GPS devices, they're everywhere, phone, car, cameras. The list is almost endless. But had you stopped for a moment to think about how reliably accurate they are. Probably not but then why would you. We have trust in what the manufacturer says and makes and it's not until we look deeper that we realise that there can be quite a big error margin in their locational accuracy. OK for general hiking this might not be a problem but for surveying it clearly is ! And what surprises me is just how much GPS equipment there is on the market making some pretty big locational accuracy claims that just can't be true when you look at the technology they use. So some basics. Please bear with me - I'll keep it light !
GPS is an abbreviation for Global Positioning System. It works through a technique called trilateration where time, location, velocity and elevation are calculated. Similar to triangulation where the angle is used but in this case it's velocity. At least 4 satellites orbiting the earth are required, each sending a signal to your GPS device. Each signal recording the time taken to travel at a certain velocity (speed). And with time and velocity known, you can then calculate the distance. Distance = Velocity x Time. Knowing the distance from 4 separate satellites allows an approximate location on the earth's surface. More satellites equals higher accuracy.
The best we can theoretically get is just under +/- 0.7m. So even best case we could be 1.4m out. In reality with atmospherics, buildings and trees +/-5m is more realistic.
More recently we've started seeing military grade dual frequency GPS devices appear on the market. Devices using what's called E1, E5a, L1, L5. You'll find these buried in your GPS settings. Have all these and you have a dual frequency and more accurate GPS device. But even this relies on a clear sky and, we live in Great Britain. So in reality claims made by manufacturers of survey equipment rely on perfect weather conditions and line of sight to all the current constellation of satellites. Furthermore the GPS must be fixed to a tripod with a large antenna not moving for anything up to half an hour. Then and only then will you get accuracy down to +/- 300mm. So yes their claims may well be theoretically true - but is this practical ?
Since early 2020 we've seen some exciting changes in the world of mobile phones and specifically locational accuracy. A new breed of mobile phones using this GPS and Glonass dual frequency Satellite technology but combined with terrestrial telecoms mast locationing.
Combining these satellite and ground based technologies can give an accuracy of down to 300mm in as little as 5 minutes in perfect conditions and for less than a few hundred pounds. Needless to say I returned my Garmin GPS MAP 65 and bought a dual frequency GPS Mobile instead. The Xiaomi Mi8 Pro.
And just in case you're wondering here is a list I compiled late last year of dual frequency mobile phones. If you're looking yourself remember you're looking for GPS and Glonass E1, E5a, L1,L5.
• Honor 20 Pro • Huawei Mate 30 Pro / P30 Pro • iQoo 3 • One Plus 7 Pro • Oppo Find X2 Pro / Reno • Realme X50 Pro / X2 Pro • Samsung Galaxy S20 & S21 and all + & Ultra models • Vivo NEX 3 • Xiaomi Mi 8 & Pro / 8 Lite / 9T Pro / RedMi K20 Pro
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