Communications: directive radio wave systems and devices (e.g. – Directive – Position indicating
Reexamination Certificate
1999-06-09
2001-10-09
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Position indicating
C342S463000
Reexamination Certificate
active
06300904
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to position determination, and in particular to the determination of the position of a transmitter without resorting to wideband signals.
BACKGROUND OF THE INVENTION
Locating items using radio location systems has been performed using many different systems. Global positioning systems are quite useful for determining the location of an item which is outdoors, or otherwise has a clear path for signals from GPS satellites. However, when items are indoors, or otherwise surrounded by other items which might reflect such signals, it is difficult to determine an accurate time of arrival of location signals transmitted or received by the items. This is referred to as a multipath problem because location signals may take multiple paths, and appear to arrive at one or more receivers at different times. RF transmitters are commonly used for such applications. They are also used on robots or other items which may be outfitted with various sensors, and move within a building or other structure. It becomes imperative to know within a small distance, where such items are located.
Methods for determining the position of an RF transmitter have been studied extensively. A common approach to these systems is to use time difference of arrival (TDOA) information. In these TDOA systems a number of receivers detect a signal that was transmitted. The receivers must determine when the signal was received. The accuracy required for the timing information depends on the precision needed in determining the transmitter's location. If a position accuracy of one meter is needed, then the accuracy of the timing information must be better than approximately 3 nanoseconds. This relationship is fundamental and is simply due to the speed of light, which is 30 cm per nanosecond. The timing information is then converted into time difference data by calculating the difference in receive times between pairs of receivers. Knowledge of the position of two receivers, and knowledge of the difference in time of arrival, determines a hyperboloid which represents the potential positions of the transmitter. More than two receivers are used to create multiple hyperboloids, whose intersection represent the actual position of the transmitter.
As these systems must typically determine the time of arrival of signals to a precision of approximately one nanosecond they have typically used wideband RF signals in order to allow the receivers to detect very fast rising edge pulses. U.S. Pat. No. 5,742,636 to Sanderford, Jr., uses a spread spectrum modulation to time stamp a received radio broadcast. Wideband approaches consume significant amounts of power, which is a precious commodity for small robot like items carrying sensors. Further, such signals are not directly compatible with the desire for narrowband RF signals that are used to carry data. There is a need for a location system that is accurate to fairly small distances to aid in locating items coupled to the source of RF signals. The system must also provide location information in real time to handle items that move. There is a further need for location systems that operate with low power requirements for the items being located. There is still a further need for such systems wherein the devices that detect the items are mobile.
SUMMARY OF THE INVENTION
Multiple pulses of narrow band signals of varying frequency are detected by multiple spaced apart receivers such that average time difference of arrivals of the signals from an item to be located are determined. The average time differences are used to calculate a position of the item to be located with a desired accuracy, such as within one meter in one embodiment. By using the average time difference at different frequencies, multipath propagation inaccuracies are minimized.
In one embodiment of the invention, the spaced apart receivers are relatively large robots, referred to as rangers. One of the rangers initiates a location process by transmitting a sync pulse. The sync pulse is received by a scout and other rangers. The scout is a small robot which acts as a transponder, sending out its own narrow band return pulse in response to the sync pulse. Each ranger then determines a difference in time between the sync pulse it receives and the return pulse generated by the scout. By using narrowband frequencies for the return pulse, power requirements are reduced.
This process is repeated multiple times, with each ranger moving a detection integration window further out in time to ensure that a leading edge of the return pulse is detected. The location process is then repeated again at different selected narrow band frequencies, and an average of the difference in time at each ranger is determined. In one embodiment, the frequency of the carrier for the return pulse is varied about approximately 900 MHz. The variation in one embodiment is in steps of 10 MHz. Measurements are taken at 900, 910, 920, 930, 940 and 950 MHz in one embodiment and an average of all measurements is used as the time difference. An alternative would be to perform various forms of statistical analysis, such as ignoring the high and low values.
In a further embodiment, each ranger is networked together via a standard wireless network, and also obtains position information via standard GPS methods. The position information and the difference information is communicated via the wireless network, and either the ranger that sent the sync pulse performs position calculations, or it transfers the information to another processing unit, either comprising a different ranger, or to a central processing unit, such as one that deployed the rangers.
An iterative calculation of the location of the scout is used to improve an estimated position solution. Time difference of arrival between two rangers is expressed as an equation with three unknown variables, x, y and z, which define a position of the scout in three dimensional space. The locus of a point that satisfies this equation is hyperbolic. Using four rangers provides three such time difference of arrival equations. Since there are inherent errors in the measurements, there is no unique solution. The iterative calculation uses a least squares approach to compute the estimated position to within a required tolerance.
In still a further embodiment, each ranger may have more than one antenna, separated by approximately at least ½ wavelength of the return pulse. The return pulse from each antenna is used to calculate time differences. A further alternative is to move the ranger between measurements. Combinations of these may also be used along with varying the frequency of the pulse to reduce the number of return pulses required to be output by the scout. This helps further conserve power. The use of RF narrowband frequencies versus wideband frequencies further simplifies the circuitry required in the scouts, which are power and space constrained.
REFERENCES:
patent: 3875570 (1975-04-01), Litchford
patent: 4740792 (1988-04-01), Sagey et al.
patent: 5099245 (1992-03-01), Sagey
patent: 5327144 (1994-07-01), Stilp et al.
patent: 5512908 (1996-04-01), Herrick
patent: 5926133 (1999-07-01), Green, Jr.
patent: 0140594A (1991-01-01), None
patent: 2250154A (1992-05-01), None
Dvorak Mark D.
Kriz Jeffrey J.
Schipper Brian W.
Blum Theodore M.
Honeywell International , Inc.
Shudy, Jr. John G.
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