At most sites the effect of groundwater, often associated with rainfall, is an important contributor to gravity variations over periods of days to years. SG groups are therefore urged to record rainfall and groundwater level at their sites as auxiliary data. Finally each station is asked to supply a log file of important events that might affect the data for each month . Following large earthquakes, GGP groups are asked to send earthquake data for a period of 1 week following the origin time of the earthquake. This data is intended to be used for free oscillation studies of the Earth.
Most SG groups monitor many other variables at their stations.
This section describes the manner in which the basic signals are recorded. In practice a 10 ms error is therefore not unreasonably stringent. GWR has designed a new gravity tide electronic board with two options for the gravity signal to be sampled at either 1s or 2s. The question of 1s versus 10s sampling rate has been actively discussed by a number of individuals, including Mansinha, Crossley, Richter, Z rn and Hinderer. With sampling at 1s or 2s, it is easier to provide a flat amplitude response for the anti-aliasing filter at 'short' normal mode periods of 10s of seconds.