1. Field of the Invention
This invention relates generally to a personnel position location and tracking system and more particularly to an in-structure three-dimensional high-accuracy position monitoring system employing low frequency radio waves.
2. Description of the Related Art
The United States Army places high importance on training for urban warfare such as Military Operations in Urbanized Terrain (MOUT). Interest in training technology for MOUT has matured over the past decade because of the accumulation of experiences in Somalia, Serbia and Afghanistan. Based on evaluations by the United States Army and other military forces, specifications were promulgated for MOUT combat training systems that include means for locating and tracking trainees (xe2x80x9cplayersxe2x80x9d) inside buildings and tunnels during simulated MOUT combat exercises. With experience, it was found that location accuracy to within one meter and tracking updating within one second satisfied the MOUT training requirements both inside and outside of structures.
The Global Positioning System (GPS) is a widely-used and very useful system for position location and tracking but the GPS relies on high-frequency radio signals from orbiting satellites that cannot penetrate structures generally. Moreover, the five meter location accuracy of the GPS is not entirely sufficient for MOUT training purposes. Alternative position tracking technologies known in the art are also generally unable to deliver the performance features required for MOUT training exercises inside structures such as rooms, tunnels and bunkers. These include ultrasonic echo-location, inertial navigation systems (INS), position sensor grids, radio frequency (VHF/UF) echo-location, and passive/active infrared (IR) detection.
Generally, these prior art systems monitor the location of a trainee or xe2x80x9cplayerxe2x80x9d by measuring the time-of-arrival (TOA) of energy transferred between the player and a plurality of synchronized emitters in the room. The player position is computed by simple trilateration using the TOA data, the propagation velocity of the energy, and the known emitter locations. Moreover, each emitter must be uniquely identified by some signal characteristic. For example, ultrasonic energy propagates at about one foot per millisecond through air at sea level and radio frequency (RF) energy propagates through the same medium at about one foot per nanosecond. Because TOA measurements made in milliseconds are inherently simpler and more precise than those made in nanoseconds, ultrasonic trilateration is simpler and cheaper than RF trilateration, for example. Of course, these prior art systems may also monitor the TOA of energy emitted by the player at a plurality of sensors stationed about the room to similar effect, relying on the reciprocity principle.
Another approach known in the art is to instrument the training facility or xe2x80x9croomxe2x80x9d with a grid of uniquely-coded sensors spaced appropriately for the required positioning precision. Player position is monitored directly by signaling with the sensor most proximate the player. Energy broadcasts, mechanical pressure, local capacitance or any other well-known and useful method may be used to trigger the proximate sensor. Disadvantageously, such a system requires the pre-installation of a large plurality of sensors (versus a few for the TOA approach) and the accurate resolution of player positions in three dimensions may impose excessive complexity on the system.
Using such systems requires regular recomputation of the player position. This may occur at the player or at the sensor/emitter stations. Ideally, computation load is places at the sensor/emitter stations to minimize the electronic power consumption aboard the trainee player. Substantial power and signal wiring may be required to interconnect all sensor/emitter stations and any related processing systems.
The INS is well-known for aircraft and missile guidance systems. The typical INS employs a gyroscope and accelerometers oriented to detect acceleration in three dimensions. Position translation may be computed by integrating the accelerations over time. Drift of INS position may be reset using the GPS when available but otherwise, position error from drift is a major disadvantage of the INS.
Table 1 compares the performance features of these prior art systems in conditions expected during MOUT training exercises:
Close examination of Table 1 demonstrates that none of the prior art technologies offers the performance features necessary for MOUT training exercises with reasonable complexity and cost. For example, the ultrasound techniques known in the art are vulnerable to inaccuracies arising from multipath interference, building obstructions and weapons noise and do not detect orientation in three-dimensions. The UHF and VHF systems are generally quite expensive and robust but their performance is vulnerable to obstructions and room ambiguity. The INS is generally robust but is very expensive and has poor stability from long term drift, for example.
There is still a strong need in the art for a player locator system adapted for MOUT training exercises that can provide the necessary performance features with reasonable complexity and cost. There is also a need for such a system for use in tracking the positions of emergency workers during fire and rescue operations in an urban structure, where conditions may be similar to those expected during MOUT training exercises. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.
This invention solves the above described problems by introducing for the first time a position location system that relies on detection by a magnetic sensor of a low frequency (LF) magnetic field from a plurality of stationary antennas. The distance between a stationary antenna and the player-borne sensor is proportional to the logarithm of the magnetic field intensity because the player remains within the xe2x80x9cnear fieldxe2x80x9d of the stationary antenna. With scheduled transmissions from six stationary antennas, the position of a player equipped with a three-axis magnetic sensor may be resolved in three-dimensions to within one foot (30 cm). Player orientation (angular position) may also be resolved in three dimensions. The LF magnetic field intensity is generally unaffected by structural obstructions, multipath distortion or any of the other performance-degrading problems discussed above in connection with Table 1.
It is a purpose of this invention to provide a player locator system adapted for MOUT training exercises that can provide the necessary performance features with reasonable complexity and cost. The performance features of this invention are summarized in Table 2:
It is another purpose of this invention to provide a position location and tracking system adaptable for use in tracking the positions of emergency workers during fire and rescue operations in an urban structure, where conditions may be similar to those expected during MOUT training exercises. It is an advantage of the system of this invention that the stationary antennas and transmitters may be permanently installed at little expense in any urban structure so that sensor-equipped emergency workers may be located and tracked within the structure during emergency fire and rescue operations.
In one aspect, he invention is a method for reporting the position of a player unit in a position locator system including a controller and one or more instrumented zones each bounded by two antennas on two generally opposite sides, including the steps of radiating a first magnetic signal from the antenna on a first side of a first instrumented zone, receiving the first magnetic signal at the player unit and generating a first sensor signal representing the first magnetic signal received at the player unit; radiating a second magnetic signal from the antenna on a second side of the first instrumented zone generally opposite the first side thereof, receiving the second magnetic signal at the player unit and generating a second sensor signal representing the second magnetic signal received at the player unit; broadcasting a player position data signal corresponding to a combination of all of the sensor signals and receiving the player position data signal at the controller.
In an exemplary embodiment, the invention is a position locator system including one or more instrumented zones, a plurality of antennas each having an orientation axis and a predetermined location on the periphery of at least one of the instrumented zones, a transmitter coupled to each antenna for producing therein a signal current, whereby a magnetic signal is radiated therefrom, and one or more player units each having a field sensor for producing a sensor signal representing the magnetic signals radiating from the antennas and a signal processor for generating player position data responsive to the sensor signal.
The foregoing, together with other objects, features and advantages of this invention, can be better appreciated with reference to the following specification, claims and the accompanying drawing.