Another Option for the Next Generation Gravity Mission Orbit Configuration

<p>ANOTHER OPTION FOR THE NEXT GEN. GRAVITY MISSION ORBIT CONFIGURATION <br /><br />For the polar pair of a next generation gravity mission, an attractive feature would be to keep the repeat period for the ground track quite short.&#160; The satellites hopefully will include a much-simplified version of the Gravitational Reference Sensors demonstrated on the LISA Pathfinder Mission, as well as laser interferometry between the two satellites.&#160; A short period repeat option for the orbits is 107 satellite revolutions in 7 sidereal days, at an altitude of 468 km.&#160; Because of the fairly high altitude, the amount of propulsion needed for drag-free operation would be relatively small.<br /><br />With this geometry, the upward passes across the equator will be separated by 3.4 deg in longitude, and will be followed 3.5 days later by downward passes at the same longitudes.&#160; In the northern hemisphere, the upward and downward passes also will cross at latitudes of 25.7 and 51.4 degrees.&#160; At all latitudes, the maximum gaps in longitude between upward and downward passes for 3 successive days will be 6.7 deg. or less.&#160; In analysing the 7-day data sets, the maximum ground track separation in longitude would be 378 km.&#160; Thus, for a point mass half way between the nearest ground tracks, the minimum distance to the satellites would be increased only from 468 km to 505 km at the most.&#160;</p> <p>A recent study by R. Spero (Adv. Space Res. 67 (2021) 1656-1664) addressed the question of how accurately rapid changes in a local mass concentration could be measured with earth gravity change mission measurements.&#160; One case that was included had about the same measurement capability as the next generation gravity mission (NGGM) assumed here.&#160; For this case, the reduction in the instrumental measurement uncertailty was dramatic.&#160; However, the actual usefulness of the results would be strongly limited by the a priory uncertainty in our ability to understand the geophysical sources of the geopotential variations.&#160; Thus, while our uncertainty in the geopotential variations at satellite altitude would be reduced at almost all frequencies, it probably would be a long time before our understanding of the geophysical sources of the variations becomes good in most regions at frequencies below about 60 cycles/rev.</p> <p>Fortunately, this limitation will be less severe in regions where one source of geopotential variation is dominant, such as river basins where the dominant variation is in the near-surface stored water level.</p>