Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2000-05-19
2002-07-16
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
Reexamination Certificate
active
06421003
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to precision attitude, position, velocity, and time measurement systems using satellite navigation system measurements. More specifically, the invention relates to systems and methods for inferring the differential path length of carrier wave signals between the satellite transmitter antenna and two or more receiver antennas for a set of satellite signals to enable accurate inference of the relative geospatial coordinates of the receiver antennas.
BACKGROUND OF THE INVENTION
The determination of the relative coordinates of points of a distributed physical system in a geospatial reference frame has application to a number of important problems. Attitude determination is a primary example. Early techniques for determining the attitude of a rigid body were based on the use of Earth-generated field phenomena. For example, one technique used sensors to detect the magnetic field direction of the Earth to determine magnetic compass heading of the rigid body and to sense the gravitational field direction of the Earth to determine the attitude of the rigid body with respect to the local gravitational vertical. These techniques have limited precision and accuracy because of complex local variations in the magnetic and gravitational fields of the Earth, difficulties in measuring such phenomena with high resolution, and unwanted coupling effects, such as acceleration effects on the sensors.
To determine the attitude of a rigid body in geospatial coordinates with high precision and accuracy, some techniques focus on measuring phenomena whose sources are more controllable than Earth-generated magnetic and gravitational fields. Optical techniques developed in surveying have achieved the desired levels of precision and accuracy, but are difficult to employ in many operating conditions, particularly with moving bodies. Recent techniques based on determining the transmission path lengths of code-modulated radio frequency signals transmitted from navigation satellite systems, such as the United States Global Positioning System (GPS) and the functionally-similar Global Navigation Satellite System (GLONASS) operated by Russia, have transformed surveying.
The geospatial location of each transmitting GPS satellite, or other signal source, as a function of time can usually be inferred from data contained in the transmitted signal. At the typical distances from the satellite, or other navigation signal source, to the surface of the Earth, the signal transmission paths can be considered nearly parallel for a physical body whose attitude is to be measured. The difference in the path lengths of the transmissions received at two satellite receiver antennas, separated by a baseline of known length, can be used to determine the attitude of the body in the plane defined by the satellite and the two separated receiver antennas. By using three or more satellite receiver antennas, arranged on the body so that they are not co-linear, the three-dimensional attitude in geospatial coordinates can be determined.
The measurement of the transmission path length from the satellite to a receiver antenna depends on the measured characteristics of the signal. The coded information imposed on the satellite carrier wave allows an unambiguous determination of the length of the transmission path. However, path length based on the coded information can only be measured to a relatively coarse resolution. Finer resolution requires the measurement of the carrier wave itself. For example, where the chip width of the coded information for GPS is approximately 300 meters, the wavelength of the GPS carrier signal is approximately 19 centimeters. This allows resolution of the transmission path length to fractions of a centimeter by phase measurement techniques within the GPS receiver.
Note that all carrier waves are identical and indistinguishable from each other. As a result, when the phase of the wave is determined at the receiver antenna, it is ambiguous which specific cycle of the carrier wave is being measured. The total path length from the satellite antenna to the receiver antenna is the sum of the GPS receiver's measured fraction of a cycle and an unknown number of integer cycles. Determining the specific wave cycle of the carrier wave that is being measured in phase is called cycle ambiguity resolution (also referred to as integer ambiguity resolution).
Typically, the information coded in the transmitted signal is used to bound the range in which the possible integer carrier cycles can exist. The code measurement for an individual signal establishes the maximum and minimum transmission path lengths possible for that signal. This bounding process is performed for all the satellite signals employed in the solution. The projection of these bounds into the solution space according to the direction vectors between the receiver antenna and the transmitter antenna establishes the geometric structure of the solution space. The correct solution for the numbers of integer cycles between the transmitter and the receiver antennas is the set of answers that satisfies all the constraints within the solution space. In practice, this process is usually done with single differences of measurements between antennas when the inter-receiver clock delays are well known, and with double difference measurements between receiving antennas and between signal transmitters when the inter-antenna time delay is not well known.
Techniques have arisen to perform the steps of determining the correct number of integer carrier cycles and measuring the fractional phase within a cycle. The performance of these techniques is evaluated based on the reliability of correctly resolving the integer values and the accuracy of the resulting solution. The existing methods are suboptimum in each of these regards.
The use of carrier cycle ambiguity resolution is not limited to the determination of body attitude. Similar techniques can be used to determine the vector offsets between two or more GPS antennas that are not rigidly fixed to the same body. In one case, the first or “reference” receiver may be fixed at a known surveyed location, while the second GPS receiver may be on a moving body. The purpose is to correct errors in the moving GPS receiver using data calibrated in accuracy by the reference receiver. The use of carrier cycle ambiguity resolution for this mode of “differential” GPS is often referred to as kinematic or real-time kinematic (RTK) GPS.
In another case, the first receiver may be at one unknown or moving location, and the second GPS receiver may be at a second unknown or moving location. In this case, the relative vector between the two GPS receivers is of interest. In yet another case, the use of carrier cycle ambiguity resolution can be focused on correcting the time measurement between a GPS reference receiver and a second remote GPS receiver.
SUMMARY OF THE INVENTION
In one aspect, the invention features a method for use in a positioning system having an array of satellite navigation system antennas simultaneously receiving signals emitted from a plurality of signal transmitters. The method determines an attitude of the array by determining a first set of potential solutions for a first baseline vector for any pair of antennas in the array and a second set of potential solutions for a second baseline vector for any other pair of antennas in the array. Each potential solution in the first set is compared with each potential solution in the second set to determine a degree of geometric consistency between each pair of compared solutions. One of the pairs of compared potential solutions is selected as representative of the attitude of the array if the corresponding baseline vectors satisfy predetermined criteria. One embodiment of the method simultaneously compares three sets of potential solutions for three different baseline vectors. Also, array attitude determinations can be improved by averaging the array attitude determined from multiple baseline vectors with correct solutions using a
Gilkey James Y.
Riley Wyatt
Phan Dao
SRI - International
Testa Hurwitz & Thibeault LLP
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