SNOWBIRDING – Why your GPS Device May Lead You Astray

December 18, 2022

April 5^th is National Read a Road Map Day.  We are encouraged to take ourselves back to a time before vehicles had a built-in GPS and everyone had a smartphone.  It is suggested that reading a simple, tangible road map induces “the thrum of adventure.”  Mention is made of the possibility of holding a road map upside down or standing over the hood of your car on the side of the road with it spread out.  Such fun!

Obviously, these map enthusiasts never lived through the nightmare of trying to find your destination with the use of a paper map in a downpour or while navigating city traffic or when your spouse refuses to simply stop and ask someone for directions.

I am a huge fan of any technology that makes life easier without threatening our freedom, and the GPS has certainly done that.  In fact, my husband and I rely on it so completely that against our better judgement we have allowed it to lead us up and down roads that resemble goat trails in California and cow paths in Texas, to no good end.  We were easily led astray because lanes leading to RV Parks we have stayed in were frequently not paved or in good condition, so we thought arrival at our destination was eminent.  We have now reached the point where when the RV Life GPS app is giving us questionable instructions, I consult another GPS App (Waze, Google Maps or Apple Maps) to see if they agree.  Sometimes we find they are sending us in opposite directions!  Why is that?  What to do?

I did some research and will share only the information that is most relevant, and which my limited abilities allow me to comprehend.  Hopefully it is enough to provide answers to these questions.

The Global Positioning System (GPS) is a network of about 30 satellites orbiting the Earth at an altitude of 20,000 km. The system was originally developed by the US government for military navigation but now anyone with a GPS device can receive the signals the satellites broadcast.

Wherever you are on the planet, at least four GPS satellites are ‘visible’ at any time. Each one transmits information about its position and the current time at regular intervals. These signals are intercepted by your GPS receiver, which calculates how far away each satellite is, based on how long it took for the messages to arrive.

Once it has information on how far away at least three satellites are, your GPS receiver can pinpoint your location using a process called trilateration. 

To be able to apply trilateration, we need to know the exact distance that our GPS receiver is from the orbiting satellites to be able to calculate an accurate position. By applying some basic mathematics, the receiver can work out this distance.

Distance = Speed x Travel Time

1. GPS signals are a radio signal, therefore they travel at the speed of light
2. If we know the time the signal was sent and the time the signal was received, we can work out travel time by subtracting the former from the latter.
3. Now we can multiply travel time by the speed of light and we can determine distance

If we know the distance between the satellite and the receiver for:

• 1 satellite, the receiver’s location is known within a sphere.
• 2 satellites, the receiver’s location is known within 3D ring
• 3 satellites, the receiver’s location is somewhere on at most 2 3D regions
• 4 satellites, the region gets smaller because of the sphere of the new satellite

Using triangulation, the GPS can deliver the location to the receiver with an accuracy of only inches. So why do GPS systems sometimes lead to the wrong locations?

There are three primary possibilities:

(1) The GPS gets its position wrong.  The system is usually accurate but can get confused near tall buildings that reflect the satellite signals.  Also, some people use GPS jammers to prevent being tracked, by generating an interference signal over a five-to-ten-meter radius that disrupts reception of the GPS satellite signal.  These jammers confuse all GPS devices around them.  Powerful jammers can disrupt signals as far away as 32 kilometers.

(2) The GPS’s map is wrong.   Sometimes the database does not contain the desired address, or it includes a duplicate street address in a different location. In other instances, a road segment is not connected within the database to the most logical access route. (Note that the routing logic can get you in trouble. Shortest route can take you on strange detours to save 200 meters.  On the other hand, programs that specify the shortest path in terms of travel time rather than distance may take you by a “technically” longer distance over a road with a higher traffic speed, to ultimately get you to your destination sooner than the most direct path.)

(3): People enter the wrong data.  If you google “sat nav errors” you will find examples of people going to Stanford Bridge, Yorkshire instead of Stanford Bridge football stadium, and a busload of Christmas shoppers going to Lille, Belgium rather than Lille in France.  Most of us have made similar errors, but with less serious outcomes. 

When we are in unfamiliar territory, we have an unfortunate and sometimes irrational tendency to place total faith in our GSP app.  That is unwise, as can be seen at https://www.hotcars.com/drivers-who-blindly-followed-the-gps-and-paid-for-it/.   However, there are things we can do to prevent such things from happening:

• Ensure your GPS Maps are always up to date.  The directions your device compiles are based on digital maps provided by a mapping and navigation company that has partnered with the device manufacturer.  Without the most up to date maps, you may be out of luck.  To be sure you always get from Point A to Point B, update your mapping software often and always bring a road map for back-up.
• If you’re driving an RV (or any large vehicle) make sure you are using a GPS app that will keep you away from bridges that are too narrow and obstacles overhead that are too low. Even then, know the height and width of your vehicle, and as soon as a bridge comes into view slow down and read the clearance details.  Sometimes it seems that GPS devices love to send trucks toward bridges they cannot possibly fit under.  If you trust your GPS app implicitly, you do so at your own peril.  See https://www.standard.co.uk/news/london/satnavs-to-blame-as-lorries-get-stuck-under-bridge-in-wimbledon-a3138696.html. 
• Don’t hesitate to ignore directions that you’re not comfortable with. The worst that can happen is that your GPS app will recalculate and route you a different way.
• Stay alert.  Sometimes signage on the road is a good indication that you are on the wrong route.  Similarly, dangers are often clearly visible and can be avoided if you’re paying attention.

While in Texas, we encountered a somewhat different problem. Whenever there was access from the service road running parallel to the highway, the GPS kept instructing us to stay left and join the highway even though we were already on the highway. Apparently, our GPS map had our location wrong and needed to be recalibrated. There are a variety of websites that give advice on how to calibrate Google Maps on an iPhone or Android, together with other basic fixes.  I followed instructions given at https://www.guidingtech.com/top-ways-to-fix-google-maps-showing-wrong-location-on-android-and-ios/. 

I hope that helps!