This system uses an American government satellite network comprised of 24 satellites in three circular orbits at a height of 20 000 km and with twelve-hour periods. At least six satellites should be visible from any point around the earth at any one time.
Table 22.5 Astra television channels Channel Astra 1-A (GHz) Channel Astra 1-B (GHz) Channel Astra 1-C (GHz) Channel Astra 1-D (GHz)1 11.21425 17 11.46425 33 10.96425 49 10.71425
2 11.22900 18 11.47900 34 10.97900 50 10.72900
3 11.24375 19 11.49375 35 10.99375 51 10.74375
4 11.25850 20 11.50850 36 10.00850 52 10.75850
5 11.27325 21 11.52325 37 10.02325 53 10.77325
6 11.28800 22 11.53800 38 10.03800 54 10.78800
7 11.30275 23 11.55275 39 10.05275 55 10.80275
8 11.31750 24 11.56750 40 10.06750 56 10.81750
9 11.33225 25 11.58225 41 10.08225 57 10.83225
10 11.34700 26 11.59700 42 10.09700 58 10.84700
11 11.36175 27 11.61175 43 10.11175 59 10.86175
12 11.37650 28 11.62650 44 10.12650 60 10.87650
13 11.39125 29 11.64125 45 10.14125 61 10.89125
14 11.40600 30 11.65600 46 10.15600 62 10.90600
15 11.42075 31 11.67075 47 10.17075 63 10.92075
16 11.43550 32 11.68550 48 10.18550 64 10.83550
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 measurement from a fourth satellite enables a correction factor to be applied.
Receivers which utilize only the short PN codes provide a resolution accurate to about 100 m. Those which can process the long codes provide a fix accurate to about 45 m.