Showing papers on "GPS/INS published in 1996"

GPS receiver utilizing a communication link

Abstract: A precision carrier frequency signal for calibrating a local oscillator of a GPS receiver which is used to acquire GPS signals. The precision carrier frequency signal is used to calibrate the local oscillator such that the output of the local oscillator, which is used to acquire GPS signals, is modified by a reference signal generated from the precision carrier frequency signal. The GPS receiver locks to this precision carrier frequency signal and generates the reference signal. In another aspect of the invention, satellite almanac data is transmitted to a remote GPS receiver unit from a basestation via a communication link. The remote GPS receiver unit uses this satellite almanac data to determine approximate Doppler data for satellites in view of the remote GPS receiver unit.

Gps relative position detection system

Abstract: A system of GPS devices which receive civilian GPS signals and provide an intuitive graphical interface for displaying the relative position of GPS devices in relation to each other, the relative position being accurate to several meters and defined as the distance to, direction of and height variance between GPS devices. A first GPS device with the person or object to be located transmits its GPS determined location to a second GPS device. This second GPS device includes a means for receiving the GPS determined position of the first GPS device, and also includes means for calculating the relative position of the first GPS device relative to the second GPS device based on a comparison of the received telemetry of the first GPS device and its own GPS determined position. The relative position of the first device is then graphically displayed on an interface of the second GPS device in a manner which eliminates the need for a map in order to travel to the location of the first GPS device. While providing an interface which displays a relative position of the first GPS device, this information remains accurate no matter how the orientation of the second GPS device changes with respect to a compass.

Hybrid GPS/inertially aided platform stabilization system

Abstract: A hybrid system for stabilizing the attitude of an instrument relative to a dynamic platform includes a plurality of (roll, pitch and yaw) inertial rate sensors, whose outputs are sampled at a rate sufficient to provide real time tracking of changes in orientation of the platform, and a global positioning system (GPS) receiver, whose precision platform attitude output is updated periodically, but at a rate less than the rate of change of attitude of the platform. The inertial rate sensors provide effectively continuous motion (e.g., angular rate) data signals representative of three-dimensional changes in attitude (position derivative signals) of the platform. The inertial rate output are integrated to provide output signals representative of the dynamic orientation of the platform. Sequential outputs of the integration-processing circuitry are also coupled to a sample buffer, which is controllably read-out in accordance with the periodic updates from the GPS receiver. The integrated inertial sensor (attitude) data is compared with the GPS update data to generate error signals which are used to adjust the inertial sensor data. By employing a sample buffer in an inertial sensor output integration feedback loop with the GPS receiver, the hybrid system of the present invention avoids what would otherwise be a staleness problem with the data provided by the GPS receiver.

Robust real-time wide-area differential GPS navigation

Abstract: The present invention provides a method and a device for providing superior differential GPS positioning data. The system includes a group of GPS receiving ground stations covering a wide area of the Earth's surface. Unlike other differential GPS systems wherein the known position of each ground station is used to geometrically compute an ephemeris for each GPS satellite, the present system utilizes real-time computation of satellite orbits based on GPS data received from fixed ground stations through a Kalman-type filter/smoother whose output adjusts a real-time orbital model. The orbital model produces and outputs orbital corrections allowing satellite ephemerides to be known with considerable greater accuracy than from the GPS system broadcasts. The modeled orbits are propagated ahead in time and differenced with actual pseudorange data to compute clock offsets at rapid intervals to compensate for SA clock dither. The orbital and clock calculations are based on dual frequency GPS data which allow computation of estimated signal delay at each ionospheric point. These delay data are used in real-time to construct and update an ionospheric shell map of total electron content which is output as part of the orbital correction data, thereby allowing single frequency users to estimate ionospheric delay with an accuracy approaching that of dual frequency users.

GPS car navigation system

Abstract: A GPS car navigation system derives GPS position update information from motion of the car along the actual track. Turns along the track are detected when they actually occur and are compared with the predicted turns so that the time and position at the actual turn can be used to update the then current GPS derived position of the vehicle. Updating position information with actual turn data improves the accuracy of GPS navigation especially during single satellite navigation.

GPS/IRS global position determination method and apparatus with integrity loss provisions

Abstract: A system for use with an inertial reference system and a global position receiver for calculating a position error after a loss of integrity by utilizing the global position system values for position and velocity at a time just before the loss of integrity and by utilizing the inertial reference system position modified by the known error in inertial reference system position as it varies with time and the position error as calculated by the global position system velocity extrapolated over the time since integrity loss.

A zero motion detection system for improved vehicle navigation system

Abstract: The improved vehicle navigation system and method uses information from a Global Positioning System (GPS) to obtain velocity vectors, which include speed and heading components, for propagating or "dead reckoning" the vehicle position from a previous position to a current position. The improved vehicle navigation system has a GPS receiver which provides the GPS velocity information which is calculated from a full set of GPS delta range measurements. GPS position data alone is not accurate enough for certain applications, such as turn-by-turn route guidance in automobile applications, because its error may be 100 m and there is considerable position drift, even when stationary. GPS velocities are much more accurate than the position data, 1 m/s or thereabouts, and can be used to propagate a known position forward and be more accurate over time than the GPS position solution. These velocities are instantaneous and not those computed from differencing two positions. The current position is calculated by adding displacements obtained from the GPS velocities to the previous position.

Integrated gps/inertial fault detection availability

Abstract: Different techniques have been proposed for combining GPS and inertial sensor information, and recent published findings seem to indicate that primary-means integrity availability is achievable with standard 2 nmi/h (95 percent) inertial sensor performance. This paper investigates and quantifies the different inertial effects that contribute to enhanced integrity of the integrated GPS/inertial system, such as coasting or Schuler feedback. A Kalman filter-based integration scheme that preserves the integrity information in an optimal fashion is presented and used to quantify integrity performance. This paper also proposes an approximate model, incorporating the main inertial effects contributing to integrity, that can be used to calculate the achievable horizontal protection level (HPL) at any geographical location and time. This model is used to estimate the availability of fault detection for an integrated GPS/inertial system. In addition, the paper compares the availability of fault detection for the GPS/inertial system with that for other augmentations to provide trade-off information.

GPS receiver having an initial adjustment for correcting for drift in reference frequency

Abstract: A GPS receiver having a rapid acquisition of a GPS satellite signal when a normal operational mode is entered after a low power standby mode. The GPS receiver includes an RF section for receiving the GPS satellite signal and providing an GPS IF signal, a correlator section for providing a correlation signal for the correlation between the GPS IF signal and an internally generated replica signal, and a microprocessor section for receiving the correlation signal and calculating a geographical location of the GPS receiver. The replica signal is based upon a reference frequency from a reference oscillator and a reference time of arrival (TOA) from a timer. In order to increase acquisition speed, the microprocessor section provides the correlator section with an initial frequency adjustment and an initial TOA adjustment to correct for drift in the reference frequency during the standby mode. The initial adjustments are based upon a learned corrections to the initial adjustments that result in acquisition of the GPS satellite signal after alternating one or more times between the standby mode and the normal mode or from stored temperature relationships and measured temperatures of the reference oscillator.

Automatic determination of traffic signal preemption using differential gps

Abstract: A traffic signal preemption system, and a related method for its use, using differential global positioning system (GPS) measurements for accurate monitoring of the position, speed and direction of an emergency or service vehicle approaching a controlled intersection. The preemption system includes a reference GPS receiver, for computing GPS measurement corrections, and a GPS receiver in each vehicle, which transmits its GPS measurements by radio to a receiver located at the controlled intersection. A computer also located at the intersection uses corrected vehicles position, and speed and direction measurements, in conjunction with previously recorded data defining approach routes to the intersection, to determine the optimum time to switch a traffic light controller to preemption mode to permit safe passage of the vehicle. GPS measurement corrections may be applied in a vehicle computer or in the computer located at the intersection. Other modes of operation of the system include a self-survey mode, whereby the reference GPS receiver determines its own true position by averaging position measurements over a period of a day or two, and a learn mode, whereby the intersection computer 'learns' unusual approach routes to the intersection as the vehicle traverses the approach routes and transmits position and velocity measurements.

Rapid development of tightly-coupled GPS/INS systems

Abstract: This paper addresses the question: "Why aren't tightly-coupled OPS/INS systems everywhere, on aircraft, ships and land vehicles?" Two barriers to the widespread use are cited. One is the high cost of the INS, and the other is the cost and complexity of tightly-coupled OPS/INS integration. One of those two barriers has recently been diminished drastically with the development of a standardized software package for tightly-coupled integration. In the past, only the largest corporations have been able to pay the initial development cost for tightly-coupled OPS/INS integration, usually with funding from a large defense program. Using the new software package, integration and van test can be accomplished in a matter of days, and this has been demonstrated with field trials. The package is intended primarily for small companies that otherwise would not be able to build tightly-coupled OPS/INS systems at all. What would have been a prohibitive 3- or 4-man year development effort is reduced to a few man weeks. To accomplish an integration, the system integrator has to find a way, through serial interfaces or by some other means, to get the INS measurements of acceleration (accumulated velocity change /spl Delta/V) and attitude rate (accumulated angle change /spl Delta//spl theta/) into a processor, along with the raw data of a GPS receiver. He also has to find a way to time tag the INS /spl Delta/V, /spl Delta//spl theta/ with GPS time. The rest of tightly-coupled OPS/INS integration is predominately accomplished in the standardized software package. That leaves the cost of the INS as the only remaining barrier to the very widespread use of OPS/INS, and invites new development of low cost inertial sensors. The focus of this paper is on the software package, and how it achieves standardization and ease of use while retaining the flexibility to produce optimal results with a variety of INS and GPS receiver types.

Inertial pointing and positioning system

Abstract: An inertial pointing and control system and method for pointing to a designated target with known coordinates from a platform, to provide accurate position, steering, and command information The system continuously receives GPS signals and corrects Inertial Navigation System (INS) dead reckoning or drift errors An INS is mounted directly on a pointing instrument rather than in a remote location on the platform for monitoring the terrestrial position and instrument attitude, and for pointing the instrument at designated celestial targets or ground based landmarks As a result, the pointing instrument and the INS move independently in inertial space from the platform since the INS is decoupled from the platform Another important characteristic of the present system is that selected INS measurements are combined with predefined coordinate transformation equations and control logic algorithms under computer control in order to generate inertial pointing commands to the pointing instrument More specifically, the computer calculates the desired instrument angles (Phi, Theta, Psi), which are then compared to the Euler angles measured by the instrument-mounted INS, and forms the pointing command error angles as a result of the compared difference

GPS transfer initialization system

Abstract: A GPS transfer initialization system for initializing a mobile unit from a base unit, includes a GPS receiver in the mobile unit for receiving a GPS signal including a time register, a frequency register, and a GPS reference oscillator for generating a GPS carrier signal of a first frequency; a mobile transmitter/receiver circuit on the mobile unit responsive to the GPS carrier signal, for generating and transmitting a transfer carrier signal of a second frequency that is a multiple of the first frequency; a base transmitter/receiver circuit on the base unit including a GPS calibrated frequency and time reference for providing a GPS calibrated signal of a third frequency; an error detection circuit for comparing the GPS calibrated signal and the transfer carder signal to generate a frequency error signal determined from the difference between them and representative of the error in the frequency of the GPS reference oscillator; and an error correction circuit responsive to the GPS calibrated frequency and time reference and to the error detection circuit for generating and transmitting the GPS time and error signal; the mobile transmitter/receiver circuit including an initializing circuit responsive to the GPS time and the error signal for adjusting the time register and frequency register to the correct GPS time and frequency.

Improved vehicle navigation system and method

Abstract: The improved vehicle navigation system and method uses information from a Global Positioning System (GPS) to obtain velocity vectors, which include speed and heading components, for 'dead reckoning' the vehicle position from a previous position. If information from the GPS is not available, then the improved vehicle navigation system uses information from an orthogonal axes accelerometer, such as two or three orthogonally positioned accelerometers, to propagate vehicle position. Because the GPS information should almost always be available, the improved vehicle navigation system relies less on its accelerometers, thereby allowing the use of less expensive accelerometers. The improved vehicle navigation system retains the accuracy of the accelerometers by repeatedly calibrating them with the velocity data obtained from the GPS information. The improved vehicle navigation system calibrates the sensors whenever GPS data is available (for example, once a second at relatively high speeds). Furthermore, the improved vehicle navigation system does not need to rely on map matching to calibrate sensors. System flexibility is improved because map matching is oblivious to the hardware, and the system hardware can be updated without affecting map matching or a change in the map database.

A method for GPS positioning

Abstract: We present an algorithm that estimates position using the Global Positioning System (GPS) C/A code measurements. We include an approximation for the covariance of the position estimate.

An Integrated Low Cost GPS/INS Attitude Determination and Position Location System

Abstract: Advances in the technology of low cost solid-state inertial sensors have brought the price for a inertial measurement unit (IMU) down to the $ 10,000 level. Such an IMU, Systron Donners’ MotionPakTM, has been purchased by the Institute of Geodesy and Navigation (BEN) in order to develop an integrated GPS/INS attitude determination system. As GPS component for the integrated system, IfEN has selected the Trimble Advanced Navigation Sensor (TANS) Vector receiver system, which is a multi antenna attitude determination and position location system. IfEN has developed a realtime navigation software to calculate position, “velocity and attitude from the outputs of the MotionPak gyroscopes and accelerometers. These computed values are integrated with position, velocity and attitude information from the TANS Vector in a Kalman filter. Test results show that this system can achieve up to 0.1 degrees attitude accuracy (RMS). This Paper describes the low cost integrated GPS/INS System, including GPS and INS hardware, data acquisition and software.

A test of precision GPS clock synchronization

Abstract: This paper describes tests of precision GPS time transfer using geodetic-quality TurboRogue receivers. The GPS data are processed with the GIPSY-OASIS II software, which simultaneously estimates the GPS satellite orbits and clocks, receiver locations and clock offsets, as well as other parameters such as Earth orientation. This GPS solution technique, which emphasizes high accuracy GPS orbit determination and observable modeling, has been shown to enable sub-1 ns time transfer at global distance scales. GPS-based monitoring of clock performance has been carried out for several years through JPL's high precision GPS global network processing. The paper discusses measurements of variations in relative clock offsets down to a level of a few tens of picoseconds. GPS-based clock frequency measurements are also presented.

Navigation sensor, filter, and failure mode simulation results using the distributed Kalman filter simulator (DKFSIM)

Abstract: Results obtained using the distributed Kalman filter simulator (DKFSIM) are given. It is a FORTRAN software package, and was run on a PC. The sensors modeled were a medium accuracy strapdown inertial navigation system (INS), a barometric pressure altimeter (BARO), the GPS, a SAR, and a terrain-aided navigation (TAN) system. The SAR system includes an electro-optical type imaging model and a precision velocity update model. The TAN system combines radar altimeter measurements with digital terrain elevation data. The mission profile for each simulation included a low-level terrain-following segment and a high dynamic combat maneuver segment typical for tactical fighter aircraft. The filter implementations used during this simulation sequence included (1) a single centralized Kalman filter incorporating all of the sensor measurements, (2) a federated Kalman filter with each sensor assigned to an independent local filter and the INS included as the system reference sensor, and (3) a cascaded Kalman filter where the GPS Kalman filter fed estimation information directly to a centralized Kalman filter. The failure modes modeled were a GPS satellite clock failure and an INS accelerometer failure. Observation of filter performance under varied conditions revealed advantages and other characteristics for each filter architecture modeled.

Compensation of gyroscope errors and GPS/DR integration

Abstract: In car-navigation, a dead-reckoning (DR) system is required for solving the problem of GPS signal blocking in urban area. The vibrating gyroscope which is widely used to determine the vehicle heading in a DR system, has a low accuracy, hence an error compensation of it is needed. In this paper, an error compensation method for the vibrating gyroscope used in DR is presented. Gyroscope errors can be characterized into two types of error, deterministic and random errors. Each error term is mathematically modeled and an indirect feedback Kalman filter is used for the error compensation. Through laboratory experiment and field-test, it is shown that the error of a vibrating gyroscope can be successfully compensated by the proposed method. Also, as an alternative method for GPS/DR integration in a case that the number of visible GPS satellites is insufficient for the correct determination of a vehicle position, the mixed-measurement algorithm is proposed. It is shown that by using this algorithm, even if the number of visible GPS satellites is 2 or 3, the position error becomes smaller than by using only DR.

Electronic timepiece and method for clock timing adjustment

Abstract: PROBLEM TO BE SOLVED: To receive a position measuring signal transmitted by a GPS satellite to realize a timepiece of sufficiently high precision with simple constitution and shorten the time from the start of receiving the signal until the output of an exact time. SOLUTION: A GPS time is calculated on the basis of the Z count and week number of a navigation message included in a GPS position measuring signal to be corrected with a prescribed correction value and an exact UTC(Universal Time Coordinated) time is found. For the correction value for delay of time of a radio wave propagating from the received GPS satellite to a receiving spot, the average distance of the whole GPS satellite of 614 sampling spots on the earth is calculated, and a time necessary for propagating the radio wave in the average distance is used. Therefore, since the same correction value can be applied regardless of a set place and a satellite number, even if the set place is not input in a timepiece, and even if a self position is not found by position measuring, the timepiece of high precision within ±20ms can be realized.

The development of an integrated GPS/INS/sonar navigation system for autonomous underwater vehicle navigation

Abstract: The need to successfully navigate in an underwater environment is rapidly becoming an important concern in the 1990's. This paper presents the development of an integrated navigation system for autonomous underwater vehicles (AUV) using GPS, INS and sonar. This paper discusses the existing problems with sub-sea navigation, the motivation for an integrated system, the mathematical derivation for an integrated GPS/INS/sonar system, and the results obtained from extensive testing.

Modeling GPS satellite attitude variation for precise orbit determination

Abstract: High precision geodetic applications of the Global Positioning System (GPS) require highly precise ephemerides of the GPS satellites. An accurate model for the non-gravitational forces on the GPS satellites is a key to high quality GPS orbit determination, especially in long arcs. In this paper the effect of the satellite solar panel orientation error is investigated. These effects are approximated by empirical functions to model the satellite attitude variation in long arc orbit fit. Experiments show that major part of the long arc GPS orbit errors can be accommodated by introducing a periodic variation of the satellite solar panel orientation with respect to the satellite-Sun direction, the desired direction for solar panel normal vector, with an amplitude of about 1 degree and with a frequency of once per orbit revolution.

System for calculating relative position through inter-vehicle communications

Abstract: PROBLEM TO BE SOLVED: To calculate the position of an approaching relatively to driver's own vehicle accurately by canceling the error of GSP radio wave propagation time SOLUTION: The system comprises a transceiver 1 for inter-vehicle communication, a GPS receiving means 2, a GPS information transmitting/receiving means 3, and means 4 for calculating a relative position based on the difference of GSP radio wave propagation time The means 4 for calculating a relative position based on the difference of GSP radio wave propagation time extracts a GPS satellite being employed commonly by an approaching vehicle and a host vehicle and determines the difference of GPS radio wave propagation time data between the approaching vehicle and the host vehicle for that GPS When the difference of GPS radio wave propagation time is obtained for three or more GPS, the position of the approaching vehicle relative to the driver's own vehicle is determined by solving simultaneous equations having the relative position as unknown COPYRIGHT: (C)1998,JPO

A Lagrangian drifter with inexpensive wide area differential GPS positioning

Abstract: Autonomous drifting buoys that acquire position data using Global Positioning System (GPS) navigation have proven valuable in Lagrangian measurement applications requiring greater accuracy and data density than is available through other technologies such as ARGOS satellite tracking. The limits on measurement accuracy in a GPS drifter are set by the geographic accuracy of the GPS position. Standard civilian receivers are affected by the purposeful degradation of GPS accuracy (termed Selective Availability or SA), which result in position measurement inaccuracies of up to +/- 100 meters. A 100 meter error over a 30 minute timespan between drifter positions results in a 6 cm/sec error in computed current velocity, a potential error which grows larger as shorter position recording intervals are used. Improvement of the accuracy of GPS positioning would allow use of GPS drifters in an even wider range of current measurement applications that demand tine spatial and temporal resolution. Differential GPS (DGPS) refers to a technique that uses GPS data from a reference station at a well known location to calculate corrections that are then used to improve the accuracy of GPS positions at a less well known location. DGPS can increase the accuracy of standard civilian receivers to +/$20 meters or better. Conventional DGPS relies on real-time telemetry of correction data from one or more reference station to the GPS receiver that requires correction. This method can be difficult, expensive, and is not available at all locations. A drifter equipped for conventional DGPS must have a telemetry receiver which adds cost and requires power. Moreover, the telemetry link must be maintained reliably during data acquisition or DGPS position accuracy is lost. If real-time results are not required, however, a Wide Area Differential GPS (WADGPS) method can be used in pose-processing to remove the effects of SA from drifter data. The requirements in the drifter are some additional data memory, minor firmware changes, and a GPS engine configured to output raw GPS satellite data (called "pseudoranges"). During a deployment, the pseudorange data is stored in drifter memory while standard GPS positions are telemetered and stored to assist in deployment logistics. After retrieval, the pseudoranges are converted to DGPS-accurate positions using software and correction data provided by the Canadian Active Control System (CACS). CACS is a global network of automated GPS reference stations pioneered by the Canadian government for survey applications. Correction information is available from CACS at a modest fee with a few days delay from real time. Dockside testing of WADGPS correction showed a reduction in 2 dimensional rms position errors from +/- 33.5 to +/$9.3 meters. Several Brightwaters Modal 104AV autonomous GPS drifters have been fitted with a WADGPS upgrade. A full field test of WADGPS correction of drifter data will be conducted in August 1996.

Evaluation of an integrated GPS/INS system for shallow-water AUV navigation (SANS)

Abstract: Many possible autonomous underwater vehicle (AUV) missions require a high degree of navigational accuracy. The Global Positioning System (GPS) is capable of providing this accuracy. However, intermittent reception caused by either wave action or deliberate submergence will cause the loss of GPS position fix information for periods extending from several seconds to minutes. The SANS system is designed to demonstrate the feasibility of using a low-cost strapped-down inertial measurement unit to navigate between GPS fixes. It is anticipated that navigational accuracy comparable to GPS is possible between fixes.

Design and Analysis of a High-Accuracy Airborne GPS/INS System

Abstract: A high-accuracy and high-rate integrated GPSANS positioning system is presented, which models accelerometer and gyro measurement errors plus position, velocity, and attitude errors. On-The-Fly ambiguity resolution techniques are applied to provide high accuracy cm-level GPS positioning. Inertial sensor errors are estimated when GPS positions are available, and used to improve INS navigation accuracy during periods when GPS positions become unavailable, due to aircraft banking or interference with the rotating blades. A number of key factors for the design and implementation of this cm-level GPS/INS system are discussed, including the INS navigation algorithm, the mathematical model of the Kalman filter, the numerically stable filtering algorithm, the INS feedback error calibration technique, and the GPS On-The-Fly ambiguity resolution technique. The performance of the integrated GPS/INS system is evaluated using data collected during the flight tests.

Traveling body position speed calculating device

Abstract: PROBLEM TO BE SOLVED: To accurately calculate position and speed by preventing the diversion of an inertial navigation calculation in a traveling body position speed calculating device for calculating position and speed regarding the traveling body (cars and people) by inertial navigation calculation. SOLUTION: An inertial navigation sensor error and attitude, speed, and position errors obtained by the inertial navigation calculation are corrected by a GPS 3, an absolute inclination angle detection means 5, and map data obtained from a map information acquisition means 6, a sensor offset error is obtained at the time of stopping by an error correction means 7 at a low speed, and the speed obtained by calculation is reset, thus reducing both sensor error and calculation error. COPYRIGHT: (C)1997,JPO

Position estimation using GPS and dead reckoning

Abstract: A method for estimating vehicle position by using GPS, a fiber optic gyro and encoders is proposed. The position estimator in this method is the optimal in the sense of least variance. The production of an experimental autonomous lawn mower and the evaluation of the performance of the method are reported. The relation between the performance and the accuracy of GPS is discussed in terms of the parallelism of the lines, the uniformity of the spaces between the lines, and the straightness of the lines.

BITAN-II: an improved terrain aided navigation algorithm

Abstract: BITAN-II ( BITAN: the abbreviation of BUAA Inertial Terrain Aided Navigation Algorithm) is a type of terrain aided navigation algorithm using the Kalman filtering theory to estimate position errors and velocity errors of the INS. Compared with BITAN, the unique feature of BITAN-II lies in its acquisition mode, which employs a bank of one state Kalman filters to correct the position error of INS indirectly, another feature is the improvement in BITAN-II's general structure. Monte Carlo simulations indicate that BITAN-II provides a higher accuracy of position fix while retaining higher immunity to false fixes.

New experimental results on GPS/INS navigation for Ocean Voyager II AUV

Abstract: This paper presents preliminary experimental results on small-sized autonomous underwater vehicle navigation in shallow water environments. The vehicle was chosen to be our second-generation Ocean Voyager II which has been integrated with on-board GPS/INS sensors. These first-cut results reveal practical problems when using raw GPS fixes to perform high-precision real-time navigation. Among these, the most damaging factor is the observed correlated noise (which has a long time constant and large magnitude) embedded in the GPS data. With such disturbance, one cannot distinguish between instantaneous signal and noise over a short time scale (especially with sporadic fixes), and consequently the position estimator will be greatly biased. However, with regular incoming fixes, one can process the error between dead-reckoned and GPS signal in order to minimize the influence of the correlated noise under the assumption that any drift due to sensor bias or currents is relatively time-invariant. This processing thus enables one to estimate the drift and drift rate over time reliably for post-processed vehicle localization. In addition to the results in this paper, a differential GPS and Doppler-velocity-log sensor are currently implemented on our Ocean Explorer which is expected to provide much improved navigational performance. The chosen differential GPS sensor has so far demonstrated successful tracking at 2 feet below water surface, thus reinforcing a potential solution for clandestine operations.