GPS_sat_constellation

THE GPS SATELLITE CONSTELLATION

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

The Block I satellites were the experimental satellites launched between 1978 and 1985 used to test the system. Eleven (11) were launched, none functioning.
The Block II satellites comprise the first nine spacecraft of the operational series.
The Block IIA satellites comprise the second 19 spacecraft of the operational series.
The Block IIR satellites comprise the replacement series.

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:

Their orbital period is approximately 11hrs 58mins, so that each satellite makes two revolutions in one sidereal day (the period taken for the earth to complete one rotation about its axis with respect to the stars).
At the end of a sidereal day (approximately 23hrs 56mins in length) the satellites are again over the same position on earth.
Reckoned in terms of a solar day (24hrs in length), the satellites are in the same position in the sky about four (4) minutes earlier each day.
The orbit groundtrack approximately repeats each day, except that there is a small drift of the orbital plane to the west (-0.03 per day).

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:

"Health status" flags transmitted as part of the Navigation Message (section 3.3.1). These are monitored automatically by the GPS receiver, and the necessary action may be initiated. Note, however, that although a satellite may be "unhealthy" for navigation purposes, it may still be suitable for phase tracking and post-processing in the GPS survey mode.
Several Bulletin Board Services, and other electronic GPS information sources on the Internet.
Specialist communication services for certain classes of users, for example, real-time differential GPS (DGPS) users are transmitted health messages within the standard data stream, aviation users will be able to receive NANUs (Notice Advisory to Navigational Users) directly via radio links, etc. It is likely that these specialist "integrity monitoring" services will expand rapidly in the coming years and be offered to survey users as well.

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

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).