The Development and Performance of the NASA/GSFC W-band (94 GHz) Solid-State Cloud Radar System

1. INTRODUCTION

The recent decade has brought rapid development in solid-state power amplifier (SSPA) technology. This has enabled the use of solid-state precipitation radar in place of high-power and high-voltage systems such as those that use Klystron or Magnetron transmitters. The NASA/Goddard Space Flight Center has recently completed a comprehensive redesign of the 94 GHz Cloud Radar System (CRS) to incorporate a solid-state transmitter.

The NASA/Goddard Space Flight Center's Cloud Radar System (CRS) is a 94 GHz Doppler radar that flies on the NASA ER-2 high-altitude aircraft. The upgraded CRS system utilizes a state-of-the-art solid-state 94 GHz power amplifier with a peak transmit power of 30 Watts. The modernized CRS system is detailed here with data results from its deployment during the 2014 Integrated Precipitation and Hydrology Experiment (IPHEX).

2. CLOUD RADAR SYSTEM PERFORMANCE

The CRS solid-state upgrade provides performance comparable-to or better-than the previous klystron-based design while reducing the weight requirements for the ER-2 aircraft to enable more payload options. The upgraded CRS system uses a 30-Watt solid-state power amplifier (SSPA) at 94 GHz produced by Quinstar Technology, Inc. The transceiver was redesigned to include mixing for both the transmit and receive waveforms, a newer low-loss receiver, and a modern internal calibration loop that monitors the transmitter and receiver performance over time. The digital receiver, designed by Remote Sensing Solutions (RSS) allows simultaneous receiving and processing of up to eight frequency diversity channels. The antenna is a half-meter reflectarray designed by NASA/GSFC and Northrop Grumman Electronic Systems under a 2010 Instrument Incubator Project (IIP), “Antenna Technologies for 3D Imaging, Wide Swath Radar Supporting ACE.”

The CRS waveform uses a frequency-diversity pulse-compression scheme similar to that developed for the NASA/GSFC's High-Altitude Wind and Rain Airborne Profiler (HIWRAP) radar that provides simultaneous pulse-compressed and short- pulse data. The transmitted chirp is 30 μs long and is surrounded by conventional short pulses with amplitude tapering to increase channel-channel isolation.

Hardware details and design considerations will be detailed along with information regarding the pulse compression performance and range sidelobes. The performance of the solid-state design as compared to a comparable high-power design will be used as a case study for the current state of cloud and precipitation radar technology.

3. IPHEX FIELD CAMPAIGN

Very soon after completion the upgraded CRS flew more than fifteen flights on the NASA ER-2 for the 2014 Integrated Precipitation and Hydrology Experiment (IPHEX) field campaign. This campaign flew CRS in a nadir-pointing mode at high altitudes (65,000 ft.) over a variety of cloud, storm, and rain conditions. During this field campaign CRS was configured to have very-high range resolution, with a 100 meter resolution sampled at intervals of 37.5 meters. Similarly, the horizontal resolution was 150 meters with spatial sampling every 50 meters.

CRS flew alongside the NASA/GSFC ER-2 X-band Radar (EXRAD) and High-Altitude Imaging Wind and Rain Atmospheric Profiler (HIWRAP) instruments. Combined these three instruments took colocated radar data at X-, Ku-, Ka-, and W-bands. This radar data was combined with on-board microwave radiometry from the COSMIR and AMPR radiometers. ER-2 flight paths were also coordinated with satellite radar overpasses from both CloudSat and GPM, and with various ground-based radar. The performance of CRS during the IPHEX campaign will be detailed with examples of co-located data at multiple wavelengths.

4. CONCLUSION

The upgraded solid-state Cloud Radar System is a unique instrument that demonstrates the mm-wave radar designs newly available due to high-power solid-state amplifiers. Relevant design decisions such as the frequency diversity pulse-compression waveform will provide insights on the use of pulse compression technology at W-band.
The performance of this instrument will be detailed with operational reflectivity and Doppler examples taken from the 2014 IPHEX field-campaign. CRS took significant high-sensitivity high-resolution data of clouds. This W-band data will be shown in context with both the X-, Ku-, and Ka-band data from the HIWRAP and EXRAD instruments also flying on the ER-2, as well as satellite radar data.