Data processing: measuring – calibrating – or testing – Measurement system – Orientation or position
Reexamination Certificate
2000-09-29
2003-04-29
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system
Orientation or position
C342S099000, C342S450000, C375S149000
Reexamination Certificate
active
06556942
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to radiolocation, and more particularly relates to a short-range radiolocation system suitable for smaller areas, indoor applications, and cost-critical products such as asset and personnel tags for providing high accuracy position data for situations where Global Positioning Satellite (GPS) systems are either ineffective or too expensive.
2. Discussion of the Related Art
The Global Positioning System (GPS) is a satellite-based navigation system known to be of great utility in many wide-area, outdoor scenarios, particularly in military and commercial navigational applications. This system lets a user with a GPS receiver determine his or her location on the earth with a high degree of accuracy, based on signals received from satellites orbiting the earth. Although this system was developed primarily for military use by the United States Department of Defense, civilian uses have exploded in recent years.
In certain navigational and positioning applications, e.g. industrial, military, transportation, and emergency assistance, there is a need for accurately determining the location of personnel, equipment, containers, personnel, and other assets in smaller areas such as plant buildings, warehouses, staging areas, storage facilities, and production line areas. The GPS generally has poor coverage inside buildings, under forest canopies or heavy foliage, or in highly developed urban areas where tall structures dominate. Multi-path effects of the radio signals from the satellites seriously deteriorate GPS accuracy in situations involving tall structures such as skyscrapers. Furthermore, known GPS receiving hardware is currently too large and costly for mass implementation applications, for example, where there is a need for identifying and locating unattended assets such as an item of equipment, a pallet or container of goods, a vehicle, etc.
GPS operates to determine the position of a user with a receiver by receiving signals transmitted by a plurality of GPS satellites orbiting the earth. The user's position on the surface of the earth is calculated relative to the center of the earth by triangulation based on signals received from multiple (usually 4: or more) GPS satellites. The distance from the user to a satellite is computed by measuring the propagation time required for a direct-sequence spread-spectrum “ranging code” signal transmitted by a satellite to reach the receiver.
A ranging code is a pseudorandom code sequence that is generated by a polynomial generator according to a known algorithm, each bit of which is called a “chip” to distinguish it from the true data bits that might form a message encoded onto the ranging code. A “chip” is a single bit in a pseudorandom code sequence used to spread the spectrum of an information signal. The pseudorandom ranging code sequence, when broadcast by radio, has a spectrum that has widely dispersed sidebands relative to the carrier frequency, and thus is referred to as a “spread-spectrum” signal. Spread-spectrum signals are known to have desirable characteristics for data security and resistance to radio-frequency (RF) interference.
Within a GPS receiver, an identical ranging code signal is generated and shifted in time (or phase) until it achieves correlation with the specific satellite-generated ranging code being acquired. The magnitude of the time shift of the identical ranging code signal within the receiver relative to the satellite transmitted ranging code provides a time differential that is related to the satellite-to-user range.
To determine user position in three dimensions, range measurements are made to a plurality of satellites, resulting in four simultaneous ranging equations that have four unknowns. These equations can be solved by computer systems to determine the values of x, y, z (the 3-dimensional location of the user's receiver), and t, which is a clock error. There are several closed-form solutions furnished in the literature for solving the equation to determine the unknown quantities.
The positioning is in general accomplished by determining the time-of-flight of the signals from at least 4 GPS satellites, and by careful processing of the real-time data from the multiple satellite clocks (and other, small corrections) the actual distances are computed; the common solution to the set simultaneous distance equations, coupled to the known satellite locations, provides the GPS receiver's position. Thus, the geometric range is given by:
r=c
(
T
u
−T
s
)=
c&Dgr;t,
(1)
where
T
s
=system time when signal left the satellite;
T
u
=system time when signal reached the receiver;
&dgr;
t
=offset of satellite clock from system time;
t
u
=offset of receiver clock from system time;
T
s
+&dgr;t=satellite clock reading when signal left satellite;
T
u
+t
u
=receiver clock reading when signal, arrived;
c=speed of light;
(x
u
, y
u
, z
u
)=position of the receiver in 3 dimensions; and
(x
j
, y
j
, z
j
)=3-dimensional position of the jth satellite (j=1 to 4).
In these terms, the pseudorange is given by:
&rgr;=
c
[(
T
u
+t
u
)−(
T
s
+&dgr;t
)]=
c
(
T
u
−T
s
)+
c
(
t
u
−&dgr;t
)=
r+c
(
t
u
−&dgr;t
) (2)
and the 4 pseudoranges are thus:
&rgr;
1
=[(
x
1
−x
u
)
2
+(
y
1
−y
u
)
2
+(
z
1
−z
u
)
2
]
½
ct
u
(3)
&rgr;
2
=[(
x
2
−x
u
)
2
+(
y
2
−y
u
)
2
+(
z
2
−z
u
)
2
]
½
ct
u
(4)
&rgr;
3
=[(
x
3
−x
u
)
2
+(
y
3
−y
u
)
2
+(
z
3
−z
u
)
2
]
½
ct
u
(5)
&rgr;
2
=[(
x
4
−x
u
)
2
+(
y
4
−y
u
)
2
+(
z
4
−z
u
)
2
]
½
ct
u
(6)
These nonlinear equations may be solved by either closed-form methods, iterative techniques based on linearization, or by Kalman-filtering (estimation) algorithms.
Although GPS radiolocation is proven and works well, it requires multiple readings to obtain positioning accuracy down to the 10-meter range, which leads to greater complexity in the receiver and longer computation time to calculate a navigational “fix”. Costly military receivers are not hampered by these limitations, but commercial receivers do not have the special encryption features required for rapid high-accuracy location determination, although recent advances in signal processing have somewhat improved the situation. In addition, GPS provides location information for receivers in the field which are typically attended by personnel. GPS does not readily adapt to situations where a central locating system is required for locating unattended assets or personnel that cannot respond by transmitting the GPS-determined location information to the central locating system via separate communication means. For these reasons, GPS is generally not suitable for radiolocation in a limited space at low cost for unattended assets. Furthermore, due to the extremely low signal strengths of the GPS satellite beacon transmitters at the GPS receiver, GPS signals are virtually always unusable indoors because of the additional attenuation of the overhead satellite signals by building roofs, upper-floors, and other overhead structures, as well as trees and dense foliage in general. In addition, in “urban canyons” and very rugged terrain, often there are too few GPS satellites in direct line-of-sight view of the receiver to obtain a sufficiently accurate position fix.
Further details of the GPS are provided in U.S. Pat. No. 4,894,662, to Counselman, “Method and System for Determining Position on a Moving Platform, Such as a Ship, Using Signals for GPS Satellites.” Further details of the GPS are also provided in Kaplan
(1)
.
One approach to a positioning system for radiolocation in a limited space is a system constructed by PinPoint Corporati
Barlow John
Gray Cary Ware & Freidenrich LLP
Le John
UT-Battelle LLC
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