Fast GPS reporting with TDMA timeslots

Fast GPS reporting with TDMA timeslots

RavTrack is very fast at reporting the positions of even large fleets over a single radio channel because each RavTrack transponder is assigned a specific time slot in which to transmit, avoiding interference which may occur if multiple devices were permitted to transmit simultaneously. The fact that all transponders share a “common clock” via the GPS satellite signals allows us to assign a unique timeslot to each device, yet maintain timing coordination over a large fleet of devices.

RavTrack timeslots are built on 50 millisecond increments by factory default, athough 10msec granularity can be achieved with newer firmware versions. For the purposes of illustration we will assume each time slot must be a multiple of 50msec. The specific size of your time slot is typically determined by the bandwidth/transmission rate of your particular transponder and the presence or absence of a repeater.

Most FCC licenses granted in the USA are for narrowband (12.5KHz) channel spacing. The associated RavTrack transponders operate quite well at 4800 baud transmission rate (factory default), although slower rates can be used. These transponders can complete a position report transmission in about 64msec, so a 100msec timeslot would be the factory default when no repeater is in the system. If your license permits wideband (25KHz) channel spacing, and your transponders are capable of wideband operation, the factory default transmission rate is 9600 baud. In this instance a position transmission can be completed in about 32msec, so a 50msec timeslot is typically used when no repeater is in the system.

If your system uses a store-and-forward repeater you need to make each timeslot longer so that once a vehicle reports, the repeater has sufficient time to receive, process, and repeat the transmission on “quiet air” before the next vehicle transmission occurs. In a 9600 baud system a 100msec timeslot may be used if you are not encrypting your transmissions, but with encryption a 150msec time slot should be used, as the repeater needs a bit more time to process an encrypted message than an unencrypted message. In a 4800 baud system using a repeater a 200msec timeslot may be used whether or not you are encrypting the position transmissions.

Timeslots are numbered starting at zero, and the zero time slot is reserved by the system. Thus you can start numbering your transponder time slots at slot 1 (0001). In a system using 100msec timeslots the first second is completed once time slots 0 through 9 are used (10 time slots total). For this reason, up to 9 vehicle transmissions can be completed in the first second, and in this example your fleet size would be limited to a total of 9 vehicles if you need the entire fleet to report each second. As the reserved zero time slot only occurs at the start of any particular TDMA cycle, cycles longer than 1 second would allow the addition of 10 more time 100msec slots for each second added to the cycle. Thus a fleet using 100msec time slots can provide reports from 19 vehicles every 2 seconds, 29 vehicles every 3 seconds, and so forth. Similar logic applies to other RavTrack timing schemes.

If your fleet is quite large and you want faster updates than the 100msec timeslot scheme allows, you can double your fleet size if local regulations allow you to use wideband transmissions that yield a 50msec time slot.  In many deployemnets a fleet can scale much larger and still preserve fast report cycles simply by using multiple frequencies.  As transponders on different frequencies will not interfere with one another even if transmitting simultaneously, a properly architected system using five frequencies can report five times faster than the same system using only one frequency.

When setting up your fleet timing it is a good idea to leave a little extra capacity in your timing scheme to allow the easy addition of new vehicles to the fleet.

Finally, if you want to use the RavTrack transponders to transmit not only position data, but extra data as well (e.g. from an on-board telemetry device), you will need longer time slots to send this extra data. Please contact us in this regard, and we are happy to help you in architecting a solid system.

For a more technical programming perspective on TDMA time slots as used by RavTrack see the following articles: 

or consult your technical manual.