### Orbital Precession Rate Change  
From 2001 to 2007, EO-1 flew in orbital formation with Landsat 7 (one minute behind). In late 2007, EO-1 began a de-orbit procedure, but barely a month into the process, NASA gave EO-1 a re-entry waiver, and all remaining fuel was used to maintain the current orbit, which by then was slightly (~5 km) lower than Landsat 7. In 2011, EO-1 ran out of maneuvering fuel, and orbital degradation began with a slowly changing rate of precession leading to increasingly earlier ground overpass times (Figure 1). A second waiver was issued for EO-1 to continue operations to study the effects of changes in precession on data quality. In 2011, EO-1 lost its ability to maintain an exact sun synchronous precession orbit when the satellite depleted its onboard maneuvering fuel and could no longer make corrective \"inclination burns\". EO-1's orbital plane is still precessing, but not at the rate required to achieve precise Sun synchronicity, resulting in nadir observation times drifting towards earlier local overpass times. Specific targets are now viewed under different illumination conditions per month of the year, as compared to earlier phases of the mission (Figure 2). Solar zenith angle (SZA) depends on local overpass time, latitude, and date. The earlier overpass times experienced in the last few years of the mission result in larger SZAs. However, these values of SZAs have already been experienced by EO-1 for imagery collected at different latitudes and seasons. As can be seen in Figure 3, approximately 60% of the time in 2016 the SZAs at EO-1 overpass times were within the previously experienced range of SZAs for that latitude. Figure 1: EO-1 changes in precession rate started in 2011. Acquisition time at the Railroad Valley Playa declined from 10:12 to 8:14, reaching 8 a.m. in October 2016. The stability of the overpass time from 2001 to 2008 is shown (within dotted box), for nadir views (open circles) as well as off-nadir views (filled circles). A 15 min change in overpass time is indicated within ellipse, after EO-1 left the Landsat-7 formation. The decline of overpass time is indicated by the downward arrow, after onboard fuel was expended. Figure 2: Solar zenith angles (SZAs), measured in degrees, of daytime Hyperion images. Due to the earlier acquisition times for the imagery, the distribution of SZAs at overpass times during 2015 shifted to larger values (red bars) than during the earlier normal mission operations (blue bars). There are several potential impacts of changing orbital precession (also referred to as \"drift\" or \"decay\") [21]:  
1. Larger SZAs due to the sun being closer to the horizon may reduce the quality of the signal by having weaker irradiances and a longer atmospheric path for radiances to traverse, which decreases the signal to noise (SNR) ratio of the data and complicates atmospheric correction procedures;
2. There is a change in the number of instances of cloudy data, which might increase in temperate zones, as early morning haze is more often present, and might decrease in tropical zones, where convective clouds dominate [22];
3. The bi-directional reflectance distribution function (BRDF) of the data changes as the influence of shadows increases in concert with the illumination angles, and somewhat larger footprints are viewed [23]. Understanding these effects of changing overpass times helps future mission planners to evaluate overpass times for sun synchronous missions as well as improving the understanding of the issues involved with integrating data from multiple satellites with different overpass times as proposed in future actual and virtual constellations. EO-1 became a natural experiment in this problem, providing unique opportunities to evaluate these questions due to its long time series to which comparisons can be made.