THE GPS SATELLITE CONSTELLATION

 


There are essentially four GPS satellite constellations in existence (see Table):

 

There are 27 Block II, IIA and IIR satellites in orbit. (In essence satellites are placed in a particular orbit pattern, with extra satellites deployed as "spares" so that they can quickly be moved to replace any of the primary satellites when they become unserviceable.) Additional satellites are "moth-balled" as replenishment spares.

The Block I satellites are not identical to the Block II/IIA/IIR satellites (HOFMANN-WELLENHOF et al., 1998), however the most important difference is that they have been launched into orbits of different inclination: 63 degrees for the Block I satellites, 55 degrees for the Block II/IIA/IIR satellites.

The operational system was to be fully deployed by the late 1980's. However, a number of factors, the major one being the Space Shuttle Challenger disaster (28 January, 1986), have meant that the full GPS system was only deployed in 1994 (24 Block II/IIA satellites). 24 orbiting satellites is considered sufficient to ensure that there will always be at least 4 satellites visible, at all sites on the globe, and at all times. The Figure below illustrates the visibility of satellites through the day for an observer in Johor Bahru, Malaysia. Note that the graph illustrates visibility down to the local horizon. At certain times of the day there are up to 12 satellites visible simultaneously.

Typical 24hr GPS coverage at Johor Bahru, Malaysia (January 1998).

 

Initial Operational Capability (IOC) was declared in July, 1993 (24 Block I, II, or IIA satellites), and Full Operational Capability (FOC) was declared on 17 July, 1995, (24 Block II or IIA satellites operating satisfactorily). The U.S. DoD will guarantee 24 satellite coverage 70% of the time, and 21 satellite coverage approximately 98% of the time. There could therefore be occasional periods of degraded satellite coverage, even under FOC conditions. At present there are more than 24 satellites available.

GPS Orbit Characteristics

The Block II GPS satellites have been deployed:

  • In 6 nearly circular orbital planes.
  • With 4 satellites equally spaced within the plane (the 3 in-orbit spares are spread across different orbital planes).
  • In orbital planes at an inclination of 55 degrees.
  • At altitudes of approximately 20200km above the earth.

 

As the GPS satellites are in nearly circular orbits, at an altitude of approximately 20200km above the earth, this has a number of immediate effects which make the prediction of satellite location comparatively easy:



The GPS constellation "birdcage".

 

The Block II satellite series have been deployed in six orbital planes at 60 intervals about the equator, with each containing 3 or 4 of the primary satellites equally spaced in the orbital plane (Table below). The orbital planes are at an inclination of 55 relative to the equatorial plane. According to deployment plans there are to be 3 active spare satellites spread evenly in addition to the primary 21 satellite configuration. However, the reality is that satellites are launched at predefined intervbals, and if the constallation health is maintained, often more than the minimum 24 satellites are available. For example, at the time of publication of these notes (September 1999) there are 27 satellites transmitting signals that users can track.

GPS Satellite Coverage

The Block II GPS satellite series coverage provides:

  • From 4 to over 12 satellites above an observer's horizon.
  • Satellites will be visible for many hours above an observer's horizon.
  • There may be short periods of degraded satellite geometry which will affect navigation users, though not surveyors.

 

The satellites are in near circular orbits at an altitude more than three times the earth's radius, a satellite will be visible above an observer's horizon for many hours, perhaps up to 6-7 hours. At various times of the day, and at various locations on the surface of the earth, the number of satellites and the length of time they are visible above an observer's horizon will vary.

With the Block II constellation, there are always at least four satellites visible, no matter where the observer is located, and sometimes more than ten. There may, however, be times when some satellites are not healthy and the satellite configuration results in rather poor geometry for navigation users (see section1.4.9 for a discussion of GDOP). These periods of degraded geometry are known as outages. To decrease the occurrences of outages, and to improve the system reliability, a number of technological solutions have been proposed, including the integration of GPS and GLONASS (the Russian equivalent of the GPS satellite system) so that receivers can track both types of satellites. This will significantly increase the number of mutually visible satellites. See Table (section 2.1.2) for a comparison of the two systems.

 

GPS Satellite Visibility

The Figures below show the coverage at four locations on the globe for the February 1998 constellation of 27 satellites. The satellite availability is expressed in the form of satellite visibility graphs, and "skyplots" showing the satellite tracks on a polar projection relative to a site's zenith.

 

GPS coverage at an Antarctic site

 

GPS coverage at a mid-USA site

 

GPS coverage at an Equatorial site

 

 

GPS Satellite Health Monitoring

There are a number of sources of information on GPS satellite health, and the general status of the system (including notification of satellite manoeuvres, status of Selective Availability, launch program, etc.). They include:


Status of the GPS satellite constellation (September 1999).
SVN PRN Launch Date Orbit Plane
Position
Useable
Block I
1
2
3
4
5
6
7
8
9
10
11

4
7
6
8
5
9

11
13
12
3

22-02-78
13-05-78
06-10-78
10-12-78
09-02-80
26-04-80
launch failure
14-07-83
13-06-84
08-09-84
09-10-85

no
no
no
no
no
no

no
no
no
no
Block II
14
13
16
19
17
18
20
21
15

14
2
16
19
17
18
20
21
15

14-02-89
10-16-89
18-08-89
21-10-89
11-12-89
24-01-90
26-03-90
2-08-90
01-10-90

E1
B2
E3
A4
D3
F3

E2
D2

15-04-89
10-08-89
14-10-89
23-11-89
06-01-90
14-02-90
no
22-08-90
15-10-90
Block IIA
23
24
25
28
26
27
32
29
22
31
37
39
35
34
36
33
40
30
38

23
24
25
28
26
27
1
29
22
31
7
9
5
4
6
3
10
30
8

26-11-90
04-07-91
23-02-92
10-04-92
07-07-92
09-09-92
22-11-92
18-12-92
03-02-93
30-03-93
13-05-93
26-06-93
30-08-93
26-10-93
10-03-94
28-03-96
16-07-96
12-09-96
06-11-97

E4
D1
A2

F2
A3
F1
F4
B1
C3
C4
A1
B4
D4
C1
C2
E3
B2
A5

10-12-90
30-08-91
24-03-92
no
23-07-92
30-09-92
11-12-92
05-01-93
04-04-93
13-04-93
12-06-93
20-07-93
28-09-93
22-11-93
28-03-94
09-04-96
15-08-96
01-10-96
18-12-97
 Block IIR
42
43
 

13
 
launch failure
23-07-97
 

F5
 

31-01-98

The orbital planes are identified by the letters A through F, and the satellite "slots" are numbered 1 to 4.
The Block I satellite orbits are different to those of the Block II satellites. Most significantly they have been launched into orbits of different inclination: 63degrees for the Block I satellites, 55degrees for the Block II satellites. There are four series of operational satellites: II, IIA, IIR and IIF (none launched yet of the IIF series).

© Chris Rizos, SNAP-UNSW, 1999