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GPS Dead Reckoning

All our car computers are available with internal Ublox-6 GPS modules with dead reckoning functionality.  This page describes what this is, how it works, and discusses installation considerations.

Dead reckoning, in the context of GPS navigation, is the process of using data from sensors, such as the vehicle odometer and a gyroscope, to continue to estimate the vehicle’s current position in the event that GPS data is either reduced in quality or no longer available.  This is especially useful when driving through tunnels, underground car parks, multi-level roads and overpasses, dense forests, or around tall buildings (urban canyons).  It can also provide resistance against GPS jamming, although the Ublox 6 GPS chipset does have anti-jamming technology built-in.  

Car PCs that are equipped with GPS dead reckoning (sometimes abbreviated to GPS DR) include the following in addition to the GPS receiver:

  • an internal gyroscope, to provide heading information
  • an input for the vehicle speed pulse, to provide speed information
  • an input for a forward/reverse signal line, to provide direction information

Under normal conditions the car’s location is determined based on a weighted average of GPS and sensor data, resulting in a more accurate estimated position than if either GPS or sensor data were used on their own.   While GPS data is available measurements from the vehicle sensors and gyro are constantly calibrated.  When GPS data is poor quality or non-existent, the module continues to provide highly accurate location data using information from the vehicle sensors and gyro.

Drive Tests

In order to demonstrate the effectiveness of the dead reckoning solution we disconnected the GPS antenna from our demo car’s PC while exiting the M48 motorway near Bristol, forcing the system to rely solely on dead reckoning, and then drove to our (now old) office 2.6 miles away.  Given that the route between the M48 and our office contains a roundabout, a B-road with multiple bends and a varying speed limit, and several junctions thereafter, we felt that this represented a tougher challenge for the dead reckoning navigation system than simply driving through a tunnel, which would typically be relatively straight and driven through at constant speed.  The results are shown below:

Dead reckoning allows navigation to continue even with no GPS signal

To summarise, having driven 2.6 miles using dead reckoning alone, without any GPS signal, the navigation system placed our demo car within less than 10 metres of its actual location, an accuracy of over 99% (although only achieved on a single drive test and therefore not representative - please see footnote* below).  By contrast, if the PC had not been equipped with dead reckoning functionality when entering the virtual tunnel then the position would have been out by 2.6 miles (0% accuracy). 

However, dead reckoning is not only useful in situations of total GPS degradation.  The image below, reproduced by kind permission of u-blox, shows how the performance of a GPS DR system is greatly superior to a GPS-only system in an urban canyon environment in Chicago.

Dead reckoning significantly improves navigation when the GPS signal is degraded

Map Matching

The above pictures show the precise locations of the car, according to the GPS and/or dead reckoning system, regardless of whether those locations are on a road.  However, navigation programs, including PC Navigator, carry out a technique called map matching which “snaps” the vehicle’s position indicator to the nearest road.  A combination of GPS, dead reckoning and map matching will result in the best navigation performance.

Installation Considerations

When installing PCs with dead reckoning functionality it is normally necessary to mount the PC horizontally, otherwise the internal gyroscope will not function correctly.  By default it is assumed that the PC will be mounted base-down, but if mounted base-up (e.g. mounted upside down to the underside of a panel) it will be necessary to apply a configuration change using preinstalled software.

If the PC needs to be mounted vertically then please contact us prior to ordering, as in many models we can mount the internal hardware accordingly.  Please note that it is only the vertical orientation that is important – left/right/forward/rear orientation does not matter, as the internal gyroscope is automatically calibrated when in use.

Two external inputs from the vehicle are necessary for the dead reckoning system to function, which connect to a DB9 plug on the outside of the PC (a break-out cable is supplied):

  • Speed pulse – either directly from the vehicle or via a CAN adapter (see below).  The speed pulse input is automatically calibrated when in use.
  • Forward/reverse signal lead.  By default this should be high (+12V) only when the vehicle is in reverse, but the behaviour can be reversed using preinstalled software.  The forward/reverse signal lead can either be taken from an existing source (e.g. reversing light feed) or from the CAN adapter detailed below.

Many modern vehicles do not include dedicated speed pulse lines, so we offer an external CAN adapter to provide this.  The CAN adapter is compatible with over 40 vehicle manufacturers covering over 300 different models, including SAE J1939 commercial vehicles, and automatically detects the vehicle type.  The CAN adapter requires its own 12V power source, but the PC can be specified with power pins for this purpose included in the dead reckoning DB9 plug.  Therefore the only cables needing to be connected from the external CAN adapter to the vehicle will be CAN-high and CAN-low.


* Although we achieved a very high accuracy over a 2.6 mile route, this was on a single drive test, and we do not mean to imply that the same level of accuracy will be achieved on every occasion, even if following the same route.  Numerous factors will affect the accuracy of the GPS DR system, including driving style, road smoothness and consistency of the vehicle speed pulse.  We suggest that the accuracy we obtained on this drive test should be taken as approaching best-case performance.