SOLID STATE TRANSMITTER UPGRADES FOR NASA GSFC HIGH-ALTITUDE AIRBORNE WEATHER RADARS

Over the past decades, a number of airborne weather radars have been developed at the NASA Goddard Space Flight Center (GSFC) for high-altitude operation at frequencies ranging from X-band to W-band, including the X-band (9.6 GHz) ER-2 Doppler Radar (EDOP), the EDOP following-up 9.6 GHz ER-2 X-band Radar (EXRAD), the Ku (13 GHz)/Ka-band (35 GHz) dual-frequency High-altitude Wind and Rain Airborne Profiler (HIWRAP) and the W-band (94 GHz) Cloud Radar System (CRS). These radars have been successfully deployed on the NASA high-altitude aircrafts, including ER-2, WB-57 and Global Hawk Unmanned Aerial System (UAS), for numerous airborne science observation campaigns or for spaceborne sensor calibration and validation.

The X-band (9.6 GHz) EDOP was built in the early 1990s based on a high peak power (25 kW) Traveling-Wave Tube (TWT) transmitter. Its follow-up X-band (9.6 GHz) EXRAD was also initially developed based on a 9 kW TWT, while the W-band (94 GHz) CRS was initially built in the late 1990s based on a 1.7 kW high-peak power Extended Interaction Klystron (EIK). The traditional tube-based radar transmitter requires a high voltage power supply and modulator. Therefore, the radar usually requires pressurization of its transmitter and RF front-end subsystem for operation in the low pressure environment at high altitudes. This usually results in a radar system with large size, heavy weight, and poor system reliability. Recent advancements in the communication industry have led to higher power Solid State Power Amplifiers (SSPA) with average output powers comparable to that of tube-based power amplifiers. Compared to tube-based transmitters, SSPAs can be built in much more compact size with lower phase noise and better output power stability. In addition, SSPAs can be operated at a much higher duty cycle (up to 100%). This enables the use of versatile waveforms, such as frequency diversity pulses for increasing the number of independent samples or long frequency-modulated chirps for implementing pulse compression.

CRS was upgraded with a 30 W W-band SSPA in the mid 2010s, then most recently with a 50 W SSPA developed by QuinStar Technology through NASA Small Business Innovation Research (SBIR) funding support. EXRAD was upgraded with a 900 W commercial SSPA along with high speed, more compact digital waveform generation and digital receiver subsystem. This presentation will provide an overview of EXRAD and CRS upgrade efforts and data samples from the recent airborne science campaigns.