SECOR satellite positioning
1. Linked to four mobile ground stations: three in accurately determined known locations, and a fourth in an unknown location
2. Measured satellite's distance from the three known stations to determine its position in space
3. Ground stations transmitted phase modulated signal to satellite, satellite received and transmitted back
4. Time taken for signal to travel multiplied by speed of light gave pseudorange distance
5. Process repeated many times to enhance accuracy, minimum 3 satellite passes sufficient