Synthetic Aperture Radar Imagery (NRCS)

jebidiah.jeffery — Thu, 17 Mar 2022 15:30:36 +0000

Spaceborne Synthetic Aperture Radar (SAR) imagery maps the surface microwave radar reflectivity at resolutions from a sub-meter to 100 m depending on the particular SAR satellite and mode. Since a radar provides its own illumination, imagery is independent of the time of day. At typical radar frequencies, SARs can image through clouds, so SARs are considered "all-weather" instruments. Several geophysical parameters can be derived from SAR including sea surface wind speed.

jebidiah.jeffery Thu, 03/17/2022 - 11:30

Spaceborne Synthetic Aperture Radar (SAR) imagery maps the surface microwave radar reflectivity at resolutions from a sub-meter to 100 m depending on the particular SAR satellite and mode. Since a radar provides its own illumination, imagery is independent of the time of day. At typical radar frequencies, SARs can image through clouds, so SARs are considered "all-weather" instruments. Several gephysical parameters can be derived from SAR including sea surface wind speed.

For side-scanning SARs, the normalized radar cross section (NRCS) of the surface is proportional to the surface roughness on the scale of the radar wave length (from 3 to 30 cm). Ocean surface roughness on this scale is dependent on the local wind speed and direction. The higher the wind speed the larger the NRCS of the surface. The highest NRCS occurs when the radar look direction and wind direction are aligned and smallest when the radar looks cross wind.

The relationship between wind speed and direction and radar cross section is called the "geophysical model function." These functions are generally empirically determined.

Although we can predict an expected NRCS value for a given wind speed and direction, the reverse is not true. A specific NRCS value can be associated with many pairs of wind speed and direction. If we know the wind direction a priori, we can infer the wind speed.

For the operational SAR-derived wind speed products, we use the model directions from NOAA’s Global Forecast System (GFS) model on a 0.5 degree, latitude-longitude grid to initiate the retrievals. The wind speed retrievals are provided at 500 m resolution. These retrievals agree with independent wind speed estimates to better than 2 m/s.

Current users of these data include the US National Ice Center, the Alaska Weather Service, and the Naval Oceanographic Office.

A good review of SAR operational wind speed retrieval can be found at: http://tos.org/oceanography/assets/docs/26-2_monaldo.pdf

Product Families

Sea Surface Roughness

Monaldo, F.M., C.R. Jackson, and W.G. Pichel. 2013. Seasat to RADARSAT-2: Research to operations. Oceanography 26(2):34–45, http://dx.doi.org/10.5670/oceanog.2013.29
SAR Marine User Manual (2004) Christopher Jackson and John Apel, Editors

Measurements

Normalized Radar Cross-Section

Sea Surface Wind Speed

Processing Levels

Level 1

Level 2

Level 3

Spatial Coverage

Selective Acquisitions

Latency Groups

0 Hours <= 24 Hours (NRT)

Spatial Resolution Groups

0 m <= 100 m

100m < 2km

Spatial Resolution Details

NRCS: Variable, 0-10 m
Winds: 500 m

Platforms

Instruments

Processing Algorithms

SAROPS

Data Providers

Copernicus

CSA

NOAA

NESDIS

OSPO

STAR

Data Tool Links

CoastWatch Tools

CoastWatch Data Portal (select Spatial Search)

Data files: https://coastwatch.noaa.gov/data/pub0022/coastwatch/

Northern & Southern Hemisphere Products

NRCS Search Tool

XPOL Search Tool

Other Search Tools

RadarSat-2 + RCM SAR Winds Search Tool (MECB)

Sentinel-1 SAR Winds Search Tool (MECB)

RadarSat-2 + Sentinel-1 SAR Winds Search Tool (OSPO) [Formally Operational]

Sample Filenames

Filenames begin with satellite, data source, year, month, day, hour, minute, second, Julian seconds, center latitude, center longitude, polarization, delimited with an underscore NRCS: S1A_ESA_2016_12_07_18_18_57_0534449937_178.66E_51.85N_VV_C5_GFS05 CDF_nrcs_level2.png
Wind: S1A_ESA_2016_12_07_18_18_57_0534449937_178.66E_51.85N_VV_C5_GFS05 CDF_wind_level3.png