3.4.7. Working of a GNSS

The GNSS operation is based on the concept of ranging and trilateration from a group of satellites, which act as precise reference points. Each of the 24 satellites in GPS emits signals to receivers that determine their locations or ranges by computing the difference between the time that a signal is sent and the time it is received.

GNSS, or Global Navigation Satellite System, operates using signals emitted from satellites to determine a receiver's location. It relies on two key principles: ranging and trilateration. Ranging involves calculating the distance from the receiver to a satellite by measuring the time taken for a signal to travel. Trilateration then uses distances from at least three satellites to pinpoint the receiver's exact location in three-dimensional space.

Imagine you are trying to find your position in a park where there are three distinct trees (like satellites). If you know how far you are from each tree, you can draw circles around them. Where these circles overlap is where you are standing.

The GNSS satellites carry precise atomic clocks that provide highly accurate time. The time information provided by atomic clocks in a GNSS receiver is placed in the codes broadcast by the satellite so that a receiver can continuously determine the time the signal was broadcasted.

GNSS satellites have atomic clocks that ensure the time signals are incredibly accurate. Each time a satellite sends out a signal, it includes information on when that signal was sent. When a GNSS receiver picks up this signal, it checks its own internal clock to determine the time delay, which is used to calculate how far away the satellite is. This precise timing is essential for accurate positioning.

Think of it like a synchronized swimming team. Each swimmer (satellite) needs to know exactly when to move compared to others to maintain formation. The atomic clocks are like their perfectly timed watches that help them stay in sync.

Each satellite broadcasts a navigation message that contains (i) the pseudo-random code, called a Course Acquisition (C/A) code, which contains orbital information about the entire satellite constellation (Almanac), (ii) detail of individual satellite’s position (Ephemeris) that includes information used to correct the orbital data of satellites caused by small disturbances, (iii) the GNSS system time, derived from an atomic clock installed on the satellite, with clock correction parameters for the correction of satellite time and delays (predicted by a mathematical ionospheric model), and (iv) A GNSS health message that is used to exclude unhealthy satellites from the position solution.

The navigation message sent by GNSS satellites includes several crucial components: 1) the C/A code, which is essential for identifying the satellite's signals; 2) ephemeris data, which provides details about the satellite's current position and how to correct for any movements; 3) system time and corrections for any delays caused by atmospheric conditions; and 4) health information about the satellite to ensure it is operational and can be trusted to provide accurate data.

Consider a GPS satellite as a librarian who not only provides books (signals) but also keeps a record of where each book is located (orbital information) and notes any books that are damaged (health message) so patrons know which books they can check out.

Once the signals are obtained, the receiver starts to match each satellite’s C/A code with an identical copy of the code contained in the receiver’s database. By shifting its copy of the satellite’s code, in a matching process, and by comparing this shift with its internal clock, the receiver can calculate how long it took the signal to travel from the satellite to the receiver.

The GNSS receiver has a stored copy of the course acquisition (C/A) code that the satellites use. To find its position, the receiver takes in signals from satellites and compares them to its stored codes. By adjusting (or shifting) its copy of the C/A code until it aligns with the incoming signals, the receiver can figure out the travel time of the signals. This time is then used to calculate the distance to each satellite.

Imagine a game of 'hide and seek' where each player (satellite) has a whistle (C/A code). If you have your own whistle and hear other players' whistles, you keep blowing yours until you hear it match. Once aligned, you know precisely how far away the players are from you.

The distance derived from this method is called a Pseudo-range because it is not a direct measure of distance, but a measurement based on time. Pseudo-range is subject to several error sources, including atmospheric delays and multipath errors, but also due to the initial differences between the GNSS receiver and satellite time references.

The calculated distance from the satellite to the receiver is termed a 'pseudo-range' as it is based on the time taken for the signal to travel, rather than a direct measurement. Various errors can affect this calculation, such as delays caused by the atmosphere or reflections from surfaces (multipath errors). Additionally, any discrepancies between the satellite and receiver clocks can introduce errors in the distance measurement.

It's similar to timing how long it takes for someone to speak a phrase across a room. The actual distance isn't just about time, but could be affected if the sound bounces off walls. You might think the person is farther away than they actually are due to that echo.

Using trilateration process, the GNSS receiver then mathematically determines its position by using the calculated pseudo-ranges and the satellite position information that has been communicated by the satellites.

Trilateration is a mathematical technique used by GNSS receivers to pinpoint their location. By knowing the distances to at least three satellites (from pseudo-ranges), and their locations in space, the receiver can accurately calculate its own position in three dimensions: latitude, longitude, and altitude. This process essentially forms spheres around each satellite, and the intersection of these spheres determines the receiver's position.

Think of it as having three friends (satellites) each standing at a different corner of a park (each having a known location). If they each shout out how far they are from you, you can figure out exactly where you are in the park by finding where the distances overlap—like how the spheres intersect in trilateration.

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