GNSS Errors RNAV

There are many different errors with GNSSGNSS —Global Navigation Satellite System, and you need to know what they are, their causes and their solutions.

• Satellite clock error is an error caused by the atomic clocks in the satellites being wrong. If this is the case, then they will send out their signals at the wrong time, causing errors in distance measurement. This is corrected by having corrections sent in the Nav Message.
• Ephemeris is an error caused by the satellites not being where they were predicted to be when they were launched into orbit because of the gravitational attraction of other celestial bodies. If this is the case, you do not know where the centre of the sphere is for the purposes of your distance calculations. This is corrected by having corrections sent in the Nav Message.
• Ionospheric error is an error caused by the radio signals changing speed as they pass through the ionosphere. If this is the case, your timings will be in error for the calculations of distance from the satellite. This is corrected by modelling the ionosphere and having corrections sent in the Nav Message.
• Relativity error is an error caused by the different speeds at which the user and satellites are moving. Because the satellites move faster, time slows down. If this is the case, they will send out messages at the wrong time, causing errors in distance measurement. This is corrected by modelling relativity and having corrections sent in the Nav Message.
• Multipath error is an error where the signal bounces off things like mountains and arrives at the user at multiple times. This is corrected by software (basically, the first signal received is the correct one, as it travelled the shortest distance!)
• User clock error is an error caused by the user clock being wrong. If this is the case, the user cannot accurately tell when the satellite signal was emitted, and so cannot correctly assess the time for the signal to travel. This is the only error that cannot be corrected in some way by the GNSSGNSS —Global Navigation Satellite System setup. To solve it, the pseudoranges (what the user calculates, which are in error by the amount of user clock error) are extended or contracted by the same amount until the spheres all intersect at one point.