Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
1999-02-24
2001-10-23
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490
Reexamination Certificate
active
06307505
ABSTRACT:
TECHNICAL FIELD
This invention relates to location determination systems. Specifically, the present invention relates to an apparatus and method for coupling data to a position determination device.
BACKGROUND ART
The Global Positioning System (GPS) consists of a constellation of orbiting satellites that transmit timing information and the satellite's ephemerides via microwave radio. Position determination devices determine position by analyzing signals received from four or more satellites. Any of a number of known methods can be used to determine position.
One frequently used method for determining position calculates pseudoranges that are then used to determine position. 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 to each satellite (pseudorange) is then determined by multiplying transit time of each received signal by the speed of radio transmissions (approximately 300,000,000 meters/second).
Signals from each GPS satellite include the satellites ephemeris. The ephemeris indicates the location of each satellite. The position of the position determination device is then 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 a more accurate determination of position is known as Differential GPS (DGPS). DGPS systems receive correction data broadcast from a DGPS reference station. DGPS reference stations are located at fixed and known locations and each DGPS reference station transmits correction data. By using receiver correction data along with signals received directly from GPS satellites, DGPS systems can accurately determine position. 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 manufacturer.
Many of the GPS reference stations broadcast in a format that conforms to standards established by the Radio Technical Commission for Maritime services (RTCM). These standards specify format, 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. 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 that 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 that 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 processes the incoming data, which is then coupled via electrical cable to DGPS processor
19
. DGPS processor
19
then uses the data from beacon receiver
13
and GPS receiver
14
to accurately determine position.
One proposed new system for correcting position determination signals from satellites is the Wide Area Augmentation System (WAAS). The WAAS is designed for use with aircraft operations. The WAAS is designed to provide a system that has sufficient integrity such that position may be determined with sufficient reliability and accuracy for aircraft operations. The WAAS includes satellites for transmitting signals and a ground network that augments GPS such that GPS may be used as a primary navigation sensor for aircraft. The WAAS augments GPS with a ranging function, (which improves availability and reliability), differential GPS corrections (which improves accuracy), and integrity monitoring (which improve safety).
Prior Art
FIG. 2
Blum Theodore M.
Trimble Navigation Limited
Wagner , Murabito & Hao LLP
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