North Sea DeltaFix
The North Sea Long-Range DeltaFix chain is operated by Fugro and consists of 6 stations, 5 of which operate continously and one of which can be switched on when required. They use the last Hyperfix hardware known to be still operational, and together with Trimble GPS receivers are used to transmit DGPS corrections. The equipment is still in fairly good condition considering its age, and at the time of writing this article (autumn 2006) it is planned to keep the chain operational for a few years yet until a better system comes along. The range of each station is 400km, and their locations are shown in map below.

Signal characteristics
Each station transmits a Phase Shift Keyed (PSK) data transmission which is keyed 2 seconds on, two seconds off. The burst is transmitted sequentially on two frequencies in the MF band; different frequency pairs are used at each station which prevents interference but also reflects the fact that DeltaFix is not intended for hyperbolic or range-range positioning. The RF frequencies used are similar to the old HyperFix allocation; around 2800 KHz for the f1 frequency and around 1900 kHz for the f2 frequency.

The map below is taken from a Fugro leaflet 'DeltaFix LR Status 2004' and I am extremely grateful to Fugro Survey for their kind permission to publish it, and also to Roger Basford for the additional technical information.