Gprs - Teletrac Prism TM Installation Information

Teletrac, Inc. - Prism TM Information and Installation Guide
Intentional Errors!
As hard as it may be to believe, the same government that spent $12 billion to develop the most
accurate navigation system in the world can cause errors by intentionally degrading its accuracy.
The policy is called "Selective Availability" or "SA" and the idea behind it is to make sure that no
hostile force or terrorist group can use GPS to make accurate weapons.
Basically the DoD introduces some "noise" into the satellite's clock data which, in turn, adds noise
(or inaccuracy) into position calculations. The DoD may also be sending slightly erroneous orbital
data to the satellites which they transmit back to receivers on the ground as part of a status
message.
Together these factors make SA the biggest single source of inaccuracy in the system. Military
receivers use a decryption key to remove the SA errors and so they're much more accurate.
Note: As of Spring 2000, the DoD eliminated the intentional error in the calculation, however, this
may come back at any time.
The Bottom Line
Fortunately, all of these inaccuracies still don't add up to much of an error, and a form of GPS
called "Differential GPS" can significantly reduce these problems.

GPRS

The second system used by the Prism TM is the General Packet Radio System, more commonly
called GPRS. This system is meant to be an invisible link from a mobile unit, such as a wireless
modem, to land line systems. The next few pages will give you an introduction to GPRS and how
it works to transmit information.
The following information was taken from information posted to
http://www.rysavy.com/Articles/GPRS2/gprs2.html
http://www.geocities.com/mobile4g/gprs.html.
A I
N NTRODUCTION TO
What is GPRS?
GPRS offers packet-switched connections to data networks via mobile technology. It is designed
to allow faster and easier Internet access with continuous connectivity, and enables applications
including multimedia messaging, wireless corporate intranet, remote control and maintenance of
appliances. It is also considered part of the migration to third generation (3G) mobile networks.
The advantages of GPRS technology allows users to stay connected to the Internet by using
packet switching technology, providing faster downloads as no time is spent attempting to access
a dial-up connection.
How does GPRS work?
GPRS transports packets between mobile devices and packet networks. Packets can be IP or
X.25, though with the Internet's popularity, operators and device vendors will probably emphasize
IP. Mobile devices will have an IP address, either static or dynamic, and, once on the network, IP
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