Removing biases in dual frequency GNSS receivers using SBAS
A method for removing biases in dual frequency GNSS receivers circumvents the need for ionosphere corrections by using L2(P) in combination with either L1(P) or L1(C/A) to form ionosphere-free ranges. A table of biases is stored in microprocessor controller memory and utilized for computing a location using corrected ionosphere-free pseudo ranges. A system for removing biases in dual frequency GNSS receivers includes a dual frequency GNSS receiver and a controller microprocessor adapted to store a table of bias values for correcting pseudo ranges determined using L2(P) in combination with either L1(P) or L1(C/A).
1. A GNSS receiver system for receiving signals from GNSS satellites, including at least one SBAS satellite, which receiver system includes:
a receiver including a nonvolatile memory adapted for storing a table of bias corrections for at least one GNSS satellite;
each bias correction including at least one of a group delay correction component and an inter-signal bias correction component; and
an ionosphere-free pseudo range being SBAS-corrected using data broadcast in the SBAS signal as well as data in the bias correction table according to the formula:
[
R
-
(
T
SV
-
τ
GD
)
-
ISC
L
1
/
CA
]
+
T
SV
GPS
-
τ
GD
GPS
+
[
T
SV
-
T
SV
GPS
+
ISC
L
1
/
CA
-
(
τ
GD
-
τ
GD
GPS
)
]
=
R
wherein:
the GPS broadcast clock model=T SV GPS ;
the broadcast group-delay=τ GD GPS ;
the superscript “GPS” indicates that T SV GPS and τ GD GPS are estimates provided by
the GPS Control Segment rather than the true values;
the GPS navigation message, which is modulated on the GPS transmitted signals, contains coefficients of a time-based polynomial fitting T SV GPS ;
the navigation message contains a single word providing the group-delay, τ GD GPS ;
each satellite sends its respective, satellite-specific values for the above quantities; and
the SBAS clock correction, T SBAS , can be broken down into its individual constituents as follows:
T
SBAS
=
T
SV
-
T
SV
GPS
+
ISC
L
1
/
CA
-
(
τ
GD
-
τ
GD
GPS
)
P
iono
_
free
_
corrected
CA
=
K
1
C
1
-
K
2
P
2
+
T
SV
GPS
+
T
SBAS
+
B
ISC_TGD
B
ISC
_
TGD
=
K
2
ISC
L
1
/
CA
+
(
τ
GD
-
τ
GD
GPS
)
;
and
P
iono
_
free
_
corrected
CA
=
R
.
2. The GNSS receiver system of claim 1 , which includes: a Kalman filter adapted for estimating the troposphere as its state.
3. The system of claim 1 , which includes:
said GNSS receiver comprising a dual frequency receiver using one of:
L2(P) and L1(P); and
L2(P) and L1(C/A).
4. The system of claim 1 , which includes means for calibrating the biases using either:
a Kalman filter or a least squares method to: estimate and remove the effects of un-modeled troposphere errors and clock errors; and to effectively time-average the data.
5. The method of claim 1 , which includes means for providing a separate B TOTAL value for each of multiple GNSS satellites.
6. A GNSS receiver system for receiving pseudo range signals from GNSS satellites, including at least one SBAS satellite, which receiver system includes:
a receiver including a nonvolatile memory adapted for storing a table of bias corrections for at least one GNSS satellite;
a microprocessor controller connected to said GNSS receiver;
each bias correction including at least one of a group delay correction component and an inter-signal bias correction component;
the pseudo range signals being bias-corrected using data broadcast in the SBAS signal as well as data in the bias correction table; and
the pseudo range being SBAS-corrected using a GNSS broadcast clock model and a broadcast group delay, each satellite sending respective satellite-specific correction values and the biases being calibrated using a Kalman filter or a least squares method to estimate and remove the effects of un-modeled troposphere errors and clock errors and to effectively time-average the data.