U.S. patent application number 11/237837 was filed with the patent office on 2007-10-04 for method and apparatus for improved l2 performance in dual frequency semi-codeless gps receivers.
Invention is credited to Gary Lennen.
Application Number | 20070230545 11/237837 |
Document ID | / |
Family ID | 35057322 |
Filed Date | 2007-10-04 |
United States Patent
Application |
20070230545 |
Kind Code |
A1 |
Lennen; Gary |
October 4, 2007 |
Method and apparatus for improved L2 performance in dual frequency
semi-codeless GPS receivers
Abstract
This invention relates to improving the signal-to-noise ratio of
semi-codeless tracking by presenting a method and apparatus which
employs an optimal W-code timing pattern, and/or provides
signal-to-noise gain by adding L1 and L2 W-code bit timing estimate
signals before combining the L1 W-code estimate signal with the L2
W-code estimate signal.
Inventors: |
Lennen; Gary; (Cupertino,
CA) |
Correspondence
Address: |
William E. Pelton;Cooper & Dunham LLP
1185 Avenue of the Americas
New York
NY
10036
US
|
Family ID: |
35057322 |
Appl. No.: |
11/237837 |
Filed: |
September 27, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09439608 |
Nov 12, 1999 |
6954488 |
|
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11237837 |
Sep 27, 2005 |
|
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60157298 |
Oct 1, 1999 |
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Current U.S.
Class: |
375/149 ;
375/E1.01 |
Current CPC
Class: |
H04B 1/7077 20130101;
H04B 2201/70715 20130101; G01S 19/32 20130101 |
Class at
Publication: |
375/149 |
International
Class: |
H04B 1/00 20060101
H04B001/00 |
Claims
1-16. (canceled)
17: An apparatus for tracking remotely generated signals
comprising: a first tracker for tracking a first component of a
first remotely generated signal and including a W-code signal
generator responsive to a second component of the first signal for
locally generating a first W-code signal from a second component of
the first signal; and a second tracker for tracking a first
component of a second remotely generated signal according to the
first W-code signal; wherein the second component of the first
signal has the same pattern as the first component of the second
signal.
18: An apparatus according to claim 17, wherein the pattern
comprises: a known pattern combined with an unknown pattern.
19: An apparatus according to claim 18, wherein timing information
about the unknown pattern is known.
20: An apparatus for tracking remotely generated signals
comprising: a first tracker for tracking a first component of a
first remotely generated signal and including a W-code signal
generator responsive to a second component of the first signal for
locally generating a first W-code signal from a second component of
the first signal; and a second tracker for tracking a first
component of a second remotely generated signal according to the
first W-code signal; wherein the second component of the first
signal has the same pattern as the first component of the second
signal; the pattern comprises a known pattern combined with an
unknown pattern; timing information about the unknown pattern is
known; and the first tracker generates a timing signal in
accordance with the timing information for improving the accuracy
of the first W-code signal.
21: An apparatus for tracking signals comprising: a first tracker
for tracking a first component of a first signal and for generating
a first estimate signal from a second component of the first
signal; and a second tracker for tracking a first component of a
second signal according to the first estimate signal; wherein the
second component of the first signal has the same pattern as the
first component of the second signal; the pattern comprises a known
pattern combined with an unknown pattern; the first signal is a GPS
LI signal; the second signal is a GPS L2 signal; the first
component of the GPS LI signal is a C/A-code component; the second
component of the GPS LI signal is a Y-code component; the first
component of the GPS L2-signal is a Y-code component; the known
pattern is a GPS P-code; the unknown pattern is a GPS W-code.
22: An apparatus according to claim 21, wherein the W-code has a
plurality of bit edges, further comprising means for determining
the precise location in time of each of said bit edges.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/157,298 filed Oct. 1, 1999.
FIELD OF THE INVENTION
[0002] This invention relates to the acquisition and tracking of
signals transmitted in accordance with the Global Positioning
System ("GPS"), and in particular, using information from the GPS
signal transmitted in the frequency band about 1575.42 Megahertz
(MHz) ("L1") to improve the signal-to-noise ratio ("SNR") for
acquiring and tracking the GPS signal transmitted in the frequency
band about 1227.6 MHz ("L2").
BACKGROUND OF THE INVENTION
[0003] Tracking the GPS L2 signal plays a significant role in
high-end GPS receivers and especially for civilian applications
such as machine control and survey equipment. The GPS L2 signal,
like the GPS L1 signal, contains-three or more separate components:
(1) a carrier signal (such as a 1227.6 MHz sinusoidal waveform);
(2) a code signal; and (3) a data signal. The code signal is
combined (modulated) with the carrier signal so that each GPS
signal source, for example a satellite, can be distinguished from
each other according to the particular code employed by that GPS
source. The data signal contains the information which allows a GPS
receiver to calculate its location, for example, the location of
the transmitting GPS source.
[0004] The GPS L1 signal can include both a coarse/acquisition
("C/A-code") code and a precision ("P-code") code. However, since
the early 1990s, the GPS satellites have been transmitting both the
C/A-code and an encrypted form of the P-code called the "Y-code".
The Y-code is generated by encrypting the P-code with another code
called the "W-code". The W-code is secret and is not available for
civilian use. Unlike the GPS L1 signal, the GPS L2 signal only
includes one code, which since the early 1990s has been the Y-code.
Because the W-code is unknown, the Y-code cannot be tracked by
conventional procedures in a civilian GPS receiver. However,
because the Y-code is formed from the P-code, in combination with
the W-code, sufficient information about the Y-code can be
ascertained by a GPS receiver to augment the GPS receiver's ability
to track GPS signals. This type of tracking is termed
"semi-codeless" because at least the contribution of the P-code to
the Y-code signal is known even if the W-code contribution is
unknown.
[0005] A number of semi-codeless tracking techniques have been
developed, see for example, U.S. Pat. No. 4,972,431 to Keegan et
al., U.S. Pat. No. 5,293,170 to Lorenz et al., U.S. Pat. No.
5,541,606 to Lennen, U.S. Pat. No. 5,621,416 to Lennen, U.S. Pat.
No. 5,663,733 to Lennen, and U.S. Pat. No. 5,610,984 to Lennen.
Each of these patents and any other documents referred to in this
specification are hereby incorporated into this application by the
respective reference.
[0006] However, none of these references apply known information
about the timing pattern of the W-code.
BRIEF SUMMARY OF THE INVENTION
[0007] One aspect of this invention is to improve the performance
of semi-codeless L2 tracking in GPS receivers by using exact
knowledge of the W-code timing information.
[0008] Another aspect of this invention is to combine estimates of
the W-code component signals from the GPS L1 signal and the GPS L2
signal, and in particular, by adding the W-code estimate signals
together before multiplying the result by the W-code estimate
signal(s) for the GPS L2 signal.
[0009] These novel techniques improve signal-to-noise ratio when
tracking the GPS L2 signal, which in turn leads to improved code
and carrier measurement capability, as confirmed by experiments
observing GPS satellites with a high gain antenna in addition to
analysis and simulation of GPS receiver designs.
[0010] One object of this invention is to provide an apparatus for
tracking signals having: (1) a first tracker for tracking a first
component of a first signal and for generating a first estimate
signal from a second component of the first signal; and (2) a
second tracker for tracking a first component of a second signal
according to the first estimate signal; such that the second
component of the first signal has the same pattern as the first
component of the second signal. This invention further provides
for: (a) the pattern to be a known pattern combined with an unknown
pattern; and/or (b) the timing information about the unknown
pattern to be known.
[0011] Another object of the invention is for the first tracker to
generate: (1) a timing signal in accordance with the timing
information for improving the accuracy of the first estimate
signal; and/or (2) a first local component signal in accordance
with the known pattern and to combine the local component signal
with a version of the first signal to generate the first estimate
signal.
[0012] An additional object of this invention is to have the second
tracker: (1) generate a second local component signal in accordance
with the known pattern and combine the second local component
signal with at least one version of the second signal to generate
at least one second estimate signal; (2) generate a timing signal
in accordance with the timing information for improving the
accuracy of the at least one second estimate signal; (3) combine
the first estimate signal with the at least one second estimate
signal to provide a tracking signal for tracking the first
component of the second signal; (4) combine the first estimate
signal with at least one second estimate signal to generate a
combined estimate signal; (5) combine the first estimate signal
with the at least one second estimate signal when the second
tracker has not locked to the first component of the second signal;
and/or (6) combine the combined estimate signal with the at least
one second estimate signal when the second tracker has locked to
the first component of the second signal.
[0013] Another object of this invention provides for: (1) the first
signal to be a GPS L1 signal; (2) the second signal to be a GPS L2
signal; (3) the first component of the GPS L1 signal to be a
C/A-code component; (4) the second component of the GPS L1 signal
to be a Y-code component; (5) the first component of the GPS L2
signal to be a Y-code component; (6) the known pattern to be a GPS
P-code; and/or (7) the unknown pattern to be a GPS W-code.
[0014] A further object of this invention is to provide a method
for tracking signals having the steps of: (1) locking to a first
component of a first signal; (2) aligning a local version of a
second component of the first signal with the first component of
the first signal; (3) aligning a local version of a first component
of a second signal with the first component of the first signal;
(4) generating a first estimate signal from a version of the first
signal and the local version of the second component; and/or (5)
applying the first estimate signal for locking to a first component
of the second signal; wherein the second component of the first
signal and the first component of the second signal have the same
pattern.
[0015] An additional object of this invention is to provide a
method for tracking signals having the steps of: (1) locking to a
first component of a first signal; (2) aligning a local version of
a second component of the first signal with the first component of
the first signal; (3) aligning a local version of a first component
of a second signal with the first component of the first signal;
(4) generating a first estimate signal from a version of the first
signal and the local version of the second component; (5)
generating a second estimate signal from a version of the second
signal and a local version of the first component of the second
signal; (6) combining the second estimate signal and the first
estimate signal to generate a combined estimate signal; and/or (7)
selectively applying either the first estimate signal or the
combined estimate signal for locking to a first component of the
second signal; wherein the second component of the first signal and
the first component of the second signal have the same pattern.
Further, this invention provides that the step of selectively
applying further includes the steps of: (1) selecting the first
estimate signal for application if the local version of the first
component of the second component has not been locked; and/or (2)
selecting the combined estimate signal for application if the local
version of the first component of the second component has been
locked.
[0016] A further object of this invention is to provide a method of
semi-codeless tracking for a GPS receiver having the steps of: (1)
receiving a GPS L1 signal and generating at least a quadrature
baseband version of the GPS L1 signal; (2) receiving a GPS L2
signal and generating baseband versions of the GPS L2 signal; (3)
multiplying the quadrature baseband version of the GPS L1 signal
with a locally generated version of a P-code used to generate the
Y-code component of both the GPS L1 and L2 signals to generate a
first estimate signal related to the W-code used with the P-code to
generate the Y-code component; (4) multiplying the in-phase
baseband version of the GPS L2 signal with a locally generated
version of the P-code to generate a second estimate signal related
to the W-code used with the P-code to generate the Y-code
component; (5) adding the first W-code estimate signal to the
second W-code estimate signal to generate a combined W-code
estimate signal; (6) applying the first W-code estimate signal to
generate tracking signals for tracking when the GPS receiver has
not locked to the GPS L2 signal; and/or (7) applying the second
W-code estimate signal to generate tracking signals for tracking
when the GPS receiver has locked to the GPS L2 signal.
[0017] Also, an object of this invention is to provide a method of
semi-codeless tracking for a GPS receiver having the steps of: (1)
receiving a GPS L1 signal and generating at least a quadrature
baseband version of the GPS L1 signal; (2) receiving a GPS L2
signal and generating baseband versions of the GPS L2 signal; (3)
multiplying the quadrature baseband version of the GPS L1 signal
with a locally generated version of a P-code used to generate the
Y-code component of both the GPS L1 and L2 signals to generate a
first wide-band estimate signal related to the W-code used with the
P-code to generate the Y-code component; (4) integrating the first
wide-band estimate signal based on known timing information of the
Y-code to generate a first narrow-band W-code estimate signal; (5)
multiplying the in-phase baseband version of the GPS L2 signal with
a locally generated version of the P-code to generate a second
wide-band estimate signal related to the W-code used with the
P-code to generate the Y-code component; (6) integrating the second
wide-band estimate signal based on known timing information of the
Y-code to generate a second narrow-band W-code estimate signal; (7)
adding the first narrow-band W-code estimate signal to the second
narrow-band W-code estimate signal to generate a combined W-code
estimate signal; (8) applying the first narrow-band W-code estimate
signal to generate tracking signals for tracking when the GPS
receiver has not locked to the GPS L2 signal; and/or (9) applying
the second narrow-band W-code estimate signal to generate tracking
signals for tracking when the GPS receiver has locked to the GPS L2
signal.
[0018] These and other objects and advantages of the present
invention will become apparent from the detailed description
accompanying the claims and attached figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 illustrates at a high-level, a GPS receiver according
to one embodiment of the invention;
[0020] FIG. 2 illustrates a channel processor according to an
embodiment of the invention;
[0021] FIG. 3 illustrates a tracker for components of a GPS L1
signal according to one embodiment of the invention;
[0022] FIG. 4 illustrates the amount of signal-to-noise ratio gain
or loss as affected by bandwidth according to one embodiment of the
invention;
[0023] FIG. 5a illustrates the W-code timing pattern as employed
according to one embodiment of the invention;
[0024] FIG. 5b illustrates the relationship between the W-code
timing pattern and the C/A-code timing pattern according to one
embodiment of the invention;
[0025] FIG. 6 illustrates the amount of signal-to-noise ratio gain
or loss by comparing different W-code integration periods;
[0026] FIG. 7 illustrates an H period generator according to one
embodiment of the invention;
[0027] FIG. 8 illustrates the relative timing of various signals
employed according to one embodiment of the invention;
[0028] FIG. 9 illustrates a tracker for components of a GPS L2
signal according to one embodiment of the invention;
[0029] FIG. 10 illustrates a P-code mixer according to one
embodiment of the invention;
[0030] FIG. 11 illustrates W-code estimate integrators according to
one embodiment of the invention;
[0031] FIG. 12 illustrates W-code estimate multipliers according to
one embodiment of the invention;
[0032] FIG. 13 illustrates the amount of signal-to-noise ratio gain
or loss relative to different selection of W-code estimate signals
according to one embodiment of the invention;
[0033] FIG. 14 illustrates a procedure for operating a GPS receiver
according to one embodiment of the invention; and
[0034] FIG. 15 illustrates a relationship between a carrier phase
error and the selection of a particular W-code estimate signal
according to one embodiment of the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0035] FIG. 1 is a high-level illustration of a GPS receiver 10 in
accordance with one embodiment of the invention. Key aspects of the
invention are provided primarily in the L1/L2 digital channel
processors (channel processors) 12. Although other portions of this
GPS receiver 10 can be similar to conventional receivers, see for
example, U.S. Pat. No. 5,541,606 to Lennen, aspects of this
invention can be incorporated elsewhere, for example, the software
which runs on the microprocessor system 14.
[0036] In operation, the antenna 16 receives the GPS L1 and L2
signals transmitted from GPS satellites and other GPS sources. The
antenna 16 outputs a corresponding antenna signal 18 which is
filtered and amplified by filter/low-noise-amplifier (LNA) 20. The
resulting filtered signal 22 is then translated (downconverted and
digitized) by a radio frequency (RF) module 24 to generate
intermediate frequency signals 26. Thus, for example, the GPS L1
signal is frequency translated to 420 kilohertz (kHz) and the GPS
L2 signal is translated to 2.6 MHz. These intermediate frequency
signals 26 are multi-bit, downconverted, digitized and sampled
versions of the GPS signals, and include: (1) an in-phase version
of the GPS L1 signal (I.sub.L1) 27; (2) a 90 degree out-of-phase
(quadrature) version of the GPS L1 signal (Q.sub.L1) 29; (3) an
in-phase version of the GPS L2 signal (I.sub.L2) 31; and (4) a 90
degree out-of-phase (quadrature) version of the GPS L2 signal
(Q.sub.L2) 33.
[0037] The frequency synthesizer 28 generates at least two signals
30, 32 from one or more local oscillators. In particular, the
frequency synthesizer 28 outputs two local oscillator signals
(LO.sub.1) 30 at 1400 MHz and (LO.sub.2) 32 at 175 MHz which are
input to the radio frequency module 24 and are used for the
frequency translation. In addition, the frequency synthesizer 28
outputs two clock signals, sclk 34 and msec 36. Sample clock signal
34 operates at 25 MHz and is used, in addition to the channel
processors 12, internally by the radio frequency module 24 to
digitally sample the filtered signal 22, after the filtered signal
is downconverted, to generate the intermediate frequency signals
26. The millisecond clock signal 36 operates at 1 kHz and is used
to time measurements in the channel processors 12. The frequency
synthesizer 28 is driven by a master oscillator signal 38 of 10 MHz
which is generated by a master oscillator 40. Thus, all the
frequency signals and clocks employed by the GPS receiver 10 for
frequency translation and measurements are derived from the same
fundamental source, master oscillator 40.
[0038] Because a single channel processor 12 is typically assigned
to operate on GPS signals emanating from a single GPS source, as
shown in FIG. 1, multiple channel processors 12 are typically
employed to allow the GPS signals from multiple GPS sources to be
observed simultaneously.
[0039] Each channel processor 12 interacts with a microprocessor
system 14 such that the microprocessor system 14 can control
operation of each channel processor including such functions, among
others, as closing the code and carrier tracking loops, reading
correlator values, and updating code and carrier numerically
controlled oscillators (NCOs). From information obtained from the
channel processors 12, the microprocessor system 14 outputs
position, velocity and time solutions to a user, or alternatively,
provides raw code and carrier phase measurements. While a user will
generally receive the outputs from the microprocessor system 14 on
a display, the outputs of the microprocessor system can also be
received by another microprocessor system or other device for
additional processing or to perform other functions, such as, for
example, controlling an automated vehicle.
[0040] FIG. 2 is a more detailed illustration of a channel
processor 12 according to an embodiment of this invention. Channel
processor 12 includes two tracker modules, an L1 tracker 52 and an
L2 tracker 54. The L1 tracker 52 observes the L1 intermediate
frequency signals I.sub.L1 and Q.sub.L1. Similarly, the L2 tracker
54 observes the L2 intermediate frequency signals I.sub.L2 and
Q.sub.L2. However, in addition to tracking the C/A-code component
in the GPS L1 signal, the L1 tracker 52 provides a W-code estimate
signal (W.sub.L1.sub.--.sub.NB) 56 which is a narrow-band
(noise-reduced) signal which estimates the W-code used by the GPS
source to generate the Y-code for the GPS L1 signal. This W-code
estimate signal 56 is employed by the L2 tracker 54 to facilitate
the semi-codeless tracking of one embodiment of the invention.
[0041] Because both the L1 tracker 52 and L2 tracker 54 are clocked
synchronously from the sample clock signal 34, the digital trackers
52, 54 are fully synchronous with each other. This, synchronicity
allows the microprocessor system 14 to place the L1 tracker 52 and
L2 tracker 54 into an exact known state using control signals 58
relative to the sample clock signal 34 and the millisecond clock
signal 36.
[0042] FIG. 3 is a detailed illustration of an L1 tracker 52 in
accordance with one embodiment of the invention. As shown in FIG.
3, the L1 intermediate frequency signals, I.sub.L1 and Q.sub.L1 at
the intermediate frequency of 420 kHz are received by a complex
mixer 62. The complex mixer 62 translates (downconverts) the L1
intermediate frequency signals, I.sub.L1 and Q.sub.L1, to baseband,
that is, the intermediate carrier frequency is removed (or nearly
removed) to generate in-phase and quadrature baseband signals
(I.sub.BB and Q.sub.BB) 63, 65. This translation employs in-phase
and quadrature local oscillator signals 67, 69 which are generated
by a carrier numerically controlled oscillator (carrier NCO)
64.
[0043] The baseband signals, I.sub.BB and Q.sub.BB, which still
retain the C/A-code and Y-code information, are then processed in a
code mixer 66. The code mixer 66 mixes each of the baseband
signals, I.sub.BB and Q.sub.BB with a locally generated C/A-code
replica signal 68 of the C/A-code. Each baseband signal, I.sub.BB
and Q.sub.BB, is also mixed with time delayed versions of the
replica signal 68. Typically, one time delayed version is early,
one is punctual, and one is late with respect to the corresponding
position in the C/A-code component of the respective baseband
signal, I.sub.BB and Q.sub.BB. In this way, the code mixer 66
generates six separate in-phase and quadrature signals, I.sub.E,
I.sub.P, I.sub.L, Q.sub.E, Q.sub.P, Q.sub.L, collectively,
(I.sub.EPL) 71 and (Q.sub.EPL) 73. These signals 71, 73 are
integrated in the correlators 72, across a period of time
determined by a C/A-code epoch signal 74. The C/A-code epoch signal
74 marks the intervals when a particular register, (the G1 register
(not shown)), used to generate the C/A-code contains all ones. The
microprocessor system 14 reads these signals, I.sub.EPL and
Q.sub.EPL, at regular intervals determined by the C/A-code epoch
signal 74, and processes the read information to determine, among
other functions, the position of the GPS receiver 10.
[0044] The code mixer 66 is provided with the C/A-code replica
signal 68 from a C/A-code generator 70. The C/A-code generator 70
is driven by a C/A-code chipping rate signal 76. The C/A-code
chipping rate, that is, the rate at which the bits (often called
"code bits" or "chips") of the C/A-code are transmitted by a GPS
source, is 1.023 MHz. The C/A-code chipping rate signal 76 is
generated by a divider 78 which divides a nominal P-code chipping
rate signal (N.sub.CLK) 80 by 10. The P-code chipping rate is the
rate at which bits of the P-code are transmitted by a GPS source
which is set at 10.23 MHz. The nominal P-code chipping rate signal
80 is generated by a code numerically controlled oscillator (code
NCO) 75. In addition to the C/A-code generator 70, the L1 tracker
52 includes a P-code generator 82.
[0045] In one embodiment of the invention, the operation and
interaction of the complex mixer 62, carrier NCO 64, code mixer 66,
C/A-code generator 70, divider 78, code NCO 75, correlators 72, and
P-code generator 82 are conventional, see for example, U.S. Pat.
No. 5,541,606 to Lennen, and "GPS Interface Control Document
ICD-GPS-200", published by Rockwell International Corporation,
Satellite Systems Division, Revision B-PR, Jul. 3, 1991. Thus,
conventional techniques can be used with the invention to acquire
GPS signals from GPS sources.
[0046] Once a GPS signal is acquired, and the GPS receiver 10 is
tracking the C/A-code component of the GPS L1 signal, that is, the
punctual signal I.sub.P is aligned (or almost aligned) with the
C/A-code component of the GPS L1 signal, a key aspect of the
invention can be activated. As discussed above, the Y-code
transmitted from the source is substantially unknown, and thus,
cannot be replicated in a civilian GPS receiver to facilitate full
Y-code tracking of that component in the GPS L2 signal. Therefore,
in this embodiment of the invention, the Y-code component of the
GPS L1 signal is used to provide a W-code estimate which replaces
an exact locally generated replica of the Y-code which would
conventionally be generated to track the Y-code component of the
GPS L2 signal.
[0047] To implement this concept in one embodiment of the
invention, as discussed below, L1 and L2 W-code estimate signals
are essentially multiplied together. However, this multiplication
is affected by noise.
[0048] The intermediate frequency signals, I.sub.L1, Q.sub.L1,
I.sub.L2, Q.sub.L2, contain versions of the components of the GPS
signals in addition to noise which arises from both the GPS source
and the travel of the GPS signals to the GPS receiver 10. The
amount of noise in the intermediate frequency signals 26 is
determined by both the signals' bandwidth and the pre-digitized
bandwidth. For a sampling rate of 25 MHz, the pre-digitizing
bandwidth is 12.5 MHz (which satisfies the Nyquist sampling
theorem, that is the minimum bandwidth to maintain the information
in the signal). This pre-digitizing bandwidth is maintained for the
baseband signals, I.sub.BB and Q.sub.BB, and subsequently the
wide-band L1 W-code estimate signal 88. Both the L1 and L2 W-code
estimate signals are substantially below the noise in the 12.5 MHz
bandwidth, for example, the signal-to-noise ratio in a 12.5 MHz
bandwidth is -25 decibels (dB). Thus, multiplying such noisy
signals together leads to an output with substantially increased
noise. In one embodiment of the invention, a 12.5 MHz bandwidth is
the wide-band bandwidth, and a 500 kHz bandwidth is the narrow-band
bandwidth.
[0049] FIG. 4 demonstrates this increased noise effect by
illustrating the loss in the signal-to-noise ratio from input to
output when multiplying the L1 W-code estimate signal by the L2
W-code estimate signal. More specifically, if the L1 W-code
estimate signal and the L2 W-code estimate signal are multiplied
together in their respective +/-12.5 MHz bandwidths, the input
signal-to-noise ratio is typically -25 dB. Thus, signal power
suffers a loss of 25 dB. However, if the input signal-to-noise
ratios of the L1 W-code estimate signal and the L2 W-code estimate
signal are improved before multiplying, this loss is substantially
reduced. Thus, for example, if the L1 W-code estimate signal and
the L2 W-code estimate signal bandwidths are reduced to 500 kHz
from 12.5 MHz the input signal-to-noise ratio will be: (-25 dB)+(14
dB)=(-11 dB), which results in an associated loss of approximately
11 dB. Hence, an important part of the invention is to optimally
process the signal-to-noise ratio of the W-code estimate signals
before the L1 W-code estimate signal and the L2 W-code estimate
signal are multiplied together.
[0050] This optimal processing in one embodiment of the invention
involves two techniques. The first technique is to ensure that
integration of the multiplied result is performed between the exact
W-code bit edges. The second technique is to add the L2 W-code
estimate signal to the L1 W-code estimate signal before multiplying
by the L2 W-code estimate signal for certain operations of the GPS
receiver 10.
[0051] More specifically, when the GPS receiver 10 is tracking the
C/A-code component of the GPS L1 signal using the in-phase baseband
signal I.sub.BB, as opposed to the quadrature baseband signal
Q.sub.BB, the in-phase baseband signal contains most, if not all,
of the C/A-code component signal power. This concentration of the
signal power occurs because the action of the C/A-code carrier
tracking loop (typically, controlled by software operating in the
microprocessor system 14) drives the carrier NCO 64 to minimize the
C/A-code power in the quadrature baseband signal Q.sub.BB, thereby
maximizing the C/A-code power in the in-phase baseband signal
I.sub.BB.
[0052] Because GPS sources transmit the C/A-code component in phase
quadrature with respect to the Y-code (or P-code) component on
respective carrier components which are also in phase quadrature
with respect to each other, minimizing the C/A-code component
signal power in the quadrature baseband signal Q.sub.BB also
maximizes the Y-code component signal power in the quadrature
baseband signal, Q.sub.BB. For this reason, according to one
embodiment of the invention, only the quadrature baseband signal
Q.sub.BB need be processed for the semi-codeless tracking of the
GPS L2 signal.
[0053] As shown in FIG. 3, the quadrature baseband signal Q.sub.BB
is provided to a multiplier 84. The multiplier 84 generates an
estimate of the W-code component of the GPS L1 signal from the
quadrature baseband signal Q.sub.BB by multiplying the quadrature
baseband signal Q.sub.BB with a locally generated replica of the
P-code component 86 of the GPS L1 signal. This P-code replica
signal 86 is generated by the P-code generator 82. As discussed
above, because Y-code is generated in GPS sources by encrypting a
known P-code with an unknown W-code, the attempt by the
multiplication in the multiplier 84 to strip the P-code component
from the Y-code component in the quadrature baseband signal
Q.sub.BB results in an estimate of the W-code component which is
provided as a wide-band L1 W-code signal (W.sub.LI.sub.--.sub.WB)
88. Furthermore, because the tracking of the L1 C/A-code signal
allows the receiver to provide a precise time estimate of the GPS
source's P-code transmission (typically, accuracies within to 2-3
nanoseconds (nsecs) of the actual transmission time), the P-code
generator 82 provides a P-code replica signal 86 which is
substantially aligned with the P-code component encoded in the
Y-code component generated by the GPS source. Because a procedure
for determining the W-code pattern has been developed for the
invention (see the discussion of the "H period" below), an H period
clock signal (H.sub.T) 90 generated by an H period generator 92
provides an exact time signal for the period of each W-code bit
which the W-bit integrator 94 uses to perform a conventional
integrate and dump operation on the wide-band W-code estimate
signal 88 to generate an improved narrow-band (that is, less noisy)
W-code estimate signal 96.
[0054] To implement the first technique of integrating the
multiplied L1 and L2 Y-code estimate signals between the edges of
the respective W-code bit, one embodiment of the invention includes
a process for determining the precise location in time of each
W-code bit edge. This process is illustrated in FIGS. 5a and 5b,
and has been ascertained by observing GPS satellites using
apparatus and methods described, for example, in U.S. Pat. No.
5,708,439 to Lennen.
[0055] The W-code timing pattern 100 as observed is the same for
all GPS sources. FIG. 5a illustrate that the W-code timing pattern
100 can be fundamentally described in terms of two chipping rates,
a first chipping rate of 20 P-code bits per W-code bit (about
0.5115 MHz) followed by a second chipping rate of 22 P-code bits
per W-bit (about 0.465 MHz) The first chipping rate is used for 11
bits of the W-code while the second chipping rate is used for the
next 21 bits of the W-code. This cycle repeats for every 32 bits of
the W-code. This 32 bit W-code cycle can be defined as the "H
period" 106. As shown in FIG. 5a, the H period 106 repeats every
682 bits of the P-code. Furthermore, as shown in FIG. 5b, the H
periods 106 are synchronized with the C/A-code periods 108 and each
C/A-code epoch corresponds to 15 H periods 106.
[0056] The information regarding the W-code timing pattern 100 does
not reveal anything about the underlying W-code bits, which remain
unknown. Therefore, the anti-spoofing capability of the Y-code is
not defeated by knowing the W-code timing pattern 100. However,
this knowledge of exact W-code timing pattern 100 allows for
optimal integration across each unknown W-code bit.
[0057] FIG. 6 shows the relative signal-to-noise ratio loss of
using 22 P-code bit or 20 P-code bit integration periods when
compared to the exact timing pattern of the H period 106. The
nominal GPS signal input signal-to-noise ratio in a 10 MHz
bandwidth is around -20 dB. Hence, from FIG. 5b, knowing and using
the exact W-code timing pattern 100 improves the signal-to-noise
ratio at the output of the W-code bit integration by approximately
0.9 dB 110 when compared to 22 P-code bit integration, and 3.5 dB
112 when compared to 20 P-code bit integration.
[0058] FIG. 7 provides a detailed illustration for an H period
generator 92 according to one embodiment of the invention. In this
embodiment, four dividers 122, 124, 126, 128 are used. These
dividers are reset to their initial conditions on the occurrence of
two events. The first event is the start of C/A-code epoch as
indicated by the C/A-code epoch signal 74 which ensures that the H
period 106 is synchronous with the C/A-code epochs. The second
event is the occurrence of each 682 bits as determined by the
nominal P-code clock signal 80. The 682-bit period signal 130
repeats at a rate synchronous with the H period 106. The H period
106 is made synchronous with the 682-bit period signal 130 and the
C/A-code epoch signal 74 by providing both signals to a logical OR
gate 132 so that either signal can trigger the reset of the
dividers 122, 124, 126, 128 with reset signal 133. More
specifically, the nominal P-code clock signal 80 at a nominal rate
10.23 MHz is divided by 22 by divider 124 to generate the second
chipping rate signal (N.sub.22) 134 and is divided by 20 in divider
122 to generate the first chipping rate signal (N.sub.20) 136. Both
the first and second chipping rate signals 134, 136 are provided to
a multiplexer 138. The multiplexer 138 can output either signal as
the H period timing signal (H.sub.T) 90. A switching signal (S) 140
determines whether multiplexer 138 should output the first chipping
rate signal 136 or the second chipping rate signal 134 based on
whether the first 11 bits of the H period's 32 bits have been
output. Because the first 11 bits of the H period 106 correspond to
220 P-code bits, the switching signal 140 is generated by dividing
first chipping rate signal 136 (20 P-code bits per W-code bit) by
11 in divider 126. The 682-bit period signal 130 is provided by
dividing the second chipping rate signal 134 (22 P-code bits per
W-code bit) by 31 in divider 128.
[0059] FIG. 8 illustrates the relative timing of the first chipping
rate signal 136, the second chipping rate signal 134, the switching
signal 140, the 682-bit period signal 130, and the H period timing
signal 90. Thus, as shown in FIG. 8, the resulting H period timing
signal (H.sub.T) 90, is a stream of pulses whose period may be 20
or 22 P-code bits wide depending on which bits of the W-code are
being observed.
[0060] FIG. 9 illustrates the L2 tracker 54 according to one
embodiment of the invention. The L2 tracker 54 processes in-phase
and quadrature intermediate frequency signals (I.sub.L2 and
Q.sub.L2) 31, 33. Similar to the L1 tracker 52, the combination of
a complex mixer 152 and carrier NCO 154 remove the carrier
component from the intermediate frequency signals, I.sub.L2 and
Q.sub.L2 to provide in-phase and quadrature baseband signals
(I.sub.BB and Q.sub.BB) 153, 155. Analogously to the removal of the
C/A-code component in the L1 tracker 52, P-code mixer 156 removes
the P-code component from the baseband signals I.sub.BB and
Q.sub.BB, using a P-code replica signal 158 generated by P-code
generator 160. The P-code generator 160 is driven by a nominal
P-code clock signal (N.sub.CLK) 162 generated by a code NCO 164
which, again, is similar to the L1 tracker 52 operation. The
wide-band W-code signals 166 output from the P-code mixer 156
contain estimated versions of the W-code component of the GPS L2
signal with wide-band noise. These estimated wide-band W-code
signals 166 include a wide-band W-code in-phase punctual signal
(W.sub.L2IP.sub.--.sub.WB) 171, a wide-band quadrature punctual
signal (W.sub.L2QP.sub.--.sub.WB) 167, and a wide-band in-phase
early-minus-late signal (W.sub.L2IEL.sub.--.sub.WB) 169. These
estimated wide-band W-code signals 166 are then integrated across
respective W-code bit periods by the W-code integrators 168 to
generate respective narrow-band versions of the wide-band signals
(W.sub.L2IP.sub.--.sub.NB) 173, (W.sub.L2QP.sub.--.sub.NB) 175,
(W.sub.L2IEL.sub.--.sub.NB) 177. These estimated narrow-band W-code
signals 170 are multiplied by the W-code estimate signal 96
generated by the L1 tracker 52 in multipliers 172. The output
signals from of the multipliers are stripped of the L2 carrier
component, the P-code, and as much of the W-code as possible
without knowing the W-code. These stripped signals (I.sub.P) 181,
(Q.sub.P) 183, (I.sub.E-L) 185 are further integrated in the
correlators 176. The correlators 176 are read by the microprocessor
system 14 at intervals determined by the L2 C/A-code epoch signal
178. The microprocessor system 14 then uses these values for code
and carrier tracking loops based on the GPS L2 signal. The code
tracking loop is closed when the microprocessor system 14 provides
the control signal 180 to adjust the P-code replica signal by
changing the output of the P-code NCO 160. Similarly, the carrier
tracking loop is closed when the microprocessor system 14 provides
the control signal 182 to adjust the carrier replica signals 157,
159 generated by the carrier NCO 154.
[0061] FIG. 10 provides a detailed illustration of the P-code mixer
156 according to one embodiment of the invention. As shown in FIG.
10, the P-code replica signal 158 is used to generate time delayed
versions 192, 194, 196 using a shift register 198. The separation
in time is determined by the sample clock signal 34, which in this
embodiment, has a period of is 40 nsecs (that is, a frequency of 25
MHz). In a first multiplier 202, the in-phase baseband signal
I.sub.BB is mixed with the punctual version 194 of the delayed
P-code replica signal 158, which results in the wide-band W-code
in-phase punctual signal 171. Similarly, a second multiplier 204
multiplies the quadrature baseband signal Q.sub.BB by the punctual
P-code replica signal 194 which results in the wide-band W-code
quadrature punctual signal 167. A third multiplier 206 multiplies
the in-phase baseband signal I.sub.BB by an early version of the
P-code replica signal 192, and a fourth multiplier 208 multiplies
the in-phase baseband signal I.sub.BB by the late version of the
P-code replica signal 196. The outputs of third and fourth
multipliers 206, 208 are subtracted from each other in subtractor
210 which results in the wide-band W-code in-phase early-minus-late
signal 169, which can be used, after further processing by the
microprocessor system 14, to close the L2 P-code tracking loop. The
W-code punctual signals 167, 171, are used by the microprocessor
system 14 to search and detect L2 signal power and then to close
the L2 carrier tracking loop. All signals at the output of the
P-code mixer 156 are in the original wide-band sampling bandwidth
of +/-12.5 MHz.
[0062] FIG. 11 provides a detailed illustration of the W-Code
integrators 168 according to one embodiment of the invention. In
this embodiment, the nominal P-code clock signal (N.sub.CLK) 162 is
provided to divider 212, which divides the nominal P-code clock
signal by 10 to generate a nominal C/A-code clock signal 214 (1.023
MHz) which in turn drives the C/A-code generator 216. In this
embodiment, the C/A-code generator 216 provides a C/A-code epoch
signal 178 which drives the H period generator 220 and the
correlators 176. The H period generator 220 in the L2 tracker 54 is
similar to H period generator of the L1 tracker 52 which has been
described above. The wide-band W-code signals 167, 169, 171 are
integrated across the respective W-code bit periods as defined by H
period signal (H.sub.T) 228 in integrators 222, 224, 226. The
output narrow-band W-code signals (W.sub.L2IP.sub.--.sub.NB,
W.sub.L2QP.sub.--.sub.NB and W.sub.L2IEL.sub.--.sub.NB) 223, 225,
227 represent optimal integrations across the L2 W-code bit
periods.
[0063] FIG. 12 provides a detailed illustration of the multipliers
172 according to one embodiment of the invention. As discussed
above, these multipliers 172 are used to multiply the optimally
integrated (that is across the respective W-code bit), the W-code
estimate signal 96 of the L1 tracker 52 by the three versions of
the narrow-band L2 W-code signals 167, 169, 171. Specifically, a
first multiplier 232 combines the W-code estimate signal 96 with
the narrow-band W-code in-phase punctual signal 171 to generate an
in-phase punctual signal 181. Also, a second multiplier 236
combines the narrow-band W-code quadrature punctual signal 167 with
the output of a multiplexer 238 to produce a quadrature punctual
signal 183. A third multiplier 242 combines the narrow-band W-code
in-phase early-minus-late signal 169 with the output of multiplexer
238 to produce an early minus late signal 185.
[0064] The multiplexer 238 output is selectable by the
microprocessor system 14 from two inputs, the W-code estimate
signal 96 from the L1 tracker 52 and the combined W-code estimate
signal 246 which is generated by adding the W-code estimate signal
96 from the L1 tracker 52 and the narrow-band W-code in-phase
punctual signal 171, which is the W-code estimate signal for the
GPS L2 signal. These W-code estimate signals 96, 171 represent the
best estimates of the W-code component for the GPS L1 and L2
signals, respectively. When searching for L2 signal power or when
not locked to the L2 carrier, the microprocessor system 14 selects
the W-code estimate signal 96 for output from multiplexer 238,
since the W-code estimate signal 171 from the L2 tracker 171 is not
yet accurate. However, once the local version of the L2 carrier
signal is phase locked with the GPS L2 signal's carrier, the
combined W-code estimate signal 246 provides the best estimate of
the W-code contribution to the GPS L2 signal.
[0065] FIG. 13 provides an illustration of the signal-to-noise
ratio gain versus relative signal power (GPS L2 signal relative to
the GPS L1 signal) in accordance with one embodiment of the
invention. FIG. 13 shows that if the received GPS L2 signal power
is identical to the received GPS L1 signal power, that is, a
relative signal power of 0 dB, the signal-to-noise ratio when
combining the W-code estimate signals 96, 171 achieves a gain of +3
dB 252 in comparison to using the W-code estimate signal 96 from
the L1 tracker 52 alone.
[0066] GPS sources nominally transmit the L2 Y-code (or P-code)
component 3 dB weaker than the L1 Y-code (or P-code) component.
Therefore, the nominal gain from using both techniques is typically
about +1.7 dB 254. However, because GPS L2 signal semi-codeless
tracking is performed at substantially weaker signal-to-noise
ratios than full code tracking, the signal-to-noise ratio gain
provided by this invention becomes significant in augmenting both
tracking ability and measurements.
[0067] The combined W-code estimate signal 246 provides an
advantage primarily if the noise in the narrow-band W-code
integrated signals 170 is uncorrelated with the combined W-code
estimate signal 246. Because the W-code estimate signal 171 from
the L2 tracker 54 is input to multiplier 232 and is also part of
the combined W-code estimate signal, the noise is somewhat
correlated. Accordingly, multiplier 232 uses the W-code estimate
signal 171 from the L2 tracker 54 instead of the combined W-code
estimate signal 246.
[0068] The microprocessor system 14 reads the correlators 176 to
track the GPS L2 signal. For example, the in-phase early-minus-late
signal (I.sub.E-L) 185 is used by the code tracking loop after it
has been accumulated in the correlators 176 as the estimate of the
code phase error and the quadrature punctual signal (Q.sub.P) 183
is driven to zero as a function of closing the carrier tracking
loop.
[0069] FIG. 14 provides a flow chart illustrating a procedure 300
for finding power and tracking the GPS L2 in accordance with one
embodiment of the invention. In the first step 310, the GPS
receiver 10 searches for and starts tracking the C/A-code component
of the GPS L1 signal which can be performed conventionally.
[0070] Once this tracking is achieved, the GPS receiver 10 is
"locked" to the L1 C/A-code and sufficient information is now
available for the second step 320, that is, setting up the P-code
generator 82. One conventional way to set up the P-code generator
82 employs conventional Z-count decoding, see for example, the
reference ICD-GPS-200, above. Once set up, the locally generated
P-code replica signal 86 is then substantially aligned in time with
the L1 C/A-code and Y-code components of the GPS L1 signal and the
Y-code component power from the GPS L1 signal appears primarily in
quadrature baseband signal Q.sub.BB. This results in the W-code
estimate signal 96 providing an estimate of the W-code component
signal power. Also, at this step 320, the H period generator 92 has
been synchronized with the W-code component of the GPS L1 signal
because the H period generator 92 has been reset by the C/A epoch
signal 74 when the GPS receiver 10 became locked to the C/A-code
component of the GPS L1 signal in step 310. Thus, the W-code
estimate signal 96 is providing an optimal estimate of the L1
W-code component.
[0071] The next step 330, sets up the carrier NCO 154 of the L2
tracker 54 with an accurate frequency extracted from the L1 carrier
tracking loop. As shown in some conventional GPS receivers, the L1
carrier frequency is first scaled by the relative carrier
frequencies, that is, L1 carrier frequency/L2 carrier frequency,
and is applied to the carrier NCO 154 in the L2 tracker 54. While
an error is introduced due to the different effect of the
ionospheric path on the two frequencies, this "carrier aiding" is
advantageous (a narrower bandwidth for the L2 tracking loop can be
used) until the L2 carrier loop has determined most of its dynamic
requirements and can properly track the L2 carrier frequency by
itself.
[0072] The next step 340, synchronizes the code NCO 164 and P-code
generator 160 of the L2 tracker 54 with the code NCO 75 and P-code
generator 82 of the L1 tracker 52, respectively. Because the
microprocessor system 14 already knows the state of the code NCO 75
and P-code generator 82 of the L1 tracker 52, this process only
involves copying the state of the L1 tracker 52 elements to the
corresponding L2 tracker 54 elements.
[0073] The next step 350 is to set up multiplexer 238 to output the
W-code estimate signal 96 from the L1 tracker 52. This selection
ensures an optimal signal-to-noise ratio during the L2 power search
step 360, that is, before the L2 carrier loop has been phase
locked.
[0074] If GPS L2 signal power is not detected, that is, the sum of
the squares of the in-phase and quadrature signals (I.sub.P) 181
and (Q.sub.P) 183 does not pass a predetermined threshold, in step
365, the code NCO 164 is phase shifted by 0.5 P-code bits and power
is rechecked. When power is found, in step 370, the L2 code and
carrier tracking loops are closed by the microprocessor system 14.
Once the carrier tracking loop for the GPS L2 signal is closed and
locked, step 380, multiplexer 238 is controlled by the
microprocessor system 14 to output the combined W-code estimate
signal 246, step 390.
[0075] In order to determine whether the L2 carrier tracking loop
is locked for step 390, the microprocessor system 14 uses the
carrier phase error. FIG. 15 provides an illustration of the
signal-to-noise ratio gain achieved when applying the combined
W-code estimate signal 246 from the multiplexer 238 with respect to
a carrier phase error between the carrier component of the GPS L2
signal and the locally generated L2 carrier signal 157. Because, as
discussed above, one can assume that the GPS L2 signal power is 3
dB less than the GPS L1 signal power, the gain provided by using
the combined W-code estimate signal 246 instead of the W-code
estimate signal 96 diminishes as the carrier phase error increases.
This effect is illustrated in FIG. 15 where the zero crossing point
for gain 410 is around the 34.degree. carrier phase error.
Therefore, using the combined W-code estimate signal 246 is
advantageous when the carrier phase error is less than 34.degree.,
but when the carrier phase error is greater than 34.degree. using
the W-code estimate signal 96 from the L1 tracker 52 is preferable.
Accordingly, microprocessor system 14 uses the correlator outputs
175, for example, I.sub.PC and Q.sub.PC to estimate the carrier
phase error either by calculating arctangent (Q.sub.PC/I.sub.PC) or
by determining the phase error value of (I.sub.PC.times.Q.sub.PC).
This estimate, or a filtered version thereof, can then be used to
generate switch signal 248 to control the multiplexer 238.
[0076] This invention allows for any number of bits to be used to
represent the signals in the GPS receiver. For example, in one
embodiment of the invention, because the intermediate frequency
signals, I.sub.L1 and Q.sub.L1, are quantized and sampled at a
sampling rate of 25 MHz, the quantization dictates the number of
binary bits used to represent each sample. Once such conventional
quantization scheme uses 1-bit quantization, while others use 2-bit
quantization. However, while multi-bit quantization is beneficial
in reducing quantization loss, decreasing loss in the
signal-to-noise ratio, and in improving jamming immunity, in most
GPS receiver designs, the number of bits used tends to increase
("fan out") as the signals are processed. For example, if the
intermediate frequency signals, I.sub.L1 and Q.sub.L1 are 2-bit
quantized, and the replica carrier signals 67, 69 are 2-bit
quantized, then the baseband signal I.sub.BB and Q.sub.BB should be
3-bit numbers. Thus, the number of bits for representing the
signals are determined by other factors in a GPS receiver design
than required for the invention.
[0077] While this invention has been described with respect to a
particular GPS receiver design, it should be understood that other
GPS receiver designs can employ this invention, for example, a GPS
receiver in which the intermediate frequency translation is
performed after the code mixer.
[0078] Thus, it is apparent that in accordance with the present
invention an apparatus and method that fully satisfies the
objectives, aims, and advantages is set forth above. While the
invention has been described in conjunct ion with specific
embodiments and examples, it is evident that many alternatives,
modifications, permutations, and variations will become apparent to
those skilled in the art in the light of the foregoing description.
Accordingly, it is intended that the present invention embrace all
such alternatives, modifications and variations as fall within the
scope of the appended claims.
* * * * *