AbstractIn 1986 the Ordnance Survey was awarded a contract by Transmanche-Link (TML) to produce a unique grid for use in the design and construction of the Channel Tunnel. L'lnstitut Geographique National (IGN) France made all the observations in France and provided an independent computation of the complete network [4]. In 1987 both IGN and OS had single frequency GPS receivers and TML requested IGN to strengthen the 1986 network by including GPS observations. Nortech were contracted by IGN to observe across the Channel with dual-frequency TI 4100 GPS sets whilst OS were to observe with Trimble receivers in England and IGN in France using their Sercel sets.