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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | Re: [gislist] realtime vehicle tracking |
| Date: |
07/05/2005 08:00:02 AM |
| From: |
Bill Diaz |
|
|
Ash,
See below:
>-----Original Message-----
>From: gislist-bounces@lists.geocomm.com
>[mailto:gislist-bounces@lists.geocomm.com] On Behalf Of
>Simmonds, Ashley (DTEI)
>Sent: Tuesday, July 05, 2005 03:58
>To: gislist@lists.thinkburst.com
>Subject: [gislist] realtime vehicle tracking
>
>sure this has been discussed before. could someone point some
>fingers at
>where some step-by-steps in this arena might be located...
>
>i've been researching APRS a bit, and am already good with GIS
>and stuff,
>but i need to get a good handle on how to transmit a GPS
>signal from a fleet
>of cars, and recieve that signal into my application.
Step by step information for commercial AVL system design is hard to come
by, since many companies use proprietary systems and methods. Methods of
transmitting and receiving a signal from a fleet can vary considerably. An
APRS like system may not be suitable for commerical application for a
variety of reasons.
APRS (Amateur Position Reporting System) is used by Radio Amateurs
World-Wide to report postions of various mobile and fixed assets. However,
it has some serious drawbacks. Position reports do not require or expect an
ack. There is no way of knowing if the position information reached the
desired destination. Often, a position report collides with a postion
report from another station, and both position reports are lost since there
is no retry mechanism for position reports.
Commercial AVL systems generally have robust protocols to ensure information
is delivered to the intended destination or is stored locally for later
transmission or download.
The key to a successful AVL systems is a dependable communications system
which covers the intended market area. If the market area is small, a single
base station may suffice and costs can be somewhat reasonable. If the
market area is larger than can be covered by a single base station, costs
can quickly escalate. Many AVL systems in the US utilize existing cellular
networks to carry information since the captial costs are mimimal compared
to a dedicated system. Most networks claim 95% coverage in the US.
Microburst and Cellemetry offer data services which utilitze the control
channel of cellular systems to allow subscribers to relay limited amounts of
data. Other commercial services are available as well.
Several AVL systems do not use radio at all. They simply log position
information into an onboard computer for later download to an AVL system.
Some are hybrid systems. They store location information until they come in
range of the AVL radio system and then send the stored information.
Asset position information can be sent or stored at regular intervals or
when specific events occur. If the interval is too short, the amount of
data can be quite large, especially when traveling long distances. Very
often the data is somewhat redundant when the vehicle is stationairy. Smart
beaconing or logging can reduce the amount of data considerably. This may
include suspension of transmissions or logging when an asset is stopped for
N minutes, and resumption when it begins to move again. It may also include
"corner pegging", or sending position information when the vehicle heading
changes N degrees. This can smooth out a vehicle track considerably.
The design of an AVL system is usually very industry specific. Over the
road transportation AVL system requirements are quite different than AVL
systems designed for Taxi-Cabs or police departments. Over the years, I
have seen many "Generic" AVL vendors go out of business because they could
not compete with industry specific AVL systems.
Delivery of required AVL information within an organizaton varies.
Operational personnel such as dispatchers may require map displays and real
time tracking data, but bean counters and most managers do not. Printed
reports which include street addresses, Date / Times, vehicle ID, driver ID
and job specific information is the norm in many industries. Transportation
companies may require information which includes only date and time vehicle
left a terminal, estimated time of arrival at destination, actual time of
arrival at destination, mileage, and loading or unloading time. Additional
related job information may include bill of lading, etc. Very often
transportation companies use GIS to provide quotes based on mileage from one
location to another.
Once specific requirements have been identified, AVL equipment can be
selected or designed. Design of the communications system is only a small
(but extremely important) element of a successful AVL system.
Bill Diaz KC9XG
APRS Working Group
>danke
>ash simmonds
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