Navigation is the process of determining own position, orientation and velocity relative to a known reference point. All living creatures have developed a natural way to determine their location and find the best possible route for their journey, whether it is for migrating or foraging for food. Humans are never an exception and the desire to travel to point B from point A has been the driving force behind going beyond the natural ways and developing systems/tools for positioning and navigating accurately and safely.
Early Radio Navigation and the Birth of Multilateration
Beginning of 20th century witnessed the technological advancement that led to the development of electronic equipment for producing, transmitting and receiving radio waves, which consequently paved the way for more sophisticated (unnatural) way for positioning and navigation. Radio waves were used to send information over long distances, early examples of which included sending out time information so that the ships could update their chronometers. Then, in the 1940s the terrestrial radio navigation system called LORAN (acronym for LOng RANge) was introduced that enabled ships to determine their position and speed using the low frequency radio signals transmitted by fixed shore based stations. LORAN operates on the principle that the difference in the time of arrival of signals from two or more stations with known locations is the measure of difference in distance from the receiver to the each of the transmitting station as seen if Fig. 1. The idea is simple; if one knows the exact location of the station and the time that the signal was transmitted, then one can record the time when it is received to find the time that the signal took to reach the station which is calculated as the difference that it was transmitted and received. Assuming that the radio signals travel at the speed of light, the distance between the station and receiver is found by simply multiplying the calculated time difference by the speed of light. Leaving the mathematical details out, now we have what we need to employ the mathematical method called multilateration in order to determine the position.
Figure : The concept of position fixing by multilateration using signals from three satellites. The user’s position is indicated by the red dot.
From Sputnik to Space-Based Navigation
While radio navigation helped overcome many problems with positioning, it is limited in terms of global availability and another technological leap was needed for globe-wide assistance with navigation. That giant leap came in late 1950s when the Russians launched and orbited the first satellite, Sputnik, in space and it was not long before the thought of carrying the radio navigation base stations into space to determine a position on earth. Remember; in radio navigation, all one needs to determine the position are the location of the stations, the time that the signal is transmitted and the time that it is received. Since the biggest problem, that is putting a satellite in space, was already, solved the rest was child’s play (well a bit more than that) for the scientists.
GNSS Evolution and Global Coverage
The development of Global Navigation Satellite System (GNSS) is probably the single most effective invention for positioning and navigation and as an added bonus for timing. The pioneering GNSS system, namely the Global Positioning System (GPS) was launched in the 1970s by the US Department of Defense for military purposes, however, the successful execution has led to extending its use to almost everyone. Currently, there are four different GNSS Systems with enough number of satellites to provide global coverage and a number of systems with regional coverage whose operating frequencies are given in the GNSS Frequency Bands and Signals blogpost. Fig. 2 illustrates the concept of position fixing by multilateration by using the range to three stations that are in space.
GNSS System Components
GNSS comprises 3 components (or segments) to provide position and timing information; i) the space segment, a constellation of satellites orbiting the Earth; ii) the control segment, made up of a group of ground control stations; and iii) the user segment, a user’s equipment or simply the receivers. Figure 3 illustrates the three segments.
Space Segment
The space segment consists of GNSS satellites, orbiting about 20,000 km above the Earth with their own “constellation” of satellites, arranged in orbits to provide the desired coverage. Table 1 shows available constellations. GPS Satellites Each satellite in a GNSS constellation transmits information about their location (current and predicted), timing and “health” through what is known as ephemeris data. This data is the used by the GNSS Receivers to calculate location relative to the satellites and thus the position on earth.
Table 1: International GNSS Satellite Constellations
| Name | Country of Origin | Fully Operational Since | Number of Satellites | Carrier Frequencies |
|---|---|---|---|---|
| GPS | USA | 1993 | 31 | L1 / L2 / L5 |
| GLONASS | Russia | 1995 | 24+ | G1 / G2 |
| Galileo | Europe | 2020 | 30 (22 current) | E1 / E5a / E5b |
| BeiDou | China | 2020 | 30 (28 current) | B1 / B2 |
| NavIC | India | 2020 | 7 | L5 |
| QZSS | Japan | 2024 | 7 (4 current) | L1 / L2 / L5 |
Control Segment
The control segment is ground-based and comprises a network stations, for controlling, data uploading monitoring. The master control station adjusts the satellites’ ephemeris (orbit parameters) and on-board high-precision clocks when necessary to maintain accuracy. Monitor stations, that are deployed over a wide geographic area, monitor the satellites through the signals transmitted. Then, this information along with the satellite status are relayed to the master control station for further analysis. The master control station, then, makes the necessary calculations and transmits orbit and time corrections back to the satellites through data uploading stations.
User Segment
The user segment consists of equipment that processes the received signals from the GNSS satellites and uses them to derive position and time information. Depending on the application, the equipment ranges from smartphones to variety of portable receivers.,
GNSS and Precision Timing
Precise time is crucial to a variety of economic activities around the world. Communication systems, electrical power grids, and financial networks all rely on precision timing for synchronization and operational efficiency. In addition to the most needed position information the GNSS is also used to disseminate precise time. Each GNSS satellite houses multiple atomic clocks that allow very precise time to be distributed through GNSS signals. GNSS Receivers then use this time information, effectively synchronizing each receiver to the atomic clocks. This enables users to determine the time to within 100 billionths of a second, without the cost of owning and operating atomic clocks. For comparison, clocks used in GNSS segments go off by a second in 100,000 years where your run of the mill quartz watch will only take about two days to lose of gain a second. GNSS time signals are also used to calculate the position more accurately.
