  Frequency-domain expressions are found for gradiometer and
satellite-to-satellite tracking measurements of a point source on the surface
of the Earth. The maximum signal-to-noise ratio as a function of noise in the
measurement apparatus is computed, and from that the minimum detectable point
mass is inferred. A point mass of magnitude M_3=100 Gt gives a signal-to-noise
ratio of 3 when a GOCE-like gradiometer passes directly over the mass. On the
satellite-to-satellite tracking mission GRACE-FO M_3=1.3 Gt for the microwave
instrument and M_3=0.5 Gt for the laser ranging interferometer. The sensitivity
of future GRACE-like missions with different orbital parameters and improved
accelerometer sensitivity is explored, and the optimum spacecraft separation
for detecting point-like sources is found. The future-mission benefit of
improving the accelerometer sensitivity for measurement of non-gravitational
disturbances is shown by the resulting reduction of M_3 to as small as 7 Mt for
500 km orbital altitude and optimized satellite separation of 900 km.
