The Global Navigation Satellite System (GNSS) encompasses a network of satellites and base stations to deliver a system that benefits numerous industries by providing location and time data. The automobile industry uses GNSS in cars, enhancing road navigation. The aviation industry uses GNSS in planes, enhancing safety and minimising fuel consumption. The logistics industry uses GNSS in delivery vehicles, minimising delivery time. The mining industry uses GNSS for vehicles and personnel, enhancing safety and productivity. The surveying industry uses GNSS in measuring equipment, enhancing accuracy and efficiency.
Figure 1 represents the three control segments associated with GNSS: monitor stations, the master control station, and antennas.
Monitor stations monitor satellites in view in medium Earth orbit by analysing continually transmitted data containing the satellite’s current time, based on an on-board atomic clock, and location. The analysed data is transmitted to the Master Control Station (MCS), which manages an entire constellation of satellites (Space Segment). The MCS will identify any discrepancies within the constellation, e.g., a satellite out of synch in time and/or position, and prepare commands to be sent by the Antenna, to said satellite to make corrections. This ensures that GNSS receivers function accurately and reliably. These stations and antennas are what make up the Control Segment. The User Segment represents the consumer, who can only receive but not transmit data to the infrastructure of the GNSS.
Table 1 lists the two most common GNSS, i.e., GPS and GLONASS.
| Characteristic | GPS | GLONASS |
| Country of origin | U.S.A | Russia |
| Satellites (total) | 31 | 24 |
| Satellites (primary) | 24 | 24 |
| Orbital planes | 6 | 3 |
| Satellites per plane | 4 | 8 |
| Orbital plane orientation | 55° | 65° |
| Altitude (km) | 20,200 | 19,100 |
| Orbital period | 11 h 58 m | 11 h 16 m |
| Accuracy (m) | 3.5 – 7.8 | 5 – 10 |
The US GPS is the most popular system due to its higher accuracy and wider spread control segment. The Russian GLONASS, however, has greater accuracy only within high-terrain environments. There are other countries that have developed their own GNSS, such as the EU (Galileo), China (BeiDou), India (IRNSS/NavIC), and Japan (QZSS). GPS utilises trilateration, the measurement of distance from a satellite to a GNSS receiver. This method is not to be mistaken for triangulation, which is the measurement of angles. Figure 2 represents an example of trilateration.
The orange dot represents the location of the GNSS receiver. Essentially, a total of four satellites are required to be in view of a GNSS receiver to provide an accurate reading of latitude, longitude, time, speed, and altitude. Three satellites provide the location of the receiver on Earth, while the fourth satellite verifies the signal integrity of the first three satellites in addition to providing an altitude reading. Note that this is just a two-dimensional representation. In reality, there would be four spheres intersecting. The speed of GNSS signals is the speed of light, as shown in Equation 1.
\begin{equation}
\tag{1}
v(t)=\frac{ds}{dt}=299,792,458 \,m\cdot s^{-1}
\end{equation}
When a GNSS receiver picks up a satellite’s signal, the signal contains information based on the satellite’s location and the time the signal was transmitted (tT), which is compared to the receiver’s time at the time of reception (tR). The difference between these two times is used to calculate the distance between the satellite and the receiver, as shown in Equation 2.
\begin{equation}
\tag{2}
s=299,792,458 \cdot (t_{T}-t_{R}) \,m
\end{equation}
Table 2 lists the three publicly available frequencies associated with GPS with wavelengths (λ) calculated as per Equations 3 & 4.
| Name | Frequency (MHz) | Wavelength (m) |
| L1 | 1575.42 | 0.1903 |
| L2 | 1227.60 | 0.2442 |
| L5 | 1176.45 | 0.2548 |
\begin{equation}
\tag{3}
v_{L1}(t)=f_{L1}\lambda_{L1}=299,792,458 \,m\cdot s^{-1}
\end{equation}
\begin{equation}
\tag{4}
\lambda_{L1}=\frac{v_{L1}}{f_{L1}}=\frac{299,792,458}{1575.42\times10^6}=0.1903 \,m
\end{equation}
The L1 frequency, in comparison to L2 and L5, is based on older technology and is well-established, meaning that a wide range of GPS receivers can pick up the signal. The downside to the L1 frequency, however, is that it is not highly effective at travelling through/around various obstacles such as heavy weather, landscapes, and buildings. The L2 frequency has a longer wavelength, meaning it can better travel through/around objects than the L1 frequency. The L5 frequency is the latest technology and can better travel through or around objects than the L1 and L2 frequencies. The L1 frequency can be used with the L2 frequency to provide greater accuracy, depending on the characteristics of the GNSS receiver.
The GNSS uses the NMEA 0183 standard, as shown in Table 3, in transmitting messages (sentences) from satellites in ASCII to be received by multiple receivers.
| Field | Description | Bytes | Example | ||
| $ | Preamble | 1 | $ | ||
| Talker ID | GP, GN, or GL (depending on NMEA ID) | 1 – 2 | GN | ||
| NMEA ID | RMC | Position, speed and time | 3 | RMC | |
| VTG | Track Made Good and ground speed | ||||
| GGA | Fix data providing 3D location (latitude, longitude, and altitude) | ||||
| GSA | Number of satellites being used by the GNSS receiver | ||||
| GSV | Satellites in view, elevation, and azimuth (horizontal angle measured clockwise from North) | ||||
| GLL | Latitude, longitude, and time | ||||
| GPTXT | Total number of messages in transmission (0 – 99) and message number in transmission (0 – 99) | ||||
| Data Field | ( , ) delimited | var. | ,124001.303,A,3351.414,S,15112.9144,E,0.37,201.50,060124,,,A | ||
| * | End of data field | 1 | * | ||
| Checksum | Hexadecimal number calculated by XOR of all characters between “$” and “*” | 2 | 53 | ||
| <CR><LF> | End of message | 2 | <CR><LF> | ||
The beginning, i.e., preamble, of a message is marked by the “$” character. The talker ID indicates what constellation is being used, i.e., GP for GPS, GL for GLONASS, and GN if more than one constellation is used in a GNSS position fix. The NMEA ID indicates what information is presented in the Data Field. The Data Field displays comma-delimited (separated by commas) information, as per the NMEA ID. The (Carriage Return) (Line Feed) indicates the end of a string, i.e., between and including “dollar” and “Checksum,” at which point the cursor returns to the beginning of a string, followed by the cursor moving to a new line for the next string to be displayed. The purpose of checksum is to detect any errors within transmission. An example of a Recommended Minimum Position (RMC) reading is shown, which provides the location, time, and speed.
Table 4 is a breakdown of the RMC message in Table 3.
| Field | Value | Description |
| UTC Time | 124001.303 | hhmmss.sss |
| Data Valid | A | V = Invalid A = Valid |
| Latitude | 3351.414 | ddmm.mmmm (degrees and minutes) |
| N/S | S | N = North S = South |
| Longitude | 15112.9144 | dddmm.mmmm (degrees and minutes) |
| E/W | E | E = East W = West |
| Speed | 0.37 | Knots |
| COG | 201.50 | Course over ground (degree) |
| Date | 060124 | ddmmyy |
| Magnetic Variation | Degree, not being output | |
| E/W | Indicator, not being output | |
| Positioning Mode | A | N = No fix A = Autonomous GNSS fix D = Differential fix |