Mounting and connecting the GPS antenna
"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.
The correct mounting of the GPS antenna is of utmost importance in ensuring that the highest accuracy is achieved.
The GPS unit requires a 3.3V active antenna (supplied) to be fitted to the “GPS1 Antenna” connection on the SPEEDBOX20. This must be mounted in a position that gives a good view of the sky. It is STRONGLY recommend that you mount it on the roof of the vehicle. 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. This is particularly important due to the fact that any alteration in vehicle course will change the satellite constellation used.
Figure 3: Correct GPS antenna mounting position on the roof of the car giving full horizon to horizon visibility
In view of the importance of the antenna mounting position, a full set of guidelines is set out below:
• The antenna must have a clear view of the sky in all directions. Note: It is NOT enough that the antenna can see vertically upwards - it must also be able to see the entire horizon as well. The GPS system actually gets very little positional or speed information from the satellites directly above - far more information is received from satellites that are on, or near, the horizon. If for example, the antenna was mounted in the bottom of a "bucket" (so that it could see upwards but could not see the horizon) then the GPS system would lock and provide positional information - but the accuracy would be very poor. In practice, this means that the antenna must be mounted on the highest point of the vehicle.
• The antenna must be mounted on a horizontal surface. It must also be mounted in a horizontal orientation facing directly upwards, as the underside of the antenna cannot receive GPS information. Similarly, do not mount the antenna on a vertical surface.
• The antenna must not be covered in tape. In particular, dark coloured tapes should not be used. 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. To be on the safe side, avoid using any tapes whatsoever.
• The antenna must not be subjected to high levels of vibration. Although the antenna is physically robust, vibration can and does affect GPS reception. To prevent this, isolate the antenna from vibration as much as possible.
• The antenna must be physically remote from sources of electrical noise, as the GPS radio signal is very weak and can easily be blocked by radio interference. To obtain a good signal, the antenna must be as far away from any source of radio interference as possible. By far the biggest source of radio interference is a gasoline engine's ignition system, so keep the antenna away from all aspects of it. Things to avoid include the engine management system, coils, high tension leads and ignition control modules.
• Avoid trapping, 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. If you do trap, pinch or cut it, then the antenna will have to be replaced. This is not covered under warranty. To avoid cable damage, do not attempt to fit the antenna cable into a shut gap that is too small, or compress it with a door or window seal.
• If at all possible, mount the antenna on a metal substrate (mounting on the roof of the car is ideal). The GPS radio signal is amplified if the antenna is mounted on a metal plate (termed a ground plane), and the bigger this is, the better. It is not essential for correct operation, but it is highly 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 – this may be the case even if no physical marks are visible. Replacements are available from Race Technology at a relatively low cost should they be required.
Note: the “GPS2 Antenna” connection is unused on the standard SPEEDBOX20, and does not need to be connected.
Race Technology currently supplies three different types of antenna:
• Standard (as supplied)
• High sensitivity
• Interference rejecting.
The standard antenna works well in almost all cases. The high sensitivity antenna can improve signal reception under some circumstances. The interference rejecting antenna should be used where there is a known RF interference problem - they give excellent interference rejection, but provide a slightly “noisier” GPS signal.
Mounting and connecting the GPS antennas – RTK option
There are two options for the RTK antenna. You can either use two separate GPS antennas, or the RTK antenna strip. For test work, the RTK antenna strip is typically the most convenient solution. It allows the antennas to be quickly placed at a known fixed baseline, with minimal risk of any installation errors. For permanent applications, it may be desirable to use two separate GPS antennas. This allows for maximum flexibility. Note: when fitting two standard antennas, it is particularly important to check that the installation guidelines have been followed – incorrect installation of the antennas will result in either no RTK lock at all, or intermittent and possibly incorrect RTK lock.
All of the points already listed regarding mounting the GPS antenna for the standard SPEEDBOX20 should be followed when mounting the antennas for the RTK unit. In addition, please bear in mind the following:
• It is doubly important for the RTK option, that the antennas must be mounted on the flattest part of the roof of the vehicle. Both antennas must see the same set of satellites for the RTK solution to work correctly.
• The rear antenna is used as the base antenna, and is connected to “GPS 1”. This is the antenna that is also used for the speed and positional calculations. The SPEEDBOX20 will not get a lock unless this antenna is connected.
• The front antenna is called the “moving” antenna, and is connected to “GPS 2”. If this antenna is not connected, the SPEEDBOX-RTK will continue to function as a standard SPEEDBOX20 as long as the base antenna is connected. It will not obtain an RTK lock without the moving antenna being connected.
• The antennas must both be mounted on the centreline of the vehicle for the yaw and slip measurements to be valid.
• All of our GPS antenna products are magnetic mounting – if the surface of the vehicle roof is not magnetic then alternative fixing arrangements will have to be made. Lock tape works well.
RTK antenna strip
The magnetic mounting antenna strip is a flexible magnetic strip. It contains two antennas that are mounted in the correct orientation and at a fixed and known distance from each other.
• The baseline of the antenna strip is a fixed 80cm, so the SPEEDBOX-RTK must be configured to use this baseline using the configuration software supplied. If the unit is not configured to this baseline, it will be unable to obtain a lock. (This configuration is the default setting for new units when they are supplied with the antenna strip).
• The antenna strip must be mounted in the correct orientation. There is a large arrow printed on it which must face towards the front of the vehicle. The wires from the two antennas are labelled “GPS 1” and “GPS 2”, and should be attached to the corresponding connections on the SPEEDBOX20.
• Please treat the cables with care. If they are damaged, then it is likely that the whole assembly will have to be replaced.
Figure 4: Mounting arrangement for the magnetic antenna strip (RTK option only)
Using two separate GPS antennas for RTK measurements
In some circumstances it may be impossible to use the magnetic strip - if for an example an obstruction exists between the front and rear antenna locations; if the available baseline is too short; if a longer baseline than 80cm is required; or if the use of antennas that are different to those on the strip is desired. In these circumstances, two separate antennas are used to provide the GPS signal to the RTK unit. As long as they are set up correctly, the results will be just as good as when using the magnetic strip. If, however, they are set up incorrectly, then the RTK solution will fail.
It is vitally important that both of the antennas used are of the same type. Using two different types of antenna may cause the RTK solution to fail.
Figure 5: Mounting arrangement for two separate antennas (RTK option only)
The general arrangement for mounting two antennas for the RTK unit is shown in Figure 5 above. Both antennas should be mounted on the roof of the vehicle, on the vehicle centreline. The rear antenna is the “base” antenna, and must be connected to “GPS 1” on the unit. The forward antenna is the “moving” antenna, and must be connected to “GPS 2” on the unit. If the antennas are connected the wrong way around, the unit will not obtain an RTK solution when the vehicle is moving. Both antennas should be mounted in the same orientation; i.e. the cable should emerge from the same side of both antennas to minimise the effect of antenna “phase centre”. It is also absolutely essential that the antennas are of the same type, model and manufacturer.