GPS speed and position readings are unaffected by mounting position of the main BRAKEBOX unit. The mounting position of the antenna, though, is of utmost importance in ensuring that the highest accuracy is achieved. The GPS unit requires a 3.3V active antenna (supplied) to be connected to the “GPS1 Antenna” connection on the BRAKEBOX, which must be mounted in a position giving a good view of the sky. On top of the vehicle is strongly recommended. Mounting the antenna on the bonnet or the boot of the vehicle may give substandard results due to the reduced number of satellites in view, and in particular the fact that any alteration in vehicle course will change the satellite constellation used, and the results from the BRAKEBOX cannot be warranted under these circumstances.
"WARNING": To avoid any possible damage to the car paintwork, please take care when mounting magnetic GPS antennas. In particular make sure that there are no dust or grit under the antenna. In some cases it maybe required to add additional protection to the paintwork prior to mounting the antenna to avoid damage.
In addition to mounting the antenna on the roof of the vehicle, it should be ensured that the test environment gives a good view of the sky. A clear open space, such as a test track or airfield is ideal. Tall trees or buildings in the vicinity of the test are not ideal, and will degrade accuracy. Any overhead obstruction, such as a bridge, is unacceptable, and will result in invalid test data.
In view of the importance of the antenna mounting position, a full set of guidelines are set out below:
- The antenna must have a clear view of the sky in all directions. Note that it is NOT enough that the antenna can see vertically upwards towards the sky, it must also be able to see all the horizons as well. The GPS system actually gets very little positional or speed information from the satellites directly above, it gets far more information from satellites on, or near, the horizon. For example if the antenna was mounted in the bottom of a "bucket", so it could see upwards but no horizons, then the GPS system would lock and provide positional information - but the accuracy would be very poor. In practice this all means that the antenna must be mounted on the highest point on the vehicle.
- The antenna must be mounted on a horizontal surface. The antenna must be mounted on a horizontal orientation facing directly up. The underside of the antenna cannot receive GPS information, similarly don't mount the antenna on a vertical surface.
- The antenna must not be covered in tape, in particular dark coloured tapes. Many tapes absorb the weak GPS radio signal. In general black tapes are the worst in this respect as they contain high amounts of carbon - however to be safe, avoid using any tapes.
- The antenna must not be subjected to high levels of vibration. Although the antenna is physically robust to vibration, it can and does effect GPS reception, so isolate it as much as possible.
- The antenna must be physically remote from sources of electrical noise. The GPS radio signal is very weak and can easily be blocked out by radio interference, so to get a good signal the antenna must be as far away from radio interference as possible. By far the biggest source of radio interference is a petrol engine's ignition system, so keep the antenna away from all aspects of it including the engine management system, coil, leads, distributor etc.
- Avoid trapping or pinching or kinking the antenna cable. The lead from the GPS antenna to the receiver is a special very high frequency cable and it is not normally practical to repair it - so if you do trap, pinch or cut it then the antenna will have to be replaced, and this isn't covered by the guarantee! - so don't try and fit it into a shut gap that is too small or compress it with a door seal etc.
- If at all possible, mount the antenna on a metal platform (mounting on the roof of the car satisfies this criterion). The GPS radio signal is amplified if the antenna is mounted on a metal plate (termed a ground plane), and the bigger the better. It is not essential for correct operation, but it is desirable.
Care must be taken not to crush the antenna lead with the vehicle window or door closure; if it is accidentally crushed then the cable may be permanently damaged even if there are no physical marks on the cable. Replacements are available from Race Technology at relatively low cost should they be required. Note that the “GPS2 Antenna” connection is unused on the BRAKEBOX, and does not need to be connected.
Figure 1. Correct GPS antenna mounting position on the
roof of the car giving full horizon to horizon visibility