Communications: directive radio wave systems and devices (e.g. – Directive – Position indicating
Reexamination Certificate
1999-03-15
2001-07-10
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Position indicating
Reexamination Certificate
active
06259404
ABSTRACT:
BACKGROUND OF THE INVENTION
This application relates generally to radio based position location systems and in particular to a generalized method which is applicable regardless of the number of base station antennas, and which compensates for unknown delays, phase shifts, and frequency shifts in received signals.
It is known that widely deployed radio communications network equipment such as cellular and paging system base stations can be used to provide position information for remote subscriber units. Such a position location system includes multiple base stations arranged in a predetermined pattern over a large region and spaced apart at relatively large distances, typically on the order of a few miles. To determine the location of an object within the region among the base stations, plural base stations receive a signal, such as a cellular phone signal, from the object. By analyzing the phase of the signals received, the location of the object is estimated.
Typical systems employ one of two common position location approaches. One of the approaches is called direction finding (DF) or angle of arrival and the other is known as pseudo ranging (PR) or time difference of arrival (TDOA). The direction finding method uses an antenna array such as a phased array at each base station to receive the signal from the object and determine its angle of arrival at the base station. By analyzing the difference in phase of the signal received at each antenna in the array, each of the plural base stations generates an estimate of the direction to the object. The object's location is estimated to be at the point of intersection of directional lines projected from each of the plural base stations at the computed angular directions.
In pseudo ranging systems, for each pair of base stations, the difference in time of arrival of the signal from the object at each base station is computed from the phase of the received signals. This time difference defines a hyperbola for each independent pair of base stations. The point at which the hyperbolas intersect provides an estimate of the location of the object.
Both of these common approaches to position location suffer from inherent inaccuracies. Since the phased arrays of antennas cannot precisely determine the angle of arrival of the signal, the direction finding approach actually does not result in several lines intersecting at one common point. Rather, the lines intersect at several points forming a region within which the object should be located. This region can be quite large depending upon certain variables such as elevation, signal strength, etc. The pseudo ranging approach is also inherently inaccurate since multiple hyperbolas do not intersect at the same point. This and other inaccuracies also result in determining a region in which the object may be located, rather than a precise position location. See, for example, Joseph P. Kennedy, et al.,
Passive High Accuracy Geolocation System and Method
, U.S. Pat. No. 5,317,323, issued May 31, 1994.
At least one position location system has applied both direction finding and pseudo ranging to determine object location. However, the two approaches are applied separately, with the direction finding or angle of arrival approach being applied only to eliminate multipath errors from the location estimate.
SUMMARY OF THE INVENTION
The present invention is directed to a position location system and method which determine the position of an object without the inaccuracies inherent in prior systems. The system of the invention includes a remote subscriber unit, typically positioned at the object or person to be located, which transmits a locating signal into a region. A plurality of base stations receive the locating signal from the remote unit. Each base station includes at least one antenna that receives the locating signal and a receiver coupled to the antenna that generates a representative signal indicative of amplitude and phase in the locating signal as it is received at the antenna. A minimal system requires at least three antennas of which two can be located at three separate base stations, or two antennas located at one base station and one antenna located at another base station.
A processor receives the representative signals from the base stations and combines information indicative of amplitude and phase in the locating signal as received at the base stations to determine the position of the remote unit.
The locating signal comprises two or more single-frequency tones. Each locating signal tone can be at a different frequency. The tones can be transmitted at is different times, or, in an alternative embodiment, they can be transmitted simultaneously. Because only single-frequency tones are used as the locating signal instead of complicated modulated signals, substantial transmission circuitry is eliminated.
More particularly, receivers receive transmissions composed of tones from the transmitting remote unit, and select certain segments of one or more of the transmitted tones to determine their respective amplitudes and phases, together called phasors. The phasors are measured by correlation with local synchronous reference tones, having the same frequency at all base stations. When the received tones are shifted in frequency, due to oscillator offsets or Doppler shifts, the resulting frequency deviation can also be measured. The phasors, the frequency deviation, and time of measurement, along with other measured values deemed necessary for other purposes such as calibration and identification, are then forwarded to a central processor.
The central processor operates on a subset (or all) of the measured values received from the base stations to estimate location. The processor uses a two step algorithm where in Step 1 the measured values received from the base stations surrounding the region are used to define a function, called the locator function, which has location coordinates as arguments. The locator function is selected to have the following key properties:
1. It depends on both amplitude and phase measurements from all base stations, antennas, and tones being used. Specifically, the coordinates of the maximum of the locator function depends on amplitude and phase of the received phasors.
2. It does not rely on knowledge of the transmitted power.
3. It has at least one local maximum near the actual location.
4. Generally, but particularly in the absence of multipath, the local maximum will be closer to the actual location when the signal to noise ratio is higher.
5. There may be several local maxima (ambiguous peaks). However, there will usually be a unique global maximum within a given region when a sufficient number of base stations, antenna elements, or tones are used. As an example, for three single-antenna base stations, and two tones transmitted from within the triangular region formed by the base stations, the function will generally have a single peak within that region when the tones are closely spaced in frequency.
6. Motion of the transmitting unit and the frequency offset caused by an imprecise local oscillator can be estimated as a by-product of the process generating the function. One way is to ignore motion and estimate the constant frequency offset of the transmitting unit by averaging the frequency deviations measured at each base station. Another way is to estimate frequency offset and motion independently and use the resulting estimates to adjust the locator function. A third way is to, at the outset, define a locator function that depends on motion and frequency offset in addition to location.
In Step 2, the processor searches for the maximum of the locator function by computing the function value at points selected by the particular search algorithm. Such algorithms are well known. This search may be aided by a priori knowledge to help speed up the search or to resolve any ambiguities that may remain. This search yields the estimated location, to compute the position of the object directly, without the need for projecting lines at angles of arrival or compu
Bussgang Julian
Parl Steen A.
Weitzen Jay
Zagami James M.
Hamilton Brook Smith & Reynolds P.C.
Issing Gregory C.
Signatron Technology Corporation
LandOfFree
Position location system and method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Position location system and method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Position location system and method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2458630