Data processing: vehicles – navigation – and relative location – Navigation – Employing position determining equipment
Reexamination Certificate
2002-02-26
2003-11-04
Nguyen, Tan Q. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Navigation
Employing position determining equipment
C701S217000, C342S457000
Reexamination Certificate
active
06643587
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to navigation systems and more particularly to a system and method of calibrating sensors in a navigation system utilizing dead-reckoning mechanisms in conjunction with GPS devices to determining the position of vehicles while the vehicle is traversing through areas of Global Positioning Satellite (GPS) signal outage.
2. Description of Related Art
It is often desirable to maintain location information on objects such as vehicles traveling through areas of partial or total GPS signal blockage or areas that present multi-path signals, or areas where external interferences cause GPS signal outage. Such areas may include urban canyons having natural and/or artificial structures causing partial or complete GPS signal blockage. For example, an airport location provides a challenge as there are generally numerous partial and total blockages due to overhead roadways, underground tunnels and densely populated artificial structures comprising the airport.
Typical navigation systems use dead-reckoning sensors to navigate through areas of partial or total GPS signal blockage. Dead-reckoning navigation systems utilize inertial measurements mechanisms, such as a flywheel, to provide navigation during partial signal outages. Sensors used in GPS-Dead-Reckoning (GPS-DR) systems include rate gyroscopes, magnetic heading sensors, accelerometers, odometers and differential odometers or wheel tick sensors. Automotive GPS applications suffer performance degradation when signals from the GPS satellites are blocked or reflected by local terrain, buildings, tunnels, and the vehicle itself. In certain applications, an automotive navigation system may still be required to provide an output even when the satellites are not visible. Use of a dead-reckoning system in these intervals is common practice. In particular, a GPS-DR system which uses a gyroscope to maintain heading and vehicle odometer input pulses to determine distance traveled is well known.
GPS and inertial sensor solutions provide a synergistic relationship when used together in hybrid navigation systems. The integration of these two types of solutions not only overcomes performance issues found in each individual solution, but produces a system whose performance exceeds that of the individual solutions. GPS provides bounded accuracy, while inertial system accuracy degrades with time.
In navigation systems, GPS receiver performance issues include susceptibility to interference from external sources, time to first fix, i.e., first position solution, interruption of the satellite signal due to blockage, integrity, and signal reacquisition capability. The performance issues related to inertial sensors are their associated quality and cost.
A primary concern with using GPS as a stand alone source for navigation is signal interruption. Signal interruption can be caused by shading of the GPS antenna by terrain or manmade structures, e.g., buildings, vehicle structure, and tunnels, or by interference from an external source. Generally, when only three usable satellite signals are available, most GPS receivers revert to a two-dimensional navigation mode by utilizing either the last known height or a height obtained from an external source. However, if the number of usable satellites is less than three, some receivers have the option of not producing a solution or extrapolating the last position and velocity solution forward in what is called “dead-reckoning” (DR) navigation. Position aiding from the inertial system can be used to help the GPS receiver reacquire the satellite signal. By sending vehicle position to the receiver, the receiver can accurately estimate the range from the given position to the satellites and thus initialize its internal code loops.
Generally, inertial sensors are of two types—gyroscopes and accelerometers. The output of a gyroscope is a signal proportional to angular movement about its input axis and the output of an accelerometer is a signal proportional to the change in velocity sensed along its input axis. A three-axis inertial measurement unit (IMU) would then require three gyroscopes and three accelerometers to inertially determine its position and velocity in free space.
One of the significant factors related to the quality of an inertial system is the drift of the gyroscopes, measured in degrees/hour. The drift of a gyro is a false output signal caused by deficiencies during the manufacturing of the sensor. In inertial sensors, these are caused by mass unbalances in a gyroscope's rotating mass and by non-linearities in the pickoff circuits as is seen in fiber-optic gyroscopes. This false signal is in effect telling the navigation system that the vehicle is moving when it is actually stationary. The manufacturing cost of gyros with low drift is approximately $1,000. An inertial unit with a drift from 1 to 100 deg/hr are currently priced from approximately $1,000 to $10,000. Inertial units providing accuracies of less than 1 deg/hr are available at prices significantly higher, ranging from ten to one hundred times the cost associated with lesser accurate units.
As can be seen, the quality of the inertial sensors has a large role in the cost effectiveness of a navigation system. If 0.0001-deg/hr gyroscopes were relatively inexpensive, GPS may not be needed today. However, in actuality, inertial sensors are expensive, and a significant result of the integration of GPS with inertial sensors is the ability to use lower performing, more cost-effective sensors. As mentioned above, during the operation of a navigation system when both GPS and inertial components are operational, the inertial navigation errors are bounded by the accuracy of the GPS solution. Thus, one significant contribution the GPS receiver makes to the operation of the inertial subsystem is the calibration of the inertial sensors. Inertial instruments are specified to meet a turn-on to turn-off drift requirement (each time a gyro is powered up, its initial drift rate differs.)
The major errors associated with inertial sensors used in conjunction with GPS systems (GPSI systems) are the gyro bias and accelerometer bias. The gyro bias and accelerometer bias typically occupy six of the states within an inertial or GPSI Kalman filter. During the operation of a GPSI system, the Kalman filter produces an estimate of these biases as derived from the velocity data received from the GPS receiver.
Integrated GPS systems, wherein GPS sensors are used in conjunction with inertial sensors (GPSI systems) have typically been accomplished by utilizing a single Kalman filter to estimate the navigation state and sensor errors. Kalman filtering is a statistical technique that combines a knowledge of the statistical nature of system errors with a knowledge of system dynamics, as represented as a state space model, to arrive at an estimate of the state of a system. In a navigation system, we are usually concerned with position and velocity, at a minimum, but its not unusual to see filters for system models with state vector dimensions ranging from six to sixty. The state estimate utilizes a weighting function, called the Kalman gain, which is optimized to produce a minimum error variance.
These designs have proven effective with high-quality inertial sensors but involve substantial costs. Unfortunately, increasing the Kalman filter state vector size involves substantial design time, tuning complexity, risk of numerical difficulties, and processing power costs. This often forces navigation designers into a harsh trade between estimating significant instrument errors and increasing state vector size.
Existing GPS-DR hybrid systems have evolved into several classes, switched and filtered. The switched and filtered GPS-DR hybrid systems are feed-forward designs which calibrate the instruments when GPS is available but do not use the dead-reckoning instruments to improve the operation of the GPS receiver.
Switched GPS-DR systems are simple and commonly available today. These systems are effect
Brodie Keith J.
Chansarkar Mangesh M.
Rounds Stephen F.
Nguyen Tan Q.
Shemwell Gregory & Courtney LLP
SiRF Technology Inc.
Tran Dalena
LandOfFree
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