Newnes Radio and RF Engineering Pocket Book, 3rd Edition

255

Each satellite transmits continuously updated information about its

orbit on two frequencies, 1227 MHz and 1575 MHz. One radio channel

carries two pseudo-random codes, one very long and the other very

short. The second channel is modulated only with the short code. The

codes enable the satellite to be positively identified, and the distance

from a receiver on, or close to, the earth to be calculated.

A receiver’s position, in three-dimensional space, is identified by

measuring and calculating the distance from three satellites. The short

codes provide the initial fix but increased precision is obtained from

the long codes. There is an error due to time variations arising from

various sources of which satellite speed is one, but taking a measure-

ment ...