GPS (Global Positioning System, United States)
GPS was the first GNSS system. GPS (or NAVSTAR, as it is officially called) satellites were first launched in the late 1970sand early 1980s for the US 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 27.
The GPS space segment is summarized in Table 2. The orbit period of each satellite is approximately 12 hours, so this provides a GPS receiver with at least six satellites in view from any point on Earth, under open-sky conditions.
Table 2: GPS Satellite Constellation
|Satellites||27 plus 4 spares|
|Orbit Inclination||55 degrees|
|Orbit Radius||20,200 km|
A GPS satellite orbit is illustrated in Figure 28.
GPS satellites continually broadcast their identification, ranging signals, satellite status and corrected ephemerides (orbit parameters). The satellites are identified either by their Space Vehicle Number (SVN) or their Pseudorandom Noise (PRN) code.
Table 3 provides further information on GPS signals. GPS signals are based on CDMA (Code Division Multiple Access) technology, which we discussed in Chapter 2.
Table 3: GPS Signal Characteristics
L1 is modulated by the C/A code (Coarse/Acquisition) and the P-code (Precision) which is encrypted for military and other authorized users.
L2 is modulated by the P-code and, beginning with the Block IIR-M satellites, the L2C (civilian) code. L2C has begun broadcasting civil navigation (CNAV) messages and is discussed later in this chapter under “GPS Modernization”.
L5, available beginning with Block IIF satellites, has begun broadcasting CNAV messages. The L5 signal is discussed later in this chapter under "GPS Modernization".
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 29.
|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, Argentina, South Africa, Bahrain, South Korea, Australia, New Zealand|
|Ground Antennas||Cape Canaveral, Ascension Island, Diego Garcia, Kwajalein|
There are 16 monitor stations located throughout the world; six from the US Air Force and ten from the NGA (National Geospatial Intelligence Agency, also part of the United States Department of Defense). The monitor stations track the satellites via their broadcast signals, which contain satellite ephemeris data, ranging signals, 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 up-loading 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 modernize the GPS space and ground segments, to take advantage of new technologies and user requirements.
Space segment modernization includes new signals, as well as improvements in atomic clock accuracy, satellite signal strength and reliability. Control segment modernization 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 modernized 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 and it delivers improved navigation accuracy. It also provides the ability to directly measure and remove the ionospheric delay error for a particular satellite, using the civilian signals on both L1 and L2. The L2C signal is expected to be available from 24 satellites by 2018.
The United States has implemented a third civil GPS frequency (L5) at 1176.45 MHz. The modernized GPS satellites (Block II-F and later) are transmitting L5.
The benefits of the L5 signal include meeting the requirements for critical safety-of-life applications such as that needed for civil aviation and providing:
- Improved ionospheric correction.
- Signal redundancy.
- Improved signal accuracy.
- Improved interference rejection.
The L5 signal is expected to be available from 24 satellites by 2021.
A fourth civilian GPS signal, L1C, is planned for the next generation of GPS satellites, Block III. L1C will be backward compatible with L1 and will provide greater civilian interoperability with Galileo. The Japanese QZSS, Indian IRNSS and Chinese BeiDou also plan to broadcast L1C, 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. It is expected that the first Block III satellites will be launched in 2016 and that there will be 24 satellites broadcasting L1C by 2026.
In addition to the new L1C, L2C and L5 signals, GPS satellite modernization includes new military signals.