# Is GNSS-based positioning trilateration or multilateration, and why?

According to the Wikipedia article on GNSS positioning calculation, GNSS-based positioning needs $t_\text{satellite}−t_\text{receiver}$ for a series of satellites, along with the satellite positions. This is an element of trilateration. However, the clock in a GNSS-receiver is not atomic, and not nearly accurate enough to determine $t_\text{receiver}$ with sufficient accuracy for trilateration. Therefore, it needs a 4th source to determine the time, such as indicated by maptoaster.com or this Yahoo Answers post. That makes sense: it simulatenously determines time, latitude, longitude, and elevation, and therefore it needs four satellites.

Why, then, do we still consider this positioning trilateration rather than multilateration? In my understanding, A GNSS receiver does not need a clock at all — after all, the clock that it has, is not nearly good enough. Isn't the implication that it's using the difference between the distances to the satellites for positioning — the principle of multilateration? Yet the latter Wikipedia article states that:

Multilateration should not be confused with trilateration, which uses distances or absolute measurements of time-of-flight from three or more sites, or with triangulation, which uses the measurement of absolute angles. Both of these systems are also commonly used with radio navigation systems; trilateration is the basis of GPS.

Is GNSS-based positioning considered trilateration or multilateration? Why?

GNSS-based positioning uses trilateration.

A GNSS receiver estimates the time-of-flight for each satellite signal received, which is used to compute the satellite pseudorange. They are call pseudoranges because it is the true range to each satellite + some bias. Pseudoranges can be computed because the satellites broadcast their position.

In multilateration, the receiver estimates the difference in distance from at least two transmitting stations with known location.

Accurately estimating this difference when the distances are very large can be challenging (here distances are on the order of thousands of km, depending on GNSS constellation, they have different orbits). You need to use signals that allow you to do so in an unambiguous way. From the signal processing point of view it becomes really challenging.

*Note that 4 satellites is the minimum required, but usually more satellites are used when computing the navigation solution (if available). The more satellites, the better.

• Actually, the GNSS satellites do not broadcast their position, but a set of parameters (called ephemeris) that allow the receiver to compute it. Jul 10, 2016 at 17:21