GPS (Global Positioning System), United States
|Orbit inclination||55 degrees|
|Orbit radius||20,200 km
GPS was the first GNSS system. GPS (or NAVSTAR, as it is officially called) satellites were first launched in the late 1970s and early 1980s for the U.S. Department of Defense. Since that time, several generations (referred to as “Blocks”) of GPS satellites have been launched. Initially, GPS was available only for military use but in 1983, a decision was made to extend GPS to civilian use. A GPS satellite is depicted in Figure 28.
The GPS space segment is summarised in Table 1. The orbit period of each satellite is approximately 12 hours, so this provides a GPS receiver with at least six satellites in view from most points on Earth, under open-sky conditions , and typically more.
A GPS satellite orbit is illustrated in Figure 29.
GPS satellites continually broadcast their signals, which contain satellite ephemeris data, ranging signals, identification, clock data and almanac data. The satellites are identified either by their Space Vehicle Number (SVN) or their Pseudorandom Noise (PRN) code.
Table 2 provides further information on GPS signals. GPS signals are based on CDMA (Code Division Multiple Access) technologies, which we discussed in Chapter 2.
|L1||1575.42 MHz||L1 is modulated by the C/A code (coarse/acquisition)
which is available to all users and the P-code
(precision) which is encrypted for military and
other authorised users. Beginning with the Block III
satellites, L1 is also modulated with the L1C (civilian)
code and is discussed later in this chapter.
|L2||1227.60 MHz||L2 is modulated by the P-code and, beginning with
the Block IIR-M satellites, the L2C (civilian) code.
L2C broadcasts civil navigation (CNAV) messages
and is discussed later in this chapter under GPS
|L5||1176.45 MHz||L5, available beginning with Block IIF satellites,
broadcasts CNAV messages. The L5 signal
is discussed later in this chapter under GPS
The GPS control segment consists of a master control station (and a backup master control station), monitor stations, ground antennas and remote tracking stations, as shown in Figure 30.
|Master control station||Schriever AFB|
|Alternate master control station||Vandenberg AFB|
|Air Force monitor stations||Schriever AFB, Cape Canaveral, Hawaii, Ascension Island,
Diego Garcia, Kwajalein
|AFSCN remote tracking stations||Schriever AFB, Vandenberg AFB, Hawaii, New Hampshire,
Greenland, United Kingdom, Diego Garcia, Guam
|NGA monitor stations||USNO Washington, Alaska, United Kingdom, Ecuador,
Uruguay, South Africa, Bahrain, South Korea, Australia,
|Ground antennas||Cape Canaveral, Ascension Island, Diego Garcia, Kwajalein|
There are 16 monitor stations located throughout the world, six from the U.S. Air Force and ten from the NGA (National Geospatial-Intelligence Agency, also part of the U.S. Department of Defense). The monitor stations track the satellites via their broadcast signals, which contain satellite ephemeris data, ranging signals, identification, clock data and almanac data. These signals are passed to the master control station, where the ephemerides are recalculated. The resulting ephemerides and timing corrections are transmitted back up to the satellites through data uploading stations.
The ground antennas are co-located with monitor stations and used by the Master Control Station to communicate with and control the GPS satellites.
The Air Force Satellite Control Network (AFSCN) remote tracking stations provide the Master Control Station with additional satellite information to improve telemetry, tracking and control.
GPS reached Fully Operational Capability (FOC) in 1995. In 2000, a project was initiated to modernise the GPS space and ground segments to take advantage of new technologies and user requirements.
Space segment modernisation includes new signals, as well as improvements in atomic clock accuracy, satellite signal strength and reliability. Control segment modernisation includes improved ionospheric and tropospheric modelling and in-orbit accuracy and additional monitoring stations. User equipment has also evolved to take advantage of space and control segment improvements.
The modernised GPS satellites (Block IIR-M and later) are transmitting a new civilian signal, designated L2C, ensuring the accessibility of two civilian codes. L2C is easier for the user segment to track than L2 P(Y) and it delivers improved navigation accuracy. It also provides the ability to directly measure and remove the ionospheric delay error for a particular satellite by using the civilian signals on both L1 and L2. The L2C signal is available from 24 satellites in 2022 and is expected to be available on all satellites by 2026.
The United States has implemented a third civilian GPS frequency (L5) at 1176.45 MHz. The modernised GPS satellites (Block IIF and later) are transmitting L5.
The benefits of the L5 signal include meeting the requirements for critical safety-of-life applications such as those needed for civil aviation and providing:
- Improved ionospheric correction
- Signal redundancy
- Improved signal accuracy
- Improved interference rejection
The L5 signal is available from 16 satellites in 2021 and is expected to be available on all satellites by 2030.
A fourth civilian GPS signal, L1C, is available from the next generation Block III GPS satellites. The European Galileo, Japanese QZSS, Indian NavIC and Chinese BeiDou systems already broadcast or plan to broadcast L1C compatible signals, making it a future standard for international interoperability.
L1C features a new modulation scheme that will improve GPS reception in cities and other challenging environments. L1C is available from five satellites as of 2022 and is expected to be available on all satellites by 2034.
In addition to the new L1C, L2C and L5 signals, GPS satellite modernisation includes new military signals.