Presentation on theme: "Mars Radar Sounders LA RIPRESA DELLE MISSIONI VERSO MARTE 198820011999199819961992 Phobos 1 & 2 URSS Mars Observer USA Mars 96 Russia Mars Global Surveyor."— Presentation transcript:
Mars Radar Sounders LA RIPRESA DELLE MISSIONI VERSO MARTE 198820011999199819961992 Phobos 1 & 2 URSS Mars Observer USA Mars 96 Russia Mars Global Surveyor USA Mars Pathfinder USA Mars Climate Orbiter USA Nozomi JPN Mars Polar Lander, USA Deep Space 1 USA Mars Odissey USA
Mars Radar Sounders Strategia di ricerca scientifica Ricerca dell acqua Geologia Vita Clima Preparazione per l esplorazione umana ACQUAACQUA Quando Dove Forma Quantità
Mars Radar Sounders Science Objectives The primary objective of the SHARAD experiment is to map, in selected locales, dielectric interfaces to a kilometer in depth in the martian subsurface and to interpret these results in terms of the occurrence and distribution of expected materials, including competent rock, regolith, water and ice. Map the thickness, extent and continuity of the layers within the polar deposits. Map the thickness, extent and continuity of sedimentary layers. Map the distribution of shallow buried channels. Identify regions on Mars for follow-up surface-based water/ice exploration. SHARAD is a radar sounder provided by ASI to NASA as a facility instrument, payload of the MRO mission What is SHARAD?
Mars Radar Sounders U.S. Programma di esplorazione di Marte
Mars Radar Sounders MRO Strumenti della missione Strumenti scientifici HiRISE (High Resolution Imaging Science Experiment) (20m/pixel) CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) (0.4-4 ) MCS (Mars Climate Sounder) (analisi atmosfera, profilo acqua, sabbia, CO 2, temperatura) MARCI (MArs Color Imager) (analisi atmosfera, nubi, ozono, albedo etc. 0.28-0.8 ) CTX (ConTeXt imager) (6m/pixel) SHARAD (SHAllow (subsurface) RADar). L italia è responsabile sia dello studio che della implementazione. Engineering Payload Electra UHF communication and navigation package Optical navigation camera experiment Ka band telecommunication experiment
Mars Radar Sounders Caratteristiche dello S/C Launch mass: 2180 kg Size: 14 m solar array tip to tip and 7 m high Array power: 2 kW in Mars orbit Maximum data rate: 5.6 Mb/s 3 m HGA and 100W TWTA Rolls to +/-30 deg. 160 Gbit solid state recorder
Bande di frequenza Risoluzione verticale ( ε r = 5) Profondità di penetrazione Risoluzione verticale 1,3-2,3 MHz: 2,5-3,5 MHz; 3,5-4,5 MHz; 4,5-5,5 MHz ~ 70 m (Banda=1 MHz) [150 m nello spazio libero] Da ~ 0,5 Km a ~5 Km 5-9 Km(along track) x 15-30 Km (across tack) I parametri di sistema
Mars Radar Sounders Le tecniche di riduzione del clutter di superficie Dual Antenna: Aggiunta di una seconda antenna con un nullo nel diagramma di radiazione in direzione nadir. Ciò consente di valutare le eco off-nadir, che possono essere sottratte da quelle dellantenna primaria. Doppler Beam Sharpening: Consiste nel ridurre lampiezza del fascio dantenna sfruttando il moto del satellite per sintetizzare unantenna di dimensione maggiore di quella reale. In tal modo si riduce lampiezza del footprint nella direzione del moto del satellite (along track) con diminuzione degli effetti di riflessione off-nadir. Dual Frequency Processing: La riflessione superficiale non dipende dalla frequenza, cosa che invece avviene per le riflessioni subsuperficiali. Lutilizzazione di due frequenze e lelaborazione delle eco relative consente la discriminazione desiderata.
Mars Radar Sounders System Parameters (from the SHARAD SFRD) Centre Frequency:20 MHz Pulse Bandwidth:10 MHz Radiated Peak Power:10 W Pulse Length:85 us Antenna Efficiency:> 10% Pulse Repetition Frequency:700 Hz, 670Hz, 775 Hz (350, 335, 387.6 Hz) alternate PRF added to cope with orbital extremes during extended phase (including topography margin) Receive window:135 us Receiver gain80 dB A/D Resolution:8 bits Downloaded sample bits8 (default), 6, 4 A/D frequency:26.67 MHz Maximum Data Rate:20.16Mbit/s (@ 700 Hz, no pre- summing) On-board pre-summing range1 to 32 samples
Mars Radar Sounders DESCRIPTION OF OPERATING GEOMETRY SHARAD is a nadir looking radar sounder with synthetic aperture capabilities
Mars Radar Sounders PRINCIPLES OF OPERATION OF A RADAR SOUNDER 2/3 In the presence of a dielectric discontinuity in the subsurface, the radar sounder will receive a second echo that is much weaker than the first surface echo. How much weaker this second echo will be depends upon the crust attenuation and the characteristics of the dielectric discontinuity. If D is the system detection dynamic range, the detection of this second echo will be possible only if its power is no more than D dB less than the first surface signal. If z is the depth of the interface and f is the frequency of the radar sounder, the instrument will be able to detect this second echo if and only if The available detection dynamic could be affected by: Surface Clutter Echoes (coming from off nadir) Noise Sidelobes and other artifacts due to the compression of the strong surface echo in presence of phase and amplitude errors Necessary dynamic Available dynamic
Mars Radar Sounders 2.3 Planning Tool General Criteria 1/2 Ionosphere f pm Sub Surface Material Magnetic Field Not Visible Zone S/N<0 Visible Zone S/N>0 Sun elevation SHARAD Mars Surface SurfaceClutter
Mars Radar Sounders HORIZONTAL RESOLUTION (ALONG-TRACK AND CROSS-TRACK) ROUGH SURFACE Limiting the synthetic length at the dimension of DPL in order to avoid RCM problems and more complicated processing, the along-track resolution will be bounded by the DPL dimension as a function of frequency and S/C height. SPECULAR SURFACE the synthetic length is limited by the first Fresnel circle diameter the Raz is a function of S/C height and frequency, The cross-track res. matches the Fresnel diameter
Mars Radar Sounders HORIZONTAL RESOLUTION (ALONG-TRACK AND CROSS-TRACK) BOUNDARY CONDITIONS
Mars Radar Sounders RANGE (DEPTH) RESOLUTION weighed compressed chirp sidelobes for four different weighting function vs. time and depth of the possible synchronous interface echo depth resolution Vs. the real part of the crust dielectric constant for different weighting function
Mars Radar Sounders OBSERVATION GEOMETRY MRO Orbit Characteristic periapsis altitude near 255 km; apoapsis altitude near 320 km; near-polar inclination of 92.6 degrees; approximate ground-track repeat cycle of 17 days
Mars Radar Sounders Doppler phase evolution It is possible to denote the following quantities with the following symbols: R 0 :Slant range of the observed point H: Orbital altitude V R : Radial Velocity of the S/C V T : Tangential Velocity of the S/C The evolution of the distance in the synthetic aperture time as a function of the orbit position, including also the surface slope θs, is given is given by:
Mars Radar Sounders Doppler phase evolution If monochromatic wave of frequency f and wavelength is transmitted the phase difference between transmitted and received waveform due to the two way travel over range R is given by: Doppler Centroid due to the radial velocity and the tangential velocity component due to surface slope Doppler Rate describes the linear frequency modulation induced by the S/C tangential motion Variable with λ: high fractional bandwidth should be considered
Mars Radar Sounders AZIMUTH PROCESSING FOR A LOW FREQUENCY WIDE BAND RADAR: FOCUSED PROCESSING Doppler rate compensation has to be done adaptively in the frequency If it is done only on the carrier the resulting azimuth compressed pulse is the following
Mars Radar Sounders AZIMUTH PROCESSING FOR A LOW FREQUENCY WIDE BAND RADAR: FOCUSED PROCESSING The same result could have been obtained considering again the maximum mismatching in the Doppler rate (as for Cook and Bernfeld chap 6) The doppler rate correction has to be performed adaptively in the frequency with a step of at least 1Mhz.
Mars Radar Sounders What is the doppler spectrum received? PRF is very high high over-sampling of the received doppler spectrum Datarate has to be optimized maintaining the SNR SHARAD antenna is a have a width beam-width Doppler spectrum is not determined by the antenna!! For very rough surfaces there will be not negligible return at off nadir As more the surface is smooth as more rapidly the returns from the surface at off-nadir angles will drop off R H z x h
Mars Radar Sounders SHARAD on Board and on Ground Processing: The on board processing of SHARAD foresee the following steps Possible compensation of the linear phase term due to the radial velocity and to the surface slope and tangential velocity; this compensation is carried on only on the carrier frequency Doppler presumming of a certain number of echoes to optimize data production rate and data volume The on ground processing include the compensation of the quadratic phase term. The latter compensation should take into account the high fractional band of the signal therefore it should be executed in the frequency domain and adaptively for each frequency in the band The more accurate reconstructed orbital parameter will allow to overcome the uncertainties on the radial and doppler velocity Techniques to overcome the surface/subsurface slope uncertainties is under study (doppler filter bank)
Mars Radar Sounders PRESUMMING LIMITATION Number of adjacent pulses that is possible to pre-sum will be strongly limited by the operative environment and by the desired performance of the radar. Pre-summing setting will be a trade off among desired performance and data production rate Two possible causes for limitation in the pre-summing : 1. Maximum residual phase shift tolerable at the edge of the synthetic aperture if the compensation of the radial velocity and surface slope is not performed (or if it is not correct) Influenced by the surface slope 2. Limit in the aliasing of the Doppler spectrum (due to off nadir clutter power) imposed by the desired detection dynamic range Influenced by the surface roughness
Mars Radar Sounders SHARAD PRESUMMING:PHASE ERRORS LIMITATIONS 1/4 what is the maximum number of pulses that is possible to pre-sum given by the useful doppler band? Limiting the maximum phase shift in a pre-summing interval (k/PRF) to π/4 and considering that the maximum phase shift occurs at the edges of the synthetic aperture:
Mars Radar Sounders Max pre-sum as a function of the surface slope correction accuracy obtainable in the on board processing 0.175 rad 10 deg 0.0175rad=1deg Vt=3410m/s, Vr=30m/sec,H=290km This limit is related to the accuracy in the knoledge of the MARS surface slope and in the polinomial approximation SHARAD PRESUMMING:PHASE ERRORS LIMITATIONS 2/4
Mars Radar Sounders SHARAD PRESUMMING:PHASE ERRORS LIMITATIONS 3/4 what is the maximum number of pulses that is possible to pre-sum given the high fractional bandwidth? As the on board compensation of the linear term is performed only on the carrier frequency (due to the difficulties to realize an on board FFT ) The residual phase shift in the signal bandwidth could be very high and could strongly influence the maximum number of presummable pulses. The error as a function of the presuming rate and of the frequency error (f- f 0= 5Mhz ) becomes :
Mars Radar Sounders Max pre-sum as a function of the surface slope due to the wide fractional bandwidth 0.175 rad 10 deg 0.0175rad=1deg Vt=3410m/s, Vr=30m/sec,H=290km This is an absolute limit resulting from the wide fractional band and the limitation in the on board processing SHARAD PRESUMMING:PHASE ERRORS LIMITATIONS 4/4
Mars Radar Sounders SHARAD PRESUMMING:CLUTTER LIMITATIONS Synthetic aperture processing requires additionally that aliasing in the observed Doppler spectrum must be avoided (SAR makes an intrinsic spatial sampling). Supposing an isotropic antenna pattern in the along track direction, and considering the clutter formulation it is possible to determine the off nadir observation angle beyond which the off nadir surface clutter returns are 30 dB or more lower than the nadir surface echo: ( )/ (0)<-30 dB The Doppler bandwidth to be observed and thus sampled by the system will then be the one enveloped by twice the calculated and therefore to satisfy the Nyquist condition: And thence
Mars Radar Sounders SHARAD PRESUMMING:PHASE ERRORS LIMITATIONS 4/4 Max number of presumable as a function of the surface roughness (to be evaluated on a scale of the order of the DPL) in the hypothesis on stationary surface of the region interested by the receiving window
Mars Radar Sounders SHARAD PRESUMMING:CONCLUSIONS There are 3 factors that have to be considered in the maximum pre-summing rate evaluation 1.A high tilt in the surface will increase the errors due to the wide fractional band 2.If the surface tilt is low but the accuracy in the knoledge/rapresentation of the slope is coarse the driving factor in the limitation will be the residual phase errors 3.If the small scale roughness of the surface is high the return doppler band will increase, consequently the equivalent PRF to be utilized will increase and the usable pre-summing rate will decrease
Mars Radar Sounders Marsis primi dati scientifici Dati Simulati
Mars Radar Sounders Operations & Science Data Processing System SHARAD operation center will be sited in Rome, Italy, under the Team Leader Institution (INFOCOM responsibility). Facility off-campus will be rented for this purpose (as for MARSIS operation center) Still TBD the availability of a university structure SHARAD operation center will accomplish the following tasks and will maintain all the related HW, SW and procedures SHARAD Planning SHARAD Commanding SHARAD Data Processing –Instrument Monitoring –Quick Look –Science Processing SHARAD SOPC reside at JPL and is be accessible remotely (SSH Login) Although slowly ASI provides the archiving facilities at its ASDC (under ASI responsibility), Frascati, Italy, site. ASDC will accomplish the data archiving, and distribution (to the PDS node and to the science team)
Mars Radar Sounders Operations & Science Data Processing System Science data distribution and archive ASDC PDS Science data products Operation planning Data processing SHARAD OPERATION CENTER: INFOCOM Processed data Commanding Telemetry Delivery System (TDS) RAW Sci Data Server (RSDS) DOM NAIF Server S/C Engineering Data Planning Files SPK CK files SOPC (JPL ) Sci Product Telemetry Engineering Data (TBC) Planning and commanding Files JPL SHARAD Team Members
Mars Radar Sounders SHARAD OBSERVATION CONSTRAINTS SSR (solid state recorder) allocation 34 GBit reserved to SHARAD It is managed cyclically (FIFO) In case of overflow data is truncated Downlink allocation Data down-linked for SHARAD is equivalent to 15% of the mission downlink Roughly from 7 to 15 Gbit per day Possible Electromagnetic incompatibility with the S/C transmission in X-band SHARAD can operate when the S/C is pointed nadir (+-10 degree)
Mars Radar Sounders Basic Assumption in SHARAD operation (1) According to its system characteristics SHARAD is in principle able to operate at any time in the orbit, independently of the sun illumination conditions. Constraints may then be those arising from the overall mission design Polar Observations SHARAD Team wants some continuous observations over poles, and other specific targets of particular scientific interest, in daylight SHARAD would get an allocation of ~200 (TBD) dedicate polar passes per pole, ~400 (TBD) total during PSP Remembering : that SHARAD is a nadir instrument that SHARAD does not require any pointing (in routine operation) that SHARAD can operate with other instrument off-nadir pointing (up to 10deg TBC) => SHARAD observation will be mostly in the nighttime therefore NIO => Dayside Observation for SHARAD will be managed as IO.
Mars Radar Sounders Basic Assumption in SHARAD operation (2) The SHARAD team will develop and maintain a database of desired targets of observation. The SHARAD team will develop and maintain a Coverage Database The operation of SHARAD will be mainly dedicated to use all the available information about the MARS operative environment to set properly the on board pre-processing parameters and optimize the data production rate: MARSIS measurements, MOLA topography, other existing science datasets and already-processed SHARAD data can be used to estimate dynamic range, the off-nadir clutter power and the predicted performance of the radar those information can be used to set properly on board processing parameters The on board parameters setting will determine the SHARAD Data rate SHARAD data volume allocation and SSR partition will determine the operative time of the instrument
Mars Radar Sounders How the instrument data rate changes with the presumming? PRF NOMINAL FM Measured Data Rate Mbps Pre-summing 4 bit6bits8 bits 110.7515.5620.36 25.377.7810.18 42.683.895.09 81.341.942.54 160.670.971.27 280.380.550.72 320.340.480.63
Mars Radar Sounders SHARAD OBSERVATION PLANNING During the mission SHARAD team will submit planning files in the form of PTF (payload target files): SHARAD PTF Shall contain in particular Latitude –Areodetic center latitude for the observation Orbit Number and Orbit Alternatives (optional). Observation Duration Setup Duration –number of seconds by which loading of the sequence precedes the start of actual data acquisition Orbital Data Table filename; Parameters Table filename ; Sequence Filename (OST) filename. MRO JPL team will: Integrate all instrument PTF Ensure there are no conflicts MRO JPL team will delivered IPTF and convert IPTF into binary ITL file and uplinks to MRO
Mars Radar Sounders Uplink Process: OST PT ODT Files ITL initiates SHARAD block and instrument file load SHARAD nominally does not uplink commands During the regular instrument programming three files for each active orbital pass are uploaded: the OST (Operational Sequence Table) file The OST, shall contain SHARAD measurement modes programming for the active portion of an orbit. the PT (Parameter Table) file Parameters contained in the PT –Configuration parameters & Calibration parameters –Operating parameters among which: –Topography polynomial coefficients –Surface Slope polynomial coefficients –Starting Latitude for each of the above coefficients. The ODT (Orbital Data Table) file. The Orbital Data Table shall contain the following 32 bit floating point values: –Latitude (or True Anomaly),Radius (Kilometres),Radius Rate (meter/sec),Tangential Velocity (meters/sec)
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