GPS observations are time delay or phase delay observation of a GPS signal transmitted from a GPS satellite and received in a network of groundbased GPS receivers. Using this network, unknowns such as satellite and receiver position and clock errors can be estimated. Another unknown is the delay caused by the atmosphere due to refraction. This Zenith Total Delay is estimated simultaneously in the processing of GPS. GPS ZTD estimates are valuable observations to be assimilated into NWP models under the condition that this data source is reliable and more or less continuous, that is without long (several hours to days) data gaps.

The current network consist of 54 stations with a core of 35 stations in the Netherlands. The GPS data from stations outside the Netherlands is downloaded from anonymous ftp servers and is processed every hour. The Dutch stations are either obtained from the AGI ftp-server at Delft University of Technology or delivered through the cooperation with Kadaster within Netpos.