Artificial satellites have transformed the world in many ways, not only in terms of relaying communication and for observing the planet in ways previously inconceivable, but also to enable incredibly accurate navigation, reports Hackaday.
Global navigation satellite system
A so-called global navigation satellite system (GNSS), or satnav for short, uses the data provided by satellites to pin-point a position on the surface to within a few centimeters.
The US Global Positioning System (GPS) was the first GNSS, with satellites launched in 1978, albeit only available to civilians in a degraded accuracy mode. When full accuracy GPS was released to the public under the 1990s Clinton administration, it caused a surge in the uptake of satnav by the public, from fishing boats and merchant ships, to today’s navigation using nothing but a smartphone with its built-in GPS receiver.
Even so, there is a dark side to GNSS that expands beyond its military usage of guiding cruise missiles and kin to their target. This comes in the form of jamming and spoofing GNSS signals, which can hide illicit activities from monitoring systems and disrupt or disable an enemy’s systems during a war. Along with other forms of electronic warfare (EW), disrupting GNSS signals form a potent weapon that can render the most modern avionics and drone technology useless.
With this in mind, how significant is the threat from GNSS spoofing in particular, and what are the ways that this can be detected or counteracted?
Ephemeral positioning
The basic concept of a GNSS is fairly straight-forward: ground-based receivers listen for the signals from the satellites that are part of the specific GNSS constellation. Each GNSS satellite encodes a collection of information into this signal, which includes the position (ephemeris) of the satellite at a given time, as well as the local time on the satellite when the signal was sent.
By taking the signals from at least four of these satellites and applying the satellite navigation solution, the absolute position of the receiver can thus be determined. This uses the principle of trilateration (distance to a known point) rather than triangulation (using angles). As can be surmised, a potential issue here involves clock drift on the side of the receiver and the satellites. Perhaps less expected is that the travel speed of the signal is also heavily affected by the atmosphere, specifically the ionosphere.
This part of the atmosphere changes in thickness and composition over the course of a day, and is heavily affected by exposure to the Sun’s radiation. As a result, part of the GNSS satellite’s message contains the required atmospheric correction parameters. Because of clock-drift and the constant changes to the Earth’s atmospheric composition, each GNSS constellation has its own augmentation system. These generally consist out of a combination of ground- and satellite-based systems that provide additional information that a receiver can use to adjust the GNSS information it has received.
For use with airplane navigation, for example, it is very common to have a ground-based augmentation system (GBAS) installed using fixed receivers. These GNSS receivers then broadcast correction parameters via the airport’s VHF communication system to the airplane, helping them navigate when they approach or depart the airport.
In addition to the GNSS satellites themselves, each GNSS constellation also has its own ground-based master controller station, from which updated information on atmospheric conditions is regularly uploaded to the satellites, along with time adjustments to compensate for the satellite’s onboard clock drift. This demonstrates that a GNSS constellation is a highly dynamic system which requires constant updates in order to function properly.
Where things get interesting, however, is when attempts are made to circumvent this system, either by jamming or actively spoofing the GNSS signals.
Did you subscribe to our daily newsletter?
It’s Free! Click here to Subscribe!
Source: Hackaday
