566 was chosen to optimize science data capture for the platform’s Dual -frequency Precipitation Radar (DPR)  and GPM Microwave Imager instruments and provide coordination with GPM Mission partner satellites .  Both the Semi -Major Axis  and Eccentric ity requirement tolerance allowes  for the geodetic height (HGT) of  the Core Observatory to be maintained within 397 km to 419 km for effective DPR operation and minimize  the altitude variation per latitude crossing over the life of the mission. T he inclined (65 degrees) non- sunsynchronous orbit allows  the observatory to sample precipitation across all hours of the day from the Tropics  to the Arctic and Antarctic Circles, and expand on the observations performed by TRMM, the Core  Observatory’s predecessor. The Flig ht Operations Team (FOT) manages  the fuel usage based on an orbital  maintenance plan derived  from these parameters [5] .  

Table 3. GPM Core Spacecraft Orbit Parameters  [5] 

Maintaining the orb it to these constraints required the FOT to perform orbit maintenance maneuvers as the  orbit experienced changes due to predictable Earth perturbations and the impact of flying in a variable drag  environment while optimizing fuel usage and actuator cycles . The first type of maneuver was  a prograde  delta- V maneuver in the velocity direction to boost GPM up into a higher orbit. This is referred to as a dragmakeup maneuver and is used to increase the SC’s altitude after atmospheric drag has caused orbit to  decay down to the bottom of the required control box. The second is due to the eccentricity requirement.  GPM has a tight eccentricity requirement such that orbit remains very near circular. They only way  eccentricity can be fixed is by preforming a maneuver, however if  the SC is already at the top of the required  control b ox then it  can’t go up any higher, and a retrograde maneuver must be done. These are done on  an as needed basis . Solar activity is a key factor . During periods of high activity  the atmosphere expands ,  resulting in higher drag and faster orbital decay. The opposite takes place when it  is low . 

Figure 15: GPM Mission Solar Array Tracking Profiles vs Beta Angle  

Hence, per pre- launch analysis, the mission was expecting to plan and perform orbit maintenance  maneuvers as frequently as once a week during high solar activity, and as infrequently as every eight weeks  during low solar activity. I ndeed, for the first few mont hs post-launch, maneuvers were performed once a  week in the higher drag conditions as predicted. The solar array tracking profile used during this period was  Profile G  as shown in Figure 15. During the on- orbit check -out phase the senior engineers determined that  the solar arrays performance was more than adequate to recharge the batteries and provide power to the  bus, thus a few of the solar array feathering profiles were tested on board to optimize the input power and  drag induced. After a few months of testing, the low drag profile know n as Profile K was selected in August  of 2014. This change, coupled with the solar flux dropping slowly as the mission months passed, allowed  for the maneuver frequency to slowly change from once a week to roughly o nce a month [5] . 

The periods of extreme low solar activity with low drag allows for more orbital eccentricity growth between  maneuvers. And for any one maneuver during this time (constrained by an upper altitude limit), the  eccentricity cannot be reduced sufficiently . Hence the Drag Make- Up maneuvers required to maintain the  Semi -Major Axis  within tolerances were both too infrequent and of insufficient size to control the eccentricity .