GOCE Workshop

1. GOCE Workshop • SATELLITE TO SATELLITE TRACKING INSTRUMENT • DESIGN • PERFORMANCE

2. HERITAGE • SSTI instrument is based on the LAGRANGE receiver architecture • LAGRANGE receiver development started in 1998 with Alcatel Alenia Space Italia S.p.A. internal funds • The product is developed, manufactured and tested by the Navigation Department in Milan plant • LAGRANGE X-prototype developed in 1999 with ASI contribution for demonstrative flight on SAC-C satellite • LAGRANGE fully space compatible design completed end 2001 • LAGRANGE is onboard the following satellites: • RADARSAT-2 (2 FM’s) • COSMO SKYMED constellation: 3 satellites (1 EQM + 6 FM’s) • OCEANSAT (1 EM + 1 FM tailored for Radio Occultation application) • Soyuz (1 FM) • LAGRANGE was onboard Soyuz mission 10S in April 2005 in the frame of ENEIDE mission

3. DESIGN 1/3 Receiver Type: Integrated GPS receiver for spaceborne applications Channels: 12 dual-frequency channels Frequency Band: GPS L1: 1575.42 MHz GPS L2: 1227.6 MHz Observables: L1CA, L1P & L2P Code phase L1CA & L2P Carrier phase L2-L1 delta range Instantaneous Doppler Time

4. DESIGN 2/3 • Receiver Unit: • Power Supply Module • Processor Module • AGGA 2 Module • RF/IF Module • Synch Module • Motherboard • RYMSA • GPS L1/L2 Antenna

5. DESIGN 3/3

6. MAIN FUNCTIONALITY Pseudorange measurements (Code Phase) • C/A Code, for GPS L1 • P Code, for GPS L1 & L2 • L1P - L2P Delta Range Integrated Doppler measurements (Carrier Phase) • L1 and L2 Carrier Phase Signal and Noise measurements • Signal to Noise Ratio C/No in dB/Hz units evaluated at the tracking loop input • AGC Setting Real-Time Orbit Determination, determined using GPS C/A signal observations and navigation messages (PVT solution through SPS and Navigation Kalman Filter) Time measurements, determined from the GPS system

7. ELECTRICAL INTERFACES POWER LINES: • N° 1 Unregulated Primary Power Supply Interface (connected to Satellite Power Bus) TELECOMMANDS: • N° 8 High Power ON/OFF Command (HPC) (Nom + Red for Receiver ON, Nom + Red for Receiver OFF, Nom + Red for Watch Dog Enabling, Nom + Red for Watch Dog disabling) TELEMETRY: • N° 2 MIL-STD-1553B I/F (Nominal + Redundant) • N° 1 Temperature Sensing Monitor I/F (TSMN) (Thermistor on the DC/DC Board) • N° 1 Analog Single Ended Monitor (ASMN) (5V Secondary Voltage Monitor) • N° 2 Relay Sensing Monitor (RSMN) (Unit ON/OFF and Watch-Dog EN/DIS Relays) SYNCHRONISATION: • N° 2 External Synchronization Pulse Per Second (PPS) (Nominal and Redundant) RF INTERFACE: • N° 1 RF Input (from Antenna)

8. PHYSICAL BUDGETS Mechanical Dimensions • 250 mm x 200 mm x 190 mm (LxHxW) including mounting feet • 29.32 mm x 212.1 mm x 225.5 including Diplexer & LNA Cables Mass: • Receiver Unit : 5.35 Kg ± 5% • Antenna: 490.6 g ± 10% • Cable to antenna: 225g ± 10% Power Consumption: • 29.3 W (steady state) • 32.1 W (OCXO warm-up)

9. SSTI PERFORMANCE SSTI Characteristics • Tight requirements in order to comply with system specifications • Performance Requirements on three large areas: • Real-Time Navigation (PVT) • Raw Measurements (errors, acquisition/tracking thresholds, biases…) • Robustness • Temperature Sensors on-board placed on sensitive points of the RF board in order to allow calibration of Rx-specific biases (IFB) • Precise Modeling/Testing of errors (Multipath, antenna CoP etc.) • Peculiar measurement sampling procedure (sample after PPS in input), typical of timing receivers • Freely drifting Rx time scale, Rx bias bound between ± 10 ms

10. SSTI PERFORMANCE SSTI Performance Test Setup:

11. SSTI PERFORMANCE • RT Navigation • Tested under Selective Availability ON and OFF (major source of RT navigation error) • Output tested in different reference frames (ECI J2000, ECI TOD, ECEF) • Orbital GOCE scenario, usually having a duration of 12 hours • Period of time representative of GPS orbital period • GOCE orbit modeled using a 70x70 gravity field model, GPS 8x8 • Drag acceleration enabled • Force model and reference frame basically limited by Spirent implementation.

12. SSTI PERFORMANCE • RT Navigation Scheme

13. SSTI PERFORMANCE • Navigation Requirements: Navigation Results: Results with S/A off (current GPS status) much better: NKF Position J2000.0 (3D 3s): ~ 30.35 m NKF Velocity J2000.0 (3D 3s): ~ 0.12 m/s

14. SSTI PERFORMANCE • Navigation Results (example)

15. SSTI PERFORMANCE • Visibility to GPS (example) Direction Of Arrival Density Plot

16. SSTI PERFORMANCE • Acquisition-Tracking thresholds & Measurements accuracy:

17. SSTI PERFORMANCE SSTI Sensitivity to Ionospheric Scintillation • A dedicated test campaign has been carried out to check robustness against ionospheric scintillation activity • Ad-hoc test set-up (Test Equipment, SSTI firmware modifications) • GISM (Global Ionospheric Scintillation Model) simulator by IEEA has been used to generate the scintillation time series in terms of amplitude and phase errors • Amplitude fades injected at RF level using a programmable RF attenuator • Phase errors injected at DSP level in the SSTI (pre-correlators carrier rotation) • Orbital scenario in the RF simulator, scintillation error applied repeatedly during a pass. • S4 values from 0 (no effect) up to 0.8. GISM version has S4 & sigma_phi linked • Analyses in terms of loss of lock on carrier phase in post-processing

18. SSTI PERFORMANCE Ionospheric Scintillation Test Setup Test Setup Phase error injection

19. SSTI PERFORMANCE Scintillation Time Series Example of amplitude and phase errors applied: Characterization in terms of loss of lock sensitivity to S4 parameter

20. SSTI PERFORMANCE Example of Results S4 = 0.76: Not OK! S4 = 0.6: OK! SUMMARY:

21. SSTI precursor: ENEIDE • SSTI-Like experience on ENEIDE experiment (May 2005): • different target application (combined GPS/EGNOS/WAAS tracking from space) but same technological base of SSTI • LAGRANGE receiver installed inside the Soyuz Orbital Module • GNSS antenna installed outside the Soyuz vehicle • Laptop installed inside the Orbital Module and connected with LAGRANGE Receiver via MIL-BUS1553 data cable • The Crew MMI provides commanding and monitoring functions • All the data acquired by LAGRANGE Receiver handled by the Laptop Software and stored on the PCMCIA Hard-Drive

22. SSTI precursor: ENEIDE ENEIDE experiment antenna LAGRANGE receiver T. Col. Vittori ENEIDE Experiment Conductor

23. SSTI precursor: ENEIDE All the analyses conducted on the flight data allow to say that the ENEIDE experiment was carried out successfully. ENEIDE proved to be a benchmark for the validation of the AAS-I receivers of LAGRANGE class, like GOCE SSTI. The different environmental conditions in which the receiver operated (Sun-pointing attitude, orbital attitude, peculiar antenna pointing constraints etc.) made the experiment an important laboratory for different applications like attitude analysis, atmospheric sounding, space weather, real-time navigation and control. • Reference: • A. Zin, S. Landenna, A. Conti, L. Marradi, M. S. Di Raimondo, “ENEIDE: an Experiment of a Space-borne, L1/L2 Integrated GPS/WAAS/EGNOS Receiver”, European Navigation Conference 2006, (ENC 2006), May 08-10 2006, Manchester, UK.

24. GOCE SSTI: FACTS • State-of-art GPS space receiver technology • Advanced performance • Twelve LAGRANGE flight models built (two SSTI FMs) • Fully qualified for space applications for several satellites • Flight proven