Real Time Location Systems
In situations where only position-location is needed, real time location systems offer a low-cost, low footprint solution. This system could be used on a variety of tasks where location of various entities is needed across a large distance.
The system uses a standard Sepura public safety grade radio programmed to report in standard Tetra Location Interface Protocol (LIP) messages transmitted at regular intervals based on the motion of the radio. LIP has the advantage of being tiny in comparison with other protocols (e.g., NMEA), carrying a 1.2 meter worldwide precision in an 11 byte message. It also is optimized for the Tetra standard, taking up only half a radio time slot, meaning that high update rates and large numbers of radios can be used to increase simulation fidelity.
The Sepura STP8000 radio is a lightweight, rugged, reliable, hand portable radio that can be worn on the belt or attached to MOdular Lightweight Load-carrying Equipment (MOLLE) vests, vehicles, or anything else that needs to be tracked. Rechargeable batteries last about a day for normal training operations, up to four days with extended batteries. For light duty applications, the PRTLS software can be configured to use the radio’s DMO mode (Direct Mode of Operation), eliminating the need for expensive interfaces into the Tetra radio network. This makes it a good candidate for home-station training applications, where reduced equipment footprints are an important factor in keeping costs under control.
The system uses the LIP protocol’s adaptive reporting rate feature to increase precision when needed. Position/location update rate is determined through a combination of time interval and motion. If an entity is stationary, the update rate defaults to a lower rate to preserve radio bandwidth. When the entity starts to move, a position/location report is generated for every few hundred meters of motion, increasing the perceived update rate and more accurately reflecting the motion of the entity. Both of these parameters can be set over the radio link, which means that the system can adapt to bandwidth congestion conditions as necessary. As well, different parameters can be set based on the type of entity. A person walking doesn’t move very fast, so a lower default timed update rate and lower movement thresholds can be selected. A vehicle might have a slightly higher default timed update rate and a higher movement threshold. Both are configurable as programmed defaults on the radio, programmed defaults in the Central Node (CN) software, and are updatable by controlling software in near real time.
The system outputs standard Distributed Interactive Simulation (DIS) protocol data units (PDUs). Postion/location is transmitted in standard DIS Entity State PDU’s and control commands are transferred by specially constructed Data PDU’s. The DIS-CN software is a modular publish and subscriber based system, that can accommodate interfaces to a variety of other simulations and systems.