Global Positioning System

Summary:

24 satellites in three orbital planes, 55° inclination.
Controlled by US Air Force based at Falcon AFB, Colorado.
Selective availability provides civilian accuracy to within 100 meters. Selective availability was turned off on May 1, 2000.
Typical accuracy is on the order of 30 to 40 meters.
Differential correction can get this down to a meter or less.
Initial Operational Capability was achieved 8 December 1993.
Full Operational Capability was achieved 27 April 1995.
Frequencies (master clock of 10.23 MHz):
- L₁: 1575.42 MHz (10.23 × 154): Coarse Acquisition (C/A) code
- L₂: 1227.60 MHz (10.23 × 120): Precise (P) or Y (encrypted) code
- L₃: 1381.05 MHz (10.23 × 135):
- L₄: 1379.913 MHz: Nuclear burst detectors Analysis Package (NAP)
- L₅: 1176.45 MHz (10.23 × 115): Civilian safety-of-life signal (proposed)
- L_?: 1841.40 MHz (10.23 × 180): Ionospheric correction (under study)
- S₁: 2227.500 MHz: S-band 1
- S₂: 2228.524 MHz: S-band 2
- S₃: 2226.476 MHz: S-band 3

The Naval Air Warfare Center at China Lake, California, has an Interactive GPS Satellite Prediction Utility which will report the satellites that should be in view given your latitude, longitude, and other relevant information.

GPS IIF features:

a new navigation signal for civil users
encrypted military code
crosslink enhancements
improved navigation signal accuracy
signal power increase
over-the-air distribution of encryption keys to properly equipped military users

Here's a synopsis of the nuclear burst detectors, taken from The Emerging Role of The US Army in Space by Arthur J. Downey, published by the National Defense University Press in 1985:

To provide attack, damage and strike assessment information, a system of improved nuclear detonation detection sensors has been developed. Formerly known as the Integrated Operational Nuclear Detection System (IONDS) and now called the Nuclear Detection System (NDS), these sensors will be deployed on board the Global Positioning System (GPS) constellation of navigation satellites scheduled to achieve full operational capability in the late 1980s. The Nuclear Detection System will provide precise location, yield, and height of burst information on any nuclear explosion worldwide. During peacetime, NDS will contribute to nuclear test ban monitoring; during the nuclear war it will also provide strike and damage assessment. If nuclear escalation is to be controlled, this system is essential to the United States as a survivable means of providing accurate, near real-time information on nuclear weapon results, both ours and the enemy's, allowing the National Command Authorities to select an appropriate response and enhancing retargeting and strategic force management. Terminals for NDS will come in tactical versions to provide the same nuclear detonation (NUDET) information to field commanders.

Further information from The U.S. Intelligence Community, 2ed (1989) by Jeffrey Richelson:

Detection of nuclear detonations is the secondary function of at least two satellite systems. Since the Defense Support Program (DSP) satellites' initial launch in 1971, their secondary function has been the detection of nuclear explosions in space or the atmosphere.
The princpal mission of DSP satellites is to provide early warning of a Soviet missile attack. Hence, they also provide notification of Soviet, Chinese, and other nations' missile tests. The satellites are cylindrical and about 23 feet long and 9 feet in diameter, weighing up to 22,000 pounds. The means of detection is a twelve-foot-long Schmidt infrared telescope that is thirty-nine inches in diameter. The telescope has a two dimensional array of lead sulfide detectors at its focus to sense energy emitted by ballistic missile exhausts during the powered stages of their flights.
[...]
The prime means for providing trans- and post-attack nuclear detonation monitoring will be the Nuclear Detonation (NUDET) Detection System (NDS), carried on board the NAVSTAR Global Positioning System (GPS) satellites. The primary function of the GPS is to provide accurate locational data for targeting and navigational purposes; the NDS will represent a major secondary function.
[...]
Beginning with the launch of GPS-8 in 1983, the NDS has been carried (and will continue to be carried) on all GPS satellites.
[...]
The two NDC packages will include x-ray and optical sensors, Bhangmeters, EMP (electromagnetic pulse) sensors, and a data processing capability that will allow the detection of nuclear weapons detonations "anywhere in the world at any time and get its location down to less than a [100 meters]." Data will be reported on a real-time basis - either directly to ground stations located at Diego Garcia; Kwajalein Atoll; Ascension Island; Kaena Point, Hawaii; Andersen Air Base, Guam; Adak, Alaska - or first to airborne terminals or other GPS satellites for subsequent downlink transmissions. All principal U.S. command posts, including E-4B and EC-135 aircraft, will receive NDS sensor data directly.

Click here for the GPS outline taken from a presentation I gave at DefCon IV in 1996.

Boeing, the contractor for the follow-on GPS IIF space vehicles, has more information on GPS.

Trimble, an equipment manufacturer, has a good introduction to GPS.

Index PRN NORAD Block NAVSTAR USA

1 2 28474 IIR-13 56 180

2 12 29601 IIRM-3 59 192

3 15 32260 IIRM-4 60 196

4 16 27663 IIR-8 51 166

5 17 28874 IIRM-1 57 183

6 18 26690 IIR-7 50 156

7 19 28190 IIR-11 54 177

8 23 28361 IIR-12 55 178

9 29 32384 IIRM-5 61 199

Index	PRN	NORAD	Block	NAVSTAR	USA
1	2	28474	IIR-13	56	180
2	12	29601	IIRM-3	59	192
3	15	32260	IIRM-4	60	196
4	16	27663	IIR-8	51	166
5	17	28874	IIRM-1	57	183
6	18	26690	IIR-7	50	156
7	19	28190	IIR-11	54	177
8	23	28361	IIR-12	55	178
9	29	32384	IIRM-5	61	199

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Updated January 23, 2010