Fleet scalability and data efficiency guidelines
RavTrack GPS tracking transponders are built for different frequencies and channel spacing. While the frequency does not affect the total bandwidth available, channel spacing does. Typically radio channels are allocated either narrowband (12.5kHz between channels) or wideband (25kHz between channels). The wideband channel provides twice the bandwidth as a narrowband channel. Bandwidth refers to the ability of a device to transmit a certain number of data bytes within a prescribed time.
| Field | Usage | Comments |
| 1 | $PRAVE | Raveon Proprietary Header |
| 2 | Form ID | The ID of the transponder that transmitted its position over the air. It is a decimal number, 0-9999 when used with Raveon M7 series of GPS transponders. It may be 1-8 decimal digits with other GPS transponders. |
| 3 | To ID | The ID that this position report was sent to. It is a decimal number, 0-9999 when used with Raveon M7 series of GPS transponders. It may be 1-8 decimal digits with other GPS transponders. |
| 4 | Latitude | dddmm.mmm format. It is signed. + is north, – is south. Empty sign means north. Note: typically there are 4 decimanl places, but as few as 0 are possible. Null field if no GPS lock. |
| 5 | Longitude | dddmm.mmm format. It is signed. + is east, – is west. Empty sign means east. Note: typically there are 4 decimal places, but as few as 0 are possible. Null field if no GPS lock. |
| 6 | UTC Time | hhmmss format. The UTC time at the time the transmission was made. Null field if no GPS lock. |
| 7 | GPS Status | 0=not a valid position. 1=GPS locked and in a valid position. 2=differential or WAAS fix. |
| 8 | Num. Satellites | The number of satellites tracked. |
| 9 | Altitude | The altitude in meters. Null field if no GPS lock. |
| 10 | Temperature | The internal temperature of the RV-M7 in degrees C. Typically this is 5-20 degrees above ambient. |
| 11 | Voltage | Input voltage to the device that sent this position. |
| 12 | IO Status | A decimal number representing the binary inputs. |
| 13 | RSSI | The signal-strength of this message as measured by the receiver in dBm. Note, if the message went through a repeater, it is the signal lever of the repeated message. |
| 14 | Speed | The speed of the device in km/h. 0-255 |
| 15 | Heading | The heading of the device. 0-360 degrees. |
| 16 | Status | Status flags received from the device. Not all products support generating all status flag codes. NULL: no alerts P: Proximity alert M: Man-down alert A: General alert C: Critical alert I: Impact alert V: Vibration S: Service required on product X: Gas fume sensor |
| 17 | Spare | A spare field, typically NULL. |
| 18 (optional Field) |
Optional Odometer | The odometer reading if this option is available. It is in kilometers and may or may not have decimal places. Most reported values typically have are one decimal place. NULL/empty or no field if reading is not available or transponder did not send it. Firmware version D1 or higher of the M7 supports this. |
| * | The * NMEA end of message identifier. | |
| Checksum | The NMEA 0183 checksum |
RavTrack uses a Time Division Multiple Access (TMDA) methodology to optimize the efficiency of any radio frequency used in a tracking system. Using the common clock signal from the GPS satellite constellation each device is given a precise time for it to transmit. In a RavTrack system using a wideband channel one vehicle or device can transmit a full GPS message in 50 msec (called a time slot). While one would expect that 20 vehicles could transmit in 1 full second, the first timeslot (slot # 0) is reserved. Thus 19 devices can transmit in the first second, 20 more in each following second, until you complete a reporting cycle and start over at the first vehicle. Using a narrowband channel, a GPS message will transmit in about 80msec. While the math is not exact this is sufficient for 11 vehicles or devices to transmit in the first second, and 12 in each subsequent second.
The table on the right shows the information that a the Raveon GPS transponder can transmit in a GPS message. The output of this information at the receiving station depends upon the configuration applied by the user.
If you notice field 12, there is some additional input information that can be included at the election of the user. These are discrete (binary on/off) input conditions signaled by voltage from an external device. This might be something like the headlights of a vehicle are on as an example. These conditions are monitored by the device by measuring the voltage on an “extra” RS232 pin. In the IP65 enclosed transponders there are two of these extra pins, in the standard transponders housing there are three extra pins. For instance, a customer tracking school buses provides 3 digital statuses from each bus: 1) flasher lights on/off, 2) stop arm up/down, and 3) passenger door open/closed.
The RavTrack PC software can be programmed by the user to interpret each status and show the condition in English such as “Bus 29. Flasher lights on”. These events can also be logged in the RavTrack PC activity log, and summarized for review at a later time.
However, what if the user wants some additional information from each vehicle? Examples can be things like oil pressure, engine RPM, or engine temperature. As these are not simple on/off conditions but rather a range of values (analog) they must be converted by a suitable external device into digital values that can be submitted by RS232, transmitted, and received at the base. While the RavTrackPC base software may not know how to process these specific values, they can be handed off to another application for logging or processing. The GPS transponders are perfectly capable of sending and receiving additional data, but it will of course take more transmission time. For instance, if you send 40 “extra” bytes of data, this will take about 50msec more time on a wideband channel, or nearly 100msec more time on a narrowband channel. In this case, each time slot would have to be longer, such as 100 msec total in the wideband example. This would give you 9 vehicle reports in the first second, and 10 more for each additional second of the reporting cycle (for details see this tech note).
The more extra data you want to send the less vehicle reports you will receive within any given time frame when using a single frequency. However, the RavTrack system is very flexible and you can expand to more vehicles and/or more data, and keep your total cycle time short, by adding additional frequencies. Divide your fleet or field of devices into two more groups and assign each group to use a different frequency, ensuring you have a suitable base station receiver allocated for each frequency. For instance in our school bus example, the customer, using just 2 narrowband frequencies, is tracking over 1,000 buses with 45 second updates from the entire fleet.
The reader should bear in mind that total transmission time will extend should the particular tracking system use one or more system repeaters. In any case, whatever your particular requirements, feel free to contact us and we can help you specify the optimal timing for your particular scenario.
