Leif J. Harcke

Locating the LCROSS Impact Craters

The Lunar CRater Observations and Sensing Satellite (LCROSS) mission impacted a spent Centaur rocket stage into a permanently shadowed region near the lunar south pole. The Sheperding Spacecraft (SSC) separated ∼9 hours before impact and performed a small braking maneuver in order to observe the Centaur impact plume, looking for evidence of water and other volatiles, before impacting itself.This paper describes the registration of imagery of the LCROSS impact region from the mid- and near-infrared cameras onboard the SSC, as well as from the Goldstone radar. We compare the Centaur impact features, positively identified in the first two, and with a consistent feature in the third, which are interpreted as a 20 m diameter crater surrounded by a 160 m diameter ejecta region. The images are registered to Lunar Reconnaisance Orbiter (LRO) topographical data which allows determination of the impact location. This location is compared with the impact location derived from ground-based tracking and propagation of the spacecraft’s trajectory and with locations derived from two hybrid imagery/trajectory methods. The four methods give a weighted average Centaur impact location of −84.6796°, −48.7093°, with a 1σ uncertainty of 115 m along latitude, and 44 m along longitude, just 146 m from the target impact site. Meanwhile, the trajectory-derived SSC impact location is −84.719°, −49.61°, with a 1σ uncertainty of 3 m along the Earth vector and 75 m orthogonal to that, 766 m from the target location and 2.803 km south-west of the Centaur impact.We also detail the Centaur impact angle and SSC instrument pointing errors. Six high-level LCROSS mission requirements are shown to be met by wide margins. We hope that these results facilitate further analyses of the LCROSS experiment data and follow-up observations of the impact region.

The DESDynI synthetic aperture radar array-fed reflector antenna

DESDynI is a mission being developed by NASA with radar and lidar instruments for Earth-orbit remote sensing. This paper focuses on the design of a large-aperture antenna for the radar instrument. The antenna comprises a deployable reflector antenna and an active switched array of patch elements fed by transmit / receive modules. The antenna and radar architecture facilitates a new mode of synthetic aperture radar imaging called ‘SweepSAR’. A system-level description of the antenna is provided, along with predictions of antenna performance.

Lunar topographic mapping using a new high resolution mode for the GSSR radar

Mapping the Moons topography using Earth based radar interferometric measurements by the Goldstone Solar System Radar (GSSR) has been done several times since the mid 1990s. In 2008 we reported at this conference the generation of lunar topographic maps having approximately 4 m height accuracy at a horizontal posting of 40 m. Since then GSSR radar has been improved to allow 40 MHz bandwidth imaging and consequently obtained images and interferograms with a resolution of about 4 m in range by 5 m in azimuth. The long synthetic aperture times of approximately 90 minutes in duration necessitated a migration from range/Doppler image formation techniques to spotlight mode processing and autofocusing methods. The improved resolution imagery should permit the generation of topographic maps with a factor of two better spatial resolution with about same height accuracy. Coupled the with the recent availability of new lidar topography maps of the lunar surface made by orbiting satellites of Japan and the United States the geodetic control of the radar generated maps products can be improved dramatically. This paper will discuss the hardware and software improvements made to the GSSR and present some of the new high resolution products.

Improved absolute radiometric calibration of a UHF airborne radar

The AirMOSS airborne SAR operates at UHF and produces fully polarimetric imagery [1]. The AirMOSS radar data are used to produce Root Zone Soil Moisture (RZSM) depth profiles. The absolute radiometric accuracy of the imagery, ideally of better than 0.5 dB, is key to retrieving RZSM, especially in wet soils where the backscatter as a function of soil moisture function tends to flatten out [2]. In this paper we assess the absolute radiometric uncertainty in previously delivered data, describe a method to utilize Built In Test (BIT) data to improve the radiometric calibration, and evaluate the improvement from applying the method.

AirMOSS P-band RF interference experience

The NASA AirMOSS instrument is a P-band (UHF) fully polarimetric synthetic aperture radar designed to measure root-zone soil moisture. The radar currently operates in the 420-440 MHz band in North America. Narrowband communications signals are secondary users of this spectrum and contribute RF interference to the radars operation. An adaptive RF interference filter based on the least-mean squares (LMS) algorithm developed for the GeoSAR UHF airborne SAR program has been adopted for use by AirMOSS. Characterization of the algorithms effects on simulated point targets finds a 1.2 dB average amplitude error. Though this is within the 1.5 dB absolute calibration specification for the AirMOSS radar, RFI removal can be a large contributor to the total radiometric error budget. Experience during the first nine months of AirMOSS operations has applied the adaptive RFI removal on an as-needed basis due to multiplicative noise (MNR) increase and calibration concerns.

Time-domain backprojection for precise geodetic coding of spaceborne SAR imagery

A new era of precise-orbit determination for space-borne SAR permits time-domain backprojection of the data for accurate geocoded image production. In this work, time-domain backprojection is applied to Level 1.0 data from the ALOS/PALSAR instrument to form imagery at two sites in southern California. The accuracy of the backprojection is verified by comparing the measured position of a corner reflector at a calibration site to its position in the formed SAR imagery. The observed offset of the corner reflector is ≪2 m in the range direction and 12 m in the cross-range or along track direction. Images backprojected in an absolute WGS-84 Cartesian system onto 1/3 arc second or 10 m posting digital elevation data exhibit no gross registration errors. This indicates that the backprojection image formation method may be useful for processing differential radar interferometry (D-InSAR) products, where topography terms must first be removed.