Global Navigation Satellite Systems
GNSS or more commonly called GPS, has become a common part of all marine navigation in the modern world. Skippers have increasingly relied more on GPS over recent years. However, it does have limitations.
It relies on the use of reliable power supplies, however a failure in the electrical system would result in a loss of the GPS system.
The accuracy of the system can vary and may actually have large errors in your position, and you would need to use other methods to verify your position.
The GNSS signals can be easily disrupted especially in times of conflict. The nations that operate GNSS systems all reserve the right to encrypt or shut the system down for a given area at any time. In which case you will need to rely on traditional navigation means.
The GPS system is based on 24 satellites in 10,900 nautical mile orbits. They
give 24 hour coverage and provide a spherical position line.
4 Satellites will give you a 3 dimensional fix useful for aircraft
because the height can be calculated as well.
The GPS system consists of:-
GNSS creates a fix by measuring the time and distance of the receiver from the satellites
(using time as the measurement). Each satellite broadcasts a message that contains information on the specific orbit of the satellite, and the time it was sent. When the receiver picks up the message it can determine how far from the satellite it is creating
a spherical position line.
Due to a number of variables, and the time to calculate the position, 4 satellite fixes are
required to give certainty to our location information and to give a 3D
fix.
Speed and direction are determined by comparing the current position with previous positions, and the receivers are constantly updating and re-calculating up to several times a second, however, the display that we use updates only once every few seconds.
The term GPS is now synonymous with all satellite navigation systems but to be correct, GPS refers to the first satellite navigation system, which was developed and owned by the military of the United States of America. There are also others, owned by different nation-states and have different approaches to the technology. Many of these systems now have improved accuracy compared to GPS, or better functionality. or higher levels of accuracy and reliability. For 99% of users, you will not be able to notice a difference.
Most satellite navigation systems are now capable of receiving signals from multiple constellations to help improve the overall accuracy of their fixes.
Accuracy increases with the number of satellites, just as with a standard 3 point fix the accuracy increases with more fixes.
The final accuracy is dependent on a number of factors including the quality of the device. A high quality sophisticated device can typically get within 10 metres about 95% of the time under ideal conditions, and good quality recreational marine ones are usually in the range of about 15-20 metres for 95% of the time. The GPS units on a mobile phone however can barely get an accuracy of within 100 metres 70% of the time under ideal conditions.
Highly sophisticated systems such as differential GPS, or fixed point compensation can give a much greater accuracy, but these are not suitable for yachts.
With the typical level of accuracy of GNNS, a significant amount of caution needs to be considered when navigating in restricted waters.
Inaccuracies in GNNS occur more commonly than you would realise.
EGNOS is an augmentation system being rolled out by the European Union that is intended to overcome the inaccuracies of a typical GNSS by accurately calculating the offsets from a highly accurate surveyed point, and transmitting the correction signals out that can be picked up by dedicated receivers. For vessels that have the correct hardware installed to receive the EGNOS signals, very high accuracy can be achieved.
Traditional methods are your most reliable backup to electronic systems, and the only independent method to confirm the accuracy of a satellite position fix. A skilled skipper or navigator should be perfectly comfortable using traditional navigation techniques alongside electronic navigation, as you never know when you are going to need it.
GNSS only tells you where you were a few seconds ago, not exactly where you are now. So based on your speed, you may have moved quite a few metres from the position shown.
A boat moving at 10 knots will cover 5 metres every second, and if the GPS display is only updated every 5 seconds, then that means you can be 25 metres away from the position the GPS is telling you, and this is not taking into account the possible accuracy which can mean you may be more than 40 metres away from where the display is indicating that you are.