PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "2008-04-08 Mike Reid, Initial version; 2008-04-15 J. Ward, Minor edit; 2008-11-26 K. Raney & M. Reid additions; 2008-12-05 J. Ward, Minor formatting; 2019-07-28 R. Simpson, updates, reorg; 2019-09-10 W. Patterson, updates" RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = LRO INSTRUMENT_ID = MRFLRO OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "MINI-RF LRO" INSTRUMENT_TYPE = "RADAR MAPPER" INSTRUMENT_DESC = " Overview: ========= Mini-RF was a hybrid dual-polarimetric side-looking synthetic aperture radar (SAR) that flew on NASA's Lunar Reconnaissance Orbiter (LRO). It was a sibling of the Mini-SAR that flew on India's Chandraay'an-1 spacecraft. Mini-RF operated in S-band (2.38 GHz) and X/C-band (7.14 GHz) imaging modes and as a communications demonstration. The designed imaging mode was monostatic (transmitter and receiver co-located); monostatic data were collected from July 2009 to December 2010, when the onboard transmitter failed. The monostatic imaging mode had two resolutions, a baseline resolution of 150 m pixels with 16 look averages and a zoom resolution with 15 by 30 m pixels at 8 looks. Mini-RF also gathered data on repeat passes in an interferometry mode. Mini-RF tested a bistatic mode in April 2011 with S-band uplinks from the Arecibo Observatory radar in Puerto Rico and transitioned to operating in that mode in 2012. In February of 2016, X-band uplinks from the Goldstone Solar System Radar DSS-13 antenna in Goldstone, CA were tested and X/C-band observations were added to the Mini-RF bistatic imaging mode in 2017. The resolution of data in the bistatic imaging mode can vary based on the observation geometry but averages approximately 100 m. The radar architecture was hybrid-polarimetric [RANEYETAL2011]; Mini-RF transmitted circular polarization and received H and V linear polarizations coherently. The antenna area was 1.1 square meters. The transmitter average power output was 25 W at 2.38 GHz (S-band) or 7.14 GHz (C-band). When in bistatic mode, Arecibo transmitted with up to 200 kW and DSS-13 transmitted with up to 80 kW. The resulting data were processed to yield Stokes parameters and circular polarization ratio (CPR). The communications demonstrations were receive-only and transmit-only, both with a cooperating Earth-based facility. The communications were at S-band and contributed to understanding the spacecraft antenna's gain and polarization properties. Scientific Objectives: ====================== The primary scientific objective of Mini-RF was to search for water-ice deposits in permanently-shadowed areas of the north and south polar regions of the Moon. From previous observations of Mercury, Mars, and the Galilean satellites, water-ice is known to cause anomalous radar reflections (high albedo and high circular polarization ratio) that differ significantly from reflections from typical non-ice surface materials. In areas not expected to have water ice deposits, Mini-RF characterized lunar surface roughness on scales of the radar wavelength (12-cm and 4-cm) from the albedo and polarization of the radar backscatter. Hardware: ========= The radar was designed as a demonstration -- to prove the concept of a 15-kg class SAR in lunar orbit which could also support communications. The instrument consisted of an antenna, a passive array of H and V elements of about 1.1 square meter area, and electronics, packaged in six sub-assemblies, as described below. --------- -------------- ---------- ---------- | Antenna |<-->| Interconnect |-->| Analog |-->| Digital | --------- | Module | | Receiver | | Receiver | -------------- ---------- ---------- ^ | Analog |<--| QDWS | | | Exciter | ---------- | ---------- -------------- | ----------- | Transmitter |<-------- | Control | -------------- | Processor | ----------- | V to s/c bus The transmitter used a microwave power module design; that is, a solid state radio frequency (RF) driver and pre-amplifier followed by a traveling wave tube (TWT) power amplifier. The combination provided the desired output power over a 3-to-1 frequency range in a compact low-mass package. The transmitter operated in burst mode for primary imaging; burst length determined azimuth resolution while burst period set the number of looks. For interferometry, a uniform pulse repetition frequency (PRF) was employed. The digital receiver and quadrature digital waveform synthesizer (QDWS) were adapted from airborne systems. The QDWS was preprogrammed with nearly 1000 waveforms, which were required to accommodate operations at varying spacecraft altitudes, incidence angles (nominally 45 degrees, but also nadir), and transmit-receive interleave cycles. The digital receiver used block adaptive quantization for compression of the downlink data. The interconnect module provided the 90 degree phase shift needed to transmit circular polarization through the H and V antenna elements, it isolated the transmit and receive paths, and it provided front-end RF filtering. The control processor accepted commands from the bus interface, sent received output to the bus, and provided control and configuration of the instrument. Calibration: ============ Amplitude calibration --------------------- The Mini-RF radar data product comprised the amplitude (or magnitude squared) of the H and the V channels of the dual-polarization receiver and the cross-product of the complex H and V amplitudes. These values are necessary and sufficient to form the 2x2 coherency matrix of the backscattered field, which is alternatively represented by the Stokes parameters (four real numbers). The first Stokes parameter represents the total backscattered power. This can be scaled to the normalized reflectivity (sigma-zero) only if the end-to-end transformation of the radar is calibrated absolutely. The starting point for this scaling is the set of pre-flight system data, coupled with in-flight specifics such as incidence and altitude. During the LRO mission, absolute calibration was maintained by imaging a lunar area whose reflectivity was well known from Earth-based radar observations. However, most lunar science measurements depend on ratios of the Stokes parameters, for which absolute amplitude calibration is not required. Rather, gain balance between the H and the V channels is the key. Relative calibration consisted of evaluating the corrective scaling constant of the H amplitude relative to the V amplitude. Calibration references (noise, tone, and chirp) were included at the beginning and end of each data take to assist in relative amplitude calibration. During the mission, relative calibration was updated by radar coverage of the lunar surface at nadir, from which the observed backscatter should have identical amplitudes seen through both the H and the V channels. Any difference could be inverted to evaluate the relative amplitude calibration constant. In addition, the mission plan called for relative amplitude calibration through a cooperative transmission and reception between the spacecraft and an Earth-based radar observatory, e.g., Arecibo. These data were included in the archive delivered to the NASA Planetary Data System (PDS) when available. Phase calibration ----------------- The cross-product of the complex H and V amplitudes is one representation of the phase to be calibrated. Under the operational assumption that the radar transmitted circular polarization, the relative phase (in the cross-product) is central to the third and the fourth Stokes parameters. Relative phase calibration consisted of evaluating the corrective phase rotation constant of the H complex amplitude relative to the V complex amplitude such that the average phase difference between them was +/-90 degrees (the sign depended on whether right- or left-circular polarization was transmitted) under the condition that the average reflecting surface was specular. The starting position for relative phase calibration was the set of pre-flight system data, coupled with in-flight measurements based on the radar's calibration references (see Amplitude Calibration, above). During the mission, relative phase calibration was updated by radar coverage of the lunar surface at nadir, from which the observed averaged backscatter should have known relative phase between the H and the V channels. Any difference could be inverted to evaluate the relative phase rotation calibration constant. In addition, the mission plan called for relative phase calibration through a cooperative transmission and reception between the spacecraft and an Earth-based radar observatory, e.g., Arecibo. These data were posted on the PDS when available. Operational Modes: ================== The Mini-RF instrument had two imaging modes: Synthetic Aperture Radar (SAR) and Interferometry. The SAR modes, at two wavelengths (12-cm and 4-cm), each had two resolutions (150 m at 16 looks, and 15 m by 30 m at 8 looks). Only one combination of wavelength and resolution could be operated at the same time. Data were initially gathered -- on a non-interference basis with respect to the other LRO instruments -- over selected sites poleward of plus/minus 80 degrees latitude. Later, the latitude restrictions were relaxed. SAR interferometric data collection took place as opportunities allowed." END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "NOZETTEETAL2009" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "RANEYETAL2011" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END