GPS accuracy [summer brief]

I'm always being asked - and have been for ages - how much confidence we can put in the GPS on our tablets and smartphones. Probably at the risk of repeating myself, I persist in asserting that our mobile devices are provided with the most accurate GPS receivers that can be found in civilian use, especially for recreational boating.

The reason is simple. GPS receiver modules built into our wonderful little mobile gadgets, Ultra miniaturized, are manufactured by tens of millions of copies for the leaders of the mobile "smart things" : Apple, Samsung, Sony, etc,.  While the receivers fitted to marine instruments designed by Garmin, Furuno, Raymarine and a few others, are sold, for the best, only a few tens of thousands units each year. And as everyone knows in this world, In economics, the price of things is inversely proportional to volume. Tablet and smartphone manufacturers can therefore obtain the best products at the lowest costs, and even the relatively low margin rates are more than offset by sales volume. While our marine electronics guys have to buy more expensive, and compensate for the low level of sales with higher margins – and therefore selling prices –. The equation is easy to understand !

What impact on the accuracy of the devices ?

Actually : very low ! Below I have compared, a few days ago, the position given on board my boat parked on the pontoon by three aircraft :

  • Furuno GP32, External antenna, End of manufacture 2003
  • MLR SP24 portable, Built-in antenna, End of manufacture 2000 (company liquidated on June 28 June 2001)
  • iPhone 8 Built-in antenna, End of manufacture 2020

Once stabilized after about ten minutes of waiting, here are the results :

To the hundredth of a minute, The position in latitude/longitude is the same, or on a scale of 18-20 meters high. MLR SP24 does not give thousandths of a minute, it cannot be compared more finely.

To the thousandth of minute, the iPhone gives a position in latitude of 0,002 mn further south than the GP32, so 3.70 meters, and longitude 0,003 MN further west, i.e. 3.64 meters at Lat 49°N (¹). This gives us a gap between the two of which is only 5 meters in the SW. Which of the two is right, I'll let you choose. However, the difference seems negligible to me, because I'd be very unwise to be shaving rocks with a GPS within 5 meters of the hazards (²) !

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(¹) After a refreshing dive into the calculation of distances in longitude and the Pythagorean theorem
(²) ENCs, Routing and sailing shaves stone
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4 Replies to "The accuracy of GPS [summer brief]”

  1. The GPS gives good information in position and speed : ground speed that Air France pilots ignored during the crash in the South Atlantic . On the other hand, the general public GPS is dubious when it comes to altitude

  2. GPS accuracy is one thing, that of the maps dates from their last survey.

    Most of our maps were surveyed before GPS was available, by triangulation methods. There have been corrections, adaptations and other adjustments. But still be more or less 5m away, It's half a boat length for most of us.

    And to add to the confusion, Let's not forget the geodetic reference grid. The one on our map is not necessarily the same as the American WGS84 grid that serves as a reference for GPS. Have you ever wondered what is the reference grid of the Chinese Beidou system or Russian GLONASS or European Galileo? Today, the receivers in our latest iPhones use at least 3 of these systems in parallel… To increase the accuracy they say.

    1. All hydro services that are members of the IHO have adopted the WGS84 coordinate system. GPS operators also for civilian frequencies which are common to GNSS, including Beidou.

  3. Thank you Francis
    new consumer GPS chips to go on sale in new smartphones, on Android for now. The accuracy will be better and above all more stable and less sensitive to environmental noise between the satellites and our equipment.
    For example, the series 8 from u-Bloc
    I'm expecting one for the end of September with a NEO-M8N GNSS chip, which, thanks to mass distribution, has this one for less than 10€ (Manufacturer price).
    I'll give you the results of my tests.

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