The Decca Navigation system consisted of a number of land-based radio beacons organised into chains. Each chain consisted of 3 slave stations, called Red, Green and Purple, positioned at the vertices of an equilateral triangle with a master station in the centre, with a typical baseline distance between the slaves and the master station being from 110 km to 225 km. Each station transmitted a continuous wave signal, with low frequencies from 70 to 129 KHz, and by comparing the phases difference of the signals, a receiver could plot a position based on the interference patterns.
The cache is placed near the entrance to one of the slave stations of the South West Africa chain. The mother station was situated to the west of Usakos.
If you look at a satellite map of the area, 60 lines are clearly visible, radiating from the central control room where the base plate of the huge mast is still to be seen, as are the anchor points for the stays of the mast. While access is easy and permitted, there are people staying at the original control room and it would be courteous to announce your presence and ask for permission to look around.
The chains were known as the South West Africa Chain, the Namaqualand Chain, the Cape Chain, the Eastern Province Chain and the Natal Chain. These local chains were finally closed on 31/3/2000. There has been some recent building activity so please use the necessary stealth when looking not to attract attention. The rocks are placed in distinct lines, so please replace the rocks carefully as you are searching, so as not to distort the lines. BYOP