Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
1998-07-22
2001-12-04
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C342S357490
Reexamination Certificate
active
06326916
ABSTRACT:
TECHNICAL FIELD
This invention relates to location determination systems. Specifically, the present invention relates to an apparatus and method for coupling correction signals to a position determination device for accurately determining position.
BACKGROUND ART
One class of position determination systems and devices determine the location of receivers using data broadcast by satellites. One constellation of satellites is the Global Positioning System (GPS). The GPS consists of a constellation of 24 orbiting satellites that transmit timing information and the satellite's ephemerides via microwave radio.
Position determination devices determine position by receiving the timing signals and ephemerides from four or more satellites. The timing information from each satellite is analyzed in order to determine the apparent distance from the position determination system to each satellite. The determination of apparent distance is made by measuring the time it takes for the signals to travel from each satellite to the receiver of the position determination system. These apparent distances are referred to as pseudoranges.
Pseudoranges are calculated by measuring the time it takes for the signal to travel from the satellite to the receiver. The satellites mark their transmissions digitally and the receiver compares the time it receives the time mark with its own time clock. The time delay, referred to as transit time, is typically in the range of about 70-90 milliseconds. Distance is then determined by multiplying transit time by the speed of radio transmissions (approximately 300,000,000 meters/second).
Since the ephemeride data includes the location of each satellite, position may be determined by a geometric calculation that uses the known satellite positions and calculated distances (pseudoranges). GPS based positions are calculated using the World Geodetic System of 1984 (WGS84) coordinate system. These positions are expressed in Earth Centered Earth Fixed (ECEF) coordinates of X, Y, and Z axes. These positions are often transformed into latitude, longitude, and height relative to the WGS84 ellipsoid.
One factor that introduces error into the process of determining location is atmospheric conditions. Another source of error results from the intentional introduction of error into the transmitted ephemerides and clock by the U.S. Air Force (referred to hereinafter as “selective availability” or “S/A”). The GPS navigation signals commonly available to civilian users are referred to as the standard positioning service (SPS). The accuracy of SPS is currently specified by the Department of Defense (DOD) to be within 100 meters horizontal position 95 percent of the time and 300 meters 99.99 percent of the time. Errors also result from atmospheric conditions. Though the specified horizontal accuracy may be adequate for some applications such as navigation of a vessel in the open ocean, other applications require an increased level of accuracy.
One method for obtaining accurate position that compensates for intentionally induced error and error due to atmospheric conditions is known as Differential GPS (DGPS). DGPS systems receive correction data broadcast from a DGPS reference station. A DGPS reference station is located at a fixed and known location. By using this information combined with the satellites' broadcast ephemerides, an actual range to each satellite is able to be determined. By differencing the received range measurement (pseudorange) with this calculated range, a correction to the pseudoranges received at other GPS receivers in the local area can be broadcast to those other receivers that are attempting to solve for their own local location. This correction includes all induced satellite clock errors and atmospheric (ionosphere, troposphere) errors.
DGPS systems typically determine position in one of two ways. Traditionally, positions have been calculated using code phase differential techniques. These are normally referred to as DGPS. More recently, carrier phase techniques have been used to determine position. These systems are referred to as Real Time Kinematic (RTK) systems.
DGPS reference stations may be dedicated facilities with permanent and/or extensive broadcast capabilities or may be simply a transient DGPS receiver with data transmitter located at a known location. DGPS reference stations transmit either their calculated corrections to the GPS signals or their actual observations of the GPS signals (raw data), or both. When transmitting calculated corrections, errors due to atmospheric (troposphere, ionosphere, etc.) and errors due to satellite timing/clock (both intentional and process noise) are represented by the correction value. The application of these corrections at a DGPS receiver will compensate for these error sources.
Differential GPS reference stations may also transmit their observations of the GPS signals for each satellite. This method of transmission is popular with RTK positioning techniques and systems due to the nature of typical RTK processing methods. When using this type of data format, errors associated with atmospherics and satellite timing/clock errors are removed at the moving/roving/differential GPS receiver. Most manufacturers of RTK systems typically broadcast this data in a format unique to the particular manufacture.
Other sources of correction data that include correction data for S/A and atmospheric conditions include broadcasts that conform to the Radio Technical Commission for Maritime services (RTCM) format. The RTCM has established standards describing format standards, communication bands, and messages for a differential correction GPS service. Correction data that conforms to the RTCM format is broadcast by the US Coast Guard and others to assist in maritime navigation. The US Coast Guard has regional DGPS reference stations that calculate and broadcast correction data using the RTCM format. The RTCM correction data broadcast by some US Coast Guard DGPS reference stations includes carrier phase observable data while data broadcast by other facilities only includes code phase correction data. However, irrespective of whether the particular US Coast Guard facility broadcasts carrier phase data or code phase correction data, the broadcast is typically in a standard RTCM format. Other agencies and port authorities throughout the world broadcast differential correction signals conforming to the RTCM format for navigation in and around coastal areas. Both raw observable data and RTCM “correction data” are referred to hereinafter as “correction data” since both forms of data allow for correction to be made to position.
FIG. 1
shows a prior art position determination system
10
for determining position using correction data originating from a DGPS Reference Station that transmits in a RTCM format. Position determination system
10
is shown to include housing
17
that contains beacon antenna
11
and beacon receiver
13
. Housing
18
is shown to include GPS antenna
12
and GPS receiver
14
. Both housing
17
and housing
18
are coupled to a third housing which contains DGPS processor
19
by electrical cable. Battery
15
is connected by electrical cable to DGPS processor
19
for providing electrical power to the components of position determination system
10
. Data logger
16
is also shown to be coupled via electrical cable to DGPS processor
19
. Data logger
16
typically includes a display and function keys so as to allow users to view output and to input data as required for the operation of position determination system
10
. In operation, beacon antenna
11
receives differential correction signals from a Reference Station that broadcasts in a RTCM format and couples the signals to beacon receiver
13
. Beacon receiver
13
demodulates the RTCM signals so as to obtain correction data which is then coupled to DGPS processor
19
. GPS antenna
12
receives signals from satellites of the GPS and couples the signals to GPS receiver
14
. GPS receiver
14
demodulates the signals from GPS satellites and proces
Blum Theodore M.
Trimble Navigation Limited
Wagner , Murabito & Hao LLP
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