Glossary

General¶

Full-polarimetric covariance matrix

Calculated by

$$[G_{3}] = \begin{bmatrix} <S_{HH}S^*_{HH}> & <S_{HH}\overline{S}^*_{HV}> & <S_{HH}S^*_{VV}> \cr <\overline{S}_{HV}S^*_{HH}> & <\overline{S}_{HV}\overline{S}^*_{HV}> & <\overline{S}_{HV}S^*_{VV}> \cr <S_{VV}S^*_{HH}> & <S_{VV}\overline{S}^*_{HV}> & <S_{VV}S^*_{VV}> \end{bmatrix}$$

The polarimetric covariance matrix $[G_3]$ is then radiometric terrain corrected and geocoded using an area-based projection algorithm, producing the GCOV matrix

Geolocation accuracy

The NISAR geolocation accuracy can be potentially affected by tropospheric delay, solid earth tides (SET) and ionospheric delay Yunjun et al., 2022.

Ionospheric delay

At altitudes above ∼50 km, radiation (mainly from the Sun) ionizes atmospheric atoms and molecules forming the ionosphere. Its refractivity is mainly controlled by the total electron content. The propagation of the microwave signal traveling through the ionosphere results in a group delay and a phase advance.

Radio Frequency Interference (RFI)

The undesired signal in the form of wideband or narrowband interference that overlaps with the in-band frequency spectrum of NISAR. The main sources of high-power RFI are electromagnetic signals from civilian and military ground-based communication or radar platforms. RFI detection is performed on L0B raw data before focusing.

Radiometric Terrain Correction (RTC)

The NISAR geocoded covariance (GCOV) product is radiometrically terrain corrected to normalize the SAR backscatter to gamma-naught $\gamma^0$. The GCOV algorithm is using an area-projection algorithm Shiroma et al., 2022 to compute RTC correction factor and to geocode the data.

Solid Earth Tides (SET)

The motion of the Earth’s surface is driven by the gravitational pull from the Sun and the Moon (solid Earth tides), the change of the rotational axis (pole tides), the loading effects from the ocean tides (OTL), the atmospheric and hydrological loading as well as the surface deformation. Displacements from tidal and loading effects are periodic with different time scales and amplitudes Yunjun et al., 2022. SET often reaches 40 and 10 cm in vertical and horizontal directions, respectively Petit & Luzum, 2010

Total Electron Content

Two components of the Total Electron Content (TEC) data from the Global Navigation Satellite System (GNSS) are sampled every 10 seconds along the NISAR orbit: the integrated TEC from surface to the GNSS orbit and the top-side TEC. The difference of the two results in sub-orbital TEC is used to estimate range delay and the azimuth shifts.

Tropospheric delay

At altitudes up to ~ 30 km, which forms the troposphere, refractivity is mainly controlled by temperature, water vapor, and dry air partial pressure Berrada Baby et al., 1988. The zenith hydrostatic delay is ~2.3 m at sea level at typical meteorological conditions, while the zenith wet delay varies from a few mm at the polar region to ∼40 cm at the equatorial region Böhm & Schuh, 2013. The tropospheric delay is corrected using a static tropospherical model (European Centre for Medium-Range Weather Forecasts) during focusing the raw data.

Radar grid (metadata cube)¶

referenceSlantRange / secondarySlantRange

The range position of the zero-Doppler grid in maters for each point of the geographical grid of the reference/secondary RSLS

referenceZeroDopplerAzimuthTime / secondaryZeroDopplerAzimuthTime

The zero-Dopple azimuth time of the zero-Doppler grid in seconds for each point of the geographical grid of the reference/secondary RSLC

coordinateX

The mapping of the zero-Doppler grid to the geographic grid (x component) in the units of the projection

coordinateY

The mapping of the zero-Doppler grid to the geographic grid (y component) in the units of the projection

losUnitVectorX

The East component of the Line-Of-Sight (LOS) unit vector from the target to the sensor in the East-North-Up coordinate system for each point of the geographic grid

losUnitVectorY

The North component of the Line-Of-Sight (LOS) unit vector from the target to the sensor in the East-North-Up coordinate system for each point of the geographic grid

losUnitVectorZ

The Up component of the Line-Of-Sight (LOS) unit vector from the target to the sensor in the East-North-Up coordinate system for each point of the geographic grid. The unit vector needs to be derived from the East and North components as: