Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
2000-11-18
2004-12-14
Chin, Wellington (Department: 2664)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S337000, C370S339000
Reexamination Certificate
active
06831911
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a system and method for receiving and processing Global Positioning System (GPS) and wireless phone signals using a combination receiver, more particularly, receiving and processing GPS signals and wireless signals during alternate time segments by suspending reception of GPS signals during times when wireless signal is received. During the time period when a wireless signal is received, a local clock keeps track of the time interval in order to maintain correlation between the stored and received PRN code.
2. Prior Art
The GPS is a satellite-based system that can be used to locate positions anywhere on the earth. GPS provides continuous (24 hours/day), real-time, 3-dimensional positioning, navigation and timing worldwide. Any person with a GPS receiver can access the system, and it can be used for any application that requires location coordinates.
The nominal GPS Operational Constellation consists of 24 satellites that orbit the earth in 12 hours. The GPS User Segment, which consists of the GPS receivers and the user community, can receive and convert satellite signals into position, velocity, and time estimates. Four satellites are required to solve the four unknowns of X, Y, Z (position) and Time.
The system works on the concept of trilateration. Trilateration is a principle that allows one to find a particular point, when the distance to other locations is known. In two dimensions one must know the distance to other points which distance is the radius of a circle, which circles overlap at the unknown point. In three dimensions the principle also works, however a point is located by determining its location on the surface of a sphere rather than a circle as in the two dimensional example.
In order to locate a point on the surface of intersecting spheres, the GPS system must be able to determine the distance from a series of satellites, i.e. the radius of each sphere. A GPS receiver calculates its distance from each satellite using a technique called satellite ranging by measuring the distance between the GPS receiver and satellite. The range calculated by the receiver is actually estimated and known as a pseudorange and is measured by the elapsed transit time for a signal from the satellite to the receiver.
Measuring the distance between the satellite and receiver requires finding the time interval it takes for the satellite signal to travel to the receiver and multiplying the signal travel time by the speed of light to compute the distance. The travel time interval of the satellite signal is determined by finding the time that the signal left the satellite and the time that the signal reaches the receiver. Determining when the signal reaches the receiver is easily accomplished by referring to an internal clock in the receiver, however to accurately calculate the travel time interval, it is necessary for the receiver to also know when the signal left the satellite. The GPS receiver's internal clock must be synchronized with the satellite's clock to calculate the correct interval. Synchronizing each GPS receiver with the satellites so that they generate the same digital code at the same time does this. The code sequence is known as a pseudorandom noise (PRN). When a GPS receiver receives a code from a satellite, it compares the received code to its own emitted code. The offset of the two codes reveals the travel time interval, from which the distance can be calculated. However the times involved are quite short and therefore great accuracy is necessary. An error of only one thousandth of a second, at the speed of light, will result in a distance error of almost 200 miles. In order to eliminate the need for this near perfect timing, a GPS receiver, performs a measurement to four satellites. Any error introduced by an error in timing can then be overcome by calculating a single correction factor that it can be applied to all timing measurements to cause the four spheres to intersect at a single point. Typically, the signal from 4 satellites is required for a position fix to determine 3 coordinates (e.g. latitude, longitude and height, or X, Y and Z) and a fourth to determine the clock offset.
In order to determine the location of a user, a GPS receiver needs to determine the pseudorange from at least 4 satellites and must receive navigation data from each satellite, which is the location of each satellite which is periodically corrected and updated. The pseudorange is determined by receiving a signal from a satellite, demodulating the signal to determine the PRN code and then measuring the time shift of the transmitted PRN code to local PRN code of the receiver. The satellite locations are determined from the navigation data transmitted by each satellite within the transmitted data frames. The navigation data is typically received from the satellites by receiving and processing a complete set of twenty-five frames of a navigation message, although it could also be downloaded into a GPS receiver using a variety of means, such as from another GPS device or through the internet.
The GPS Navigation Message consists of time-tagged data bits marking the time of transmission of each subframe at the time they are transmitted by a satellite. As shown in
FIG. 1
, a data bit frame consists of 1500 bits
101
divided into five 300-bit subframes
102
-
106
. A data frame is transmitted every thirty seconds. Three six-second subframes contain orbital and clock data. Corrections of the satellite clocks are sent in subframe one
102
with precise satellite orbital data sets (ephemeris data parameters) for the transmitting satellite sent in subframes two
103
and three
104
. Subframes four
105
and five
106
are used to transmit different pages of system data. An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over a 12.5 minute period. Data frames (1500 bits) are sent every thirty seconds. Each frame consists of five subframes. Data bit subframes (300 bits transmitted over six seconds) contain parity bits that allow for data checking and limited error correction. Each subframe includes a header having a telemetry word
107
and a handover word
108
.
As shown in
FIG. 2
, each GPS satellite transmits two microwave carrier signals. The L1 frequency
201
(1575.42 MHz) and L2 frequency
205
(1227.60 MHz). Three binary codes
202
-
204
shift the L1 and/or L2 carrier phase.
The GPS receiver produces replicas of the C/A and/or P (Y)-Code. Each PRN code is a noise-like, but pre-determined, unique series of bits. The receiver produces the C/A code sequence for a specific satellite with some form of a C/A code generator. The receiver of the current invention can either store a complete set of precomputed C/A code chips in memory for each satellite, or a hardware shift register, implementation can also be used. In a shift register implementation the code chips are shifted in time by slewing the clock that controls the shift registers. In a memory lookup scheme the required code chips are retrieved from memory.
It is also necessary for making accurate timing calculations to know exactly where each satellite is in relation to the earth, therefore all GPS satellites occupy high orbits that are very predictable. The Department of Defense measures any variations in a satellite orbit. The satellite orbit information along with any orbit error information is sent to the satellites, which is then transmitted along with the timing signals to the receiver.
The GPS satellite signal includes GPS data bits where each data bit is modulated by a pseudorandom noise (PRN) code sequence that is distinct for each GPS satellite. The GPS data bits are twenty milliseconds long and have a time-of-transmission from the GPS satellite of a GPS-based clock time of 00 hours, 00 minutes, 00 seconds, and 000 milliseconds and each twenty milliseconds thereafter each day. The PRN code sequence is one millisecond long and includes 1023 chips. Each GPS data bit is mo
Sridharan Guruswami M.
Sridharan Kartik M.
Ashvattha Semiconductor Inc.
Chin Wellington
Goodwin & Procter LLP
Schwaltz William
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