1 / 139

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

quynh
Télécharger la présentation

SHARAD: SHALLOW SOUNDER

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related