The NASA S-band Dual Polarimetric Radar (NPOL) is a research grade S-band, scanning dual polarimetric Doppler radar. It has equivalent capabilities to the Doppler radars of the U.S. National Weather Service’s Next Generation Weather Radar (NEXRAD) network. However, in contrast to the stationary NEXRAD radars, NPOL is one of two completely transportable research-grade S-band systems in the world. The radar operates atop a pedestal and 5 intermodal containers (i.e., sea containers) and is stored inside these containers when being transported for deployment. NASA’s NPOL weather radar has been and continues to be used for various NASA field campaigns. It resides and operates near NASA’s Wallops Flight Facility in Newark, MD when not on field campaign deployment.
Weather radars use electromagnetic (EM) radiation to detect atmospheric targets. The radar’s transmitter emits a short “pulse” of radiation energy toward a volume of precipitation particles. The transmitter is then paused while the receiver “listens” to detect the radiation that is reflected back (i.e., backscattered) toward the radar where the signal will be measured.The rate at which the pulse is transmitted by the radar, known as the pulse repetition frequency (PRF), can vary. For NPOL, the PRF can range from 500 - 1000 Hz. These measurements give information such as the amount and size of the atmospheric targets.
The “S-band” ranges from 2 to 4 GHz and lies in the microwave region of the EM spectrum, a region commonly used for weather observations. NPOL uses a 850 kW magnetron transmitter and an 8.5 m parabolic antenna to transmit radiation at 2.7 to 2.8 GHz (S-band) with a 0.95° beamwidth. “Dual polarimetric” means that the NPOL can transmit pulses at two orientations, horizontal and vertical. These measurements allow the radar to identify precipitation type and other microphysical characteristics. NPOL is also a Doppler radar, meaning it can resolve wind velocities within precipitation. The speed and direction of movement by precipitation particles will affect the backscattered signal, revealing information about the air motions relative to the radar.
NPOL’s antenna can be maneuvered to complete three different types of radar scans including Plan Position Indicator (PPI) where a 360 degree sweep of the antenna is made; PPI Sector (PPS) where the sweep of the antenna is limited to a specific azimuth range; and Range Height Indicator (RHI) in which scans are pointed at a specific azimuth and the antenna tilts upward to get vertical profile information. Range gates are specific distance intervals from the radar in which the radar return signal is measured. These gates are used to differentiate return signals originating at different distances from the radar.
Measurements
Measures radar reflectivity, Doppler velocity, differential reflectivity, correlation, differential phase, rainfall rate, particle size distribution, water contents and precipitation type. When the radar antenna is wet, this can increase signal attenuation and lead to data quality issues. Self-consistency and disdrometer comparisons are used to estimate the absolute calibration of NPOL. Also, the pointing angle of the radar dish is monitored with software that uses the sun’s position to calibrate the elevation and azimuth angles.
Applications
- Satellite validation
- Precipitation studies
- Global water cycle
- Climate
- Precipitation microphysics
| Frequency | Spatial Extent | Beam Width | Accuracy | Pulse Repetition Frequency (PRF) | Antenna | Gain | Pulse Width |
|---|
2.7 - 2.9 GHz
(10.65 cm wavelength) | 150 km range | 0.95° | 0.1° pointing accuracy | 500 - 1000 Hz | 8.5 m | 45.8 dB
(+/- 0.3 dB) | 0.8 - 2.0 ms |