The Global Positioning Systems’ constellation is a mix of old and new satellites and it is constantly evolving, as new satellites replace older ones. (This table by the National Coordination Office for Space-Based Positioning, Navigation, and Timing summarizes the features of the current and future generations of GPS satellites.) The minimum number of satellites for a full constellation is 24 operational 95% of the time, but the target number is 33 and decommissioned satellites are kept in reserve to maintain the numbers. The first GPS satellite was launched in 1978 and the U.S. Air Force declared that the constellation had reached “full operational capability” in July 1995. As of today, 72 GPS satellites have been launched and 31 are in orbit and operational, not including the decommissioned, on-orbit spares.
Therefore, GPS modernization is an ongoing process, which involves the complete replacement of legacy GPS satellites and ground systems with newer, more capable ones. For example, GPS IIR satellites brought in a new kind of time-keeping system technology and GPS IIF brought in a better rubidium atomic frequency standard. However, in recent years, the term modernization has been used mostly to refer to the third generation of GPS satellites, or GPS III, which will introduce new capabilities, including new navigation signals. Modernization also includes continuous upgrades to the ground-based GPS control segment, which is necessary to command and control the newer GPS satellites and to enhance cybersecurity. The U.S. government started gearing up for GPS III in 1998 and Congress funded the effort in 2000.
GPS III will have three times better accuracy and up to eight times improved anti-jamming capabilities. Spacecraft life will extend to 15 years, 25 percent longer than the newest GPS satellites on-orbit today.
On the military side, GPS III will fully implement the high-bandwidth M-Code and introduce a new high-gain directional antenna exclusively for use with M-code transmissions. This will enable the U.S. military to boost the power of military GPS signals by 100 times in particular regions, making military GPS even harder to jam.
On the civilian side, in addition to the legacy L1 C/A civil signal, the government is in the process of phasing in three new signals— L2C, L5, and L1C—for a total of four civil GPS signals. These new signals will be of limited use until they are broadcast from 18 to 24 satellites and users will have to upgrade their equipment to benefit from them. The higher power for civilian GPS signals means they should be less subject to interference and jamming. Current GPS signals are low power, and GPS receivers have to be notoriously sensitive to accurately pick up GPS signals. However, high-sensitivity receivers are very easy to jam. The GPS III design, and the L5 signal in particular, are designed to make GPS more resistant to jamming.
For surveyors and other high-precision civil GPS users, the biggest improvement will be the availability of the L2C signal, which will provide improved corrections through the ionosphere. For consumers, L2C will significantly increase the strength of the GPS signal, making it easier to pick up in urban canyons, under thick tree canopy, indoors, and in other challenging environments. Additionally, by comparing the L1 C/A and L2C signals, receivers will be able to compensate for ionospheric delay error, which is the biggest cause of errors in the existing L1 C/A signal, making them more accurate. Receivers work around it today by using other sources of information, such as the FAA’s Wide Area Augmentation System (WAAS) or the locations of nearby Wi-Fi hotspots.
L5, dubbed the “safety-of-life” signal, is intended mainly for aviation and transportation. It is twice as powerful as the L2C signal, has much greater bandwidth, can operate over huge ranges (with augmentation), and has an enhanced signal structure so that it can carry more data. Receivers that can handle L1 C/A, L2C, and L5 should be able to leverage all three signals with a technique called trilaning to get location accuracies to less than one meter.
The L1C signal expands international cooperation in the Global Navigation Satellite System (GNSS) arena because it is interoperable with Russia’s GLONASS, Europe’s Galileo, Japan’s QZSS, India’s IRNSS, and China’s BeiDou. Therefore, in the future, users of civilian GPS receivers will be able to connect to L1C from multiple GNSS constellations.
In September, the Air Force accepted the first GPS III satellite and declared it “available for launch,” with launch expected in 2018 (four years behind the initial launch schedule). The second one is completing its testing and eight more are in various stages of production assembly by Lockheed Martin Space Systems. Harris Corporation, a Lockheed Martin subcontractor, has already provided the navigation payload for the first ten. Payloads for satellites 11 and beyond will bring further signal transmit capabilities, making the signals more powerful and more flexible to adapt to advances in GPS technology and changes in mission needs.
The upgrade to GPS III will greatly benefit a host of Applanix products, which integrate GPS technology for improved accuracy.