Multiple Base Receivers vs Repeaters in a RavTrack System

Often times deploying one base receiver with a highly mounted omni-directional antenna will provide adequate coverage for a RavTrack vehicle tracking station. .  However, in some circumstances, especially as the area of operation increases in size, this is not sufficient to cover the entire tracked area. Additionally other factors may compel you to install multiple base receivers, or one or two system repeaters, with antennas located at distributed points around your tracked area..

First, before you determine that one base receiver location is not sufficient, consider your area for tracking.  RavTrack radio coverage can roughly be compared to a 5 watt  UHF or VHF radio system.  The Ravtrack system coverage is typically similar to the voice system coverage within the same frequency band, although not necessarily identical.

Working from your primary base station try mounting the base station antenna as high as possible to cover the desired territory.  In some cases this may mean moving the antenna away from the control room to achieve the best coverage.  A skilled RF technician familiar with your area can probably determine a good antenna location, sometimes by performing a site survey, or simply by experience.

Be careful to minimize the length of antenna cable that links the antenna to the receiving modem, and by all means avoid looping or coiling the cable.  To do this may cause you to relocated the receiving modem from nearby the tracking PC.  RS232 serial communication links have strict distance limitations.  However, if you can  use Ethernet/IP communications between the base modem and the computer then you can use a ‘terminal server’ (AKA telnet server, serial-IP convertor) and install the base receiving modem onto the Ethernet/IP backbone.  In this configuration the GPS messages received by the base station are carried across the network from the base receiving modem to the tracking PC as a telnet type of IP service..

Using an IP backbone is a common approach in distributing several base receiving stations around your area to provide thorough radio coverage of the area in question..  You will need an Ethernet/IP backbone with points of presence at all base receiving sites to collectively link all these to your tracking PC.  It is best to check with your I.T. department and allow them to use certain network hardware that they prefer.  Please note that the Ethernet/IP backbone does not necessarily need to be wired (copper or fiber) as a wireless backbone may be suitable as well.  There are a number of topologies available.

Ensure your tracking software can acquire the telnet traffic when using this approach, and in some cases, if the PC cannot ascribe to the telnet service directly you may use another telnet server at the PC location to interface with the IP backbone and reconvert the GPS messages back to their original serial output state. Also ensure that you can ascribe to multiple message feeds as you will have a message feed coming from each base receiver station.  Raveon’s RavTrack PC tracking software supports up to 6 base station modems either through direct connections or telnet, and nearly unlimited base receiver connections by utilizing the Raveon CiGorn gateway.

For much more detail on using terminal servers see application note 140 “Using an Terminal Server to expand your Ccverage area” at the following link.

http://ravtrack.com/pdf_appnotes/AN140(RavTrackTermServ).pdf

If this approach is not practical, which can especially be the case if no IP backbone is available,  you may consider using, one or more repeaters..  For a RavTrack system, the same model of transponder that you use in your vehicles, or as a base station, can readly be programmed as a store-and-forward repeater.  As a repeater may transmit a lot, the only physical alternation you may need to make as a transponder is to increase the cooling efficiency of the transponder perhaps by adding a heat sink or CPU cooling fan atop the case. Remember that a system repeater, just as a base receiver, will need power, and repeaters will draw more power than receivers as they transmit quite frequenctly.  Try to optimize the height of the antenna above the ground in considering repeater locations.  For more information on antennas see the following tech blog “Antennas for a RavTrack vehicle tracking system”:

http://ravtrack.com/GPStracking/2009/antennas-for-a-ravtrack-vehicle-tracking-system/

A RavTrack system repeater will repeat all the GPS messages it receives back out, to be received elsewhere within its area of coverage, so you will always need at least one transponder configured as a base receiver.  It is possible to configure a transponder to simultaneously act as a receiver and repeater.  This is sometimes done to support a second tracking computer  physically located away from a primary computer.   Remember as well that in- vehicle transponders will transmit their own position information will receiving similar information from from other fleet members within radio range, a feature fairly unique to RavTrack.  If you will install tracking displays inside your vehicles to take advantage of this feature, the vehicle-to-vehicle tracking range will benefit from a system repeater even if only one base receiver is used, so you may with to configure your receiver as a repeater as well.

Please consider that a store-and-forward repeater first receives GPS messages very briefly and then repeats out that same transmission.  So any RavTrack repeater will take time to receive then repeat.  Because of this the total duration of a transmission should be extended by lengthening the duration of each transmission slot in the system, which reserves TDMA system time for each repeated message.   System timing parameters and overall system performance should be considered.  Deploying multiple repeaters can further impact system-wide timing, so be judicious in their deployment.   For more information on system timing with and without repeaters please refer to the tech blog  “TDMA Time Slots”:

http://ravtrack.com/GPStracking/2009/tdma-time-slots/

If you believe your scenario will require you to install either multiple base receiver stations or one or more system repeaters we encourage you to contact us so we can help you work towards the best approach for your particular system.