Radar

The APR-3 is a three frequency (13, 35, and 94 GHz), Doppler, dual-polarization radar system. It has a downward looking antenna that performs cross track scans, covering a swath that is +/- 25 to each side of the aircraft path. Additional features include: simultaneous dual-frequency, matched beam operation, simultaneous measurement of both like- and cross-polarized signals at both frequencies, Doppler operation, and real-time pulse compression (calibrated reflectivity data can be produced for large areas in the field during flight, if necessary).

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.

HIWRAP (High-Altitude Imaging Wind and Rain Airborne Profiler) is a dual-frequency radar (Ka- and Ku-band), dual-beam (300 and 400 incidence angle), conical scan, solid-state transmitter-based system, designed for operation on the high-altitude (20 km) Global Hawk UAV. HIWRAP characteristics: Conically scanning; Simultaneous Ku/Ka-band & two beams @30 and 40 deg; Winds using precipitation & clouds as tracers; Ocean vector wind scatterometry; Map the 3-dimensional winds and precipitation within hurricanes and other severe weather events; Map ocean surface winds in clear to light rain regions using scatterometry.

The APR-2 is a dual-frequency (13 GHz & 35 GHz), Doppler, dual-polarization radar system. It has a downward looking antenna that performs cross track scans, covering a swath that is +/- 25 to each side of the aircraft path. Additional features include: simultaneous dual-frequency, matched beam operation at 13.4 and 35.6 GHz (same as GPM Dual-Frequency Precipitation Radar), simultaneous measurement of both like- and cross-polarized signals at both frequencies, Doppler operation, and real-time pulse compression (calibrated reflectivity data can be produced for large areas in the field during flight, if necessary).

The Center has been developing a wideband radar altimeter that operates over the frequency range from 13 to 17 GHz. The primary purpose of this radar is high precision surface elevation measurements over polar ice sheets. The data collected with this radar can be analyzed in conjunction with laser-altimeter data to determine thickness of snow over sea ice. The radar has been flown on a NASA DC-8 aircraft, and the NSF provided a Twin Otter aircraft.

The Center for Remote Sensing of Ice Sheets has developed an ultra-wideband radar that operates over the frequency from 2 to 8 GHz to map near-surface internal layers in polar firn with fine vertical resolution. The radar has also been used to measure thickness of snow over sea ice. Information about snow thickness is essential to estimate sea ice thickness from ice freeboard measurements performed with satellite radar and laser altimeters. This radar has been successfully flown on NASA P-3 and DC-8 aircraft.

UAVSAR, a reconfigurable, polarimetric L-band synthetic aperture radar (SAR), is specifically designed to acquire airborne repeat track SAR data for differential interferometric measurements.

Differential interferometry can provide key deformation measurements, and is important for studies of earthquakes, volcanoes and other dynamically changing phenomena.

Using precision real-time GPS and a sensor controlled flight management system, the system can fly predefined paths with great precision (to be within a 10 m diameter tube about the desired flight track).

The radar is designed to be operable on a UAV (Uninhabited Aerial Vehicle), but will initially be demonstrated on a NASA Gulfstream III. The radar is fully polarimetric, with a range bandwidth of 80 MHz (2 m range resolution), and a range swath greater than 16 km.

The antenna may be electronically steered along track to assure that the antenna beam can be directed independently, regardless of speed and wind direction.

Other features supported by the antenna include elevation monopulse and pulse-to-pulse re-steering capabilities that will enable some novel modes of operation. The system will nominally operate at 41,000 ft (13800 m).

The program began as an Instrument Incubator Project (IIP) funded by NASA Earth Science Technology Office (ESTO). Since 2018, UAVSAR facility instrument suite has been enhanced with two additional bands: P-band (AirMOSS) and Ka-band (GLISTIN-A). The P-band capability was originally added in 2012 to support the EVS-1 AirMOSS mission to observe sub-canopy and subsurface root zone soil moisture. The modification was accomplished by replacing UAVSAR's L-band front-end electronics and antenna with components that operate at P-band (420-440 MHz). The Ka-band single-pass interferometric SAR capability (GLISTIN-A) was added through NASA's Advanced Instrument Technology Transition program (AITT). The horizontally polarized GLISTIN-A (35.62-35.70 GHz) instrument generates high-precision, high resolution, large-swath digital surface models for ice surface topography mapping.

* Detection of ice bottom surface requires frequencies in the range of 0.1 to 0.3 GHz to overcome the high dielectric loss in complex scattering medium.

* Detection of ice top surface is achieved with higher range of frequencies 0.8 to 1.2 GHz.

* Utilization of wide radar bandwidth (0.1 - 1.2 GHz) enables fine vertical resolution.

* Complementary sensors include laser altimeter (detects snow surface), snow radar (snow depth, 4-8 GHz) or radar altimeter (ice top surface).

The Second Generation Precipitation Radar (PR-2) is a dual-frequency, Doppler, dual-polarization radar system.

The airborne PR-2 system includes a real-time pulse compression processor, a fully-functional control and timing unit, and a very compact LO/IF module, all of which could be used in spaceborne applications.

The RF circuitry can be divided into two categories: circuits operating at frequencies of less than 1.5 GHz and circuits operating at frequencies above 1.5 GHz. The lower frequency (below 1.5 GHz) circuitry is all contained in a single unit, the local oscillator / intermediate frequency (LO/IF) module. This unit converts transmit chirp signals from 15 MHz up to 1405 MHz and downconverts received IF signals from 1405 MHz to 5 MHz. The unit contains both upconversion channels and all four receive channels and fits into the equivalent of a double wide 6U-VME card.

The RF front-end electronics are unique to the airborne PR-2 design and consist of five units: one local oscillator / up converter (LO/U) unit, two TWTAs and two waveguide front end (WGFE) units. In the DC-8 installation, the two TWTAs are stacked vertically in a standard rack with the LO/U in between them and the two WGFEs are mounted on top of the antenna pressure box, near the antenna feed. A calibration loop is included for each channel. This feeds some of the transmit power to the receiver, allowing in-flight variations of the transmit power and receiver gain to be monitored and removed from the data.

The digital electronics consists of a control and timing unit (CTU), an arbitrary waveform generator (AWG), and a data processor. The CTU generates the pulse timing and all other timing signals. It also provides control signals to RF. The AWG is loaded with a digital version of the linear FM chirp that is to be transmitted. The data processor is based on FPGA technology. It performs pulse compression and averaging in real-time.

The 4 MHz bandwidth received signals are sampled at 20 MHz, then digitally downconverted to complex samples, resulting in I and Q samples at 5 MHz rate. The data processor also includes pulse-pair Doppler processing. The output of the processor is the lag-0 (power) and lag-1 (complex Doppler data) for the co- and crosspolarized channels at each frequency. A VME-based workstation runs the radar, including ingesting and saving the processed data. Following calibration on the ground, the PR-2 data are stored in an HDF format.