GPS tracking vehicles and people
How accurate is GPS?
GPS or Global Positioning System is a satellite-based radio navigation system. This satellite system provides geolocation and time information to a GPS receiver which can be controlled by an end user. Common systems that use this type of technology can be, and is not limited to, car sat nav’s, mapping applications such as Google maps, and tracking devices both human (Tag) or vehicular.
The technology used requires clear line of sight to record the most accurate readings and requires four or more satellites to be visible for navigation. Obstacles such as mountains and buildings block the relatively weak GPS signals therefore, accuracy of the data being received should always be a consideration.
GPS readings can be affected by a process called multipath, the bouncing of signals off large obstacles such as buildings. This explains the frequent shifts in GPS position and therefore can affect conclusions drawn from saved readings. GPS devices typically need to receive signals from at least 7 or 8 satellites to calculate location to within about 10 meters. With fewer satellites contributing, the amount of uncertainty and inaccuracy increases.
With fewer than 4 satellites, many GPS receivers are unable to produce any location estimates and will report "GPS signal lost". The GPS unit is usually looking to find and acquire signals from more satellites, so should eventually recover from situations where not enough satellites are found.
When we investigate GPS evidence, there are many factors that need to be considered. For example, if you are faced with GPS evidence that suggest a vehicle was travelling from A to B at a speed of 60mph, and this speed was ascertain from GPS, then this data needs to be investigated.
The speed correlation between 2 GPS points is based on a straight line. Therefore, if there is any deviation off the straight line between these two points, this will unduly affect to average speed required to get there, therefore rendering the presented data potentially unreliable.