GPS receiver capable of calculating accurate 2DRMS

Details GPS receiver capable of calculating accurate 2DRMS GPS receiver capable of calculating accurate 2DRMS

2001-03-23

2002-06-18

Phan, Dao (Department: 3662)

Communications: directive radio wave systems and devices (e.g.,

Directive

Including a satellite

C342S357490, C701S216000

Reexamination Certificate

active

06407701

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a GPS (Global Positioning System) receiver which provides evaluation values for evaluating accuracy of data obtained by GPS measurement, and the invention also relates to a navigation system in which such a GPS receiver is implemented.
In general, a GPS receiver provides data indicative of a position (a GPS position) and a velocity (a GPS velocity), in real time, by performing the GPS measurement. The data (the GPS position and/or the GPS velocity) obtained by performing the GPS measurement is also referred to as a GPS solution. In the GPS measurement, the GPS position is calculated using a range, or a distance between the GPS receiver and GPS satellite, which is measured using a satellite signal.
It is well known that the range measured using the satellite signal includes an error due to, for example, Satellite clock stability, Ephemeris prediction error, Ionospheric delay, Tropospheric delay and other error sources. Therefore, the range measured using satellite signal is called a pseudo-range.
Further, the GPS receiver calculates an evaluation value for evaluating accuracy of the GPS position in real time. In general, 2DRMS (2×Distance Root Mean Square), i.e., 2&sgr; value (2×standard deviation) of the horizontal error in the GPS position, is used as the evaluation value. Typically, the 2DRMS is a radius of a circle which contains 95% of all possible GPS positions.
Conventionally, 2DRMS is defined by the equation (1):
2
DRMS=
2
·HDOP·&sgr;
UERE
  (1)
where HDOP is a horizontal dilution of precision, and &sgr;
UERE
(user equivalent range error) is a root-sum-square value of each 1&sgr; error included in the measured pseudo-range described above. Typically, the &sgr;
UERE
is a constant, for example, 8.0 m.
The DOP (Dilution Of Precision) is a factor to relate an error in pseudo-range with an error in GPS position. The DOP changes according to the satellite geometry.
In general, a Kalman filter, which is well-known in the art, is employed in the GPS receiver or the navigation system. A publication, “Understanding GPS: principles and applications”, E D. Kaplan ed., Artech House, 1996, describes the use of the Kalman filter in the GPS receiver and calculation of the HDOP and the conventional 2DRMS, teachings of which are incorporated herein by reference.
Treating the GPS solution as a dynamic system, the Kalman filter calculates an estimate of the GPS solution and an error covariance matrix of the estimate. In the mathematical process of the Kalman filter, the estimate and the error covariance matrix of the estimate obtained in the prior estimation are referred to in the succeeding estimation.
The mathematical processes of the Kalman filter includes; provisionally estimating a system state based on a state equation to obtain a provisional estimate; and updating the estimated system state (the estimate) using a difference between a measurement value (a GPS solution) and the provisional estimate.
The updating process for a simple model is given by the equation (2):
X
(
t
)=
x
(
t
)+
K
(
t
)[
Y
(
t
)−
x
(
t
)]  (2)
where X(t) is the estimate, x(t) is the provisional estimate, Y(t) is the measurement value, and K(t) is a Kalman gain.
The following is an example of a computing process in the GPS receiver. The GPS receiver first performs the GPS measurement using the Kalman filter. Next, the HDOP is calculated based on geometry of GPS satellites used in the GPS measurement, and the 2DRMS is calculated according to the equation (1). Then, the GPS solution and the 2DRMS are outputted from the GPS receiver as a part of a GPS message. This GPS message is received and used by the navigation system which functions as a host to the GPS receiver.
In the navigation system, a CPU (Central Processing Unit), which executes a navigation application program, performs estimating a location of a vehicle by using both the GPS solution and a result of a dead-reckoning (a DR solution) computed based on signals outputted by dead-reckoning sensors. The signals from the dead-reckoning sensors include, for example, a gyro output signal, a speed pulse signal and a back signal.
The following is an example of a process of the navigation application program. Initially, the GPS solution and 2DRMS are transferred from the GPS receiver to the CPU in the navigation system. Then, the signals outputted by the dead-reckoning sensors are received and the DR solution is calculated by the CPU. Also, an evaluation value which indicates an error included in the DR solution is calculated.
By making a comparison of the evaluation value of the DR solution and the 2DRMS, the CPU select the GPS solution or the DR solution as a location of the vehicle. Finally, the location selected according to the process described above is compensated using a map-matching.
Thus, the 2DRMS plays an important role in avoiding an undesirable effect from the error included in the DR solution and/or the GPS solution, and in obtaining the location of the vehicle with high accuracy.
However, there may be a case where, the 2DRMS, based on the conventional definition, expressed in the equation (1), takes discrete data values, because the HDOP used for calculating the 2DRMS varies depending on an instantaneous GPS satellites geometry. In particular, in the case of receiving the GPS signal at a mobile station (i.e., a vehicle), since geometry of available satellites, from which GPS signals are receivable, varies from moment to moment, a correlation of the 2DRMS, calculated based on the conventional definition, with respect to time becomes weak.
A complication arises from such nature of the 2DRMS, as follows.
FIG. 1
is a graph showing a relation between the 2DRMS based on the conventional definition and a real error
61
included in the GPS solution.
In
FIG. 1
, relatively long time period of up to t
0
represents a state in which the GPS receiver can not receive GPS signals due to the fact that, for example, the vehicle, with which the GPS receiver is equipped, goes through a tunnel. Hereafter, this state is referred to as “on-GPS-measurement-state”.
At time t
0
, it becomes possible to receive a plurality of GPS signals (i.e., to use a plurality of GPS satellites) required for performing the GPS measurement, and the GPS receiver obtains navigation data, such as ephemeris. The GPS receiver starts to perform the GPS measurement at t
0
. Hereafter, this state in which the GPS signals can be received is referred to as “GPS-measurement-state”.
That is,
FIG. 1
shows progression of 2DRMS and the real error
61
of the GPS solution over time, after the GPS receiver goes into the GPS-measurement-state from the non-GPS-measurement-state.
At time t
0
, since the Kalman filter does not have historical data (past GPS solutions), the real error included in the GPS solution outputted by the GPS receiver (i.e., the estimate of the Kalman filter) becomes relatively large as shown in FIG.
1
. Then, the Kalman filter converges the estimate to a real location of the vehicle with the passage of time. As the estimate converges, accuracy of the estimate increases. Accordingly, the real error
61
included in the estimate decreases.
The 2DRMS can take a low value at time to when the GPS-measurement-state starts because the 2DRMS is a variable which depends on only an instantaneous GPS satellites geometry. The real error
61
falls under the 2DRMS at time t
1
.
Considering that the navigation system makes the selection described above (i.e., the navigation system selects the GPS solution or DR solution by comparing the evaluation value of DR solution with the 2DRMS) during a time period t
0
-t
1
, where the 2DRMS is smaller than the real error
61
(i.e., the 2DRMS does not reflect the real error
61
during the time period t
0
-t
1
). In this case, even though real accuracy of the DR solution is higher than real accuracy of the GPS solution, the GPS solution may be selected as a location of the vehicle because the 2DRMS is smaller than the ev

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