Communications: directive radio wave systems and devices (e.g. – Directive – Position indicating
Reexamination Certificate
2002-02-15
2004-04-13
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Position indicating
C342S448000
Reexamination Certificate
active
06720921
ABSTRACT:
BACKGROUND OF THE INVENTION
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 (“players”) 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 “player” 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 “room” 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:
TABLE 1
Prior Art Position Monitoring Technology
Performance Feature
Ultrasound
Sensor Grid
VHF
UHF
Infrared
Inertial
Freq (MHz)
0.05
N/A
900
30,000
10
10
N/A
Wavelength (cm)
0.7
N/A
33
1
0.0001
N/A
Accuracy
Excellent
Depends
Good
Good
Good
Fair
Stability
Fair
Excellent
Excellent
Excellent
Excellent
Poor
Measures Orientation?
No
No
No
No
No
Depends
Position Resolved at?
Either
Building
Either
Either
Either
Player
Multipath Resistance
Excellent
Robust
Poor
Fair
Good
Robust
Room Ambiguity Resistance
Poor
Robust
Poor
Fair
Good
Good
Gunfire/Noise Resistance
Poor
Depends
Excellent
Excellent
Excellent
Fair
Smoke/Fog Resistance
Good
Depends
Excellent
Excellent
Poor
Excellent
Resistance to Obstructions
Fair
Depends
Fair
Poor
Poor
Excellent
Thermal Imager Compatibility
Excellent
Depends
Excellent
Excellent
Poor
Excellent
Live Fire Damage Resistance
Poor
Poor
Poor
Poor
Poor
Excellent
Player Unit Complexity/Cost
Low
Low
High
High
Moderate
Very High
Building Site Complexity/Cost
High
Very High
High
High
Moderate
Low
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.
SUMMARY OF THE INVENTION
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 “near field” 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:
TABLE 2
Performance Feature
LF System of This Invention
Freq (MHz)
0.1
Wavelength (cm)
300,000
Accuracy
Excellent
Stability
Excellent
Measures Orientation?
Yes
Po
Ripingill, Jr. Allen E.
Robinson David A.
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