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34 A Deep Integration Estimator for Urban Ground Navigation Dale Landis, Tom Thorvaldsen, Barry Fink, Peter Sherman, Steven Holmes Copyright © 2006, IEEE. Presented at IEEE PLANS, San Diego, CA, April 25-27, 2006 abstract The objective of the Personal Navigator System (PNS) is to construct a wearable navigation system that provides accurate position over extended missions in a deprived Global Positioning System (GPS) environment. The prototype multisensor navigator included a set of micromechanical inertial sensors, a three-axis miniature radar, a selective availability antispoofing module (SAASM) GPS receiver, and a barometric altimeter. Real-time embedded software sampled sensor data, controlled GPS receiver tracking loops, and hosted a multisensor optimal estimator whose output position was transmitted via wireless link to a highresolution personal data accessory (PDA) tracking display. The fully packaged system was field tested in Cambridge, Massachusetts under realistic, GPS-stressed conditions. This paper focuses on the deep integration (DI) algorithm design used for the optimal estimation of both position and receiver tracking control. The algorithm was tailored here for intermittent GPS visibility on the ground and in outdoor-indoor-outdoor maneuvers. DI has been used previously for missile guidance, navigation, and control with clear sky view. The PNS required an optimal estimator that combined the nonlinear GPS/inertial DI algorithm with measurements from other sensors. The mission duration here was much longer, and the satellite environment over the ground track was highly variable compared with earlier DI applications. This required the development of strategies for dropping satellites from track after long blockage times and for taking control of newly visible satellites under DI tracking. Here, the advantage of DI tracking is the ability to extract GPS pseudorange information almost instantly if a satellite reappears momentarily from a blockage. This paper reviews the DI approach with stress on the receiver correlator power measurements, nonlinear filter equations, and the calculation of numerically-controlled oscillator (NCO) commands. Specific problems encountered, such as clock error recalculation and numerical issues, will be mentioned. Urban canyon performance data demonstrating accurate navigation under sparse GPS availability are also described.
Introduction The PNS is a small package containing a Draper Laboratory micromechanical inertial measurement unit (IMU), a Rockwell Collins GPS receiver, a triad of Doppler radar velocity sensors, a barometric altimeter, a PDA that allows human user interface, and a processor that contains Draper-developed, real-time navigation software. This package is wearable in a front-mounted configuration by a foot soldier, and its objective is to provide long-term accurate coordinates in both outdoor and indoor environments, including significant periods of GPS signal deprivation. The software comprises strapped-down navigation algorithms combined with Draper’s deep integration (DI) nonlinear filter for processing GPS correlator outputs, based on previous Draper munitions shell applications. Doppler updates, navigation initialization, and satellite line-of-sight (LOS) error estimation were among the many features added for the application. Demonstration in a full hardware mode was done in Spring 2005. An example is shown in Figure1. Up (ft) Position NEU Recorded 3-D Track In-Building GPS-Denied 20 0 -20 Finish -40 -60 -80 200 450 400 150100 Rooftop Start 350 300 50 Sky in Full View 250 0 200 150 East (ft) -50 100 50 North (ft) -100 0 Figure 1. Real-time PNS test results in Technology Square, Cambridge. The test illustrated involved an outdoor phase followed by an indoor GPS-deprived period. Figure 1 shows that position accuracy was maintained, even during the indoor phase. An overlay of the recorded track onto geolocated floor plans showed very good registration with hallways. In the vertical direction, stairwell landings are clearly seen. The PNS effectively locates the user to the correct floor. The tests also showed that on return to the outdoor environment, GPS resumed almost immediately. Results of this test program were reported at JNC’05. [1] The algorithm that accomplished this performance is surveyed in subsequent sections, with emphasis on the components that required fresh techniques. Navigation algorithm and related Calculations The inputs to the navigation algorithm are 100-Hz sampled specific force (accelerometers) and rate (gyroscopes) in a PNS orthogonal body-fixed frame that is designated by b in this paper. The core of the PNS navigation algorithm is a standard strapped-down integration algorithm comprising IMU compensation, quaternion third-order integration, gravity compensation of accelerometer outputs, and velocity and position integration in earth-fixed, earth-centered (ecef or e) coordinates. For future reference, the navigation major outputs are: = position e frame = velocity e frame q = quaternion b to e = direction cosine matrix Navigation initialization, omitting many details, is as follows. The receiver begins with conventional acquisition and tracking, downloads ephemeris, and sends a position and velocity to navigation. A crude azimuth estimate is made by assuming an initial north and level orientation (accuracy of 10 deg in azimuth is sufficient). Once the receiver enters deep integration mode (less than a minute), the wearer moves horizontally, and the filter is able to refine the attitude estimates sufficiently for continued operation using the difference between IMU- and GPSdetermined accelerations. Current work at Draper includes more advanced forms of attitude initialization that impose less artificial restraints on the PNS wearer. In addition to navigation proper, there are calculations that keep track of optimal estimates of other quantities, primarily the following: dtR = user (receiver) clock bias d = user clock frequency error db k = LOS delay error satellite k An error filter based on perturbation of the navigation algorithm is used to process all the measurements. The filter states are listed for future reference in Table 1. Table 1. PNS Filter States. Error States Units Position dr 3 chips Velocity dv 3 chips/s Attitude y 3 rad Gyro Bias Shift 3 rad/s Gyro Bias Markov 3 rad/s Accel. Bias Shift 3 chips/s 2 Accel. Bias Markov 3 chips/s 2 User Clock Bias 1 chips User Clock Frequency 1 chips/s Doppler Misalignment (6) 6 rad Satellite Delay 12 chips Altimeter Bias 1 chips A Deep Integration Estimator for Urban Ground Navigation 35
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