Sect_1.2.8

1.2.8 System Ingredients

OVERVIEW: Essential Characteristics

From a study of the design requirements of an ideal "Global Positioning System", and as a consequence of the feasible technological features of such a system, the essential characteristics of the system can now be identified.

(1) System Configuration:

A multi-satellite system at high altitude, but not in a geostationary orbit. The number of satellites to be visible to a user is dependent upon the observation type to be employed and the positioning strategy adopted.
A Control Segment responsible for tracking the satellites, and computing the ephemerides.
Satellites broadcast their ephemerides to all users.

(2) Satellite Technology:

The system should concentrate as much complexity into the satellites as possible.
The system should be passive (one-way) as far as the user is concerned, with the satellites transmitting the signals necessary to support position determination at the user station. No receiving function is to be performed by the satellites.

(3) Measurement Technology:

A one-way ranging system based on microwave transmissions would satisfy the requirements for a listen-only, high precision, simple-to-use positioning system.
To make such a system work, separate clocks must be used (in the satellites and within the user equipment), and they must be synchronised in some way.
The satellites should somehow broadcast time-of-transmission information to the user.

What remains to be established is:

the positioning principle to be used (related to the measurement technology and the characteristics of the satellite constellation), and

the residual errors remaining in the system (after application of the best available technology), and the development of strategies to overcome any unacceptably high error sources.