Few everyday technologies have had the kind of revolutionary impact that the Global Positioning System (GPS) has. From civilians to the military, from precision scientific studies to urban planning and disaster risk estimation, GPS has significantly changed our expectations of where we are and our sense of place.
What is GPS?
The U.S. Department of Defence started the GPS programme in 1973 and launched the first satellite in 1978. The modern GPS satellite constellation consists of 24 satellites moving around the earth in six orbits. Each satellite completes two orbits in a single day. The overall programme has three main components: the space segment, the control segment, and the user segment.
The space segment of course consists of the 24 satellites. The six orbits they occupy are all 20,200 km above the earth, and each orbit has four satellites at all times. In this configuration, anyone on the earth will be able to ‘see’ at least four satellites at a time, which is a crucial requirement.
The control segment consists of a global network of ground-based control stations and antennae that track the 24 satellites, make sure their performance is as expected at all times, and transmit commands. The services provided by the GPS system are designed to meet the Standard Positioning Service (SPS) performance standard, the latest edition of which was published in April 2020. In essence, the SPS standard tells application developers and users anywhere in the world what they can expect from the GPS system. The control segment ensures these commitments are kept.
The master control station is located at Schriever Air Force Base, Colorado, and the alternate master control station is at the Vandenberg Air Force Base, California. The ground antennae are in Florida (Cape Canaveral), Ascension Island, Diego Garcia island, and Kwajalein Atoll. There are monitoring and tracking stations in Hawai’i, Alaska, New Hampshire, Washington, D.C., Colorado, and Florida in the U.S., and in Greenland, Ecuador, Uruguay, the U.K., South Africa, Bahrain, South Korea, Guam, Australia, and New Zealand.
The user segment pertains to the use of GPS in various sectors and applications. The major sectors include agriculture, construction, surveying, logistics, telecommunications, power transmission, search and rescue, air travel, meteorology, seismology, and military operations. In 2021, according to one estimate, there were 6.5 billion global navigation satellite system (GNSS) devices installed worldwide. The figure is expected to rise to 10 billion by 2031.
How does GPS work?
Each GPS satellite continuously broadcasts a radio signal containing information about its location in orbit, operational status, and the time at which the signal is emitted. The signals are transmitted at the L1 (1,575.42 MHz) and the L2 (1,227.6 MHz) frequencies at 50 bits/second. The signals are encoded with code-division multiple access. This allows multiple signals to be transmitted in the same channel and for a receiver to be able to disentangle them. There are two encoding types: the coarse/acquisition mode, which civilians can use to access coarse GPS data, and the precise mode, which is encrypted and is for military use.
Being an electromagnetic signal, the radio waves travel at the speed of light. On your smartphone, a GPS receiver picks this signal up and uses it to calculate its precise distance from the satellite. The distance is equal to the speed of light times the signal’s travel time. The signal’s travel time is equal to the time on the receiver’s clock minus the time at which the signal was emitted. If the receiver has access to signals from four satellites, it will have the information required to calculate its location in four dimensions (three of space plus one of time relative to the satellite clock) – and can thus accurately triangulate its location on the ground. This informs the need for every point on the earth being able to ‘see’ four satellites at a time.
Some adjustments are required to ensure the measurements are as error-free as possible. For example, the satellites around the earth are in a region of weaker gravitational potential, so their onboard clocks run 38 microseconds faster than those on the ground. This is explained by the general theory of relativity. The special theory of relativity requires engineers to account for the relative velocities of the satellite and the receiver.
How do the satellites keep time?
Good timekeeping is essential to ensure the GPS system works as well as possible. For example, not adjusting for the 38-microsecond offset between the clocks on the satellites and on the ground could lead to an error of 10 km within a single day. An offset of one millisecond can lead to an error of a full 300 km. For this reason, the satellites are all equipped with atomic clocks. In 1974, the U.S. Naval Research Laboratory first launched an atomic clock into space on board the NAVSTAR NTS-1 satellite.
The clocks onboard the modern-day GPS constellation are all synchronised to within just 10 nanoseconds of each other, and with reference clocks on the ground.
An atomic clock takes advantage of a simple but profound fact. The atoms of all elements have some number of electrons around the nucleus. Each of these electrons can have a specific amount of energy, no more and no less. Imagine these amounts of energy to be steps on a staircase. An electron can occupy only these particular steps; it can’t have some energy in between two steps. The size of these steps is the same for all atoms of a given element. For example, all caesium atoms in the universe have the same jump size between steps 2 and 3.
When radiation containing the exact amount of energy these electrons require to jump between two states – called the resonant frequency – is supplied, the electrons absorb it and jump. If too much or too little energy is supplied, fewer electrons will jump. So scientists begin with a radiation source and keep fine-tuning it to the frequency that causes the maximum number of electrons to jump. Once they have the frequency, they use it to measure time. For example, if it is 50 Hz, then one second will have passed when the radiation has completed 50 cycles. This is how an atomic clock works.
Simply put, the electrons tell the source which frequency is ‘correct’. The scientists can check the frequency at regular intervals to make sure that the source producing it isn’t drifting off and losing/gaining time.
Do other countries have GNSS?
According to the U.S. Space-Based Positioning, Navigation, and Timing Policy, the GPS system will cooperate with the operation of other GNSS. Such systems are currently operated by Australia, China, the European Union, India, Japan, South Korea, Russia, and the U.K. Of these, Russia’s GLONASS, the E.U.’s Galileo, and China’s BeiDou systems are global.
Officials of the U.S. government and their counterparts in other countries meet regularly to ensure their respective technologies are compatible with each other. There is also an International Committee on GNSS, operating under the United Nations Office of Outer Space Affairs. According to its website, it “promotes voluntary cooperation on matters of mutual interest related to civil satellite-based positioning, navigation, timing, and value-added services”.
India mooted its own Indian Regional Navigation Satellite System in 2006, later rechristened Navigation with Indian Constellation (NavIC). Its space segment consists of seven satellites: three in geostationary orbits and four in geosynchronous orbits. As of May 2023, the minimum number of satellites (four) could facilitate ground-based navigation. The master control facilities are located in Hassan in Karnataka and Bhopal in Madhya Pradesh.
The NavIC satellites use rubidium atomic clocks and transmit data in the L5 (1,176.45 MHz) and the S (2,492.028 MHz) bands, with newer satellites also transmitting in the L1 band. They include a messaging interface that can receive messages from control stations and transmit them to specific areas, like warning fishers about being close to international borders, etc.
India also operates the GPS-Aided Geo Augmented Navigation (GAGAN) system, which was developed and established by the Indian Space Research Organisation (ISRO) and the Airports Authority of India. According to the ISRO website, GAGAN’s primary purpose is “safety-of-life civil aviation applications catering to the Indian airspace” and for providing “correction and integrity messages for GPS”.
