Data processing: vehicles – navigation – and relative location – Navigation – Employing position determining equipment
Reexamination Certificate
2000-12-26
2002-07-23
Arthur, Gertrude (Department: 3661)
Data processing: vehicles, navigation, and relative location
Navigation
Employing position determining equipment
C701S207000, C701S213000, C342S357490, C342S450000, C342S451000
Reexamination Certificate
active
06424914
ABSTRACT:
FIELD OF THE PRESENT INVENTION
The present invention relates generally to a global positioning system and inertial measurement unit (GPS/IMU) integrated positioning and navigation method and system, and more particularly to an improved fully-coupled integration method and system of the global positioning system (GPS) receiver and the inertial measurement unit (IMU), which allows the mutual aiding operation of the GPS receiver and the inertial navigation system (INS) at an advanced level with features of inertial aiding global positioning system satellite signal tracking, fuzzy logic for attitude determination, master-slave relative positioning, robust attitude determination, and on-the-fly resolution of GPS carrier phase integer ambiguities and real-time positioning in the differential GPS mode.
BACKGROUND OF THE PRESENT INVENTION
The GPS user equipment, which comprises an antenna, a signal processing unit, and associated electronics and displays, receives the signals from the GPS satellites to obtain position, velocity, and time solutions. There are two types of GPS observables: code pseudoranges and carrier phases. Phase measurements are based on two L-band carrier frequencies. One is the L
1
carrier with frequency 1575.42 MHz and the other is the L
2
carrier with frequency 1227.60 MHz. For pseudorange measurements, there are two basic types of Pseudo Random Noise (PRN) code measurements. One is known as the C/A (Coarse/Acquisition) code modulated on the L
1
frequency only and the other is known as the P (Precise) code modulated on both L
1
and L
2
frequencies. In addition to the above information in the GPS signals, the GPS signals also modulate the navigation message, which includes GPS time, clock corrections, broadcast ephemerides, and system status, on both L
1
and L
2
frequencies.
Because of the navigation message transmitted by the GPS satellites, the positions and velocities of the GPS satellites can be computed. Therefore, the propagating time of a GPS signal can be determined. Since the signal travels at the speed of light, the user can calculate the geometrical range to the satellite. In this way, the code pseudorange measurements can be determined and is degraded by errors, such as ephemeris errors, user and satellite clock biases (including selective availability (SA)), atmospheric effects (ionosphere and troposphere), and measurement noise (receiver error and random noise). These errors not only affect pseudorange measurements but phase measurements. The most obvious difference between both measurements is the measurement error. For phase measurements, the measurement noise is of the order of a few millimeters and for pseudorange measurements, the measurement noise is accurate to about 30 centimeters (for the P code) or 3 meters (for the C/A code).
The Global Positioning System, GPS, contains a number of error sources: the signal propagation errors, satellites errors, and the selective availability. The user range error (URE) is the resultant ranging error along the line-of-sight between the user and the global positioning system satellite. Global positioning system errors tend to be relatively constant (on average) over time, thus giving the global positioning system long-term error stability. However, the signals of the global positioning system may be intentionally or unintentionally jammed or spoofed, or the global positioning system (GPS) receiver antenna may be obscured during vehicle attitude maneuvering, and the global positioning system signals may be lost when the signal-to-noise ratio is low and the vehicle is undergoing highly dynamic maneuvers.
In addition to the unavoidable errors (such as ionospheric delay, tropospheric delay, clock biases, and measurement errors) and the intentional error (such as SA), the GPS measurements (pseudorange and phase) may also be affected by the environment surrounding a GPS user antenna. Like the multipath effect, because of an object nearby the user antenna, the antenna receives not only a direct signal from a GPS satellite but also a second or more reflected or diffracted signals from the object. For a highly dynamic vehicle, the onboard GPS receiver may lose the lock of a GPS signal because the signal-to-noise ratio (SNR) is low or the GPS signal is blocked by the body of its own vehicle.
Typically, the navigation solution is estimated by using the pseudorange measurements. Since the satellite clock biases are provided by the navigation message, for three-dimensional position determination, in addition to the three unknowns in position, the receiver (user) clock bias also needs to be estimated, i.e., there are four unknowns for the navigation solution. As a result, for a stand-alone receiver, the position determination usually needs a minimum of four visible GPS satellites, and the estimated position is accurate to about 100 meters with SA on. In order to improve the accuracy of the estimated position, the phase measurements will be used. Also, to eliminate the most of SA and other common errors (for example, receiver and satellite clock biases), the differential GPS will be employed. As a result, the accuracy of the estimated position is of the order of a few centimeters. However, to achieve the centimeter accuracy, one of the key steps is to resolve carrier phase integer ambiguities.
An inertial navigation system (INS) comprises an onboard inertial measurement unit (LIU), a processor, and embedded navigation software(s), where the components of the IMU include the inertial sensors (accelerometers and gyros) and the associated hardware and electronics. Based on measurements of vehicle specific forces and rotation rates obtained from onboard inertial sensors, the positioning solution is obtained by numerically solving Newton's equations of motion.
The inertial navigation system is, in general, classified as a gimbaled configuration and a strapdown configuration. For a gimbaled inertial navigation system, the accelerometers and gyros are mounted on a gimbaled platform to isolate the sensors from the rotations of the vehicle and then to keep the measurements and navigation calculations in a stabilized navigation coordinate frame. Generally, the motion of the vehicle can be expressed in several navigation frames of reference, such as earth centered inertial (ECI), earth-centered earth-fixed (ECEF), locally level with axes in the directions of north-east-down (NED), and locally level with a wander azimuth. For a strapdown inertial navigation system, the inertial sensors are rigidly mounted to the vehicle body frame. In order to perform the navigation computation in the stabilized navigation frame, a coordinate frame transformation matrix is used to transform the acceleration and rotation measurements from the body frame to one of the navigation frames.
In general, the measurements from the gimbaled inertial navigation system are more accurate than the ones from the strapdown inertial navigation system. And, the gimbaled inertial navigation system is easier in calibration than the strapdown inertial navigation system. However, the strapdown inertial navigation systems are more suitable for higher dynamic conditions (such as high turn rate maneuvers) which can stress inertial sensor performance. Also, with the availability of modem gyros and accelerometers, the strapdown inertial navigation systems become the predominant mechanization due to their low cost and reliability.
Inertial navigation systems, in principle, permit pure autonomous operation and output continuous position, velocity, and attitude data of the vehicle after initializing the starting position and initiating an alignment procedure. In addition to autonomous operation, other advantages of an inertial navigation system include the full navigation solution and wide bandwidth. However, an inertial navigation system is expensive and is degraded with drift in output (position and velocity) over an extended period of time. It means that the position and velocity errors increase with time. This error propagation characteristic is primarily caused
American GNC Corporation
Arthur Gertrude
Chan Raymond Y.
David and Raymond Patent Group
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