## Appendix A The MISR Instrument and MISR-HR Products
### The MISR Instrument  
MISR became operational on 24 February 2000 and is still working as of this writing. This instrument measures the amount of solar radiation reflected by the Earth, providing a unique opportunity to monitor climate and environmental changes with the same, carefully calibrated instrument over this period. Remote sensing from space traditionally relies on the analysis of spectral, spatial and temporal variations to characterize the state and evolution of the environment. MISR enhances this approach by also collecting quasi-simultaneous multi-angular observations through its nine separate cameras, each acquiring spectral measurements in four spectral bands (blue: 446 nm, green: 558 nm, red: 672 nm, and near-infrared (NIR): 867 nm): Four cameras are pointing forward (Df: 70.3\({}^{\circ}\), Cf: 60.2\({}^{\circ}\), Bf: 45.7\({}^{\circ}\), Af: 26.2\({}^{\circ}\)), one camera is pointing at nadir (An: 0.1\({}^{\circ}\)), and the last four cameras are pointing aft (Aa: 26.2\({}^{\circ}\), Ba: 45.7\({}^{\circ}\), Ca: 60.2\({}^{\circ}\), Da: 70.6\({}^{\circ}\)) of the platform. MISR, therefore, measures the intensity of the solar radiation reflected by the planet in 36 data channels ([45; 107; 108; 109; 110], as well as a recent issue of the MDPI journal _Remote Sensing_ dedicated to that instrument (See [[https://www.mdpi.com/journal/remotesensing/special_issues/misr_rs](https://www.mdpi.com/journal/remotesensing/special_issues/misr_rs)]([https://www.mdpi.com/journal/remotesensing/special_issues/misr_rs](https://www.mdpi.com/journal/remotesensing/special_issues/misr_rs)), last accessed on 14 October 2020.)). The native spatial resolution (technically the ground sampling distance) of MISR is 275 m in all 36 data channels (actually 250 m at nadir, before the resampling implemented at Level 1). However, the resulting data rate exceeds the download bandwidth assigned to that instrument. To meet that constraint, 12 of those data channels are actually downloaded at the full spatial resolution (the 4 spectral bands of the nadir-pointing camera, and the red spectral channels of the 8 off-nadir-pointing cameras). The remaining 24 data channels, i.e., the non-red spectral channels from the off-nadir-pointing cameras, are averaged on-board the satellite and downloaded at the spatial resolution of 1100 m, achieving a 16:1 compression ratio in those channels. This is known as the MISR Global Mode (GM) of operation. MISR can also occasionally be operated in Local Mode (LM), whereby all 36 data channels are acquired at the full spatial resolution of 275 m but only over a limited region of about 300 km along the track of the platform. As a result, all standard MISR Level-2 data products available from NASA are also provided at the spatial resolution of 1100 m (or coarser). They are openly accessible and permanently archived at the NASA Atmospheric Science Data Center (ASDC) in Hampton, VA, USA. Each location on Earth is successively observed by all nine cameras in less than seven minutes. To combine and analyze the data from multiple cameras, these observations must be projected onto a common geographical grid, in this case, the orbit-specific Space Oblique Mercator (SOM) projection. This projection minimizes the distortion resulting from assigning raw data (from the detectors) into acommon grid to co-register the outputs of the various cameras. All MISR data products at Level 1B2 and Level 2 are geo-referenced to the SOM appropriate for the orbit. Since the Terra platform repeats its orbital sequence every 16 days, the instruments observe the same locations from the same angular position every 233 orbits. These repeating orbital patterns are called Paths in MISR parlance. An Orbit refers to one Terra satellite revolution around the planet, numbered sequentially since launch. The elementary MISR data granule is a full Orbit (from pole to pole), but data are logically assigned to successive Blocks to ease the ordering and processing.