Data processing: vehicles – navigation – and relative location – Navigation – Employing position determining equipment
Reexamination Certificate
1999-12-10
2001-09-11
Cuchlinski, Jr., William A. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Navigation
Employing position determining equipment
C701S207000, C342S357490, C342S357490, C342S357490, C340S988000
Reexamination Certificate
active
06289280
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to locating the position of devices, and specifically to a method and apparatus for determining the position of a device based upon information provided from Global Positioning System (GPS) satellites and associated position location systems.
BACKGROUND OF THE INVENTION
Recent developments in Global Position System (GPS) and terrestrial mobile communications make it desirable to integrate GPS functionality into mobile communications devices such as cellular mobile stations. The cellular geolocation problem can be solved using either network-based methods or using handset-based methods.
Terrestrial Location
Network-based solutions rely on the signal transmitted from the mobile station and received at multiple fixed base stations. This can be accomplished by measuring the Time of Arrival (TOA) of the mobile station signal at the base stations. The mobile will lie on a hyperbola defined by the difference in time of arrival of the same signal at different base stations. An accurate position estimate depends on accurate synchronization and signal structure (bandwidth, etc.).
GPS-based Location
GPS-based location relies on a constellation of 24 satellites (plus one or more in-orbit spares) circling the earth every 12 hours. The satellites are at an altitude of 26,000 km. Each satellite transmits two signals: L1 (1575.42 MHz) and L2 (1227.60 MHz). The L1 signal is modulated with two Pseudo-random Noise (PN) codes-the protected (P) code and the coarse/acquisition (C/A) code. The L2 signal carries only the P code. Each satellite transmits a unique code, allowing the receiver to identify the signals. Civilian navigation receivers use only the C/A on the L1 frequency.
The idea behind GPS is to use satellites in space as reference points to determine location. By accurately measuring the distance from three satellites, the receiver “triangulates” its position anywhere on earth. The receiver measures distance by measuring the time required for the signal to travel from the satellite to the receiver. However, the problem in measuring the travel time is to know exactly when the signal left the satellite. To accomplish this, all the satellites and the receivers are synchronized in such a way that they generate the same code at exactly the same time. Hence, by knowing the time that the signal left the satellite, and observing the time it receives the signal based on its internal clock, the receiver can determine the travel time of the signal. If the receiver has an accurate clock synchronized with the GPS satellites, three measurements from three satellites are sufficient to determine position in three dimensions. Each pseudorange (PR) measurement gives a position on the surface of a sphere centered at the corresponding satellite. The GPS satellites are placed in a very precise orbit according to the GPS master plan. GPS receivers have a stored “almanac” which indicates where each satellite is in the sky at a given time. Ground stations continuously monitor GPS satellites to observe their variation in orbit. Once the satellite position has been measured, the information is relayed back to the satellite and the satellite broadcasts these minor errors “ephemeris” along with its timing information as part of the navigation message.
It is very expensive to have an accurate clock at the GPS receiver. In practice, GPS receivers measure time of arrival differences from four satellites with respect to its own dock and then solve for both the user's position and the clock bias with respect to GPS time.
FIG. 1
shows four satellites
101
,
102
,
103
,
104
and a GPS receiver
105
. Measuring time of arrival differences from four satellites involves solving a system of four equations with four unknowns given the PR measurements and satellite positions (satellite data) as shown in FIG.
1
. In other words, due to receiver clock error, the four spheres will not intersect at a single point. The receiver then adjusts its clock such that the four spheres intersect at one point.
Hybrid Position Location System
The terrestrial location solution and the GPS solution complement each other. For example, in rural and suburban areas not too many base stations can hear the mobile station, but a GPS receiver can see four or more satellites. Conversely, in dense urban areas and inside buildings, GPS receivers may not detect enough satellites. However, the mobile station can see two or more base stations. The hybrid solution takes advantage of cellular/PCS information that is already available to both the mobile station and the network. Combining GPS and terrestrial measurements provides substantial improvements in the availability of the location solution. The hybrid position location system may combine Round-trip Delay (RTD) and Pilot phase measurements from the terrestrial network with GPS measurements.
The hybrid approach merges GPS and network measurements to compute the location of the mobile station. The mobile station collects measurements from the GPS constellation and cellular/PCS network. These measurements are fused together to produce an estimate of the mobile station position.
When enough GPS measurements are available, it is unnecessary to use network measurements. However, when there are less than four satellites or, in the case of bad geometry, four or more satellite measurements, the measurements must be complemented with network measurements. The minimum number of measurements for obtaining a solution will be equal to the number of unknowns. Since the system has four unknowns (three coordinates and GPS receiver time bias) the minimum number of measurements to obtain a solution will be four. For any satellite measurements that are not available, round trip delay (RTD) measurements may be used to determine the range to a base station. RTD measurements may also be used to provide time aiding information. In addition other information, such as PN offset pseudo-ranges (if time bias is the same as for satellites), PN offset differences (if time bias is different) and altitude aiding may be used to provide additional information and thus increase the number of equations that include the unknowns being sought (i.e., x, y, z, and time offset). As long as the total number of equations is larger than four it will be possible to find a solution.
Round Trip Delay (RTD)
The pilot timing on the forward link of each sector in the base station is synchronized with GPS system time. The mobile station time reference is the time of occurrence, as measured at the mobile station antenna connector of the earliest arriving usable multipath component being used in the demodulation. The mobile station time reference is used as the transmit time of the reverse traffic and access channels.
FIG. 2
shows one terrestrial transceiver station
201
and a mobile station
202
. As shown in
FIG. 2
, the mobile
202
uses the received time reference from the serving base station
201
as its own time reference. Accounting for its own hardware and software delays, the mobile station transmits its signal such that it is received back at the serving base station
201
delayed by a total of 2&tgr;, assuming that the forward and reverse links have essentially equal propagation delays. The total delay is measured at the base station by correlating the received signal from the mobile station
202
with the referenced signal at time T
sys
. The measured RTD corresponds to twice the distance between the mobile
202
and the base station
201
(after calibration of base station side hardware delays).
Note that knowledge of the PN of the serving base station can also be used (due to sectorization as a rough angle of arrival (AOA) measurement) to help with resolving ambiguity.
Pilot Phase Measurements
The mobile station is continuously searching for active and neighboring pilots. In the process, it measures the PN offset of each pilot it receives. If the time reference is the same on both PN offset and satellite measurements then the bias on these measurements (as measured at
Agashe Parag A.
Fernandez-Corbaton Ivan J.
Soliman Samir S.
Vayanos Alkinoos Hector
Beaulieu Yonel
Brown Charles
Cuchlinski Jr. William A.
Greenhaus Bruce
Qualcomm Incorporated
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