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.