SHARAD Signal Attenuation and Delay Offsets Due to the Martian Ionosphere

Campbell, B. A.; Putzig, N. E.; Foss, Frederick J.; Phillips, R. J.

doi:10.1109/lgrs.2013.2273396

Cited by 33 publications

(41 citation statements)

References 15 publications

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“…For groups of observations, typically spanning about 35 km along the track, the autofocus method optimizes the SNR, and the derived compensation is applied to all radargram frames within the region. The scalar coefficient, E , has a close correlation with delay offsets at hundreds of orbit crossover locations observed at different solar zenith angles, showing that a very good approximation for the TEC is 0.29 E [ Campbell et al , ]. This “calibration” of the TEC values is accurate enough to reduce the RMS errors in surface echo vertical positioning to just a few range cells (i.e., <50 m).…”

Section: Ionospheric Distortion Compensation and Tec Estimationsupporting

confidence: 75%

“…MARSIS and SHARAD signals can be approximated by a linear frequency ramp over the duration of the chirp. In reality, the transmitted signal has variations in amplitude (and perhaps phase) due to the imperfect impedance match between the signal and the dipole antenna and matching network, but neglecting these aspects appears to still allow stable ionospheric modeling results [e.g., Campbell et al , ]. The MARSIS “up‐swept” chirp can be expressed as an instantaneous angular frequency, φ, that varies with time, t , from the start of the pulse:

φ ((), t) = 2 π ((), F_{L} + a t)

where F L is the low‐frequency end of the signal bandwidth and a is the chirp rate (in Hz/s).…”

Section: Ionospheric Distortion Compensation and Tec Estimationmentioning

confidence: 99%

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Phase compensation of MARSIS subsurface sounding data and estimation of ionospheric properties: New insights from SHARAD results

Campbell

Watters

2016

J. Geophys. Res. Planets

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Subsurface radar sounding observations by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) and Shallow Radar (SHARAD) instruments are affected by ionospheric phase distortions that lead to image blurring and delay offsets. Based on experience with SHARAD image correction, we propose that ionospheric blurring in MARSIS radargrams may be compensated with a model of smoothly varying quadratic phase errors along the track. This method yields well-focused radargrams for geologic interpretation and allows analysis of the validity range for models used to derive total electron content (TEC) from phase distortion terms in previous MARSIS studies. The quadratic term appears to be a good proxy for TEC at solar zenith angles >65°for MARSIS Band 4 (5 MHz) and >75°for Band 3 (4 MHz). Comparison of MARSIS-and SHARAD-derived TEC values from 2007 to 2014 reveals correlations in seasonal behavior and in the characterization of ionospheric activity due to coronal mass ejections. We also present SHARAD and MARSIS evidence for a persistent region of anomalous radar scattering south of Arsia Mons. These echoes have been previously suggested to arise from refraction of the radar signal by electron density variations. There are no strong signatures, however, in the quadratic image compensation term correlated with the anomalous scattering, suggesting either that electron density variations responsible for refracted signal paths occur primarily in regions offset from the spacecraft track or that these density changes have a minimal impact on the integrated phase distortion of the subspacecraft footprint. We suggest observations and analyses to better constrain the mechanism and timing of such echoes.

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Section: Ionospheric Distortion Compensation and Tec Estimationsupporting

confidence: 75%

φ ((), t) = 2 π ((), F_{L} + a t)

where F L is the low‐frequency end of the signal bandwidth and a is the chirp rate (in Hz/s).…”

Section: Ionospheric Distortion Compensation and Tec Estimationmentioning

confidence: 99%

Phase compensation of MARSIS subsurface sounding data and estimation of ionospheric properties: New insights from SHARAD results

Campbell

Watters

2016

J. Geophys. Res. Planets

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“…Their main conclusion was that model‐data comparisons were reasonably successful. Similarly, Campbell et al [] in a first approximation compared this data set with the TEC obtained from the subsurface radar SHAllow RADar (SHARAD) [ Seu et al , ] on board Mars Reconnaissance Orbiter probe, concluding that the maximum average TEC value from SHARAD observations for solar zenith angles near 30 and 90° agrees well with Mouginot et al's TEC measurements from Mars Express.…”

Section: Marsis Tec Discrepancy: Description Of the Problemmentioning

confidence: 68%

Total electron content in the Martian atmosphere: A critical assessment of the Mars Express MARSIS data sets

et al. 2015

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The total electron content (TEC) is one of the most useful parameters to evaluate the behavior of the Martian ionosphere because it contains information on the total amount of free electrons, the main component of the Martian ionospheric plasma. The Mars Express Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) radar is able to derive TEC from both of its operation modes: (1) the active ionospheric sounding (AIS) mode and (2) the subsurface mode. TEC estimates from the subsurface sounding mode can be computed from the same raw data independently using different algorithms, which should yield similar results. Significant differences on the dayside, however, have been found from two of the algorithms. Moreover, both algorithms seem also to disagree with the TEC results from the AIS mode. This paper gives a critical, quantitative, and independent assessment of these discrepancies and indicates the possible uncertainty of these databases. In addition, a comparison between the results given by the empirical model of the Martian ionosphere developed by Sánchez-Cano et al. (2013) and the different data sets has been performed. The main result is that for solar zenith angles higher than 75°, where the maximum plasma frequency is typically small compared with the radar frequencies, the two subsurface algorithms can be confidently used. For solar zenith angles less than 75°, where the maximum plasma frequency is very close to the radar frequencies, both algorithms suffer limitations. Nevertheless, despite the solar zenith angle restrictions, the dayside TEC of one of the two algorithms is consistent with the modeled TEC.

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“…The SHARAD signal is subject to distortion and attenuation by the ionosphere of Mars, effects largely confined to dayside observations that can be corrected in processing [ Campbell et al ., , ]. In addition to providing additional coverage, the inclusion of dayside observations is important to our analysis because they form a data grid with the crossing nightside observations, thereby allowing confident mapping of specific subsurface returns.…”

Section: Radar Observationsmentioning

confidence: 99%

“…In addition to providing additional coverage, the inclusion of dayside observations is important to our analysis because they form a data grid with the crossing nightside observations, thereby allowing confident mapping of specific subsurface returns. During SHARAD operations, it was established that an increase in the signal‐to‐noise ratio up to 6 dB can be achieved by acquiring observations with the spacecraft in a rolled configuration and (on the nightside) positioning the solar array in a specific orientation [ Campbell et al ., ]. We included these rolled observations in our search for subsurface returns, but they are omitted in the derivation of roughness to avoid complicating comparisons of changes in relative surface reflectivity [ Campbell et al ., ].…”

Section: Radar Observationsmentioning

confidence: 99%

SHARAD soundings and surface roughness at past, present, and proposed landing sites on Mars: Reflections at Phoenix may be attributable to deep ground ice

Putzig

Phillips

Campbell

et al. 2014

J. Geophys. Res. Planets

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We use the Shallow Radar (SHARAD) on the Mars Reconnaissance Orbiter to search for subsurface interfaces and characterize surface roughness at the landing sites of Viking Landers 1 and 2, Mars Pathfinder, the Mars Exploration Rovers Spirit and Opportunity, the Phoenix Mars lander, the Mars Science Laboratory Curiosity rover, and three other sites proposed for Curiosity. Only at the Phoenix site do we find clear evidence of subsurface radar returns, mapping out an interface that may be the base of ground ice at depths of~15-66 m across 2900 km 2 in the depression where the lander resides. At the Opportunity, Spirit, and candidate Curiosity sites, images and altimetry show layered materials tens to hundreds of meters thick extending tens to hundreds of kilometers laterally. These scales are well within SHARAD's resolution limits, so the lack of detections is attributable either to low density contrasts in layers of similar composition and internal structure or to signal attenuation within the shallowest layers. At each site, we use the radar return power to estimate surface roughness at scales of 10-100 m, a measure that is important for assessing physical properties, landing safety, and site trafficability. The strongest returns are found at the Opportunity site, indicating that Meridiani Planum is exceptionally smooth. Returns of moderate strength at the Spirit site reflect roughness more typical of Mars. Gale crater, Curiosity's ultimate destination, is the smoothest of the four proposed sites we examined, with Holden crater, Eberswalde crater, and Mawrth Vallis exhibiting progressively greater roughness.

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SHARAD Signal Attenuation and Delay Offsets Due to the Martian Ionosphere

Cited by 33 publications

References 15 publications

Phase compensation of MARSIS subsurface sounding data and estimation of ionospheric properties: New insights from SHARAD results

Phase compensation of MARSIS subsurface sounding data and estimation of ionospheric properties: New insights from SHARAD results

Total electron content in the Martian atmosphere: A critical assessment of the Mars Express MARSIS data sets

SHARAD soundings and surface roughness at past, present, and proposed landing sites on Mars: Reflections at Phoenix may be attributable to deep ground ice

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