Track model constraint for GPS position

Details Track model constraint for GPS position Track model constraint for GPS position

2001-06-22

2004-04-27

Nguyen, Thu V. (Department: 3661)

Data processing: vehicles, navigation, and relative location

Navigation

Employing position determining equipment

C342S357490

Reexamination Certificate

active

06728637

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to the use of a track model to constrain a GPS position.
2. Description of the Related Art
Technologies for tracking moving objects are in demand. For example, systems are used to track airplanes, automobiles, persons, objects at sporting events and other objects of interest. One technology that has become popular for tracking objects is the use of the Global Positioning System (GPS). GPS is a satellite based navigation system operated and maintained by the U.S. Department of Defense. GPS consists of a constellation of GPS satellites providing worldwide, 24 hour, three dimensional navigational services. By computing the distance to GPS satellites orbiting the earth, a GPS receiver can calculate an accurate position of itself. This process is called satellite ranging. The position being tracked is the position of the antenna of the GPS receiver.
Each GPS satellite carries an atomic clock to provide timing information for the signals transmitted by the satellites. Internal clock correction is provided for each satellite clock. Each GPS satellites transmits two spread spectrum, L-band carrier signals-an L
1
signal with carrier frequency f
1
=1575.42 MHz and an L
2
signal with carrier frequency f
2
=1227.6 MHz. These two frequencies are integral multiples f
1
=1540f
0
and f
2
=1200f
0
of a base frequency f
0
=1.023 MHz. The L1 signal from each satellite uses binary phase shift keying (BPSK), modulated by two pseudorandom noise (PRN) codes in phase quadrature, designated as a C/A code and P code. The L2 signal from each satellite is BPSK modulated by only the P code.
A GPS receiver measures distance using the travel time of radio signals. To measure travel time of a GPS signal from the satellite to a receiver, the receiver will generate the same pseudo-random code as the satellite and compare the generated code with the received code to determine the shift between the two codes. The travel time is multiplied by the speed of light to determine the distance between the satellite and the receiver. Along with distance, a GPS receiver needs to know exactly where the satellites are in space. A calculation of a three dimensional location generally requires valid data from four satellites. GPS receivers can also provide precise time information.
The above described method of computing position requires very accurate synchronization of the satellite and receiver clocks used for the time measurements. GPS satellites use very accurate and stable atomic clocks, but it is economically infeasible to provide a comparable clock in a receiver. The problem of clock synchronization is circumvented in GPS by treating the receiver clock error as an additional unknown in the navigation equations and using measurements from an additional satellite to provide enough equations for a solution for time as well as for position. Thus, the receiver can use a less expensive clock for measuring time. Such an approach leads to the pseudorange measurement:
&rgr;=c
(
t
rcve
−t
xmit
)
where t
rcve
is the time at which a specific, identifiable portion of the signal is received, t
xmit
is the time at which that same portion of the signal is transmitted, and c is the speed of light. Note that t
rcve
is measured according to the receiver clock, which may have a large time error. The variable t
xmit
is in terms of GPS satellite time.
If pseudorange measurements can be made from at least four satellites, enough information exists to solve for the unknown position (X, Y, Z) of the receiver antenna and for the receiver clock error C
b
. The equations are set up by equating the measured pseudorange to each satellite with the corresponding unknown user-to-satellite distance plus the receiver clock error:
ρ
1
=
(
x
1
-
X
)
2
+
(
y
1
+
Y
)
2
+
(
z
1
+
Z
)
2
+
c
b
ρ
2
=
(
x
2
-
X
)
2
+
(
y
2
+
Y
)
2
+
(
z
2
+
Z
)
2
+
c
b
ρ
3
=
(
x
3
-
X
)
2
+
(
y
3
+
Y
)
2
+
(
z
3
+
Z
)
2
+
c
b
ρ
4
=
(
x
4
-
X
)
2
+
(
y
4
+
Y
)
2
+
(
z
4
+
Z
)
2
+
c
b
where &rgr;
i
denotes the measured pseudorange of the ith satellite whose position in ECEF coordinates at t
xmit
is (x
1
, y
1
, z
1
). There are four equations depicted above. The unknowns in this nonlinear system of equations are the receiver position (X,Y,Z) in ECEF coordinates and the receiver clock error C
b
. If more than four satellites are used, there will be an equation for each satellite.
There are a number of errors that are associated with GPS ranging, including errors due to the Earth's ionosphere and atmosphere, noise, multipath satellite clock, and ephemeris errors. Additionally, basic geometry itself can based on the configuration of the satellites in the sky can magnify the errors. The dilution of precision, a measure of error, is a description of the uncertainty of particular GPS data.
One enhancement to standard GPS technology includes the techniques of differential GPS, which involves a reference GPS receiver that is stationary and has its position accurately surveyed. To understand differential GPS, it is important to know that satellite signals have errors which have a high spatial and temporal correlation. So, if two receivers are fairly close to each other, the signals that reach both of them will have traveled through virtually the same slice of atmosphere, and will have virtually the same errors. With differential GPS, the stationary reference receiver is used to measure errors. The reference receiver then provides error correction information to the other receivers (e.g. roving receivers). This way, systemic errors can be reduced. The reference receiver receives the same GPS signals as the roving receivers. Instead of using timing signals to calculate its position, the reference receiver uses its known position to calculate timing. It figures out what the travel time of the GPS signals should be, and compares it to what they actually are. The difference is used to identify the error information (also called differential corrections or differential GPS data). The reference receiver then transmits the differential corrections to the roving receivers in order to correct the measurement of the roving receivers. Since the reference receiver has no way of knowing which of the many available satellites a roving receiver might be using to calculate is position, the reference receiver quickly runs through all the visible satellites and computes each of their errors. The roving receivers apply the differential corrections to the particular satellite data they are using based on information from the reference receiver. The differential correction from the reference receiver improves the pseudorange position accuracy because its application can eliminate to varying degrees many of the spatially and temporally correllated errors in the pseudorange measured at the rover receiver. A differential GPS reference receiver can also transmit its carrier measurements and pseudoranges to the roving receiver. The set of measurements and pseduoranges transmitted from the reference receiver can be used to improve the position accuracy through the use of differential carrier positioning methods.
Despite the use of differential GPS, many land applications which use GPS are hampered by the restrictions imposed by buildings and natural impediments to the transmitted GPS signals. Often the GPS geometry is too poor to provide the geometrical strength required to generate the position accuracy that an application requires. One particular example of an environment for which the above described GPS technology does not provide sufficient accuracy and reliability is the real-time tracking of automobiles (or other objects) during a race, which requires extreme positioning accuracy and reliability in conditions of reduced satellite visibility and a highly dynamic environment. In an environment such as a professional auto race, the visibility of all satellites is severely reduced at some point on the track due to the existence of obstacles such

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