GPS Reliability and Precision

The use of a satellite-based navigation system has grown tremendously in the last decade. Almost every phone, many cars, and even fitness watches have an inbuilt GPS receiver these days.

As already mentioned previously, there are many safety benefits when using a GPS-based flight controller, however, there can also be some issues using this technology though. Some reasons that relying solely on the accuracy of the GPS position may be problematic will be listed and explained in this article. GPS Systems in principle calculate the position in space in 3 Dimensions using satellite data-independent if it is the Global Positioning System (GPS), the European Galileo, or the Russian GLONASS system. To do so they acquire the signal of geostationary satellites. We know their position at any given time- pretty much like sailors relying on the position of the stars for their calculations. The GPS receiver figures out how far each satellite it receives is away. It does so by using a precise time signal. By simple trigonometric calculations, the position of the GPS receiver can be calculated. The position estimation gets better the more satellites a GPS receiver receives. To do so it requires the “LOS” line of sight to the satellites. Data of at least 3 satellites are needed for calculating a 2-dimensional Position (LAT/LON). If 4 or more satellites are available a 3-dimensional Position (LAT/LON/ALT) can be determined. Today from most places in the world there should be at least 4 up to 12 satellites in a usable position in the sky. Not is only the number of satellites is important, but also the relative position of the satellites to each other. The further apart in the sky they are the better. Due to the need for “LOS” to the satellites worse reception can be expected near high obstacles like buildings- this has to be kept in mind when using GPS for navigation. Furthermore, other factors like reflections from buildings can also influence the precision of the signal. The good news is- the higher you fly, the more satellites the GPS will “see” and fewer reflections impair the signal. In general, the horizontal accuracy of less the 3.5m should be achievable with a high-quality GPS receiver according to FAA data. Interesting for the user to rate the accuracy at any given time is the HDOP value. It stands for Horizontal Dilution of Precision, a value for the accuracy of the position estimation. Values between 0 and 50 are possible with zero being the best value. An HDOP value of 0 would mean there is no position deviation on the measured and the real position of the GPS receiver. Good accuracy is still given when the HDOP value is smaller than 2.5. For general navigation purposes values up to 8 are considered as “usable”. For use with an aerial platform values smaller than 2 are desirable. Most GPS receivers are able to display this value somehow. Ardupilot for example displays this value in the Mission Planner´s "Quick Tab" and will not allow to arm the flying platform if the value is higher than 2.0. On their long way from the satellite to the receiver, the GPS signals travel through the Earth’s ionosphere. The charged particles in the atmosphere influence the GPS radio signal. The GPS systems compensate for the average influence of the ionosphere to achieve the best accuracy under these conditions. During a heavy space weather event, like a big solar storm, the average model cannot compensate for the influence anymore. The wrongness in the position estimation can increase to tens of meters or even more. Besides the space weather, there is another severe influence factor caused by the atmosphere. The so-called Ionospheric Scintillations are caused by the interaction of the radio waves with free electrons in the Ionosphere. Due to the density distribution of the earth’s magnetic field, these Scintillations are more prevalent at low and high latitudes, so it is not a phenomenon that influences Europe and the US too often. This atmospheric effect can make it impossible to calculate the correct position. In general, the influence of scintillations will cause the position estimations to be less accurate. Considering these factors it should be easier to know how much to trust the GPS position calculations under certain circumstances. It should be clear now that flight modes that rely on GPS should always be chosen with discretion. As a rule of thumb, one should be cautious if the KP-Index is higher than 4 or the HDOP factor is bigger than 2.5. In these cases, one should not rely solely on GPS. Written by RomanRadtke

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