revisit from a 705-km orbit altitude. Fortuitously, for many years we have enjoyed simultaneous coverage by both Landsat 5 and Landsat 7 (and now by Landsats 7 and 8), yielding an 8-day coverage pattern. However, flying two Landsat satellites in the future would likely be prohibitively expensive, except in cases where an earlier satellite exceeds its design life. Thus consideration should be given to increasing the swath width to reduce revisit time at far less cost than increasing the number of satellites. Landsat 8 can point its sensors off nadir ±15 degrees by a spacecraft yaw maneuver. This capability is implemented to enable data collection only for major disaster relief and recovery or other high-priority imaging.
ESA plans to fly a moderate-resolution multispectral system, Sentinel-2, with a 290-km swath width, which could improve revisit time to about 10 days with a single satellite and 5 days7 with the planned two satellites flying concurrently. With the current 185-km swath, the nadir view angle at the swath edge is 7.5 degrees, the sensor view angle (different because of Earth’s curvature) is 8.3 degrees, and the relative atmospheric path length is 1.010. With a 290-km swath, the corresponding angles are 11.6 and 12.9 degrees, and the path length is 1.026—a minor impact to angular viewing geometry at the edges of the field of view and, of course, no impact at all within the central 185-km swath for those applications that are particularly sensitive to angular viewing geometry. Generally the bidirectional reflectance distribution of most surfaces shows significant angular features at angles beyond 1 degrees from the nadir.8 Thus, the possibility of increasing the swath width for future U.S. systems needs to be explored in more depth, as it could help considerably with the goal of a shorter revisit time at lower cost.
Historically, every Landsat has included the full Landsat sensor suite of the time. Improved revisit times required more Landsats. Fortunately the extended life of Landsat 5 provided an 8-day revisit time, even though the original Landsat requirement was a 16-day revisit time. However, nothing compels future missions to involve only a single satellite, or for each satellite to contain the full sensor suite.
Smaller satellites can offer many benefits, either as an augmentation to a “mother ship,” such as Landsat 8 (with a full sensor suite), or as an ultimate replacement. RapidEye and the Disaster Monitoring Constellation (DMC) are already examples of less costly (though less complete) land imaging satellites that could augment SELIP by providing more frequent revisit times. A small satellite carrying only a thermal infrared sensor, placed in a phased orbit with the primary Landsat, could cut revisit time in half for much less than the cost of a duplicate Landsat, with the benefit of estimating evapotranspiration for practical water resource management. Alternatively, a small satellite carrying only a simple land imaging instrument, such as a slightly enhanced Multispectral Imager (MSI), routinely flown on the DMC of imaging small satellites, would cut revisit time in half for the nonthermal imaging channels. Two such small satellites, one with thermal and the other with VNIR and SWIR, flying in conjunction with the primary Landsat, might be able to provide near full capability at half the revisit time for dramatically less cost than two full Landsats.
Small satellites also offer several other benefits. They are intrinsically resilient, enabling intelligent trade-offs of redundancy at the constellation level, as opposed to requiring full redundancy in each spacecraft, allowing for lower cost. By being simpler (often single-payload), they have lower systems engineering, integration, and test costs. Their smaller size can enable them to fly as secondary payloads, reducing launch costs. They offer improved revisit because one can afford to acquire more satellites,9 so engineering teams can be continuously tasked instead of being organized and then dismantled for every mission. And by having more satellites, there are more opportunities for gradual introduction of new technology, enabling continuous improvement at lower cost and risk than wholesale replacement.
7 The planned revisit time is 5 days over the equator and 2 to 3 days over mid-latitudes. See European Space Agency, ESA-NASA Collaboration Fosters Comparable Land Imagery, February 13, 2013, available at http://www.esa.int/Our_Activities/Observing_the_Earth/GMES/ESA_NASA_collaboration_fosters_comparable_land_imagery.
8 M. von Schönermark, B. Geiger, and H.P. Röser, eds., Reflection Properties of Vegetation and Soil—With a BRDF Database , Wissenschaft und Technik Verlag, Berlin, 2004.
9 The RapidEye constellation of five small satellites cost $160 million, including launch (Space News, May 22, 2006, available at http://www.spacenews.com/archive/archive06/Briefs_052206.html).