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SHARAD: SHALLOW SOUNDER

SHARAD: SHALLOW SOUNDER. Science & Technical Meeting Rome, 19 - 20 - 21 March 2002. Table of Contents (1/2). Design Status: Overview Summary of SHARAD Requirements Summary of SHARAD System Parameters SHARAD Architecture SHARAD Operative Modes SHARAD Budgets

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SHARAD: SHALLOW SOUNDER

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  1. SHARAD: SHALLOW SOUNDER Science & Technical Meeting Rome, 19 - 20 - 21 March 2002

  2. Table of Contents (1/2) • Design Status: Overview • Summary of SHARAD Requirements • Summary of SHARAD System Parameters • SHARAD Architecture • SHARAD Operative Modes • SHARAD Budgets • Design Status: Analysis and Trade-offs • Design Status: Description • DES - Digital Section • TX and RX - Radiofrequency sections • ANT - Antenna • Thermo-Mechanical & S/C Collocation, Budgets

  3. Table of Contents (2/2) • Design & Development Approach • Model Philosophy • Integration and Testing Flow • Ground Testing Approach • Calibration and Validation • Management & Schedule • Team Composition • Schedule • Receivables • Deliverables • Product Assurance (Mission Assurance)

  4. Design Status:Overview - System architectureR. CROCIE. ZAMPOLINI FAUSTINI

  5. Overview - Summary of Radar requirements • Vertical Resolution: 10 - 20 metres • Penetration Depth: 300 - 1000 metres • Horizontal resolution: 300 - 1000 metres (along track) • Observation Geometry: Nadir looking • Desired centre frequency: @ 20 MHz

  6. Overview - Preliminary System Parameters • Antenna Efficiency: > 10% • Centre Frequency: 20 MHz • Radiated Peak Power: 10 W • Pulse Length: 300 ms (single chirp) • Pulse Bandwidth: 10 MHz • Pulse Repetition Frequency: 200 Hz • Vertical Swath Range: 60 ms (9 Km - free space, 3km – e=9) • Topographic Margin: 40 ms (6 Km - free space) • Receive window: 400 ms (300+60+40) • A/D Resolution: 8 bits • A/D frequency: 26.6 MHz • Maximum Data Rate: 17 Mbit/s

  7. Overview - Design status • System Analysis: “Analysis and Trade-off” Document Released • Antenna: Under final trade-off • Transmitter: Preliminary Requirements Specification Document (“Mini Spec”) released. • Receiver: Preliminary Requirements Specification Document (“Mini Spec”) released • DES: Preliminary Requirements Specification Document (“Mini Spec”) released • Open Points: • On-board Processing • Tracking on board: Open vs Close loop • Data Rate/Volume • ADC number of bits • PRF

  8. Overview - SHARAD Block Diagram • SHARAD interfaces with S/C by means of: • Power I/F (from Bus to SHARAD, used also as on/off switch) • Command I/F (from OBDH to SHARAD) • Telemetry I/F (from SHARAD to OBDH, option not baselined) • TC/TM I/F (between OBDH and SHARAD, option under evaluation) • Science Data I/F (from SHARAD to SSR, used also as HK Telemetry)

  9. Overview - Architecture block diagram

  10. Overview - Architecture baseline • The SHARAD sounder is composed by the following main building blocks. • The DES, in charge of all the digital functions, i.e • Instrument command & control • Signal generation by means of a Digital Chirp generator • Handling and formatting of the digitised received signal • The Transmitter & front-End. • Amplify Tx signal • Provide matching vs the antenna load • Provide duplexing function • The Rx (packed together with DES in the RDS) • Amplifies, filter, digitises the echo signal • The Antenna • radiates the Tx signal and pick-up the echo • can allocate part of the matching network (TBC)

  11. Overview - Architecture considerations (1) GENERAL • Considering the involved frequencies, it has been chosen to avoid frequency conversions in both Tx (generation of chirp directly on a 20 MHz carrier) and in Rx. • The Rx foresees direct sampling at the RF frequency. Sampling is performed at fs < fmax/2, with controlled aliasing (fs > 2 times filter bandwidth) • This approach allows to avoid down conversion and/or sin/cos demodulation, while keeping the sampling rate low. • The current baseline for ADC resolution is 8 bit, with possibility to select a smaller number of bits for transmission to ground.

  12. Overview - Architecture considerations (2) RECEIVER - UNIFORM SAMPLING ON THE CARRIER • If the received echo is directly sampled on the carrier at a frequency fS such as fS=4 f0 /(2M-1) (M integer), achieved samples will be alternatively I and Q samples multiplied by +/- 1. • Samples re-alignment may be accomplished in the digital section using DSP or FPGA: the latter to be preferred in the high speed digitisation for its possibility to implement parallel architectures (subject to trade-off ). • Uniform Sampling on the Carrier may be applied to a 10 MHz Chirp (on 20 MHz Carrier) at a rate of 26.6 MHz.

  13. Overview - Architecture considerations (3) DE-RAMPING TRADE-OFF • Use of de-ramping receiver has been considered to reduce the sampling rate and the overall data volume • PROsPost de-ramping bandwidth is proportional to the width of the tracking window -> significant reduction for small windows (< 100 msec) • CONsEffects of amplitude/phase distortions cannot be compensated after de-ramping: therefore, increase of side lobes introduced by hardware non-idealities cannot be compensated by ground processing. • For this reason, the baseline foresees no de-ramping.

  14. Overview - Architecture considerations (4) ON-BOARD PROCESSING/TRACKING TRADE-OFF • Current baseline does not foresee on-board processing, with possible exception on data coherent pre-summing to limit the data rate. • Availability of raw data on ground would allow to optimise the compensation function (to recover HW errors) and processing parameters without risk of data loss: the drawback is a larger data volume for a give operating time. • Similarly, in order to simplify design and limit the risk areas, it has been assumed an open-loop control for the tracking window positioning, based on platform-supplied orbit data (no tracking of the surface). • Even if surface tracking has to be implemented, requiring a limited on-board processing (i.e., range compression), recommended approach is to use compressed data for the tracker only, and send the raw data to ground.

  15. Design Status:Overview - OperationsE. ZAMPOLINI FAUSTINI

  16. Overview - Operations: Support Modes • CHECK/INIT • HW and SW initialisation of the Digital Electronics Section DES with RX and TX Off. • STANDBY • RX and TX Off; Digital Electronics Section generates Housekeeping Telemetries and accept Macrocommands. • WARM-UP1 • The Receiver, in addition to DES, is switched On. • WARM-UP2 • The Transmitter , in addition to DES and RX, is switched On. • SAFE/IDLE • Used after recovery actions due to severe hardware or software anomalies possibly encountered in any of the available modes (either belonging to Support or Operation classes).

  17. Overview - Operations: Operational Modes • Measurement Modes • SHARAD performs scientific measurements. A variable data rate will be produced depending on the specific processing setup. Sub-Modes are defined to accommodate different types of processing and data rates: SS LOW, SS HIGH, RAW DATA. • CALIBRATION • SHARAD acquires unprocessed sounding data. • RECEIVE ONLY • SHARAD performs passive measurements (mainly during the cruise phase).

  18. Overview - Operations • SHARAD operating modes transition diagram

  19. Overview - Operations: How SHARAD operates GENERALITIES • Heaters Control will be performed by the S/C • Heaters typically OFF during the Operative Part of the Orbit • SHARAD will be turned ON and OFF every Orbit (TBC) • SHARAD will go through its SUPPORT and OPERATION Modes autonomously. • This way to operate will be accomplished by means of automatic mode transitions (SUPPORT Modes have a fixed duration) or following the instructions listed in the OPERATIONS SEQUENCE TABLE

  20. 4 19 4 Mode Duration in PRI Mode Sel 1 Param 1 DCG Config. Param 2 Spares N Mode Duration in PRI Mode Sel DCG Config. Param 1 Param 2 Spares Overview - Operations: How SHARAD operates OPERATIONAL SEQUENCE TABLE • Two different type of Operational Sequence Table (OST) are envisaged: • Default Operation Sequence Table (DOST) stored in EEPROM • Orbit Dedicated Operation Sequence Table (ODOST) loaded in RAM by dedicated MCMD • OST Composition (Data Field of dedicated Serial MCMD):

  21. Overview - Operations: How SHARAD operates SHARAD COMMANDABILITY • The instrument can be commanded using packet MCMD • SHARAD can receive TC Packets only in STANDBY Mode • SHARAD can be switched ON/OFF by the spacecraft in any operative mode without any special reactivation procedure other than the nominal switch-on procedure. SHARAD OBSERVABILITY • 4 Analog Temperature Sensor Channels • Packet TM covering both HK (current mode, time, voltages, MCMD Accepted/Refused, etc.) and Events (Mode Transitions and Anomalies)

  22. Overview - Operations: What SHARAD needs ON-BOARD • Heater Lines Controlled by S/C • Command to load Orbiter Time (mandatory for each flyby) • Command to load the PARAMETERS TABLE (mandatory for each flyby) • Command to load the OPERATIONS SEQUENCE TABLE (optional but typically used for each flyby) ON-GROUND (every orbit) • Accurate Orbit Prediction/Reconstruction (time to p, H, Vt, Vr) • Predicted/Reconstructed Attitude • Knowledge of solar zenith angle

  23. Design Status:Overview - BudgetsR. CROCIL. MARINOE. ZAMPOLINI FAUSTINI

  24. Overview - Mass Budget Unit masses (Kg) Note: these estimations are still preliminary (waiting for feedback from unit suppliers) and do not include instrument contingency - up to 15 kg . More reliable figures can be provided following feedback from unit suppliers.

  25. Overview - Power Budget Power Consumption (W) + Replacement heater Power: currently allocated 10 W (not simultaneous to operating power)

  26. Overview - Data Volume

  27. Design Status:Analysis and Trade-offsL. MARINO

  28. Analysis and Trade-Off - Contents • Observing Geometry • Principles of Operation • Investigation Approach • Preliminary Parameters Sizing • Surface Clutter and Penetration Depth • Surface Received Power Levels • Surface Signal to Noise Ratio • Subsurface Received Power Levels

  29. v H Zmax Height Along Track PR range presentation time Isorange Contour time PD range presentation time time Surface Clutter Echoes dynamic range after signal compression & SAR Processing Cross Track Isodoppler Contour time Echo from subsurface Analysis - Observation Geometry & SAR Concept

  30. Analysis - Key Design Elements • System Key design elements: • Center frequency: 20 MHz • One frequency modulated radar pulse: • length 300 s; • bandwidth 10 MHz. • Radiated Peak Power: 10 W • System Key design elements Subjected to Trade-Off: • Pulse Repetition Frequency: 200 Hz • Open Loop control for the tracking window positioning • A/D Resolution: 8 or a selectable lower number of bits • A/D frequency (on the Carrier Frequency): Max 30 Mhz • On ground, echoes are processed through SAR based techniques to enhance the azimuth resolution and therefore clutter reduction.

  31. Analysis - Investigation Approach (1/3) • The ability of SHARAD to achieve its science objectives will be largely dependent on the electrical properties of the soil. • A few representative category of the Martian surface composition has been selected as most meaningful: • The scenarios employed for the modelling of the detection subsurface are: I/W)Ice/water interface detection scenario: the pores are filled with ice from the surface down to a depth below which liquid fills the pores. D/I) Dry/ice interface detection scenario: the pore-filling material is considered to be gas up to a depth below which ice fills the pores.

  32. Analysis - Investigation Approach (2/3) • To assess the interface detection performance the back scattering cross sections of concurrent echoes coming from the surface and subsurface layers have to be evaluated. • Fractal geometric description of the surface in the classical Kirchhoff approximation: H=1 Geometric Optic model (Simpson and Tyler, 1982): H=0.5, Hagfors’ model (Hagfors, 1964):

  33. Analysis - Investigation Approach (3/3) • Fresnel reflectivity for a subsurface layer located at a depth z:

  34. Analysis - Preliminary Parameters Sizing (1/3) • For a chirp radar (as required in planetary missions due to the low power available), range resolution is a function of the transmitted bandwidth: z=c/(2 B e) • In terrestrial dry rocks, values of e usually range between 4 and 10, decreasing for increasing porosity. • Requirement: 10 meters. • Except for the II-1 category, just a 7 MHz Chirp bandwidth met the requirement.

  35. Analysis - Preliminary Parameters Sizing (2/3) • Along-Track Resolution: the 300 - 1000 m requested, imply the use of synthetic aperture processing techniques for resolution enhancing. RAZ=  H / 2LS • Cross-Track Resolution: limited by the radar vertical resolution. Pulse-limited resolution : R=2 x (c H/B) ~ 5.3  6.9 Km for 10 MHz bandwidth; ~ 6.3  8.3 Km for 7 MHz Bandwidth. • In presence of specular surfaces, the horizontal resolutions will be limited by the Fresnel zone size: r= (H /2) ~ 1.5 Km 1.6 s < Tint < 2.9 s for RAZ=300 m 0.5 s < Tint < 0.8 s for RAZ=1000 m H ~400 km Vsc~3360 m/s H ~ 230 km Vsc~3440 m/s

  36. Analysis - Preliminary Parameters Sizing (3/3) • Aliasing in the observed doppler spectrum must be avoided. • PRF has to be sized over the off nadir observation angle beyond which the surface clutterreturns are 30 dB lower the nadir surface echo: • The doppler bandwidth to be sampled by the system will be enveloped by twice the  angle: PRF 200 Hz  Martian Surface Coverage 35% PRF 150 Hz  Martian Surface Coverage 28%

  37. Analysis - Surface Clutter and Penetration Depth (1/3) • Surface return echoes are considered as clutter, because the subsurface discontinuity detection may be reduced from the surface back-scattering. • Synthetic aperture processing improves the subsurface detection capability thanks to the reduction of the off-nadir surface clutter power: typical improvement: ~ 10 dB.

  38. Analysis - Surface Clutter and Penetration Depth (2/3) • Improvement Factor coming from Processing Doppler at a depth z:

  39. Analysis - Surface Clutter and Penetration Depth (3/3)

  40. Analysis - Surface Received Power Levels (1/2) • Rough surfaces cannot have returns higher than specular ones: maximum back-scattered power is provided by Fresnel Model. • Minimum Power Level: • Geometric model; • wavelength scale r.m.s. slope 0.06; • Martian Crust Category II-1 (sediments); • Scenario D/I (porosity 50%); • Satellite altitude 400 Km. • Maximum Power Level: • Fresnel model; • Martian Crust Category II-3 (basalts); • Scenario: interface I/W (porosity 20%); • satellite altitude 230 Km.

  41. Analysis - Surface Received Power Levels (2/2) • Surface Power Levels for all the Scenarios and Categories of the presented Martian Crust materials and for the worst case of wavelength-scale r.m.s. slope of 0.06.

  42. Analysis - Galactic and Extra-Galactic Noise • The graphic shows Galactic background measurements at 5.2, 9 , 15.6, 23 MHz using half wave dipoles together with those made by Novaco and Brown (1978). The smooth curve is derived from a simple model of the Galaxy: Ig() and Ieg(): brightness contributed by the Galaxy and by the extra-galactic sources; (): optical depth for the absorption.

  43. Analysis - Surface Signal to Noise Ratio (1/2) • For the Fresnel Model the Doppler Processing Gain is negligible. • Synthetic Aperture will be limited by the Fresnel Circle  number of integrable pulses 4.

  44. Analysis - Surface Signal to Noise Ratio (2/2) • SNR for all the Scenarios and Categories of the presented Martian Crust materials and for the worst case of wavelength-scale r.m.s. slope of 0.06.

  45. Analysis - Subsurface Received Power Levels (1/2) • Subsurface Dynamic represents the attenuation which the signal is subjected to when crosses the Martian subsurface and is reflected at depth z. • Subsurface and Crust Materials have been assumed with the same characteristics: category, scenario, r.m.s. rugosity slope.

  46. Analysis - Subsurface Received Power Levels (2/2) • Subsurface Power Levels for all the Scenarios and Categories of the presented Martian materials and for the worst case of wavelength-scale r.m.s. slope of 0.06. • Power Levels have been computed at 300 meters of penetration depth.

  47. Analysis - Sampling Technique • If it exists the relationship (with M integer): • samples of the signal s(t) are: • Samples re-alignment may be accomplished by DSP or FPGA (the latter to be preferred for its parallel architectures - subject to trade-off ). • Signal Spectrum of a 7 MHz Chirp on 20 MHz Carrier, after Uniform Sampling on the Carrier at a rate of 16 MHz:

  48. Analysis - Data Volume • The SHARAD Radar Sounder is able to work 30 minutes per orbit and four orbits per day. • Avoiding De-ramping (availability of raw data on ground) allows to optimise processing parameters without risk of data loss. • Coherent pre-summing may be foreseen in order to limit data rate. • In order to simplify design and limit the risk areas, it has been assumed an Open-Loop control for the Sampling Window positioning, based on platform-supplied orbit data.

  49. Design Status:DescriptionF. BERNARDINIR. CROCIM. MAPPINIM. MARCOZZIP. NOSCHESE

  50. Introduction • The SHARAD system is composed of four units: • DES: Digital Electronics Section • TX: Transmitter section • RX: Receiver section • ANT: Antenna DES and RX are mechanically joined to form a single section: • RDS: Receiver and Digital Section

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