1. Field of the Invention
The present invention relates generally to a GPS receiver, and more particularly to a GPS handset.
2. Description of the Background Art
The Global Positioning System (GPS) is a satellite-based system developed by the U.S. Department of Defense to give accurate positional information to a GPS receiver anywhere in the world. A properly equipped GPS receiver may therefore be used in any setting in which position is desired, and typically yields positional coordinates in three dimensions. The GPS system is enabled by a satellite orbital constellation made up of 24 or more satellites orbiting the earth in 12 hour orbits. The satellites are arranged in six orbital planes, each containing four satellites. The orbital planes are spaced sixty degrees apart, and are inclined approximately fifty-five degrees from the equatorial plane. This constellation ensures that from four to twelve satellites will be visible at any time at any point on earth with a clear view of the sky.
The GPS satellites transmit data to be used by GPS receivers, including satellite position data (ephemeris data) and satellite clock correction data. The GPS signal includes a carrier signal that is bi-phase modulated with a 1023 bit long Gold spreading code at a 1.023 Mhz chip rate (0.001 second repeat interval). It is also modulated by data bits at a 50 bits per second (BPS) rate (transmitted at a rate of twenty milliseconds per data bit). The 50 BPS data includes information for determining a GPS-based time (i.e., a clock time of the GPS satellite) and information for determining geographical location.
Detailed information on the data contained within the GPS signal is available in Interface Control Document ICD-GPS-200, revised in 1991, published by Rockwell International Corporation and incorporated herein by reference.
The clock time included in the GPS message is an absolute time signal that is precisely synchronized at the satellite. That is, all satellites in the GPS constellation are synchronized by ground reference stations that take into account the signal propagation time from the satellite to the ground station. In this manner, every satellite in the constellation is time synchronized at the satellite, the absolute time error is precisely controlled to within a few nanoseconds to tens of nanoseconds.
The absolute time signal may be used by a GPS receiver in order to accurately determine position. Once position is approximately known, the absolute time in the receiver can also be known by offsetting the precise time observed by the receiver in the satellite broadcast message by the computable propagation delay between the receiver and the satellite.
Knowledge of absolute time in the receiver is an important parameter because the GPS satellites move at approximately four meters per millisecond. If the range measurement time is in error by one millisecond, then the measured ranges can be in error by as much as four meters. This range error then multiplies by a geometry factor (GDOP, or Geometric Dilution Of Precision) to translate into additional positional error that can be many times the four meters of additional range error.
A Time Of Week (TOW) data field included in the 50 BPS data, in conjunction with the absolute time signal, allows a GPS receiver to accurately and reliably determine a local time. The TOW data is transmitted by all satellites at six second intervals. The detection of the TOW data is dependent on the signal magnitude. Below a certain signal magnitude level it is possible to obtain a range measurement, but it is not possible to decode the TOW data. For example, for signal levels below approximately 30 dB-Hz, it becomes impossible to decode individual message bits of the 50 BPS message. It is possible, however, to obtain signal correlation at signals substantially below 30 dB-Hz, down to levels below 20 dB-Hz. Techniques employed by Motorola applications Ser. Nos. 09/253,318, 09/253,662, and 09/253,679 can be used to extend the sensitivity of correlation detection to these levels. Thus, what is needed is a method of determining time at signal levels below 30 dB-Hz.
The GPS receiver cannot always reliably determine a local time from the GPS satellite broadcast data. GPS functions are very useful and as a result have been incorporated into a variety of devices, including, for example, cellular phones, and other hand-held electronic devices. Due to their portable nature, such devices are often in vehicles traveling in urban canyons, or carried into buildings or other obstacles. As a natural result, the GPS signal may be blocked or poorly received. This may make the reception of the 50 BPS absolute time signal unreliable. In such cases, it is desirable that a GPS handset obtain an accurate time measurement so that positional measurements are available. Because GPS satellites move at a rate of approximately 4 meters per millisecond, if a time of a range measurement is not known with accuracy, the range measurement and therefore the resulting positional measurement will have a proportional error. For example, if the measurement time contains an error of 20 milliseconds, the range measurement may be in error by as much as 80 meters, and the resulting position measurement may be in error by several hundreds of meters depending on the geometry.
In some cellular telephone devices, such as CDMA (code division multiple access) cellular phones, the infrastructure is synchronized and every base station receives a precise time from a network GPS receiver. The CDMA base station then synchronizes mobile devices by transmitting the time to the mobile devices, allowing a time in the CDMA phone handset to be known to an accuracy on the order of one microsecond (plus a transmission delay).
In non-synchronized GPS devices, such as, for example, Global System for Mobile communications (GSM) cellular phones, precise time information is desirable but generally not available from the signals emanating from the network. However, it is impractical and expensive to modify existing networks in order to synchronize them by adding appropriate hardware and/or software.
The related art has attempted to solve the problem in a variety of ways. A first scheme is given in Krasner, U.S. Pat. No. 5,812,087. Krasner uses a digital snapshot memory in a handset to capture a random swath of data and transmit it to a base station. The base station uses a conventional GPS receiver to measure the time of arrival of a portion of the 50 BPS data bits captured in the digital snapshot memory. The data bits measured by the base station have a known time of arrival and are correlated against the unknown data bits captured in the digital snapshot memory. When a maximum correlation is obtained, the data capture time in the mobile memory can be determined.
However, the drawback in Krasner is that a remote base station GPS receiver is required. The remote base station GPS receiver must measure a current 50 GPS data sequence from one or multiple GPS satellites and then transmit it to the mobile unit. Alternatively, the mobile unit may transmit post-detection correlation data to the base. In either manner, the scheme requires that a large number of bits be transmitted between the base station and the mobile unit (for example, 12 satellites time 50 bits each), and it requires that the handset store a sample of the received data for subsequent correlation with the pattern observed and communicated from the base station.
In a second related art scheme, Location Measurement Units (LMUs) are placed throughout an unsynchronized network. The purpose of the LMU is to measure the time offset between a time as kept by an unsynchronized communication network and GPS time. An individual LMU measures the time of arrival of message bits from each base station and determines the relative time offset of each base station. This is accomplished by a GPS receiver inside an LMU, and with knowledge of the location of each base station and the location of the LMU (for purposes of determining a propagation time).
The second related art scheme, however, is costly because it requires additional components in a communication network. It is also a complicated solution.
In a third related art scheme, the 50 BPS data sequence is observed at a fixed master site. The time data is transmitted from the master site to a mobile unit. The mobile unit may use the known data pattern and its known time of arrival (observed by the master site) to narrow the acquisition bandwidth below the 50 BPS data rate.
Again, a major drawback is that the measured 50 BPS data sequence must be transmitted to the mobile unit, and the mobile unit must be able to receive and demodulate the data.
There remains a need in the art, therefore, for a time-of-measurement determination in a GPS handset that is part of a non-synchronized network, such as the GSM system.
A method for determining time in a GPS receiver is provided according to a first aspect of the invention. The method comprises the steps of capturing GPS data from a GPS message for a predetermined period of time, locating a expected data pattern in the captured data, and determining a time offset between a data capture start time and a time of arrival of the expected data pattern.
A method for determining time in a GPS receiver is provided according to a second aspect of the invention. The method comprises the steps of capturing post-correlation coherent GPS data for a predetermined period of time, locating a expected data pattern in the captured data, and determining a time offset between a data capture start time and a time of arrival of the expected data pattern.
A method for determining time in a GPS receiver is provided according to a third aspect of the invention. The method comprises the steps of down converting the GPS message to a predetermined intermediate frequency, sampling the intermediate frequency to produce a plurality of samples, storing the plurality of samples into a memory, determining a code phase for each visible GPS satellite signal, determining a Doppler shift for each visible GPS satellite signal in the memory, selecting a satellite GPS data, measuring and storing an in-phase coherent characteristic of a post correlation data over a plurality of predetermined time periods, measuring and storing a quadrature coherent characteristic of a post correlation data over a plurality of predetermined time periods, coherently correlating the in-phase and the quadrature characteristics to a expected data pattern, and determining a time offset from a correlation maximum, with the correlation maximum occurring the time offset after a beginning of the capturing step.
A method for determining time in a GPS receiver is provided according to a fourth aspect of the invention. The method comprises the steps of down converting the GPS message to a predetermined intermediate frequency, sampling the intermediate frequency to produce a plurality of samples, non-coherently correlating the plurality of samples against each visible satellite PRN and each visible satellite Doppler, determining a real-time code phase and a real-time Doppler shift for each visible GPS satellite signal, selecting satellite GPS data, measuring and storing a real-time in-phase coherent characteristic of post-correlation data over a plurality of predetermined time periods, measuring and storing a real-time quadrature coherent characteristic of post correlation data over a plurality of predetermined time periods, coherently correlating the in-phase and the quadrature characteristics to a expected data pattern, and determining a time offset from a correlation maximum, with the correlation maximum occurring the time offset after a beginning of the non-coherent correlating step.
An apparatus for determining time in a GPS receiver is provided according to a fifth aspect of the invention. The apparatus comprises a GPS down converter for down converting a GPS signal to an intermediate frequency, the down converting capable of being connected to a GPS signal source, an analog-to-digital converter connected to the down converter for converting intermediate frequency data to digital data, a digital IF switch connected to the analog-to-digital converter and connected to a clock, the digital IF switch is controlled by the clock to periodically close for a predetermined sample time period, an IF memory connected to the digital IF switch and capable of storing a plurality of digital IF data available when the digital IF switch is closed, a non-coherent GPS correlator connected to the IF memory and capable of determining a code phase characteristic, determining a Doppler characteristic, and determining a signal strength characteristic for the plurality of digital IF data, a coherent GPS correlator connected to both the IF memory and the non-coherent GPS correlator and producing in-phase and quadrature coherent measurements, a memory storage switch connected to the coherent GPS correlator, the memory storage switch periodically closing to produce a plurality of in-phase and quadrature measurements, a samples memory connected to the memory storage switch and capable of storing the plurality of in-phase and quadrature measurements, a correlator connected to the samples memory and to a expected data pattern storage, the correlator coherently correlating the plurality of in-phase and quadrature measurements to a expected data pattern to produce a correlation output, a coherent integrator connected to the correlator, the coherent integrator integrating the correlation output to produce an integration output, a magnitude determiner connected to the coherent integrator, the magnitude determiner detecting a magnitude of the integration output and generating a plurality of integration magnitudes, a magnitude versus delay memory connected to the magnitude determiner and storing the plurality of integration magnitudes, and a maximum integration magnitude determiner connected to the magnitude versus delay memory and capable of determining a maximum integration magnitude, wherein a delay period of a maximum signal magnitude is a time offset.
An apparatus for determining time in a GPS receiver is provided according to a sixth aspect of the invention. The apparatus comprises a GPS down converter for down converting a GPS signal to an intermediate frequency, the down converting capable of being connected to a GPS signal source, an analog-to-digital converter connected to the down converter for converting intermediate frequency data to a plurality of digital data, a non-coherent GPS correlator connected to the analog-to-digital converter and capable of determining a code phase characteristic, determining a Doppler characteristic, and determining a signal strength characteristic for the plurality of digital data, a coherent GPS correlator connected to both the analog-to-digital converter and the non-coherent GPS correlator and producing in-phase and quadrature coherent measurements, a memory storage switch connected to the coherent GPS correlator, the memory storage switch periodically closing to produce a plurality of in-phase and quadrature measurements, a samples memory connected to the memory storage switch and capable of storing the plurality of in-phase and quadrature post-correlation measurements, a correlator connected to the samples memory and to a expected data pattern storage, the correlator coherently correlating the plurality of in-phase and quadrature measurements to a expected data pattern to produce a correlation output, a coherent integrator connected to the correlator, the coherent integrator integrating the correlation output to produce an integration output, a magnitude determiner connected to the coherent integrator, the magnitude determiner detecting a magnitude of the integration output and generating a plurality of integration magnitudes, a magnitude versus delay memory connected to the magnitude determiner and storing the plurality of integration magnitudes, and a maximum integration magnitude determiner connected to the magnitude versus delay memory and capable of determining a maximum integration magnitude, wherein a delay period of a maximum signal magnitude is a time offset.
The above and other features and advantages of the present invention will be further understood from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.