X-Band Lightweight Rainfall Radiometer

LRR-X at a glance

The LRR-X instrument

The X-Band Lightweight Rainfall Radiometer using Synthetic Thinned Aperture Radiometer technology (LRR-X STAR) is a technology testbed instrument for the Global Precipitation Measurement (GPM) Mission, an international satellite precipitation mission led primarily by NASA and NASDA. One of the main objectives of GPM is high frequency global sampling of rainfall. In order to achieve this objective, the NASA Goddard Space Flight Center and the University of Michigan have developed LRR-X as an aircraft microwave sensor using the synthetic thinned aperture approach.

First Airborne Flight LRR-X installed on the NASA DC-8 aircraft


The first airborne flight of the LRR-X instrument took place in May 2003.


The LRR-X instrument has successfully flown on the NASA DC-8 research aircraft on several engineering check flights and one science campaign to investigate cloud microphysics and radiative transfer in the freezing layer of precipitating stratiform clouds.

Measurement Objectives

GPM logo

The LRR-X instrument is designed to measure rainfall and use rain retrievals to improve predictions of climate, weather, and hydrometeorology. The instrument will provide remote sensing of the vertical profile of precipitation and the near-surface wind speed over the ocean. These measurements will improve the tracking of the global water cycle and release of latent heat, and contribute to our understanding of cloud microphysical processes. LRR-X will aid in hazardous flood forecasting, seasonal draught-flood outlooks, fresh water resources prediction, and hurricane research and forecasting.

LRR-X aircraft


/LRR-X slotted waveguide antenna array and ground plane

The LRR-X instrument uses interferometric principles to measure rainfall. By spacing slotted waveguides, each equipped with its own receiver on the back plane, at selected intervals across an aluminum antenna frame mounted on an aircraft or satellite perpendicular to the earth-pointing axis, cross-track convolution images at desired frequencies and polarizations can be obtained from a rigid-mount payload. The parallax between waveguides provides the phase-shift interferogram information while the waveguide spacing covers all essential multiple-half wavelength modes of a transform image. HIRad uses solid state MMIC receiver technology and ultra low power digital quatdrature demodulators and cross correlators, making it a less expensive, lower weight, and lower power alternative to conventional scan-type radiometers. It operates at 10.7 GHz with 86+ simultaneous 2.1o HPBW antenna beams distributed over a ±45° cross track field of view to permit pushboom imaging. The design allows for multiple channels and polarizations, while the antenna can be oriented in either a nadir-oriented cross-track configuration or a canted cross-track configuration to replicate either cross-track or conical scanning orientations typically used for conventional scanning radiometers.

STAR antenna test plate

10.7 GHz slotted waveguide antenna


Chris Ruf – Science investigator and system engineer
Steven Gross – Digital signal processing electrical engineer
Steven Musko – Control and data handling software engineer
Boon Lim – Thermal control electrical engineer

SPRL engineers work on LRR-X

Technical Specifications

10.7 GHz -- HPol
Synthetic Aperture 1 meter²
Cross-track imaging
Spatial resolution @ 11 km altitude
                381 x 466 m (nadir)
                436 x 484 m (17° cross track)
                1079 x 629 m (45° cross track)
NEDT of 0.3K

Engineering Feats of Note

The synthetic thinned aperture radiometer technology used in LRR-X gives the system many strategic advantages over mechanically scanned systems, including an inexpensive design with no moving parts. Because it does not rely on an antenna scan-drive assembly with mechanical failure points, LRR-X is a low risk system. LRR-X allows for multiple frequencies, polarization separation, and nadir or conical imaging. The instrument also has lower weight and power requirements than a mechanically scanned system, as well as superior calibration stability.

Two LRR-X prototypes

LRR-X prototype undergoing noise