Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2000-10-24
2002-10-22
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C342S357490, C701S215000
Reexamination Certificate
active
06469663
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to Global Positioning System (GPS) receivers and more particularly to a method and an apparatus for computing a precise location using differential carrier phases of a GPS satellite signal and a Wide Area Augmentation System (WAAS) or similar satellites.
2. GPS Background
The Global Positioning System (GPS) was established by the United States government, and employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit microwave L-band radio signals in two frequency bands, centered at 1575.42 MHz and 1227.6 MHz., denoted as L
1
and L
2
respectively. These signals include timing patterns relative to the satellite's onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites. GPS receivers process the radio signals, computing ranges to the GPS satellites, and by triangulating these ranges, the GPS receiver determines its position and its internal clock error. Different levels of accuracies can be achieved depending on the techniques deployed. This invention specifically targets the sub-centimeter accuracies achievable on a remote and possibly mobile GPS receiver by processing carrier phase observations both from the remote receiver and from one or more fixed-position reference stations. This procedure is often referred to as Real-Time-Kinematic or RTK.
To gain a better understanding of the accuracy levels achievable by using the GPS system, it is necessary understand the two types of signals available from the GPS satellites. The first type of signal includes both the Coarse Acquisition (C/A), which modulates the L
1
radio signal and precision (P) code, which modulates both the L
1
and L
2
radio signals. These are pseudorandom digital codes that provide a known pattern that can be compared to the receiver's version of that pattern. By measuring the time-shift required to align the pseudorandom digital codes, the GPS receiver is able to compute an unambiguous pseudo-range to the satellite. Both the C/A and P codes have a relatively long “wavelength,” of about 300 meters (1 microsecond) and 30 meters ({fraction (1/10)} microsecond), respectively. Consequently, use of the C/A code and the P code yield position data only at a relatively coarse level of resolution.
The second type of signal utilized for position determination is the carrier signals. The term “carrier”, as used herein, refers to the dominant spectral component which remains in the radio signal after the spectral content caused by the modulated pseudorandom digital codes (C/A and P) is removed. The L
1
and L
2
carrier signals have wavelengths of about 19 and 24 centimeters, respectively. The GPS receiver is able to “track” these carrier signals, and in doing so, make measurements of the carrier phase to a small fraction of a complete wavelength, permitting range measurement to an accuracy of less than a centimeter.
In stand-alone GPS systems that determine a receiver's position coordinates without reference to a nearby reference receiver, the process of position determination is subject to errors from a number of sources. These include errors in the satellite's clock reference, the location of the orbiting satellite, ionospheric induced propagation delay errors, and tropospheric refraction errors. A more detailed discussion of these sources of error is provided in U.S. Pat. No 5,828,336 by Yunck, et al. A large portion of the satellite's clock error, referred to as Selective Availability (SA) is purposefully induced by the U.S. Department of Defense to limit GPS accuracy to non-authorized users. SA can cause positioning errors exceeding 40 meters, but even without SA, errors caused by the ionosphere can be tens of meters.
To overcome the errors of the stand-alone GPS system, many kinematic positioning applications make use of multiple GPS receivers. A reference receiver located at a reference site having known coordinates receives the satellite signals simultaneously with the receipt of signals by a remote receiver. Depending on the separation distance, many of the errors mentioned above will affect the satellite signals equally for the two receivers. By taking the difference between signals received both at the reference site and at the remote location, these errors are effectively eliminated. This facilitates an accurate determination of the remote receiver's coordinates relative to the reference receiver's coordinates.
The technique of differencing signals is known in the art as differential GPS (DGPS). The combination of DGPS with precise measurements of carrier phase leads to position accuracies of less than one centimeter root-mean-squared (centimeter-level positioning). When DGPS positioning utilizing carrier phase is done in real-time while the remote receiver is potentially in motion, it is often referred to as Real-Time Kinematic (RTK) positioning.
One of the difficulties in performing RTK positioning using carrier signals is the existence of an inherent ambiguity that arises because each cycle of the carrier signal looks exactly alike. Therefore, the range measurement based upon carrier phase has an ambiguity equivalent to an integral number of carrier signal wavelengths. Various techniques are used to resolve the ambiguity, which usually involves some form of double-differencing. Some prior art related to this is U.S. Pat. No. 4,170,776 by MacDoran, U.S. Pat. No. 4,667,203 by Counselman, U.S. Pat. No. 4,963,889 by Hatch, U.S. Pat. No. 5,296,861 by Knight, and U.S. Pat. No. 5,519,620 by Talbot et al. Once ambiguities are solved, however, the receiver continues to apply a constant ambiguity correction to a carrier measurement until loss of lock on that carrier signal.
Regardless of the technique deployed, the problem of solving integer ambiguities, in real-time, is always faster and more robust if there are more measurements upon which to discriminate the true integer ambiguities. Robust means that there is less chance of choosing an incorrect set of ambiguities. The degree to which the carrier measurements collectively agree to a common location of the GPS receiver is used as a discriminator in choosing the correct set of ambiguities. The more carrier phase measurements that are available, the more likely it is that the best measure of agreement will correspond to the true (relative to the reference GPS) position of the remote GPS receiver. One method, which effectively gives more measurements, is to use carrier phase measurements on both L
1
and L
2
. The problem though is that it is relatively difficult to track L
2
because it is modulated only by P code and United States Department of Defense has limited access to P code modulation by encrypting the P code prior to transmission. Some receivers are capable of applying various cross-correlation techniques to track the P code on L
2
, but these are usually more expensive receivers that L
1
only capable receivers (see, for example U.S. Pat. No. 5,293,170 Lorenz, et al.).
Other approaches have been employed to gain additional measurements on GPS receivers in an attempt to enhance RTK. Hatch, for example in “Pseudolite-aided method for precision kinematic positioning” (U.S. Pat. No. 5,177,489) suggests the use pseudolites, which due to their proximity exhibit rapid changes in relative location causing the apparent affect of additional satellites. A derivation of this, which uses Low Earth Orbit (LEO) satellites, that travel across the sky much more rapidly than GPS satellites is presented by Enge, et al. in “Method and receiver using a low earth orbiting satellite signal to augment the global positioning system” (U.S. Pat. No. 5,944,770). The use of the GLObal NAvigation Satellite System (GLONASS) satellites from the former USSR has been spelled out in “Relative position measuring techniques using both GPS and GLONASS carrier phase measurement
Feller Walter
Whitehead Michael L.
Blum Theodore M.
Cantor & Colburn LLP
CSI Wireless Inc.
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