Subsystem support feasibility for formation flight measuring Bi-directional Reflectance

Abstract

Distributed Spacecraft Missions can be used to improve science performance in earth remote sensing by increasing the sampling in one or more of five dimensions: spatial, temporal, angular, spectral and radiometric. This paper identifies a gap in the angular sampling abilities of traditional monolithic spacecraft and proposes to address it using small satellite clusters in formation flight. The angular performance metric chosen to be Bi-directional Reflectance Distribution Function (BRDF), which describes the directional and spectral variation of reflectance of a surface element at any time instant. Current monolithic spacecraft sensors estimate it by virtue of their large swath (e.g. MODIS, POLDER), multiple forward and aft sensors (e.g. MISR, ATSR) and autonomous maneuverability (e.g. CHRIS, SPECTRA). However, their planes of measurement and angular coverage are limited. This study evaluates the technical feasibility of using clusters of nanosatellites in formation flight, each with a VNIR (visible and near infra-red) imaging spectrometer, to make multi-spectral reflectance measurements of a ground target, at different zenith and azimuthal angles simultaneously. Feasibility is verified for the following mission critical, inter-dependent modules that need to be customized to fit specific angular and spectral requirements: cluster geometry (and global orbits), guidance, navigation and control systems (GNC), payload, onboard processing and communication. Simulations using an integrated systems engineering and science evaluation tool indicate initial feasibility of all listed subsystems.

A framework for orbital performance evaluation in Distributed Space Missions for earth observation

“…The corresponding equations (also COWPOKE) are more complex than (8 and have not been analyzed in this paper. Parallel literature [11], [13] has shown that slightly eccentric orbits are beneficial in compensating for the drift in the along track direction and aid in keeping the formation together after 7-8 months of operation. It would be valuable to find the optimal chief and differential eccentricity that would allow a good azimuthal spread via free orbit ellipses as well as be beneficial to maintain.…”

“…Different inclinations cause the J2 forces on each satellite to be different causing the RAAN to rotate differently. A 20 deg view zenith angle spread takes less than 3 months to break up [11] because increasing RAAN differential causes the angular spread at the poles to decrease and equator to increase and eventually the latter is too large for the satellites to see each other. Figure 15 shows a drift not only in the along-track direction (as seen in Figure 14) but also in the radial direction, indicative of the formation breaking (similar to semi major axis difference).…”

“…The cluster can make multi-spectral measurements of a ground spot at multiple 3D angles at the same time as they pass overhead either using narrow field of view (NFOV) instruments in controlled formation flight (Figure 1-a) or wide field of view (WFOV) instruments with overlapping ground spots providing integrated images at various angles (Figure 1-b). Parallel studies have demonstrated the technical feasibility of subsystems [11], suitability of payload development [7] to support such a mission, availability of science models to quantify the performance of such DSMs [2], [12], [13] as well as open-source flight software to continually update satellite capability for staged, scalable deployment [14], [15]. This paper focuses on generation of feasible formation flight and constellation architectures for all types of multi-angular measurements on the earth (Figure 1).…”

“…The simplest imaging mode [13] for formation flight is assumed for this paper where a constant satellite is chosen as reference and looking nadir, while others point to the ground spot below the reference. Trades and design of the payload [7], subsystem capabilities to support such a mission [11], appropriate cost models [42] and coupling with appropriate OSSE models [2], [12], [13], [43] have been discussed in other literature. The science evaluation model [2], [13], [44] is based on complex BRDF models and validated against data collected by the airborne Cloud Absorption Radiometer (CAR) instrument, developed and operated by NASA Goddard Space Flight Center (GSFC) [16] [20].…”

“…Three satellites is the minimum required for the BRDF OSSE models and eight corresponds to NASA ARC's Edison Demonstration [3], currently the highest number of commissioned satellites in any DSM. Since studies [11][46] have shown that the only differential Keplerian elements maintainable using small sat technology are RAAN and TA, only they will be considered LVLH variables. For any given number of satellites (say, N), N-1 RAAN-TA differential combinations are picked from the 8 available ( 8 C N-1 ) in Table 1.…”