Global Navigation Satellite System (GNSS) is a constellation of satellites, held at specific orbits, that provide accurate location and timing to users. The creation of a GNSS has been the purview of nation states, with the most prominent being Global Positioning System (GPS) for the U.S., GLOSSNAS from Russia, BeiDou from China, and Gallello from the European Union. To cover the entire globe, these systems require a minimum of 18 satellites in medium earth orbit. Gallello, the latest entrant, reached this milestone in 2014.
Figure 1. The big four GNSS systems globally
A satellite receiver needs to “find” four satellites to acquire its timing and location. Why then do GNSS require 18 satellites to be viable? This is more than a math question. Receivers require four signals for trilateration, the math necessary to pinpoint a location using four inputs, to work. A GNSS with 18 satellites provides a minimum global coverage. The satellites have to contend with variations in terrain. Having an increased number of satellites allows the GNSS receiver to switch to more readily as it loses signal due to terrain masking and allows each satellite to transmit a stronger, narrower signal. This narrow signal provides more accurate data.
Each GNSS system uses specific bands, or frequencies, to transmit data.
| GPS | GLONASS | Galileo | BeiDou |
| L1: 1575.42 MHz | L1: 1602 MHz | E1: 1575.42 MHz | B1: 1561.098 MHz |
| L2: 1227.6 MHz | L2: 1246 MHz | E5: 1176.45 MHz | B2: 1207.14 MHz |
| L5: 1176.45 MHz |
Trilateration not triangulation?
Why do modern GNSS use trilateration and not triangulation? The short answer is triangulation requires measuring angles to determine a precise location, and this is not how GNSS functions. Instead, the system uses trilateration, which requires three satellite spot beams. These beams act as a circle error of probability. Or, that the receiver must be located within the beam’s radius. Once the receiver finds a second beam it knows that it can only be found at one of the two intersection points between the beams. The third beam confirms which of the two intersection points is the receiver’s location. The fourth satellite is used for increased accuracy and to ensure timing in the system.
Figure 2. Trilateration. The intersection point of all three satellites’ beams is the receiver location
Timing
One of the key factors for positioning and many systems is timing. GNSS receivers pull timing from their satellite constellations. A receiver that uses multiple GNSS for location and timing must calculate the variability between the different systems. Failure to do this can introduce issues with location and timing in the system. The GNSS clock synchronization provides the basis for many important functions in the modern economy and military and has potential vulnerabilities.
The most obvious use of GNSS technology is for platform navigation. Navigation is not possible without reliable time. Planes, ships, and ground forces use GNSS signals to continuously update their positions. The use of these signals and the accurate timing allow longer range deconfliction of airspace and real-time command and control of force. Many of these platforms also rely on precision-guided munitions, like JDAMs, to accurately employ weapons. The use of GNSS enabled weapons allows the U.S. military to reduce the risk of collateral damage. These are the most obvious uses of a robust GNSS constellation of signals. More opaque is the role GNSS systems play in providing timing across data links, satellite communications, and command and control devices.
The uses of GNSS extend far beyond military applications. Ports, aircraft, and corporate shipping companies rely on GNSS to track the movement of goods and people across countries to keep the U.S. and global economy moving. It also underpins much of the emergency service network and critical infrastructure in the U.S. Telecommunications networks, power grids, and industrial safety SCADA tools all rely on GNSS to function properly.
Limitations and Vulnerabilities
With any technology, GNSS and the receivers have physical limitations that stem from natural and manmade interference. GNSS receivers can correct many of the most common errors with a system of checks based on data from across the GNSS. However, more pernicious is the vulnerabilities from space, cyber, and physical attack. Despite the narrow bands many GNSS satellites transmit, they use weak signals across specific bands. Outside of specific military GNSS bands the signals are not encrypted. Earth based jammers can use their proximity to the GNSS receivers to block signals. Satellites are vulnerable to targeting by modern militaries, and the technology to spoof, or send false location and timing data across the unencrypted bands has proliferated.
Solutions to these vulnerabilities include developing robust GNSS receivers that incorporate inertial navigation systems and include real time kinematics to reduce errors and eliminate threats of jamming and spoofing. Redundant systems are not just insurance from attack; they help to reduce the surface area an enemy can exploit to harm the nation. TAG are experts in building, designing and testing assured-position, navigation, and timing devices. We have developed a deep understanding of GNSS technology, its weaknesses, and how we can make a more capable network to support the military and economy.
