Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2000-02-04
2001-06-26
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490
Reexamination Certificate
active
06252545
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to satellite navigation systems, and, more particularly, to satellite navigation systems employing modulation schemes to enhance signal-detection sensitivity.
2. Description of the Related Art
A satellite navigation system, such as the Global Positioning System (GPS), comprises a constellation of satellites that transmit GPS signals that can be used by a wireless terminal to determine the wireless terminal's position.
FIG. 1
is a schematic diagram of GPS system
100
of the prior art. In prior art system
100
, one or more satellites
101
of a satellite constellation transmit GPS signals
102
that are received by a wireless terminal
103
. As is known in the field, the positioning operation is performed by receiving GPS signals
102
from three or more satellites. The basic method of determining position is based on knowing the time difference for each of the satellites. The time difference for each satellite is the time required for a GPS signal
102
initiated at the satellite to be received by wireless terminal
103
. When GPS signals
102
from three satellites are simultaneously received, a “two-dimensional” position (latitude and longitude) can be determined. When GPS signals
102
are received from four or more satellites simultaneously, a “three-dimensional” position (latitude, longitude, and altitude) can be determined.
Each satellite
101
orbits earth at a known speed and is located at a known distance apart from the other satellites. Each satellite
101
transmits a unique GPS signal
102
which includes a carrier signal with a known frequency f modulated using a unique pseudo-random noise (PN) code and navigational data associated with the particular satellite
101
, wherein the PN code includes a unique sequence of PN chips and navigation data includes a satellite identifier, timing information and orbital data, such as elevation angle &agr;
j
and azimuth angle &phgr;
j
.
Wireless terminal
103
generally comprises a GPS receiver
105
for receiving GPS signals
102
, a plurality of correlators
107
for detecting GPS signals
102
and a processor
109
having software for determining a position using the navigation data. GPS receiver
105
detects GPS signals
102
via PN codes. Detecting GPS signals
102
involves a correlation process wherein correlators
107
are used to search for PN codes in a carrier frequency dimension and a code phase dimension. Such correlation process is implemented as a real-time multiplication of a phase shifted replicated PN codes modulated onto a replicated carrier signal with the received GPS signals
102
, followed by an integration and dump process.
In the carrier frequency dimension, GPS receiver
105
replicates carrier signals to match the frequencies of the GPS signals
102
as they arrive at GPS receiver
105
. However, due to the Doppler effect, the frequency f at which GPS signals
102
are transmitted changes an unknown amount &Dgr;f
j
before GPS signal
102
arrives at GPS receiver
105
—that is, each GPS signal
102
should have a frequency f+&Dgr;f
j
when it arrives at GPS receiver
105
. To account for the Doppler effect, GPS receiver
105
replicates the carrier signals across a frequency spectrum f
spec
ranging from f+&Dgr;f
min
to f+&Dgr;f
max
until the frequency of the replicated carrier signal matches the frequency of the received GPS signal
102
wherein &Dgr;f
min
and &Dgr;f
max
are a minimum and maximum change in frequency GPS signals
102
will undergo due to the Doppler effect as they travel from satellites
101
to GPS receiver
105
, i.e., &Dgr;f
min
£&Dgr;f
j
£&Dgr;f
max
.
In the code phase dimension, GPS receiver
105
replicates the unique PN codes associated with each satellite
101
. The phases of the replicated PN codes are shifted across code phase spectrums R
j
(spec) until replicated carrier signals modulated with the replicated PN codes correlate, if at all, with GPS signals
102
being received by GPS receiver
105
, wherein each code phase spectrum R
j
(spec) includes every possible phase shift for the associated PN code. When GPS signals
102
are detected by correlators
107
, GPS receiver
105
extracts the navigation data ND from the detected GPS signals
102
and uses the navigation data to determine a location for GPS receiver
105
, as is well-known in the art.
Correlators
107
are configured to perform parallel searches for a plurality of PN codes across the frequency spectrum f
spec
and the code phase spectrums R
j
(spec). In other words, each of the plurality of correlators
107
are dedicated to searching for a particular PN code across each possible frequency between f+&Dgr;f
min
to f+&Dgr;f
max
and each possible for that PN code. When a correlator
107
completes its search for a PN code, correlator
107
searches for another PN code across each possible frequency between f+&Dgr;f
min
to f+&Dgr;f
max
and each possible phase shift for that PN code. This process continues until all PN codes are collectively searched for by the plurality of correlators
107
. For example, suppose there are twelve satellites
101
thus there would be twelve unique PN codes. If GPS receiver
105
has six correlators
107
, then GPS receiver
105
would use its correlators
107
to search for two sets of six different PN codes at a time. Specifically, correlators
107
search for the first six PN codes, i.e., a first correlator searches for PN−1, a second correlator searches for PN−2, etc. Upon completing the search for the first six PN codes, correlators
107
search for the next six PN codes, i.e., a first correlator searches for PN−7, a second correlator searches for PN−8, etc.
For each PN code being searched, correlator
107
performs an integration and dump process for each combination of frequency and phase shifts for that PN code. For example, suppose the frequency spectrum f
spec
includes 50 possible frequencies for the carrier signal and the code phase spectrum R
j
(spec) for a PN code includes 2,046 possible half-chip phase shifts. To search for every possible combination of frequency and half-chip phase shifts for the PN code, the correlator
107
would then need to perform 102,300 integrations. A typical integration interval for correlators
107
is 1 ms, which is generally sufficient for GPS receiver
105
to detect GPS signals
102
when the wireless terminal has a clear view of the sky or a direct line-of-sight to satellites
101
. Thus, for the above example, 102.3 seconds would be required for one correlator
107
to search every possible combination of frequency and half-chip phase shifts for a PN code.
One disadvantage of the prior art is that, if GPS signal
102
is attenuated by a building or other obstacles, it may become impossible for a wireless terminal to receive sufficiently strong GPS signals from the minimum number of satellites needed to determine the position of the wireless terminal. This results in an interruption of the position determination. To compensate for weaker GPS signals and enhance detection of GPS signals
102
, correlators
107
can be configured with longer integration intervals. In other words, detection is more accurate with longer integration intervals.
However, the presence of the navigation data limits the signal-detection capabilities of a wireless terminal by limiting the length of the integration interval to 20 ms.
SUMMARY OF THE INVENTION
If the wireless terminal has an independent knowledge of the navigation data, it can utilize this knowledge to extend its integration interval beyond 20 ms. For example, if the wireless terminal can receive navigation data from a source other than the GPS signal source (i.e., the transmitting satellite), the wireless terminal can use this information to perform a “data wipe-off” operation. The “data wipe-off” is simply a sign operation on the incoming raw RF data before the integration and dump process. The “data wipe-off” operation is ba
Da Ren
Vannucci Giovanni
Lucent Technologies - Inc.
Mendelsohn Steve
Phan Dao
LandOfFree
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