In spaceborne gravimetry we usually have a very high number of both observations and the unknowns we want to derive from these observations.
The GRACE satellites have now been in orbit for over 13 years. This means that we have about 87 million intersatellite range observations, 43 million GPS positions for each satellite, as well as accelerometer and attitude measurements. Considering global daily gravity field solutions we estimate about 2600 parameters per day, resulting in 13 million parameters for the whole time series. Processing such a huge amount of data in a sensible time frame (read: days) is quite challenging from two perspectives: