# Determining the AMSR-E SST Footprint from Co-Located MODIS SSTs
## Data  
### Amsr-E  
AMSR-E was a National Space Development Agency (Japan) (NASDA) instrument launched on the Aqua satellite in May 2002. It failed when its antenna stopped rotating in October 2011. AMSR-E was a dual-polarized, conical-scanning, passive microwave radiometer. The wide swath (1450 km) of the instrument and the near-polar orbit in which it was flown provided near global coverage twice daily, once on the sun-side of Earth (daytime, ascending, 1330 local Sun time (LST)) and once on the dark side (nighttime, descending, 0130 LST). AMSR-E, a twelve channel instrument, measured the vertical and horizontal polarization in each of six frequency bands. The AMSR-E antenna footprint, which was unique for each channel and increased with decreasing frequency, was determined by the antenna gain pattern [11]. The antenna gain pattern footprint describes how much the emissions from a particular receive direction contributes to the observed value. The Japan Aerospace Exploration Agency (JAXA) provided measured antenna patterns for the AMSR-E sensor at all center frequencies. The nominal size of the footprint for each of the spectral bands is reproduced from [11] in Table 1. For additional details with regard to the footprint, please see [12]. As noted in the introduction, there is likely a difference between the SST footprint and the footprint of the primary spectral channel contributing to the retrieval. The scan geometry of the instrument in orbit is shown in Figure 1. The satellite orbit height was 705 km, the ground incidence angle was 55\({}^{\circ}\), and the observation angle range was from -61\({}^{\circ}\) to +61\({}^{\circ}\). In one scan, AMSR-E sampled 243 cell footprints for each of the frequency bands except for the 89 GHz band, for which it sampled at twice the frequency. In this study, we use SST fields obtained with version-7 (v7) of the AMSR-E algorithm developed by RSS. The data were acquired from the Physical Oceanography-Distributed Active Archive Center (PO-DAAC) server at: [[https://podaacjpl.nasa.gov](https://podaacjpl.nasa.gov)]([https://podaacjpl.nasa.gov](https://podaacjpl.nasa.gov)). This algorithm simultaneously determines water vapor, cloud liquid water, precipitation, sea surface WS and SST using brightness temperature measurements from all spectral channels except for those at 89 GHz. SST, the variable of interest here, is derived primarily from the 6.93 GHz channel. As shown in Table 1, the ground resolution of the 6.93 GHz channel was approximately 75 km in the direction defined by a vector from the pixel to the nadir point of the satellite and approximately 43 km normal to this vector [11]. Thus, the retrieved SST was of coarse spatial resolution. The distance between adjacent observations for all channels, except those of 89 GHz, was approximately 10 km across-track and 10 km along-track [11]; i.e., the 6.93 GHz channel was substantially oversampled, with the remaining channels being less so. (Note that because the AMSR-E antenna rotates about a vertical axis with a beam direction 47.5\({}^{\circ}\) relative to this axis, a scan-line describes an approximate circle on the Earth's surface so, as one moves away from nadir, the across-track direction was not orthogonal to the along-track direction.) To work simultaneously \begin{table}
\begin{tabular}{c c c} \hline \hline
**Frequency** & **Beam Width** & **Footprint** \\
**(GHz)** & **(\({}^{\circ}\), Nominal)** & **(km, Across-Track \(\times\) Along-Track)** \\ \hline
6.925 & 2.2 & 43.2 \(\times\) 75.4 \\
10.65 & 1.5 & 29.4 \(\times\) 51.4 \\
18.7 & 0.8 & 15.7 \(\times\) 27.4 \\
23.8 & 0.9 & 18.1 \(\times\) 31.5 \\
36.5 & 0.4 & 8.2 \(\times\) 14.4 \\
89 & 0.2 & 3.7 \(\times\) 6.5 \\
89 & 0.2 & 3.5 \(\times\) 5.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Nominal footprint size (reproduced from ([11], pp. 2–31)). with all spectral channels, the RSS retrieval algorithm resampled the brightness temperatures for each of the spectral channels to match the resolution of the 6.93 GHz channel [13].