The navigational aspects of the requirements for a medium altitude satellite communication system are discussed. Twelve satellites have to be injected into a circular equatorial orbit with very great precision. It is not practicable to navigate the launching vehicle accurately enough to obtain the correct orbit at first injection and the satellite itself has to be controlled and adjusted until finally it is correctly positioned with the right period.

The navigation of the third-stage rocket is discussed as this effects the limits of adjustment required for the satellite. Some error in period is necessary at injection in order to position the satellite correctly in orbit, in a reasonable time. A suitable tolerance on the period at injection is 3 minutes in 8 hours. This must be reduced, by fine adjustment of the satellite speed, to less than 30 milliseconds. Methods of measuring the orbital period to this accuracy are discussed.

When the 12 satellites are finally in orbit and equally spaced around the orbit they should remain for up to 5 years with the pattern substantially undisturbed.

This paper was presented at an Ordinary Meeting of the Institute held in London on 18 January.

The basis of navigation is the finding of present position and velocity so that the necessary action can be taken to guide the craft or vehicle to its destination.