NASA - National Aeronautics and Space Administration

The NASA/JPL Airborne Rain MApping Radar (ARMAR) was developed for the purpose of supporting future spaceborne rain radar systems, including the TRMM PR. ARMAR flies on the NASA DC-8 aircraft and operates at 13.8 GHz (Ku-band); it has Doppler and multi-polarization capabilities. It normally scans its antenna across track +/- 20 degrees but can also operate with its antenna pointing at a fixed angle. In addition to acquisition of radar parameters, it also spends a small fraction of its time operating as a radiometer, providing the 13.8 GHz brightness temperature. ARMAR is a pulse compression radar, meaning that it transmits an FM chirp signal of relatively long duration. The raw data is recorded directly to a high speed tape recorder. Post-processing occurs in two steps; first, the raw data is compressed by correlating it with the transmitted chirp, giving data comparable to a conventional short pulse radar. These data are used to form various second-order statistics, which are averaged over at least 100 (often several hundred) pulses. The second processing step takes the pulse-compressed and averaged data and performs calibration. This step uses data acquired by the system calibration loop during flight to convert the measured power to the equivalent radar reflectivity factor Ze. It also produces Doppler velocity and polarization observables, depending on the mode of operation during data collection.

JPL's WINDRAD system measures brigthness temperatures with multiple polarimetric 17, 19, and 37 GHz Ku- and Ka-band passive microwave radiometers.

SLIT is a high-resolution slit-spectrgraph which is fed by optical fiber attached to window assembly telescope. Its objective is to resolve shock emissions in the near-UV.

The instrument consists of a computer controlled slit-spectrograph which is fed by an optical fiber and a small telescope assembly at the window. A co-aligned camera provides pointing capability, detecting stars to magnitude +7. The camera is an EM CCD Andor DU97 IN, back illuminated UV enhanced CCD, with 1600x400 pixels (16x16 micron) and 25.6 x 6.4 mm image area. The Spectrometer is an Acton Sp300i imaging spectrometer with 300 mm focal length F/D 4.5. The telescope assembly focussing is performed with a 90 degree off-axis parabolic mirror of 50 mm diameter with a focal length of 100mm. The F/D~2 was chosen to meet the numerical aperture of the fibre optics yielding an angle of view of 0.45 degree. A bundle of 50 quartz fibres of 100 µm diameter are chosen. On the telescope side the fibres form a round cross section of 0.8 mm diameter, on the spectrometer side they are oriented in a row which can be used like a slit with a height of 5.6 mm and a width of 100 micron. If sufficient amounts of light are available, a slit can be used in addition to improve the spectral resolution.

The Sandia National Laboratory 2-channel radiometer uses two narrow-band (10 nm) filters in the red and near-IR at a sampling rate of many thousands per second. It measures the total radiative output of the SRC during entry in the 380-600 nm band and the 600-900 nm band and detect rapid fluctuations of light output from spacecraft rotation, instabilities in the shock layer, and ablation.

This instrument consists of two photometers, each equipped with a filter of choice: here a low-pass and high-pass cut-off filter. Each photometer measures the sky over a large ~15 degree field of view, at about 5 - 35 degree elevation.

The miniature Echelle spectrograph provides gigh-resolution Echelle spectroscopy, whereby the spectral range (360 - 900 nm) is folded into shorter segments and projected on a CCD camera for simultaneous exposure.

This instrument consists of a 100 mm f4.5 UV Nikkor lens and Catalina Scientific Corp. "Echellette" Spectrograph with Visible Module or UV Module coupled to a Q-Imaging "Intensified Retiga" blue-enhanced image intensified CCD camera.

Scientific objective: Spectral resolution of shock layer radiation. Resolve spectral lines of air plasma emissions at optical wavelengths for the measurement of excitation temperatures. Provide highest possible spectral resolution.

ASTRO utilizes slit-less spectroscopy with transmission grating, a long focal length lens, and a cooled CCD camera detector.

This instrument consists of a Richardson Grating Laboratory 11 x 11 cm plane transmission grating (35-54-20-660), an AF-S Nikkor f2.8/300 mm Nikon 300D IF-ED lens, and a two-stage thermoelectrically cooled back-illuminated 1024 x 1024 pixel Pixelvision CCD camera. An optional order separation filter.

Scientific objective: Spectral resolution of shock layer radiation. Resolve spectral lines of air plasma emissions at optical wavelengths for the measurement of excitation temperatures. Provide high spectral resolution and absolute calibration at high dynamic range. Limitation: only one measurement made in a brief time interval during the point of peak brightness.

Remote sensing of soil moisture using C- and X-band microwave frequencies provides less penetration of vegetation and soil probing depth than L-band, but is more amenable to implementation using airborne or spaceborne antennas of practical size. The Japanese AMSR-E imaging radiometer on board the NASA EOS Aqua satellite is one such sensor capable of retrieving soil moisture using a microwave channel at 6.9 GHz with ~75 km spatial resolution. Aqua was launched in May 2002, and will provide a global soil moisture product based on AMSR-E data. The C-band channels on the future NPOESS Conical Microwave Imager and Sounder (CMIS) will provide new operational capabilities for mapping soil moisture. Sea surface temperature is also observable under most cloud conditions using passive microwave C-band radiometry.

To provide airborne mapping capabilities for measuring both soil moisture and sea surface temperature a second operational PSR scanhead was built incorporating fully polarimetric C- and X-band radiometers inside a standard PSR scanhead drum. The C-band radiometer in PSR/CX provides vertically and horizontally polarized measurements within four adjacent subbands at 5.80-6.20, 6.30-6.70, 6.75-7.10, and 7.15-7.50 GHz. In addition, the radiometer provides fully polarimetric measurements at 6.75-7.10 GHz. The use of four subbands and polarimetric capability has provided a unique means of demonstrating and optimizing algorithms for RFI mitigation.

PSR/CX was originally implemented using only a C-band radiometer (as PSR/C) in preparation for SGP99. In preparation for SMEX02 an X-band radiometer was added to provide vertically and horizontally polarized measurements within four bands at 10.60-10.68, 10.68-10.70, 10.70-10.80, and 10.60-10.80 GHz. Fully polarimetric measurements are provided within 10.60-10.80 GHz. The combined dual-band system provides additional information on soil moisture, along with the capability to measure precipitation and the near-surface wind vector over water backgrounds. The X-band channels also provide additional RFI mitigation capability.

Applications of PSR/CX include ocean surface emissivity studies, soil moisture mapping, sea ice mapping, and imaging of heavy precipitation.

The PSR/A scanhead provides either full-Stokes vector or tri-polarimetric sensitivity at the radiometric bands of 10.7, 18.7, and 37 GHz, and thus is well suited for the NPOESS Integrated Program Office’s internal government (IG) studies of ocean surface wind vector measurements. PSR data has been used to demonstrate the first-ever retrieval of ocean surface wind fields using conically-scanned polarimetric radiometer data. The results have suggested that the NPOESS specification for wind vector accuracy will be achievable with a polarimetric two-look system.

The Polarimetric Scanning Radiometer (PSR) is a versatile airborne microwave imaging radiometer developed by the Georgia Institute of Technology and the NOAA Environmental Technology Laboratory (now NOAA/ESRL PSD) for the purpose of obtaining polarimetric microwave emission imagery of the Earth's oceans, land, ice, clouds, and precipitation. The PSR is the first airborne scanned polarimetric imaging radiometer suitable for post-launch satellite calibration and validation of a variety of future spaceborne passive microwave sensors. The capabilities of the PSR for airborne simulation are continuously being expanded through the development of new mission-specific scanheads to provide airborne post-launch simulation of a variety of existing and future U.S. sensors, including CMIS, ATMS, AMSU, SSMIS, WindSat, TMI, RAMEX, and GEM.

The basic concept of the PSR is a set of polarimetrc radiometers housed within a gimbal-mounted scanhead drum. The scanhead drum is rotatable by the gimbal positioner so that the radiometers (Figure 2.) can view any angle within ~70° elevation of nadir at any azimuthal angle (a total of 1.32 pi sr solid angle), as well as external hot and ambient calibration targets. The configuration thus supports conical, cross-track, along-track, fixed-angle stare, and spotlight scan modes. The PSR was designed to provide several specific and unique observational capabilities from various aircraft platforms. The original design was based upon several observational objectives:

1. To provide fully polarimetric (four Stokes' parameters: Tv, Th, TU, and TV) imagery of upwelling thermal emissions at several of the most important microwave sensing frequencies (10.7, 18.7, 37.0, and 89.0 GHz), thus providing measurements from X to W band;
2. To provide the above measurements with absolute accuracy for all four Stokes' parameters of better than 1 K for Tv and Th, and 0.1 K for TU and TV;
3. To provide radiometric imaging with both fore and aft look capability (rather than single swath observations);
4. To provide conical, cross-track, along-track, and spotlight mode scanning capabilities; and
5. To provide imaging resolutions appropriate for high resolution studies of precipitating and non-precipitating clouds, mesoscale ocean surface features, and satellite calibration/validation at Nyquist spatial sampling.

The original system has been extended - as discussed below - to greatly exceed the original design objectives by providing additional radiometric channels and expanded platform capabilities.

The PSR scanhead was designed for in-flight operation without the need for a radome (i.e., in direct contact with the aircraft slipstream), thus allowing precise calibration and imaging with no superimposed radome emission signatures. Moreover, the conical scan mode allows the entire modified Stokes' vector to be observed without polarization mixing.

The Millimeter-wave Imaging Radiometer (MIR) is a cross-track-scanning radiometer that measures radiation at nine frequencies. In every scanning cycle of about 3 seconds in duration, it views two external calibration targets. MIR responds predominantly to atmospheric parameters like water vapor, clouds, and precipitation.