Autofocus Correction of Phase Distortion Effects on SHARAD Echoes

Campbell, B. A.; Putzig, N. E.; Carter, L. M.; Phillips, R. J.

doi:10.1109/lgrs.2011.2143692

Cited by 40 publications

(33 citation statements)

References 22 publications

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“…SHARAD results suggest a generally smooth, cosine‐like drop in TEC with solar zenith angle, as predicted by the model of Chapman [], with broad fluctuations of 10–20% due to localized nightside effects possibly linked with remnant crustal magnetic fields [ Campbell et al , ] and dayside conditions modulated by these fields and other mechanisms [ Withers , ]. In thousands of SHARAD tracks processed to date, we also observe that changes in the total electron content of the ionosphere are smooth down to the scale of the radargram postings (about 500 m).…”

Section: Ionospheric Distortion Compensation and Tec Estimationsupporting

confidence: 75%

“…The compensation for products delivered by the U.S. instrument team to the PDS is based on autofocusing, with the “downswept” linear chirp modified by an empirically derived function given by

normalΦ ((), t) = E {[F_{H} - a t]}^{- 1.93}

where F H is the maximum chirp frequency (25 MHz) and E is a scalar parameter optimized by the autofocusing. The fixed power law exponent means that the coefficients of the quadratic ( β ) and cubic ( χ ) phase distortion terms have a fixed ratio [ Campbell et al , ], and this approximation appears valid over the SHARAD frequency band. 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.…”

Section: Ionospheric Distortion Compensation and Tec Estimationmentioning

confidence: 99%

“…Comparison of total electron content unit estimates for the period from 6 December 2006 to 30 September 2007 from MARSIS [ Mouginot et al , ] and SHARAD [ Campbell et al , , ] data. Note that MARSIS TECU estimates exceed those from SHARAD by up to about 10% and exhibit progressively greater variance with smaller solar zenith angle.…”

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%

normalΦ ((), t) = E {[F_{H} - a t]}^{- 1.93}

Section: Ionospheric Distortion Compensation and Tec Estimationmentioning

confidence: 99%

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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“…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%

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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“…Focused synthetic‐aperture processing is used to increase the coherent gain of the returned signals and narrow their along‐track resolution to 300–500 m [ Seu et al ., ]. SHARAD echoes also contain information on the roughness and dielectric properties of Mars, the range between the spacecraft and surface, and the total electron content of the intervening ionosphere [ Campbell et al ., ; Safaeinili et al ., ]. Here we focus on the recovery of surface physical properties from the peak power and delay width of the SHARAD echoes.…”

Section: Introductionmentioning

confidence: 99%

Roughness and near‐surface density of Mars from SHARAD radar echoes

et al. 2013

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[1] We present a technique for estimating Mars topographic roughness on horizontal scales from about 10 m to 100 m using Shallow Radar (SHARAD) sounding data. Our results offer a view of surface properties complementary to Mars Orbiter Laser Altimeter (MOLA) pulse-width or baseline roughness maps and can be compared to SHARAD peak-echo properties to infer deviations from the average near-surface density.Latitudinal averaging of SHARAD-derived roughness over Arabia and Noachis Terrae shows good agreement with MOLA-derived roughness and provides clear evidence for latitude-dependent mantling deposits previously inferred from image data. In northwestern Gordii Dorsum, we find that bulk density in at least the upper few meters is significantly lower than in other units of the Medusae Fossae Formation. We observe the same behavior indicative of low near-surface density in wind-eroded crater fill in the southern highlands. Combining surface-properties analysis, subsurface sounding, and high-resolution optical images, we show that the Pavonis Mons fan-shaped deposit differs significantly from lobate debris aprons which SHARAD has shown to be ice-cored. There are no internal radar reflections from the smooth-facies portion of the Pavonis Mons fan-shaped deposit, and we suggest that these deposits are either quite thin or have little dielectric (i.e., density) contrast with the underlying terrain. Future application of these techniques can identify other low-density units across Mars, assist in the mapping of regional volatile-rich mantling units, and provide new constraints on the physical properties of the polar layered terrain.

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Autofocus Correction of Phase Distortion Effects on SHARAD Echoes

Cited by 40 publications

References 22 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

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

Roughness and near‐surface density of Mars from SHARAD radar echoes

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