Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
1999-09-07
2002-10-01
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
Reexamination Certificate
active
06459405
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to wireless communication systems and, in particular, to satellite-based location systems.
BACKGROUND OF THE RELATED ART
Satellite-based navigational systems provide accurate, three dimensional position information to worldwide users. Prior art satellite-based navigational systems, however, utilize a time consuming search process for determining position information. Time consuming search processes are undesirable in navigational systems particularly when the user is moving or in an emergency situation requiring immediate assistance.
FIG. 1
depicts a well-known satellite-based navigational system referred to as Global Positioning System (GPS)
10
. GPS
10
comprises a plurality of satellites
12
-j and at least one GPS receiver
14
, where j=1,2, . . . ,n. Each satellite
12
-j orbiting earth at a known speed v
j
and being a known distance apart from the other satellites
12
-j. Each satellite
12
-j transmits a GPS signal
11
-j which includes a carrier signal with a known frequency f modulated using a unique pseudo-random noise (PN-j) code and navigational data (ND-j) associated with the particular satellite
12
-j, wherein the PN-j code includes a unique sequence of PN chips and navigation data ND-j includes a satellite identifier, timing information and orbital data, such as elevation angle &agr;
j
and azimuth angle &phgr;
j
.
FIG. 2
depicts a typical
20
ms frame of the GPS signal
11
-j which comprises twenty full sequences of a PN-j code in addition to a sequence of navigation data ND-j.
GPS receiver
14
comprises an antenna
15
for receiving GPS signals
11
-j, a plurality of correlators
16
-k for detecting GPS signals
11
-j and a processor
17
having software for determining a position using the navigation data ND-j, where k=1,2, . . . ,m. GPS receiver
14
detects GPS signals
11
-j via PN-j codes. Detecting GPS signals
12
-j involves a correlation process wherein correlators
16
-k are used to search for PN-j 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-j codes modulated onto a replicated carrier signal with the received GPS signals
11
-j, followed by an integration and dump process.
In the carrier frequency dimension, GPS receiver
14
replicates carrier signals to match the frequencies of the GPS signals
11
-j as they arrive at GPS receiver
14
. However, due to the Doppler effect, the frequency f at which GPS signals
11
-j are transmitted changes an unknown amount &Dgr;f
j
before GPS signal
11
-j arrives at GPS receiver
14
— that is, each GPS signal
11
-j should have a frequency f+&Dgr;f
j
when it arrives at GPS receiver
14
. To account for the Doppler effect, GPS receiver
14
replicates the carrier signals across a frequency spectrums 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
11
-j, wherein &Dgr;f
min
and &Dgr;f
max
are a minimum and maximum change in frequency GPS signals
11
-j will undergo due to the Doppler effect as they travel from satellites
12
-j to GPS receiver
14
, i.e., &Dgr;f
min
≦&Dgr;f
j
≦&Dgr;f
max
.
In the code phase dimension, GPS receiver
14
replicates the unique PN-j codes associated with each satellite
12
-j. The phases of the replicated PN-j codes are shifted across code phase spectrums R
j
(spec) until replicated carrier signals modulated with the replicated PN-j codes correlate, if at all, with GPS signals
11
-j being received by GPS receiver
14
, wherein each code phase spectrum R
j
(spec) includes every possible phase shift for the associated PN-j code. When GPS signals
11
-j are detected by correlators
16
-k, GPS receiver
14
extracts the navigation data ND-j from the detected GPS signals
11
-j and uses the navigation data ND-j to determine a location for GPS receiver
14
, as is well-known in the art.
Correlators
16
-k are configured to perform parallel searches for a plurality of PN-j codes across the frequency spectrum f
spec
and the code phase spectrums R
f
(spec). In other words, each of the plurality of correlators
16
-k are dedicated to searching for a particular PN-j code across each possible frequency between f+&Dgr;f
min
to f+&Dgr;f
max
and each possible for that PN-j code. When a correlator
16
-k completes its search for a PN-j code, the correlator
16
-k searches for another PN-j code across each possible frequency between f+&Dgr;f
min
to f+&Dgr;f
max
and each possible phase shift for that PN-j code. This process continues until all PN-j codes are collectively searched for by the plurality of correlators
16
-k. For example, suppose there are twelve satellites
12
-j, thus there would be twelve unique PN-j codes. If GPS receiver
14
has six correlators
16
-k, then GPS receiver
14
would use its correlators
16
-k to search for two sets of six different PN-j codes at a time. Specifically, correlators
16
-k search for the first six PN-j codes, i.e., correlator
16
-
1
searches for PN-
1
, correlator
16
-
2
searches for PN-
2
, etc. Upon completing the search for the first six PN-j codes, correlators
16
-k search for the next six PN-j codes, i.e., correlator
16
-
1
searches for PN-
7
, correlator
16
-
2
searches for PN-
8
, etc.
For each PN-j code being searched, correlator
16
-k performs an integration and dump process for each combination of frequency and phase shifts for that PN-j code. For example, suppose the frequency spectrum f
spec
includes 50 possible frequencies for the carrier signal and the code phase spectrum R
f
(spec) for a PN-j 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-j code, the correlator
16
-k would then need to perform 102,300 integrations. A typical integration time for correlators
16
-k is 1 ms, which is generally sufficient for GPS receiver
14
to detect GPS signals
11
-j when antenna
15
has a clear view of the sky or a direct line-of-sight to satellites
12
-j. Thus, for the above example, 102.3 seconds would be required for one correlator
16
-k to search every possible combination of frequency and half-chip phase shifts for a PN-j code.
GPS receivers, however, are now being incorporated into mobile-telephones or other types of mobile communication devices which do not always have a clear view of the sky. Thus, GPS receiver
14
will not always have a clear view of the sky. In this situation, the signal-to-noise ratios of GPS signals
11
-j received by GPS receiver
14
are typically much lower than when GPS receiver
14
does have a clear view of the sky, thus making it more difficult for GPS receiver
14
to detect the GPS signals
11
-j. To compensate for weaker signal-to-noise ratios and enhance detection of GPS signals
11
-j, correlators
16
-k can be configured with longer integration times. A sufficient integration time, in this case, would be approximately 1 second. Thus, for the example above, 102,300 seconds would be required for a correlator
16
-k to search for every possible combination of frequency and half-chip phase shifts for a PN-j code. Longer integration times result in longer acquisition times for detecting GPS signals
11
-j. Longer acquisition times are undesirable.
Wireless assisted GPS (WAG) systems were developed to facilitate detection of GPS signals
11
-j by GPS receivers configured with short or long integration times. The WAG system facilitates detection of GPS signals
11
-j by reducing the number of integrations to be performed by correlators searching for GPS signals
11
-j. The number of integrations is reduced by narrowing the frequency range and code phase ranges to be searched. Specifically, the WAG system limits the search for GPS signals
11
-j to a specific frequency or frequencies and to a range of code phases less than the code phase spectrum R
Da Ren
Vannucci Giovanni
Goo Jimmy
Lucent Technologies - Inc.
Mull Fred H
Tarcza Thomas H.
LandOfFree
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