA) Source ESA 11
Space Service Volume GNSS systems were originally designed for use on earth : ICD defines min. received power on earth but not for space Limited issue for LEO (within GNSS main lobe) A possible shortcoming for use on high altitude and GTO : no received power level guaranteed as coverage relies on GNSS Tx secondary lobes Definition of Space Service Volume, as a minimum received level in space at least up to GEO, would foster applications at high altitudes and beyond MEO orbit (GEO, HEO, etc.) GPSIII s L1C ICD proposes a minimum level for geo orbit Similar approach is to be encouraged from other GNSS providers Measurements campaign of received levels and secondary lobes once satellite is deployed (eg. SSTL GIOVE measurements) are useful if not mandatory 12
What s next? GNSS for space applications reaches a good level of maturity Wide scope of applications alike terrestrial ones Existing technologies allow to meet most challenges and needs. GNSS becomes a standard for most spacecraft Improvements now aim MCV and costs Larger series, lower costs and size are required for large constellations like OneWeb, GEO, nanosatellites. Solutions are already identified (~2005) and investigated thanks to progress in space digital electronics Integration into Spacecraft On-Board Computer thanks to highly integrated SoC More powerful and integrated SDR, thanks to large scale FPGA (and low cost commercial grade FPGA) Expected future of GNSS space receivers GNSS in OBC for most spacecraft operations use case (OD, POD, Timing, etc.), GNSS SDR for science and remote sensing applications Low cost, more powerful SoC and SDR will also foster : GNSS use beyond MEO orbit (more powerful signal processing, less integration constraints) Flexibility to accommodate any missions profile at minimum cost and delay Development of new applications for remote sensing : high density GNSS-RO, GNSS reflectometry, etc. Interface standardization and Space Service Volume definitions remain to be achieved to make GNSS receivers in space as common as in mobile phones 13
What s (next)²? Thank you for your attention! 14