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02 - VB3i GPS Antenna Placement

Single Antenna Systems

The GPS antenna supplied with the VBOX is a 5V active antenna. For the best possible signal quality, it is important to maintain a clean connection between the antenna and the VBOX. Before fixing the antenna to the VBOX, ensure that there are no dust particles in either connector. Replacement antennae are available by contacting your VBOX distributor.

 

The antenna is a magnetic mounting type for quick and simple mounting to the vehicle roof. For optimum GPS signal reception, make sure that the antenna is fitted to the highest point of the vehicle away from any obstructions that may block satellite reception. The GPS antenna works best with a metal ground plane underneath (a metallic vehicle roof is perfect for this).

 

Please also note that when using any GPS equipment, a clear sky view is important. Objects in the surrounding area such as tall buildings or trees can block the GPS signal causing a reduction in the number of satellites being tracked, or introducing reflected signals that can decrease the accuracy of the system. Note that clouds and other atmospheric conditions do not affect the VBOX’s performance.

 

GPS contact.jpg

 

GPS antennas require a ground plane to operate correctly. This helps to reduce unwanted reflections of the GPS signal caused by nearby objects, and usually the metal roof of a vehicle performs this function. However, if a test requires an antenna to be placed either off the vehicle, or on a vehicle that does not have a metallic roof, a special ground plane antenna must be used. This has an internal ground plane and can operate perfectly without the need for mounting on a metal surface. Ground plane antennas are available from your VBOX distributor.

Satellite Elevation Mask

This feature can be used to improve GPS signal quality when nearby obstacles like trees and building are reflecting or temporarily obscuring the signal from satellites at low elevation. Raising the mask will cause the GPS engine to ignore satellites below the mask angle, so must be used carefully as it also reduces the total number of received satellites. 

The elevation mask angle can be changed in VBOX Tools Setup, VBOX Setup and by using VBOX Manager.

 

Sat_elevation_mask.png

Dual Antenna Systems

For further info on slip/pitch and slip/roll setups, click here.

 

When testing using dual antenna mode, the greater the antenna separation, the greater the accuracy of the dual antenna derived data channels.

Slip Angle Accuracy

Pitch/Roll Angle Accuracy

<0.2° rms at 0.5 m antenna separation
<0.1° rms at 1.0 m antenna separation
<0.067° rms at 1.5 m antenna separation
<0.05° rms at 2.0 m antenna separation
<0.04° rms at 2.5 m antenna separation

<0.14° rms at 0.5 m antenna separation
<0.07° rms at 1.0 m antenna separation
<0.047° rms at 1.5 m antenna separation
<0.035° rms at 2.0 m antenna separation
<0.028° rms at 2.5 m antenna separation

Some vehicle roofs limit the potential separation value. In this case a roof mount (RLACS171) can be utilised to increase separation.

 

Antennas should be positioned so that the gold antenna connector of primary and secondary antennas (A+B) are pointing in the same direction. This matching positioning ensures that the separation measurement is relative. 

10.png

 

We recommend you measure separation from outer edge of antenna connector A, to same outer edge of antenna connector B.  Note that accurate entry of antenna separation is essential for dual antenna operation.
 

_MGL0450_OSS.jpg

 

  • In pitch alignment, the primary antenna (ANT A) should be placed towards the rear of the vehicle, and the reference antenna (ANT B) placed at the front.
  • When in roll alignment, the primary antenna (ANT A) should be placed to the left of the vehicle, and the reference antenna (ANT B) placed to the right.

 

When mounting the antennas directly to the vehicle roof, ensure that the antenna placement still follows the guidance of the single antenna above (i.e. clear ground-plane, away from obstruction).

The two antennas must be on a similar plane!  If there is a degree of separation greater than 10°, the system will not obtain dual antenna lock.

GPS Coldstart

A GPS coldstart may be required when:

  • The GPS engine has locked up.
  • VBOX is struggling to acquire satellite lock

 

A GPS coldstart forces the GPS engine to reset its downloaded almanac of current satellite positions. This can be useful if the VBOX3i is having trouble locking onto satellites, which typically occurs if the VBOX3i has not been used for several weeks or if it was last used a long distance (over one thousand miles) away from the current location.

 

After performing a GPS coldstart leave the VBOX3i powered up in a static location where the antenna has an unobstructed view of the sky until the ‘GPS’ LED becomes solid green.

 

Once the VBOX3i has downloaded the new almanac it will reacquire satellites in noisy situations (such as near trees, buildings and under bridges) much quicker. It will also acquire satellite much quicker on power-up.

 

To perform a GPS coldstart on the VBOX3i perform the following actions:

 

  • Press and hold the ‘LOG’ button on the front of the VBOX3i for five seconds until a long beep is sounded.
  • When the button is released the ‘GPS’ LED will start to flash red showing that the coldstart has been performed and the GPS engine is now not locked onto any satellites.
  • After approximately 30-45 seconds the ‘GPS’ LED will start to flash green indicating that satellite lock has been achieved and indicating the number of satellites that it is locked onto.

 

Important notes

When a GPS coldstart is carried out, please note that this reset the DGPS and RTK mode to 'none'. Also, if using a dual antenna system, the VBOX 3iSL will reset dual antenna separation to default settings (1 m). 

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