John F. Raquet

Closed-form solution for determining emitter location using time difference of arrival measurements

A direct and short derivation of an algorithm based on the closed-form solution of the nonlinear equations for emitter location using time difference of arrival (TDOA) measurements from N + 1 receivers, N /spl ges/ 3, is given.

Magnetic field navigation in an indoor environment

This paper describes a method that has been developed to aid an inertial navigation system when GNSS signals are not available, by taking advantage of the uniqueness of magnetic field variations. Most indoor environments have many different features (ferrous structural materials or contents, electrical currents, etc.) which perturb the Earths natural magnetic field. The variations in the magnetic field in indoor environments can be used as a way to identify a users position, and possibly orientation, because the 3-dimensional magnetic field varies significantly as a function of position. Using relatively inexpensive 3-axis magnetic field sensors, it is possible to estimate a users location in an indoor environment.

Broadcast vs Precise GPS Ephemerides: A Historical Perspective

The Navstar Global Positioning System (GPS) Operational Control Segment (OCS) generates predicted satellite ephemerides and clock corrections that are broadcast in the navigation message and used by receivers to estimate real-time satellite position and clock corrections for use in navigation solutions. Any errors in these ephemerides will directly impact the accuracy of GPS based positioning. This paper compares the satellite position computed using broadcast ephemeredes with the precise position provided by the International GPS Service for Geodynamics (IGS) Final Orbit solution. Similar comparisons have been undertaken in the past, but for only short periods of time. This paper presents an analysis of the GPS broadcast ephemeris position error on a daily basis over the entire operational lifetime of the GPS system. The comparison was undertaken from 14 November 1993 through to 31 December 2002. The statistics of these errors were also analyzed.

Bandwidth Efficient Cooperative TDOA Computation for Multicarrier Signals of Opportunity

Source localization, the problem of determining the physical location of an acoustic or wireless emitter, is commonly encountered in sensor networks which are attempting to locate and track an emitter. Similarly, in navigation systems that do not rely on the global positioning system (GPS), ldquosignals of opportunityrdquo (existing wireless infrastructure) can be used as ad hoc navigation beacons, and the goal is to determine their location relative to a receiver and thus deduce the receivers position. These two research problems have a very similar mathematical structure. Specifically, in either the source localization or navigation problem, one common approach relies on time difference of arrival (TDOA) measurements to multiple sensors. In this paper, we investigate a bandwidth efficient method of TDOA computation when the signals of opportunity use multicarrier modulation. By exploiting the structure of the multicarrier transmission, much less information needs to be exchanged between sensors compared to the standard cross correlation approach. Analytic and simulation results quantify the performance of the proposed algorithm as a function of the signal-to-noise ratio (SNR) and the bandwidth between the sensors.

Stochastic constraints for efficient image correspondence search

The navigation state (position, velocity, and attitude) can be determined using optical measurements from an imaging sensor pointed toward the ground. Extracting navigation information from an image sequence depends on tracking the location of stationary objects in multiple images, which is generally termed the correspondence problem. This is an active area of research and many algorithms exist which attempt to solve this problem by identifying a unique feature in one image and then searching subsequent images for a feature match. In general, the correspondence problem is plagued by feature ambiguity, temporal feature changes, and occlusions which are difficult for a computer to address. Constraining the correspondence search to a subset of the image plane has the dual advantage of increasing robustness by limiting false matches and improving search speed. A number of ad hoc methods to constrain the correspondence search have been proposed in the literature. A rigorous stochastic projection method is developed here which constrains the correspondence search space by incorporating a priori knowledge of the aircraft navigation state using inertial measurements and a statistical terrain model. The stochastic projection algorithm is verified using Monte Carlo simulation and flight data. The constrained correspondence search area is shown to accurately predict the pixel location of a feature with an arbitrary level of confidence, thus promising improved speed and robustness of conventional algorithms

Non-GNSS radio frequency navigation

There are many situations in which global navigation satellite systems (GNSS) such as the global positioning system (GPS) cannot provide adequate navigation performance (such as indoors or in urban canyons). This paper describes the technical challenges of non-GNSS radio frequency navigation, with particular emphasis on signals of opportunity (i.e., signals that are intended for purposes other than navigation). Advantages and disadvantages of signal of opportunity navigation are described, along with the dominant issues that must be dealt with in order to make such systems a practical reality.

Multipath-adaptive GPS/INS receiver

Multipath interference is one of the contributing sources of errors in precise global positioning system (GPS) position determination. This paper identifies key parameters of a multipath signal, focusing on estimating them accurately in order to mitigate multipath effects. Multiple model adaptive estimation (MMAE) techniques are applied to an inertial navigation system (INS)-coupled GPS receiver, based on a federated (distributed) Kalman filter design, to estimate the desired multipath parameters. The system configuration is one in which a GPS receiver and an INS are integrated together at the level of the in-phase and quadrature phase (I and Q) signals, rather than at the level of pseudo-range signals or navigation solutions. The system model of the MMAE is presented and the elemental Kalman filter design is examined. Different parameter search spaces are examined for accurate multipath parameter identification. The resulting GPS/INS receiver designs are validated through computer simulation of a user receiving signals from GPS satellites with multipath signal interference present The designed adaptive receiver provides pseudo-range estimates that are corrected for the effects of multipath interference, resulting in an integrated system that performs well with or without multipath interference present.

Real-Time UWB-OFDM Radar-Based Navigation in Unknown Terrain

We present a signal processing algorithm and simulation study for aerial navigation with an ultrawideband orthogonal frequency division multiplexed (UWB-OFDM) radar in Global Positioning System (GPS)-denied environments. Stationary scatterers are detected and tracked using an M/N detector and modified global nearest neighbor (GNN) tracker. The radar measurements to the scatterers are combined with inertial navigation system (INS) measurements in an extended Kalman filter (EKF) to compute the aircraft position. The estimation error of the proposed algorithm is analyzed through computer-based simulations with/without radar measurements from the scatterers and with varying signal-to-noise ratio (SNR).

Localized three-dimensional adaptive spatial-temporal processing for airborne radar

Radar space-time adaptive processing (STAP) techniques have classically focused on azimuth-Doppler adaptivity while placing minimal emphasis on elevation. Elevation adaptivity offers significant clutter suppression improvement, allowing further suppression of interference sources having identical Doppler and azimuth as the expected target. This work incorporates elevation adaptivity using two approaches: (1) a factored approach and (2) a joint domain approach, both greatly improving clutter suppression performance. The proposed concepts are validated using results based on simulated range ambiguous airborne radar data. Target detection improvements on the order of 8 dB and 10 dB (as compared to standard 2D-JDL processing) are demonstrated for the factored and joint domain approaches, respectively, using an 8 /spl times/ 8 non-uniform rectangular array.

Enhanced feature detection and tracking algorithm for UWB-OFDM SAR navigation

There is a great need to develop non-GPS based methods for position and navigation in situations where GPS is not available. This paper presents an improved feature extraction and tracking algorithm for a novel navigation sensor, an Ultra-Wideband Orthogonal Frequency Division Multiplexed radar. The radar is onboard an aerial vehicle and collects data periodically in a side-looking stripmap Synthetic Aperture Radar configuration flying over unknown terrain. An M/N detector is first used to extract range/Doppler observations from SAR features in the terrain. The observations are then associated with existing tracks using Global Nearest Neighbor. The associated measurements are then used as updates to a previously developed EKF which combines the radar observation tracks with INS measurements to compute an optimal aircraft position estimate. The filter performance is evaluated and compared with that of a previously developed threshold-based method using simulations. The algorithm performance dependence on the quality of INS measurements are also evaluated based on INS error models of commercial-grade, tactical-grade, and navigation-grade INS systems.