Method and system for providing a user with precision location information

A method and system for providing a user with correction information for use in accurately determining the location of the user. A plurality of monitor stations are disposed at a plurality of known fixed locations for monitoring a plurality of navigation data messages transmitted by Global Positioning Systems (GPS) Space Vehicles (SVs) and for determining a plurality of corresponding correction factors. A central ground station is operatively coupled to each of the plurality of monitor stations for receiving the plurality of correction factors from each of the plurality of monitor stations. A GPS transceiver receives at least one of the plurality of navigation data messages at an unknown location and determines a general location of the user. The general location of the user is then transmitted to the central ground station. The central ground station determines user location correction data based on the navigation data received by the user and the plurality of correction factors transmitted by each of the plurality of monitor stations. The central ground station then transmits the user location correction data to the user for use in determining the location of the user with sub-meter precision. An accounting record may be created for the user for receiving the precision correction information so that the user can be subsequently billed a fee for the services provided by the central ground station.

TECHNICAL FIELD 
This invention relates to a Global Positioning System (GPS) and, more 
particularly, to a method and system for providing a user with precision 
location information utilizing the GPS for use in accurately determining 
the location of the user. 
BACKGROUND ART 
The United States Department of Defense created the Global Positioning 
System (GPS) to allow military ships, aircraft and ground vehicles to 
determine their location anywhere in the world. GPS consists of a 
satellite segment, a ground control segment, and user receivers. The 
satellite segment consists of 24 satellites placed asymmetrically in six 
orbital planes where each plane is inclined by 55.degree. relative to the 
equatorial plane. Each satellite continuously broadcasts direct-sequence, 
spread-spectrum signals on which passive receivers can perform precise 
ranging measurements. Each broadcast is also modulated with a navigation 
data message, which is developed by the ground control station and 
provides data to the user such as the satellite location and clock error. 
For each of several satellites, the user equipment measures a "pseudo 
range" and demodulates the navigation message. The pseudo range is equal 
to the true range from the receiver to the satellite plus errors, either 
intentionally or unintentionally, introduced in the range measurement. 
Pseudo range measurements to four well-spaced satellites are sufficient to 
solve for the user's three-dimensional position and clock offset. 
GPS includes the standard positioning service (SPS) which provides civilian 
users with 100 meter accuracy. Civilian users are guaranteed access to a 
1.023 MHz spreading code (the C/A code) which modulates a signal at a 
frequency of 1575.42 MHz. Errors are introduced, however, due to several 
slowly varying biases arising from ionospheric refraction, tropospheric 
refraction, stability of the satellite clock, predictability of the orbit 
of the satellite, stability of the receiver, direct sequence tracking, and 
selective availability. Selective availability is the largest error source 
and is intentionally introduced by the Department of Defense for national 
security reasons. With selective availability enabled, the SPS provides 
100 meter horizontal accuracy. Additional information regarding GPS can be 
found in "The Global Positioning System: Signals, Measurements, and 
Performance" by Per K. Enge, INTERNATIONAL JOURNAL OF WIRELESS INFORMATION 
NETWORKS, Volume 1, No. 2, 1994. 
Another approach to providing precision navigation involves the use of the 
Wide Area Augmentation System (WAAS) utilized by the Federal Aviation 
Agency (FAA) and available to the general public. The WAAS system improves 
the integrity, accuracy, availability and continuity of the GPS Standard 
Positioning Service (SPS). The WAAS system includes a terrestrial network 
of stations (wide area reference stations) monitoring the performance of 
GPS satellites, and a WAAS geostationary satellite. The reference stations 
continuously report to regionalist master stations (wide area master 
stations). The master stations process the observed data to determine SPS 
signal corrections and whether a system performance fault has occurred. 
The SPS signal corrections and a system performance fault, if any, is 
up-linked to a geostationary satellite via a command antenna (ground earth 
station) for immediate rebroadcast by the satellite to users using the 
WAAS service. 
The WAAS system provides correction data to all users that have a WAAS 
receiver. The WAAS system, however, fails to provide a user with precise 
location information since all users throughout the Continental United 
States receive the same SPS correction signals. Furthermore, this system 
does not have a way to create an accounting record so the user can be 
subsequently charged for receiving the correction data. In addition, there 
is a delay in providing the user with signal integrity since a system 
fault has to first be determined by the master stations and then 
transmitted to the geostationary satellite before it is broadcasted to the 
user. 
DISCLOSURE OF THE INVENTION 
It is thus a general object of the present invention to provide a method 
and system for providing a user with precision correction information for 
use in accurately determining the location of the user. 
It is another object of the present invention to provide a method and 
system for creating an accounting record for a user receiving precision 
information in determining the location of the user. 
In carrying out the above objects and other objects, features and 
advantages, of the present invention, a method is provided for providing a 
user with precision correction information so the user can accurately 
determine the location of the user. The method includes the steps of 
receiving at least one of a plurality of navigation data messages, 
including position estimates and navigation data, at an unknown location 
and determining a general location of the user based on the at least one 
of a plurality of navigation data messages. The method further includes 
the step of transmitting a user data message including data corresponding 
to the general location of the user to a central ground station. Still 
further, the method includes the step determining user location correction 
data based on the user data message and a plurality of correction factors 
transmitted by a plurality of monitor stations disposed at known fixed 
locations. The method concludes the step of transmitting the user location 
correction data to the user for use in determining the location of the 
user with sub-meter precision. 
In further carrying out the above objects and other objects, features and 
advantages, of the present invention, a system is also provided for 
carrying out the steps of the above described method. The system includes 
a plurality of monitor stations disposed at a plurality of known fixed 
locations for monitoring the navigation data messages transmitted by the 
GPS and for determining a plurality of corresponding correction factors. 
The system also includes a central ground station operatively coupled to 
each of the plurality of monitor stations for receiving the plurality of 
correction factors from each of the plurality of monitor stations and for 
determining an integrity of the navigation data messages. Still further, 
the system includes a first transceiver for receiving at least one of the 
plurality of navigation data messages at an unknown location and 
determining a general location of the user accordingly and for generating 
a corresponding user data message. The system also includes a second 
transceiver for transmitting the user data message to the central ground 
station. The central ground station determines user location correction 
data based on the user data message and the plurality of correction 
factors transmitted by each of the plurality of monitor stations. The 
central ground station further transmits the user location correction data 
to the user for use in determining the location of the user with sub-meter 
precision. 
The above objects and other objects, features and advantages of the present 
invention are readily apparent from the following detailed description of 
the best mode for carrying out the invention when taken in connection with 
the accompanying drawings.

BEST MODES FOR CARRYING OUT THE INVENTION 
Turning now to FIG. 1, there is illustrated a block diagram of the system 
of the present invention, denoted generally by reference numeral 10. The 
system 10 includes a plurality of Global Positioning System (GPS) Space 
Vehicles (SVs) 12 orbiting in space. The GPS SVs 12 broadcast 
pseudo-random navigation data messages to earth. The navigation data 
messages include three-dimensional position estimates of the SVs 12, clock 
correction information, and navigation data. 
The system 10 also includes a plurality of monitor stations (MS) 16 
disposed at known fixed locations throughout a predetermined area, such as 
the United States. Since the SPS of GPS SVs 12 induces a 100 meter ranging 
error, each of the monitor stations 16 monitor the navigation data 
messages broadcasted by the GPS SVs 12 and determine a correction factor 
based on the known location of the monitor station 16. That is, based on 
the navigation data messages broadcasted by the GPS SVs 12, the monitor 
station 16 determines its location and compares the determined location to 
the known location of the monitor station 16. Based on the comparison, 
each of the monitor stations 16 computes a correction factor. 
Each of the monitor stations 16 are operatively coupled to a central ground 
station 18 for receiving the correction factors from each of the monitor 
stations 16. The central ground station 18 determines correction factors 
for each of the monitor stations 16 and points in between the monitor 
stations 16. The monitor stations 16 and the central ground station 18 are 
typical computer-based systems capable of receiving the pseudo-random 
signals of SPS and generating a position or location signal. 
A user seeking to determine his/her location receives the navigation 
signals from the GPS SVs 12 via a GPS transceiver 20. A GPS navigation 
receiver, such as a Scout.TM. or Scout Master.TM. GPS receiver 
manufactured by Trimble, may be used. The GPS SVs 12 provide the user with 
signals which may be used to determine his general location within 100 
meters without additional corrections. The GPS transceiver 20 generates a 
user data message including a three-dimensional position representing the 
general location of the user based on the navigation data messages 
broadcast by the GPS SVs 12. The user data message also includes 
information regarding which SVs 12 was used by the user, the user's time 
estimate and pseudo-range measurement data. In order to obtain more 
accurate information regarding the user's position, the user transmits the 
user data message to the central ground station 18 via a transceiver 22. 
The transceiver 22 may be either a wireline-based telephone system or a 
wireless telephone system, such as a cellular or satellite telephone 
system. In the case of a satellite telephone system, the system 10 
includes a communication SV 14 for providing satellite communication. The 
transceiver 22 may also be a unique transceiver in which the GPS 
transceiver 20 and the transceiver 22 are combined into a single unit. 
The central ground station 18 then determines location correction data to 
transmit to the user based on the user data message. Based on the 
navigation data message received from the GPS SVs 12 or the information in 
the user data message, either the central ground station 18 or the 
plurality of monitor stations 16 can also determine whether or not the 
messages broadcasted by the GPS SVs 12 to the user are good or bad, i.e., 
the integrity of the signal. This information can be determined and sent 
to the user as quickly as one second, since the information is transmitted 
directly to the user rather than to a geostationary satellite which 
ultimately broadcasts the information to the user. The central ground 
station 18 determines the location correction data in one of two ways: 1) 
utilizing the correction factor of a monitor station 16 located nearest 
the user; or 2) determining an intermediate correction factor based upon 
the correction factors of two or more monitor stations 16 if the user is 
located between two or more monitor stations. The central ground station 
18 then transmits the user location correction data to the user via the 
transceiver 22. The user location correction data includes data regarding 
the integrity of the navigation signal and the correct location of the 
user. 
The location correction data is then transmitted to the GPS transceiver 20 
via the transceiver 22 which then determines the location of the user with 
sub-meter accuracy. 
An accounting record may be created by the central ground station 18 so the 
user can be subsequently charged for receiving the more accurate position 
information. This accounting record can be created based on the 
communication link established by the user to the central ground station. 
The user is charged for this sub-meter accuracy based upon the amount of 
time the user was tied into the system 10 in the same way that a user is 
charged a fee for telephone services. The user may be charged by 
determining the total time the user is connected to the central ground 
station 18, or the user may be charged a per-transaction fee in which the 
total number of times the user connects himself to the central ground 
station 18 is determined. 
Turning now to FIG. 2, there is shown a flow diagram illustrating the 
sequence of steps associated with the method of the present invention. At 
least one of a plurality of navigation data messages are received by a 
user at an unknown location, as shown at block 30. 
The GPS transceiver 20 then determines the general location of the user 
based on the at least one navigation data message, as shown at block 31, 
and generates a corresponding user data message. To obtain more accurate 
information, the user transmits the user data message to the central 
ground station 18, as shown at block 32. 
The central ground station 18 then determines user location correction data 
based on the user data message and a plurality of correction factors 
transmitted by the plurality of monitor stations 16, as shown at block 34. 
Either the plurality of monitor stations 16 or the central ground station 
18 may also determine integrity of the at least one navigation data 
message based on the known fixed location of the plurality of monitor 
stations 16 and the user data message, as shown at block 35. 
Next, the central ground station 18 transmits user location correction data 
indicating the integrity of the navigation signal and the correct location 
of the user to the user as shown at block 36. Finally, the user is able to 
determine his location with sub-meter precision, as shown at block 37, 
based on the user location correction data. 
An accounting record may be created for the user for receiving the user 
location correction factor from the central ground station 18, as shown at 
block 38. The accounting record is then utilized for billing the user for 
the services provided by the central ground station 18, as shown at block 
40. The amount of the fee charged to the user is based on the amount of 
time that has elapsed between the transmission of any navigation data 
message to the central ground station 18 and the transmission of any 
navigation data to the user. The amount charged can also be determined 
based on a per-transaction basis as described above. 
The present invention may also be used in conjunction with INMARSAT 
(International Maritime Satellite) and the FAA Wide Area Augmentation 
System programs. Potential commercial applications include airplanes that 
need to do "type 3" landings, surveyors, and other users who require high 
accuracy position and velocity type information. 
While the best modes for carrying out the invention have been described in 
detail, those familiar with the art to which this invention relates will 
recognize various alternative designs and embodiments for practicing the 
invention as defined by the following claims.