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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96

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Combining Location Devices with Wireless WANs

Location devices identify your position on the Earth’s surface in terms of latitude and longitude. Traditionally, products that provide this type of information have been relatively expensive; therefore, they have mainly been used for providing navigation information to aircraft and ships. In the last few years, however, the size and cost of location devices have shrunk dramatically.

Location devices are based on Global Positioning System (GPS) technology—a worldwide, satellite-based radio navigation system providing three-dimensional position, velocity, and time information to users having GPS receivers anywhere on or near the surface of the Earth. Today, you can purchase a handheld position indicator for several hundred dollars. With this device and a good map, you’re unlikely to get lost. GPS devices tell you your position on earth within a few meters. Plus, many products, such as mapping software, are starting to incorporate GPS technology.

The GPS was developed by the U.S. Department of Defense and provides two levels of service—a Standard Positioning Service (SPS) and a Precise Positioning Service (PPS). SPS is a positioning and timing service available to all GPS users as a continuous, worldwide service with no direct charge. SPS provides the capability to obtain horizontal positioning accuracy within 100 meters and vertical positioning accuracy within 140 meters. The PPS is a highly accurate service used by the military to obtain positioning, velocity, and timing information.

The GPS satellites operate on two L-band frequencies: L1 (1575.42 MHz) for SPS and L2 (1227.6 MHz) for PPS. The system uses spread spectrum modulation and provides a great deal of resistance to interference. Each satellite transmits a navigation message containing its orbital elements, clock behavior, system time, and status messages. A user’s GPS receiver can determine its position by obtaining time information from three satellites. Altitude determinations require more than three satellites.


NOTE:  

The GPS consists of space, control, and user segments. The space segment consists of 24 satellites in six orbital planes. The satellites operate in circular 10,900 nautical mile (20,200 km) orbits at an inclination angle of 55 degrees and with a 12-hour period. The control segment consists of Monitor Stations and Ground Antennas. There are five Monitor Stations with one located in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs. There are three Ground Antennae, one at Ascension Island, Diego Garcia, and Kwajalein. The monitor stations gather range data by tracking all satellites within view. The user segment consists of antennas and receiver-processors, providing positioning, velocity, and precise timing to the user.


GPS/Wireless Applications

The combination of wireless WAN technologies and location devices offer some interesting applications. The joining of these technologies makes it possible for a mobile element to communicate its exact location to other elements. Many companies are combining CDPD and paging with the Global Positioning System (GPS) in their products, mainly for vehicle tracking.

ETE’s MobileTrak

ETE has a product, MobileTrak, which is a portable, low-cost, Automatic Vehicle Locating (AVL) system that works with ETE’s line of wireless communications products. MobileTrak combines wireless, two-way packet data communications with GPS to create a low cost, off-the-shelf, AVL system. The MobileTrak system depicts vehicle location on a full color map on either a laptop, desktop PC, or Macintosh that can be integrated with wireless dispatch products from ETE and other companies.

The MobileTrak system consists of the following three main components:

  MobileTrak Remote. A compact, self-contained position tracking and reporting system that includes a wireless digital packet radio and a GPS receiver. You can mount a MobileTrak Remote easily in any vehicle. The system connects to a vehicle’s power system or can operate for up to 10 hours with rechargeable Nickel-Metal Hydride batteries.
  MobileTrak Mapping Diplay Software. Displays vehicles or other mobile assets equipped with MobileTrak Remotes on a full color, real-time, moving map display. The software has many features, such as vehicle tracking, zooming, panning, address location, heading and velocity labels, and two-way chat messaging. The Mapping Display Software can also remotely control a MobileTrak Remote via commands sent over the wireless data communications network.
  MobileTrak Wireless Agent. A software product supporting wireless communications between the MobileTrak Mapping Display Stations and vehicles or assets equipped with MobileTrak Remotes. The MobileTrak Wireless Agent supports up to 100 MobileTrak Display Stations.

In the United States, the MobileTrak Wireless Agent supports both RAM Mobile Data and ARDIS wireless WAN services. Internationally, the MobileTrak Wireless Agent supports DataTAC and Mobitex wireless networks.


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