Deltafix is a development of Hyperfix that enables DGPRS corrections to be transmitted using the data time slot 'D' in the transmission. By using the corrections to enhance the positions given by GPS, highly accurate results are possible. To understand this process, we need to understand a little about GPS itself.

Global Positioning by Satellite
The GPS system is a 'constellation' of 24 satellites in a high orbit (20,200km) that gives them a orbital period of about 12 hours. A receiver can provide a fix when it receives signals from at least 2 satellites, but the more satellites it can see, the more accurate the fix will be.

The GPS satellite system is operated by the US government, and although made available for civilian purposes the US government wanted to limit the usefulness of GPS to all but the US military. A technical solution was used to achieve this, call 'Selective Availability' (SA). This introduced a pseudo-random error into GPS, degrading the accuracy of the fixes obtained to around 100m- unless of course you knew what that error was at any one time i.e. you were the US military.

On 1st of May 2000, President Bill Clinton announced that the selective availability would be terminated with effect from midnight on that day. This meant that the basic accuracy of GPS was instantly improved to around 15 metres (see footnote 1). This is still, however, not anywhere near as good an accuracy as positioning systems such as Hyperfix could achieve and is why the MF positioning systems continued to thrive.

Differential GPS (DGPS)
Whilst SA itself could not be defeated, some development work was done to see if it was possible to minimise the effect of it. The concept of 'Differential' GPS (DGPS) is simple. Two receivers are required, one located in fixed, known location; and the other is mobile and used for positioning. At the fixed site, the GPS fix is compared with the 'real' position of the site, which is accurately known. Any difference must of course by an error in the GPS fix. As long as the mobile station isn't too far away from the fixed receiver, the same error will theoretically apply, and when subtracted from the mobile's fix will yield an accurate position. Obviously, some means of communicating the error to the mobile is needed, and in various applications different means are typically used for this varying from short range UHF or VHF data links through to LF broadcasting stations. The net effect however was to improve the accuracy of the system by an order of magnitude, to around 10 metres.

When SA was switched off, the accuracy of GPS with DGPS correction increased to around 20cm. This is better than MF positioning systems could achieve, and effectively meant the end of an era for them. However, taking into account the need to transmit the DGPS corrections, the possibility existed of using the existing Hyperfix hardware to do this.

DeltaFix station equipment
The Deltafix reference station used a modified Hyperfix receiver/controller unit with the addition of a GPS receiver. The transmitter and ATU were standard. The receiver/controller had different EPROMS on its two processor cards (Micro 1 and Micro 2). The GPS receiver was manufactured by Trimble; it's external 10MHz input could be fed from a rubidium oscillator. Such a unit was available in the Hyperfix range of units. The pictures below are by kind permission of William McKibben.

Picture showing DGPS reference station at Bronnysund. On top of the PCs to the left is the Trimble GPS receiver- looks like a DSM232 model. The monitor is standing on the Rubidium standard.


Picture showing the GPS antennas at a DeltaFix DGPS reference station in Brazil.

Footnotes:

1. Books have been written about the accuracy of GPS; it is a hotly debated subject. The closest I could easily find to a definitive figure is that stated in the text, which is given by Magellan (A GPS receiver manufacturer) for the guidance of their customers.