AVL – Data Communications

9 Nov 2010

Passive Vs. Active GPS Tracking

There are many variables to take into consideration in the feature sets and cost/benefit equation to meet a customer’s needs – many of which change over time.  Where a “passive”, data-logging system (gather and store data for later communication) will fit the need for some or many units in a fleet, there will be a number of units that require “active” real-time or near real-time wireless communications – of location data or text messaging or telemetry data. A “real-time” unit means there is close to instantaneous – usually measured in under a few (<10) seconds communication of data, where “near real time” represents a delay of 30 seconds up to several minutes, before the data is transmitted and received.

Passive Data Collectors

As mentioned above, some systems are passive data collectors that store the GPS and Telemetry data, and then require a physical connection (typically via a cable) to communicate the data, to a unit such as a handheld PDA. While typically the most inexpensive, this approach has broad operational issues to manage. These include, who is responsible to upload, how do they do it, using what hardware, owned and maintained by whom, and when is the data to be periodically uploaded. Additionally, the collection of the uploaded data from the PDAs and added into a single database from many different physical locations would be a very difficult logistics function given many fleet users implementation needs – many vehicles, many different users, units don’t park at common locations etc.

Wireless Data Transmission

There are quite a number of different wireless communication methods, all requiring varying levels of infrastructure, from relatively lower cost, VHF and UHF RF (radio frequency) to cellular tower or satellite based systems.

Radio Frequency (RF) – Simplex, Repeater or Trunking Radio Systems

Many organizations already have and use private or other type RF systems.  Examples of this include public safety applications such as police and fire organizations, municipal, schools and other two-way radio users. Many of these systems can also be used to transmit data by connecting to an installed radio that has a data input capability. A data unit is then connected to a base radio to separate and collect the data transmissions and forward them to a computer for use by the customer’s application programs. If a customer has already invested in the RF infrastructure (antennas, towers, base stations, etc) and has the appropriate radio equipment this can provide a cost effective communications method for invehicle GPS, Telemetry and text data transfer.

RF System Implementation

There are significant issues related to designing, system selection implementation and operation of an RF data communications system. These include, reliable signal coverage, available bandwidth, are the frequencies to be utilized licensed or un-licensed – as this can have major implications on reliability of data communications, especially if multiple users have access to the same frequency, antenna space leasing and management, availability of antenna space in the desired geographic areas, among others.  By way of example, many government entities, utilize private radio systems. Quite often these are in the 400 to 800 Mhz RF spectrums. It is very common for a government to have to invest many millions of dollars to implement a reliable, private radio system that provides reliable coverage to the majority of the land/road areas in an area – let alone “full” coverage.

Challenges & Issues Relating To An RF System

A key element to RF systems is specific but limited geographic coverage.  While this is usually not an issue, this type of system couldn’t track fleets out of the defined area. As a result, there would be no tracking for units used not “in area”. Especially important is the availability of sufficient bandwidth to allow for reliable data communications. The amount of bandwidth primarily controls how much data can be transmitted per second. The impact of this is not evident in small installations of a few to a hundred or so vehicles, but when there are several hundred to a thousand or more individual units vying for “space” to communicate data on a limited RF frequency bandwidth, the number of failed transmissions rise dramatically.  In a requirement where a majority (all) of the units will be transmitting data in a compressed period of time – such as several hundred school buses, from 6:30AM to 9:20 AM and again a bandwidth peak demand from 2:30 PM to 5 PM, the requirement for a highly effective, very reliable system is paramount – otherwise there will be many failed transmissions and the basic data will not be available.  By way of example, a non-data specific RF implementation, where data is transmitted by some units and voice is transmitted by others, it is common for a single data transmission to take between – second and up to 2 seconds, depending on several variables. It is simple math to determine that if 400 to 600 vehicles were attempting to communicate only 20-40 times per route, based on particular ‘events” such as crossing a route geo-boundary, or a door switch opening with approximately 820 routes per AM & 820 per PM shifts, a standard RF frequency bandwidth (one or even several 12.5 kHz channels) would be overwhelmed and few of the transmissions would get through. And, very importantly, there would be no bandwidth available for the voice users. If an interval transmission method were used, where a unit transmitted a set number of seconds – e.g. every 10 seconds for example, and there were approx. 1640 routes per day, this equates to approximately 300,000 GPS message transmissions per day if the average route were 30 minutes in duration – 200,000 with 20 minute average routes ( our calculations indicate an overall average of 21.1 minutes per route). This would equate to an average of over 9 messages per second, and with overhead and message collision, the effective message per second rate the system would be required to process, is likely to be over 30+ per second.  Even a private system dedicated to data only will be required to be extremely robust with considerable infrastructure to accomplish a short interval reporting frequency. Reliable communications of data over RF is subject to appropriate bandwidth availability and infrastructure.

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