G-Band Radar for Humidity and Cloud Remote Sensing

Abstract

VIPR (Vapor In-cloud Profiling Radar) is a tunable G-band radar designed for humidity and cloud remote sensing. Using all-solid-state components and operating in a frequencymodulated continuous-wave (FMCW) radar mode, VIPR's transmit power is 200-300 mW. Its typical chirp bandwidth of 10 MHz over a center-frequency tuning span of 167-174.8 GHz results in a nominal range resolution of 15 m. The radar's measured noise figure over the transmit band is between 7.4-10.4dB, depending on its frequency and hardware configuration, and its calculated antenna gain is 58 dB. These parameters mean that with typical 1 ms chirp times, single-pulse cloud reflectivities as low as -26 dBZ are detectable with unity signal-to-noise at 5 km. Experimentally, radar returns from ice clouds above 10 km in height have been observed from the ground. VIPR's absolute sensitivity was validated using a spherical metal target in the radar antenna's far field, and a G-band switch has been implemented in an RF calibration loop for periodic recalibration. The radar achieves high sensitivity with thermal noise limited detection by virtue of both its low-noise RF architecture and by using a quasioptical duplexing method that preserves ultra-high transmit/receive isolation despite operation in an FMCW mode with a single primary antenna shared by the transmitter and receiver.

Validation of a G-Band Differential Absorption Cloud Radar for Humidity Remote Sensing

“…In addition to being the first water vapor DAR, VIPR is the first all-solid-state G-band cloud radar, and is operated in frequency-modulated, continuous-wave (FMCW) mode to increase the transceiver sensitivity relative to a pulsed system. An early version of the VIPR system, discussion of FMCW radar detection, and demonstration of the DAR measurement principle were previously described in detail in Cooper et al (2018) and Roy et al (2018). Since this early work, a higher-power transmitter and higher-gain optics have been installed, greatly increasing the sensitivity of the system, and the hardware has been modified for aircraft deployment.…”

“…To acquire calibrated measurements of cloud reflectivities, the calibration factor C( f ) must be known. By comparing the standard weather radar equation (Doviak and Zrnić 1993) with the reflectivity factor definition Z obs 5 l 4 h obs /[p 5 jK w ( f )j 2 ], we find an expression for this calibration factor in terms of radar system parameters:…”

First Light Multi-Frequency Observations with a G-band radar

“…VIPR is a first-of-its-kind solid-state G-band differential absorption radar (DAR). It's technical specifications are described in detail in Cooper et al (2020) and Roy et al, (2020).…”

“…8c). In some instances, we noted that strong radar returns from close-range rain caused an increase in the system noise floor of up to 20 dB stemming from broadband phase noise in the transmitted signal (Cooper et al, 2020). At 20:41 UTC on 25 February,…”

“…It took 30 years before we saw the development of the next generation of G-band radars. In 2018, thanks to significant technological advancements in radar front ends, mixers and low-power wide-bandwidth solid state G-70 band sources, the Jet Propulsion Laboratory (JPL) developed a highly sensitive non-Dopplerized frequency-modulated continuous-wave (FMCW) G-band radar tunable from 167 to 174.8 GHz (i.e., DAR;Cooper et al, 2020;Roy et al, 2018). The system, named Vapor In-Cloud Profiling Radar (VIPR), was deployed during 7-days over April 2019 at the ARM Southern Great Plains facility to evaluate VIPR's ability to exploit differential absorption signatures to retrieved in-cloud humidity profiles (Roy et al, 2020).…”