Design - GPS Interface

The Precision Lightweight GPS Receiver, or PLGR, is the most widely used GPS receiver used in the United States military today. The PLGR is a self contained, hand held, five channel, single frequency GPS receiver that provides accurate Position, Velocity and Timing (PVT) data to individual warfighters and integrated platform users. PLGR supports position location, target location, rendezvous and en route and terminal navigation.

The Precision Lightweight GPS Receiver incorporates a security module. When operated with the US military COMSEC to access the Precise Positioning Service (PPS) signal, the device provides access to Selective Availability, with anti spoofing and anti jam capabilities.

Only a military grade GPS receiver is acceptable for our project due to the security features enable by the military itself. The GPS satellite network is operated and maintained by the United States Air Force 50th Space Wing and is provided free for civilian use as a public good. For fear of hostile use of hostile forces exploiting this highly accurate service, military operators decided to degrade the signal to be less accurate, a condition the military has labeled SA, or selective availability. The degradation, however, is constant over a wide area. This consistency has allowed the civilian market to develop methods to correct the degraded signal. Fixed land positions receive the degraded signal from GPS satellites and compare it to there on location, calculating the percent error. These fixed land stations then broadcast the corrections to nearby civilians using GPS receivers. Their receivers then take the correction and factor it into the signals they receive from satellites giving them a somewhat accurate bearing.

Military grade GPS receivers, however, overcome the problem of selective availability in a much more direct way. As the military operates the GPS satellite force, they have a direct access to its accurate capabilities. Along side the civilian-used degraded GPS signal, the satellites also transmit an encoded signal. This secondary signal is not degraded at all and is therefore very accurate. Military GPS receivers can decode these additional signals, thereby negating the need for additional land based installations for correction. This feature makes the military GPS independent when it comes to calculating bearings. This is a desirable trait for the On Target beacon as it may be employed in some of the most remote areas in the world and therefore must be able to function on its own, independent from support.

Our GPS Receiver
As military GPS technology is classified and regulated by the U.S. government, a final receiver for the On Target beacon would not be selected until after final production was approved. One example of a military beacon that could be compatible with our beacon, however, would be the Trimble's Trimpack™ III receiver. Having been used in such conflicts as Desert Storm, it was designed to meet the requirements of rugged, lightweight, and accurate. Similar to the FM transmitter located on the On Target beacon, this receiver uses a military technology know as frequency hopping. A single radio frequency can be jammed or even listened to, so the Trimble's Trimpack™ III, as well as its host (the GPS satellite system), change frequencies constantly to disturb such interferences. To still transmit data, however, the host and the receiver must synchronize both the frequencies they jump to as well as at what time. This complex schedule is mapped out ahead of time and is provided via download to the receiver. The Trimble's Trimpack™ III can perform frequency hopping as well as contains further classified defenses against interferences such as jamming. It is capable of operating in a range of temperatures from -30 to 65 O C. Its rugged plastic shell helps its absorb heavy abuse from the soldiers that use it.

The Trimble's Trimpack™ III can be located internally on the On Target beacon. It operates using its on internal antenna as opposed to the externally mounted antenna for the FM transmitter. It provides the operator with accurate information as to the location of the beacon. As the beacon is unlikely to move, constantly using the receiver solely to confirm an already known position can seem redundant and would drain the power source faster. To this end, the receiver will only power to confirm its location at set time intervals such as one hour. Yet on the battlefront, situations change and soldiers are often required to improvise. In keeping with the On Targets theme of adaptability, the beacon can increase this interval either manually or by itself. For example, if once activated, the On Target beacon senses it has been moved since its last location update, it will immediately increase the frequency of the updates to once every five minutes.