NASA - National Aeronautics and Space Administration

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.

PolSCAT is a Ku-band polarmetric scanning scatterometer operating at 13.95 GHz. with an approved NASA license. The transmitting polarizations of PolSCAT, alternating between Vertical and Horizontal, from pulse to pulse. Two receivers detect the V and H polarized radar echoes simultaneously allowing for measurements of VV, HH, VH, and HV radar responses. It provides scalable resolution, between 3,000 and 20,000 feet AGL.

The PolSCAT antenna assembly includes two axis gimbals for conically scanning, parabolic antenna, which is controlled from 0° (nadir) to 65 degrees. It was designed and built to investigate the benefits of active microwave for the remote sensing of high resolution snow-water-equivalent (SWE).

PolSCAT’s flexible design is compatible with many aircraft. It has flown on the NCAR C-130, NASA’s DC-8, P-3, and Twin Otter International’s, Twin Otter. Flown more than 500 hours in support of NASA’s Cold Land Process (CLPX) campaigns, PolSCAT is a very mature instrument.

In July 2005, the Johns Hopkins University, Applied Physics Laboratory began “Pathfinder Airborne Radar Ice Sounder (PARIS)” funded under the NASA Instrument Incubator Program (IIP). The primary objective of this project was the first feasibility demonstration of successful radar sounding of ice sheet layering and bottom topography from a high-altitude platform. Major contributing factors included a high-fidelity 150-MHz radar, supported by along-track partially- coherent processing. “High-fidelity” implies very wide dynamic range, extreme linearity, and very low sidelobes generated by the transmitted pulse. “Partially- coherent processing” implies the delay-Doppler technique, previously proven in airborne radar altimeter and low-altitude radar ice sounding embodiments. The radar was mounted on the NASA P-3, and deployed on a mission over the Greenland ice sheet in the spring of 2007. Data were recorded on board as well as displayed in flight on a quick-look processor. The data subsequently were processed in the laboratory to quantify performance characteristics, including dynamic range, sidelobe level control, and contrast improvement from the delay-Doppler algorithm.

The transmit waveform is a 5-MHz bandwidth chirp at a 150-MHz operating frequency with a trapezoidal envelope. Such severe weighting is essential to reduce the ringing commonly associated with the initial on-off transition of weakly-weighted waveforms. The 180-W (peak) linear-FM pulse has ~6 MHz bandwidth. The amplifier is class AB to help ensure the high linearity needed to suppress the internal clutter (sidelobes) generated by the chirp waveform. Laboratory measurements of the driver and power amplifier show excellent linearity with a two-tone third-order inter-modulation of at least -26 dBc at peak power.

There is no down conversion or IF signal within the receiver, greatly simplifying the design, and eliminating most potential sources of distortion and intermodulation. Upon reception, the radar A/D operates on the RF signal directly out of the LNA. The sample rate is well below Nyquist, but it is chosen so that the resulting spectra shift an alias of the main signal to offset baseband in a clear channel. The receiver includes variable attenuators to adjust the voltage range of the signal input to the analog-to- digital converter as well as sensitivity time control (STC) to increase the effective dynamic range of the response as a function of depth of penetration. The overall noise figure of the receiver is less than 5.5 dB with a gain of over 60 dB and a 45 dBm third-order intercept point.

The digital components consist of a field programmable gate array (FPGA) radar synchronizer, a direct digital synthesizer (DDS), and an under-sampling analog-to-digital converter (ADC). All components of the digital subsection are clocked by a stable 66.6 MHz reference oscillator. The radar data are time-tagged by reference to GPS. The flights included passes over the summit ridge, from which results show internal layering, and the bottom profile at several km depth.

The Center for Remote Sensing of Ice Sheets (CReSIS) has developed radars (MCoRDS) that operate over the frequency range from 140 to 230 MHz with multiple receivers developed for airborne sounding and imaging of ice sheets. MCoRDS radars have an adjustable radar bandwidth of 20 MHz to 60 MHz. Multiple receivers permit digital beamsteering for suppressing cross-track surface clutter that can mask weak ice-bed echoes and strip-map synthetic aperture radar (SAR) images of the ice-bed interface. With 200 W of peak transmit power, a loop sensitivity > 190 dB is achieved. These radars are flown on twin engine and long-range aircraft including NASA P-3 and DC-8.

Clouds are a key element in the global hydrological cycle, and they have a significant role in the Earth’s energy budget through its influence on radiation budgets. Climate model simulations have demonstrated the importance of clouds in moderating and forcing the global energy budget. Despite the crucial role of clouds in climate and the breadth of our current knowledge, there are still many unanswered details. An improved understanding of the radiative impact of clouds on the climate system requires a comprehensive view of clouds that includes their physical dimensions, vertical and horizontal spatial distribution, detailed microphysical properties, and the dynamical processes producing them. However, the lack of fine-scale cloud data is apparent in current climate model simulations.

The Cloud Radar System (CRS) is a fully coherent, polarimeteric Doppler radar that is capable of detecting clouds and precipitation from the surface up to the aircraft altitude in the lower stratosphere. The radar is especially well suited for cirrus cloud studies because of its high sensitivity and fine spatial resolution.

EDOP is an X-band (9.6 GHz) Doppler radar nose-mounted in the ER-2. The instrument has two antennas: one nadir-pointing with pitch stabilization, and the other forward pointing. The general objectives of EDOP are the measurement of the vertical structure of precipitation and air motions in mesoscale precipitation systems and the development of spaceborne radar algorithms for precipitation estimation.

EDOP measures high-resolution time-height sections of reflectivity and vertical hydrometeor velocity (and vertical air motion when the hydrometeor fall speed and aircraft motions are removed). An additional capability on the forward beam permits measurement of the linear depolarization ratio (LDR) which provides useful information on orientation of the hydrometeors (i.e., the canting angle), hydrometeor phase, size, etc. The dual beam geometry has advantages over a single beam. For example, along-track horizontal air motions can be calculated by using the displacement of the ER-2 to provide dual Doppler velocities (i.e., forward and nadir beams) at a particular altitude.

EDOP is designed as a turn-key system with real-time processing on-board the aircraft. The RF system consists of a coherent frequency synthesizer which generates the transmitted and local oscillator frequencies used in the system, a pulse modulated (0.5 to 2.0 micro-second pulse) high gain 20 kW Traveling Wave Tube Amplifier which is coupled through the duplexer to the antenna, and the receiver which is comprised of a low-noise (~1dB) GaAs preamplifier followed by a mixer for each of the receive channels. The composite system generates a nadir oriented beam with a co-polarized receiver and a 350 forward directed beam with co- and cross- polarized receivers. The antenna design consists of two separate offset-fed parabolic antennas, with high polarization isolation feed horns, mounted in the nose radome of the ER-2. The antennas are 0.76 m diameter resulting in a 30 beamwidth and a spot size of about 1.2 km at the surface (assuming a 20 km aircraft altitude). The two beams operate simultaneously from a single transmitter.

Critical to progress in understanding and modeling ice sheets are a better characterization of what ice sheets are doing at present, how fast they are changing, what are the driving processes controlling these changes, and how we can better represent these processes in numerical models to derive more realistic predictions of the evolution of glaciers and ice sheets in the future. Chief among these measurements, are detailed, enhanced and sustained measurements of ice sheet elevation, at high spatial resolution, with high vertical accuracy, over the entire ice sheets. These measurements provide critical information about long-term ice sheet dynamics (mass balance trends) and short-term variability (precipitation, ablation events, surface lowering of an accelerating glacier, etc.).

Ideally suited to making these measurements is GLISTIN, a Ka-band single pass interferometric synthetic aperture radar (InSAR). Proposed also as a spaceborne mission concept [1], the airborne GLISTIN-A serves as a proof-of-concept demonstration and science sensor. Key features include:

1. The Ka-band center frequency maximizes the single-pass interferometric accuracy (which is proportional to the wavelength), reduces snow penetration (when compared with lower frequencies), and remains relatively impervious to atmospheric attenuation.

2. Imaging capabilities that are important for mapping large areas. Imaging allows features to be tracked with time for estimation of ice motion and reduces data noise when measuring topographic changes over rough surfaces of glaciers and coastal regions of ice sheets.

GISMO is a concept for a spaceborne radar system designed to measure the surface and basal topography of terrestrial ice sheets and to determine the physical properties of the glacier bed. Our primary objective is to develop this new technology for obtaining spaceborne estimates of the mass of the polar ice sheets with an ultimate goal of providing essential information to modelers estimating the mass balance of the polar ice sheets and estimating the response of ice sheets to changing climate. Our technology concept employs VHF and P-band interferometric radars using a novel clutter rejection technique for measuring the surface and bottom topographies of polar ice sheets. Our approach will enable us to reduce signal contamination from surface clutter, measure the topography of the glacier bed, and paint a picture of variations in bed characteristics. The technology will also have applications for planetary exploration including studies of the Martian ice caps and the icy moons of the outer solar system. We have recently shown that it is possible to image a small portion of the base of the polar ice sheets using a SAR approach. Through the concept developed here, we believe that, for the first time, we can image the base and map the 3-dimensional basal topography beneath an ice sheet at up to 5 km depth.

GISMO is a NASA Instrument Incubator Project.

D2P is a JHU/APL (Johns Hopkins University/Applied Physics Laboratory) designed, built, and operated airborne radar instrument sponsored by NASA's Instrument Incubator Program (start in 1998). The goal of this project is to demonstrate the use of two enhancements to satellite radar altimetry and to reduce the risk to a future flight program that would employ an enhanced altimeter.

The D2P system is composed of two logical portions: the flight system, and the ground system. The flight system has also two logical sections:

• A set of RF and digital components that create, amplify and transmit the radar pulse, and receivers to capture the returning radar echoes

• A set of computers that control the operation of the RF/digital components as well as digitizing and recording the received data.

The Airborne Synthetic Aperture Radar (AIRSAR) was an all-weather imaging tool able to penetrate through clouds and collect data at night. The longer wavelengths could also penetrate into the forest canopy and in extremely dry areas, through thin sand cover and dry snow pack. AIRSAR was designed and built by the Jet Propulsion Laboratory (JPL) which also manages the AIRSAR project. AIRSAR served as a NASA radar technology testbed for demonstrating new radar technology and acquiring data for the development of radar processing techniques and applications. As part of NASA’s Earth Science Enterprise, AIRSAR first flew in 1988, and flew its last mission in 2004.