  The GRACE mission has been providing valuable new information on time
variations in the Earth's gravity field since 2002. In addition, the GRACE
Follow-On mission is scheduled to be flown soon after the end of life of the
GRACE mission in order to minimize the loss of valuable data on the Earth's
gravity field changes. In view of the major benefits to hydrology and
oceanography, as well as to other fields, it is desirable to investigate the
fundamental limits to monitoring the time variations in the Earth's gravity
field during GRACE-type missions. A simplified model is presented in this paper
for making estimates of the effect of differential spurious accelerations of
the satellites during times when four successive revolutions cross the Pacific
Ocean. The analysis approach discussed is to make use of changes in the
satellite separation observed during passages across low latitude regions of
the Pacific and of other oceans to correct for spurious accelerations of the
satellites. The low latitude regions of the Pacific and of other oceans are the
extended regions where the a priori uncertainties in the time variations of the
geopotential heights due to mass distribution changes are known best. In
addition, advantage can be taken of the repeated crossings of the South Pole
and the North Pole, since the uncertainties in changes in the geopotential
heights at the poles during the time required for four orbit revolutions are
likely to be small.
