Pointing Mirror Assembly (PMA) and Modes

OCO-3 Observation Modes

The PMA allows for target mode observations, similar to those taken by OCO-2, typically at Total Column Carbon Observation Network (TCCON) ground sites for use in validation. The PMA provides the ability to scan large contiguous areas (order 80 km by 80 km), such as cities and forests, on a single overpass. This mode is known as "snapshot" mode and allows for fine scale spatial sampling of CO₂ and SIF variations, unlike what can be done with any current satellite system. OCO-3 is using the same instrument as OCO-2, but it has been adapted to work on the ISS. The instrument functions in four modes in flight: Nadir viewing (straight down), glint (reflected), SAM (snapshot area mapping), and a pointing mode for target sites. Unlike OCO-2, which performs complex maneuvers of the entire satellite bus to observe ground targets, the OCO-3 instrument is fitted with an agile 2-D pointing mechanism, i.e., a Pointing Mirror Assembly (PMA) that allows for rapid transitions between nadir and glint mode (less than 1 minute).

Unlike OCO-2, which flies in a polar, sun-synchronous orbit around the Earth, OCO-3 is onboard the ISS, and follows a precessing orbit. This means that overpasses progress earlier and earlier in local time of day for a given point on the earth over periods of days. In about 30 days, at a given location, measurements progress from late in the day to early in the day. For some locations at the higher latitudes, there are periods where measurements are taken both in the morning and in the afternoon of the same day. This variable time of day sampling has implications with respect to the diurnal cycle of both clouds and aerosols (known contaminants when observing XCO₂), and studies of the carbon cycle, which itself has a strong diurnal variation. The precession in time-of-day sampling is especially informative for the SIF observations with respect to studying the biosphere response (both natural and anthropogenic) to changes in sunlight. OCO-2 and OCO-3 operating concurrently, collect overlapping and complimentary data. The primary method of comparing OCO-2 and OCO-3 data is through the TCCON measurements.

Additional details can be found in the paper: Eldering et al.

OCO-3 Pointing Mirror Assembly

This system relies on a single pair of matched mirrors in an orthogonal configuration that impart less than 0.05% change to the polarization (Mishchenko et al., 2007). For the OCO-3 PMA, the concept additionally includes a 2-axis pointing system - one controlling the azimuthal (cross-track) angle, and the other controlling the elevation (along-track) angle. Although the PMA itself does not change the polarization of the light more than 0.1%, there are polarization implications because the slit image is rotated as a function of the change in the PMA, driven primarily by the elevation (along-track) angle. It is worth noting that reflected sunlight is naturally polarized by its interaction with the Earth’s surface and atmosphere, especially over water. The PMA is required to allow non-nadir (straight down) observations from the fixed position on the ISS. Two important design requirements of the PMA were 1) to allow quick movement through a large range of angles, and 2) that the movement not affect the measurements through angular dependent polarization or radiance changes. OCO-3 meets these objectives by incorporating a variation of the pointing system designed for the Glory Aerosol Polarimetry Sensor (APS) (Persh et al., 2010).