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Dual-frequency Precipitation Radar (DPR) - Instrument Details


One of the prime instruments for the GPM Core Observatory is called the Dual-frequency Precipitation Radar (DPR). The DPR consists of a Ku-band precipitation radar (KuPR) and a Ka-band precipitation radar (KaPR). The KuPR (13.6 GHz) is an updated version of the highly successful unit flown on the TRMM mission (shown below). The KuPR and the KaPR will be co-aligned on the GPM spacecraft bus such that that the 5 km (3.1 mile) footprint location on the earth will be the same.

A JAXA scientist standing next to the DPR (image credit: JAXA)

DPR demonstration and DPR assembly at GSFC cleanroom

Over view of the Dual-frequency Precipitation Radar “DPR”

Data collected from the KuPR and KaPR units will provide 3-dimensional observations of rain and will also provide an accurate estimation of rainfall rate to the scientific community. The DPR instrument will be allocated 190 Kbps bandwidth over the 1553B spacecraft data bus.

The DPR is a spaceborne precipitation radar capable of making accurate rainfall measurements. The DPR is expected to be more sensitive than its TRMM predecessor especially in the measurement of light rainfall and snowfall in high latitude regions. Rain/snow determination is expected to be accomplished by using the differential attenuation between the Ku-band and the Ka-band frequencies. The variable pulse repetition frequency (VPRF) technique is also expected to increase the number of samples at each IFOV to realize a 0.2 mm/h sensitivity.

The Dual-frequency Precipitation Radar (DPR) aboard the GPM core observatory is composed of two precipitation radars (PRs), the KuPR on the Ku-band (13.6 GHz) and the KaPR on the Ka-band (35.5 GHz.)

The KaPR, whose main purpose is to improve sensitivity, is useful for detecting light rain and snow that cannot be measured by the KuPR. Conducting measurements simultaneously with the KuPR, which can detect strong rain, the two radars can observe both strong rain in the tropical zone and light rain and snow in high latitude areas. In general, strength of precipitation echoes is affected by attenuation due to precipitation on those frequencies, but the amount of attenuation depends on the frequency and the size of raindrops.

The KuPR and KaPR, therefore, match their radar beam positions and transmission pulse timings of each other to estimate a size of a raindrop (Raindrop Size Distribution, DSD) by calculating the difference in precipitation attenuations. This information cannot be acquired through only one-frequency radar like the TRMM’s PR, hence the accuracy of precipitation volume estimation will be significantly improved.

Please Note: The tables on this webpage will be updated soon to reflect changes due to the Nov 07-08, 2023 GPM Satellite Orbit Boost

Top-level general design specifications are as follows:

Item	KuPR	KaPR
Swath Width	245 kilometers (km)	120 kilometers (km)
Range Resolution	250 meters (m)	250/500 meters (m)
Spatial Resolution	5 km (Nadir)	5 km (Nadir)
Beam Width	0.71 degrees	0.71 degrees
Transmitter	128 Solid State Amplifiers	128 Solid State Amplifiers
Peak Transmit Power	1013 Watts (W)	146 Watts (W)
Pulse Repetition Freq. (In nominal operations mode)	4100 to 4400 Hertz	4100 to 4400 Hertz
Pulse Width	two 1.667 microseconds (µs) pulses	two 1.667 microseconds (µs) pulses in matched beams two 3.234 microseconds (µs) pulses in interlaced scans
Beam Number	49	49 (25 in matched beams and 24 in interlaced scans)

GPM Microwave Imager (GMI) - Instrument Details

The Global Precipitation Measurement (GPM) Microwave Imager (GMI) instrument is a multi-channel, conical- scanning, microwave radiometer serving an essential role in the near-global-coverage and frequent-revisit-time requirements of GPM.

GMI is a primary microwave sensor onboard both the GPM core satellite and constellation satellites. The core satellite flies in a 407-km circular orbit with a 65o inclination angle. The constellation will be a Sun-synchronous (TBD) satellite flying at altitudes of about 635 km.

The instrumentation enables the Core spacecraft to serve as both a precipitation standard and as a radiometric standard for the other GPM constellation members.

The GMI is characterized by thirteen microwave channels ranging in frequency from 10 GHz to 183 GHz. In addition to carrying channels similar to those on the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), the GMI carries four high frequency, millimeter-wave, channels about 166 GHz and 183 GHz.. Except for the heritage hot load and cold load that are commonly used for linear sensor radiometric calibrations, a hot noise diode and a cold noise diode are implemented in the GMI to determine the non-linearity and noise levels of the measurements.

With a 1.2 m diameter antenna, the GMI will provide significantly improved spatial resolution over TMI.

Scan Geometry

The off-nadir-angle defining the cone swept out by the GMI is set at 48.5 degrees which represents an earth-incidence-angle of 52.8 degrees. To maintain similar geometry with the predecessor TMI instrument, the-earth-incidence angle of GMI was chosen identical to that of the TMI. Rotating at 32 rotations per minute, the GMI will gather microwave radiometric brightness measurements over a 140 degree sector centered about the spacecraft ground track vector. The remaining angular sector is used for performing calibration; i.e. observation of cold space as well as observation of a hot calibration target.

The 140 degree GMI swath represents a swath of 904 km (562 miles) on the Earth's surface. For comparison, the DPR instrument is characterized by cross-track swath widths of 245 km (152 miles) and 120 km (75 miles), for the Ku and Ka-band radars respectively. Only the central portions of the GMI swath will overlap the radar swaths (and with approximately 67 second duration between measurements due to the geometry and spacecraft motion). These measurements within the overlapped swaths are important for improving precipitation retrievals, and in particular, the radiometer-based retrievals.

Scan geometry of GMI (image credit: NASA)

GMI Footprint Changes Due to 35 km Altitude Boost

The GMI sensor data include the following:

Nominal altitude: 407 km.

Orbital inclination: 65 deg.

Spin rate: 32 rpm.

Scan time: 1.875 sec.

Swath width: 885 km.

Earth viewing sector: 140 deg.

Earth samples: 202.

Integration time: 3.6 msec = (140/360) x 1.875 sec/202.

Dish size: 1.22 m.