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SHARAD Trade off and conclusions

SHARAD Trade off and conclusions. Trade off. The full coverage of the scientific objectives requires high performance of the radar in terms of range and azimuth resolution and penetration capability.

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SHARAD Trade off and conclusions

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  1. SHARAD Trade off and conclusions

  2. Trade off The full coverage of the scientific objectives requires high performance of the radar in terms of range and azimuth resolution and penetration capability. The complex model of the Mars surface and subsurface roughness (amplitude, slope and correlation length) and composition (complex permittivity) imposes high receiver sensitivity and dynamics, clutter cancellation capability and constrains few performance on SAR behaviour

  3. Trade off Range resolution impact on chirp bandwidth (and therefore on antenna complexity) and is a function of subsurface permittivity Azimuth resolution (along track and cross track) are respectively constrained by the surface roughness and by the range resolution and spacecraft altitude (DPL/Fresnel diameter dimensions). The penetration depth capability is a function of absolute transmitted frequency and of the complex permittivity of surface and subsurface and is almost clutter limited.

  4. Trade off The range resolution of 10 m, obtainable with 10 MHz chirp bandwidth, could be reduced due to the inapplicability of this chirp bandwidth if multiple matching network are needed. A value of 5-7 MHz bandwidth can be considered feasible, with only one matching network, imposing the resonance of the antenna on the middle of the band (15-25 MHz). In this way a minimum efficiency of 10% of radiated power at the edge of the BW is guaranteed. Loop antenna could overcome this problem. If the surface and subsurface permittivity are greater than 4, a range resolution of 10 m is achievable with the reduced bandwidth as is shown in the following graph for three different values of the bandwidth 10, 7 and 5 MHz (the widening of the compressed pulse due to weighting function has not been considered).

  5. Trade off

  6. 2 </4 Trade off • Fresnel behaviour for surface roughness less than /4 • DPL behaviour for surface roughness higher than /4. Clutter due to surface external to 2 contribute to reduction of SCR Fresnel DPL

  7. Trade off Modelsl characterising the Mars surface:Two scale model and Fractal: Large scale model:rms slope within 0.01,0.1 rad; correlation length 200,20000 m Small scale model:0.1,0.6 rad; rms height 0.1,1m Fractal model:the properties (as s()) are strongly dependent by the linear dimension of the considered surface of analysis (absence of reference value). In this way the average value of the sample is not an intrinsic value of the overall surface. The model is quite similar to the two scale model. • DPL (Rough surface) s() 0.010.1 rad H  0.5 1 (H=0.5 Hagfors model, H=1 geometric optic model) rms height>/4 • Fresnel (Specular behaviour) s()0.010.1 rad coherent scattering rms height</4

  8. Trade off Limiting the processing to DPL, the maximum obtainable along track resolution, when the Mars surface presents a roughness greater than /4 and a correlation length much lower than DPL, is constrained by the maximum permissible antenna lengths that, in this case, is given by: The obtainable resolution is given by: The following table summarises the resolutions obtainable with different values of Bc evaluated for =15 m.

  9. Trade off Resolution in DPL condition

  10. Trade off When operating in Fresnel domain the Mars surface appears as a mirror and SAR operation can be performed considering the antenna length equal to Fresnel diameter given by: In this case the along track and cross track resolution are shown in the following table evaluated for =15m

  11. Trade off Resolution in Fresnel condition

  12. Trade off In order to obtain better along track resolution the synthetic antenna can be designed longer than DPL. In this case the along track resolution will be behind the scientific requirements but the processing will be more sophisticated. Therefore this aspect will be analysed during the design phase. In the next table is shown the SNR evaluated in different condition at a S/C height of 315 km. To obtain the values of SNR at 400 and 230 km is enough to respectively subtract 2.5 and add 3.5 dB.

  13. Trade off

  14. Trade off

  15. Conclusion • 15-25 Mhz bandwidth has been selected as good trade off between required resolution, penetration capability, low sensitivity to ionosphere and physical feasibility of the antenna. • The scientific requirements have been satisfied excluding few requirements, depending mainly by the Mars topography, unachievable given the S/C and mission constraints. The limitations in terms of performance are relevant to some reduction of resolution and signal dynamic (penetration depth capability) in some particular cases of composition and shape of the surface/subsurface of the Mars model. • Considering the obtainable values of SNR it appears that a processing limited to DPL (wrt to Fresnel) is more effective for the along track resolution. Analysing the cross track resolution and the clutter cancellation improvement obtainable the Fresnel behaviour is more efficient. • Fresnel behaviour presents better performance in terms of cross track resolution and clutter insensitivity also if there is a reduction of along track resolution.

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