Microwave imaging radiometers operating in the 50–183 GHz range for retrieving atmospheric temperature and water vapor profiles from airborne platforms have been limited in the spatial scales of atmospheric structures that are resolved not because of antenna aperture size, but because of high receiver noise masking the small variations that occur on small spatial scales. Atmospheric variability on short spatial and temporal scales (second/ km scale) is completely unresolved by existing microwave profilers.
The solution was to integrate JPLdesigned, high-frequency, low-noiseamplifier (LNA) technology into the High-Altitude MMIC Sounding Radiometer (HAMSR), which is an airborne microwave sounding radiometer, to lower the system noise by an order of magnitude to enable the instrument to resolve atmospheric variability on small spatial and temporal scales.
HAMSR has eight sounding channels near the 60-GHz oxygen line complex, ten channels near the 118.75-GHz oxygen line, and seven channels near the 183.31-GHz water vapor line. The HAMSR receiver system consists of three heterodyne spectrometers covering the three bands. The antenna system consists of two back-to-back reflectors that rotate together at a programmable scan rate via a stepper motor. A single full rotation includes the swath below the aircraft followed by observations of ambient (roughly 0 °C in flight) and heated (70 °C) blackbody calibration targets located at the top of the rotation.
A field-programmable gate array (FPGA) is used to read the digitized radiometer counts and receive the reflector position from the scan motor encoder, which are then sent to a microprocessor and packed into data files. The microprocessor additionally reads telemetry data from 40 onboard housekeeping channels (containing instrument temperatures), and receives packets from an onboard navigation unit, which provides GPS time and position as well as independent attitude information (e.g., heading, roll, pitch, and yaw). The raw data files are accessed through an Ethernet port. The HAMSR data rate is relatively low at 75 kbps, allowing for real-time access over the Global Hawk high-data-rate downlink. Once on the ground, the raw data are unpacked and processed through two levels of processing. The Level 1 product contains geo-located, time-stamped, calibrated brightness temperatures for the Earth scan. These data are then input to a lD variational retrieval algorithm to produce temperature, water vapor, and cloud liquid water profiles, as well as several derived products such as potential temperature and relative humidity.
The addition of a state-of-the-art LNA to the 183-GHz receiver front-end and the upgrade of the 118-GHz LNA provide excellent low-noise performance, which is critical for microwave sounding retrievals. The data system is upgraded to provide in-flight data access through the Global Hawk data link, making it possible to relay data to the ground in real time. This is particularly relevant for hurricane observations where HAMSR can provide real-time information on tropical storm structure, intensity, and evolution.
This instrument is the first to demonstrate the value of the technology through atmospheric water vapor measurements. The receiver noise was reduced by an order of magnitude compared to the previous receiver. A ground-based measurement campaign demonstrated unprecedented measurements of small-scale water vapor variability, resolving atmospheric fluctuations on meter and second space and time scales. Subsequent airborne measurements on the Global Hawk UAV showed similar results over a 40-km swath. This is a critical step in space-qualifying these receivers.