Changdon Kee

Wide area augmentation of the Global Positioning System

The Wide Area Augmentation System (WAAS) is being deployed by the Federal Aviation Administration (FAA) to augment the Global Positioning System (GPS). The WAAS will aid GPS with the following three services. First, it will broadcast spread-spectrum ranging signals from communication satellites. The airborne WAAS receiver will add these new ranging signals to the GPS constellation of measurements. By so doing, the augmented position fix will be less sensitive to the failure of individual system components, thus improving time availability and continuity of service. Second, the WAAS will use a nationwide ground network to monitor the health of all satellites over our airspace and flag situations which threaten flight safety. This data will be modulated on to the WAAS ranging signals and broadcast to the users, thereby guaranteeing the integrity of the airborne position fix. Third, the WAAS will use the ground network to develop corrections for the errors which currently limit the accuracy of unaugmented GPS. This data will also be included on the WAAS broadcast and will improve position accuracy from approximately 100 m to 8 m. When complete, the augmented system will provide an accurate position fix from satellites to an unlimited number of aircraft across the nation. It will be the primary navigation system for aircraft in oceanic routes, enroute over our domestic airspace, in crowded metropolitan airspaces, and on airport approach.

Wind Estimation and Airspeed Calibration using a UAV with a Single-Antenna GPS Receiver and Pitot Tube

This paper proposes a method that uses an aircraft with a single-antenna GPS receiver and Pitot tube to estimate wind speed and direction and to calibrate the airspeed. This sensor combination alone does not determine the true attitude of the aircraft, so the wind parameters cannot be obtained directly from the measurements. However, if the aircraft flies at different headings, such as in banking turns or circle maneuvers, the wind magnitude and direction can be estimated from the geometrical relation between the wind and the measurements. An extended Kalman filter (EKF) is applied to estimate wind parameters. The EKF can also estimate the scaling factor used to convert dynamic pressure to airspeed. This is useful for the operation of small unmanned aerial vehicles (UAVs) because of difficulty in determining the airspeed scaling factor of a low-cost UAV. Simulations are performed for a constant 2-D wind. To test the effectiveness of the proposed method, flight tests of a small UAV are conducted. Simulations and flight test results show that the proposed method is effective.

Geometric Descent Algorithms for Attitude Determination Using the Global Positioning System

Thispaperdescribesasetofnumericalgradient-basedoptimizationalgorithmsforsolvingtheGlobalPositioning System (GPS)-based attitude determination problem. We pose the problem as one of minimizing the function tr(H NH T Qi 2H W) with respect to the rotation matrix H , where N, Q, and W are given 3 £ 3 matrices, and tr(¢ ) denotes thematrix trace. Both the method of steepest descent and Newton’ s method are generalized to the rotation group by taking advantage of its underlying Lie group structure. Analytic solutions to the line search procedure are also derived. Results of numerical experiments for the class of geometric descent algorithms proposed here are presented and compared with those of traditional vector space-based constrained optimization algorithms.

Wide area differential GPS (WADGPS): future navigation system

Conventional differential Global Positioning System (DGPS) usually has accuracies of 1-3 within 100 km of the reference station wide area differential GPS (WADGPS) can achieve the same accuracy as DGPS in much wider area while reducing the number of reference stations and increasing the integrity substantially. Test results have shown 1.5-4 m positioning accuracy for a dual frequency user with 10-20 s of age using WADGPS, which in this case has six monitor stations with a 1632 km minimum baseline.

Vision-based real-time target localization for single-antenna GPS-guided UAV

This paper discusses a real-time method to calculate the three-dimensional location of a fixed target detected by a gimbaled camera in a fixed-wing experimental unmanned aerial vehicle (UAV) equipped with a single-antenna GPS receiver on board. The camera was operated by an algorithm that automatically tracked the detected target, and the location was obtained using triangulation. The algorithm, however, suffers from a bias in target localization, because the aircraft Euler angles are not measured directly, but rather inferred from GPS velocity. To enhance the algorithm, the influence of the wind was taken into consideration to improve the localization accuracy, which cannot be deduced using a single-antenna GPS. Using a special maneuver, the circular level flight, wind velocity was estimated by the periodic variation of the aircrafts velocity and angle of attack (AOA) using the camera angles. Then, the pseudopitch angle was compensated for using the estimated results. The experimental data show that applying the pitch angle calibration successfully eliminated almost 90% of the horizontal localization error and the final 3D accuracy was less than 10 m.

Fully automatic taxiing, takeoff and landing of a UAV using a single-antenna GPS receiver only

This paper presents automatic taxiing, takeoff and landing of a UAV based on a single-antenna GPS receiver. In this paper, inertial sensors such as gyros and accelerometers are not used at all to show the full potential of a single-antenna GPS receiver based attitude determination system. DGPS is implemented to give high accuracy position information for automatic taxiing, landing and takeoff on the runway. For a fixed wing aircraft, under the assumption of coordinated flight, the attitude information called as pseudo-attitudes can be estimated from the measurements of a single-antenna GPS receiver. So full state variables for the automatic control can be obtained from single-antenna GPS receiver. In addition to GPS receiver, only an airspeed sensor is added because the velocity relative to the air is very important during landing and takeoff. The forward velocity is replaced with the airspeed obtained from Pitot tube. From linearized equations of motions around the steady state, LQR controllers for takeoff and landing are built, m particular, the flare controller that controls the pitch, altitude and airspeed of a UAV is designed. During flight tests, the aircraft taxies and takes off the runway, follows the predefined waypoint path, and then lands on the runway along the curved approach path, all fully automatically. Based on flight test results, a single-antenna GPS receiver can be used as a main sensor for a backup or a low-cost control system of UAVs.

Indoor Navigation System using Asynchronous Pseudolites

The Global Positioning System (GPS) is now attracting worldwide attention as a navigation sensor. One of the advantages of GPS is that people can find their position using a GPS receiver wherever they are except in obstructed environments. Existing GPS receivers do not work in an obstructed environment despite there being many potential applications. This paper shows the possibility of navigation using GPS technologies indoors in a blocked environment. The paper describes the pseudolite-based indoor navigation system that was developed and implemented at Seoul National University (SNU). The system, which uses GPS technologies, has some different characteristics to normal outdoor GPS systems. The differences cause some practical problems, such as near/far, time-tag, multipath and unknown transmission position problems, which need to be solved to implement the indoor navigation system. The paper introduces various methods for solving such problems. The paper then shows the experimental results and the system accuracy. The satisfactory experimental results show that the RMS static error is 1 mm(la) horizontally and 2 mm(la) vertically and the RMS dynamic error is 5.6 mm(1σ) horizontally and 15 mm(la) vertically. The paper also shows the result of a field application test in the ocean engineering basin of the Korea Research Institute of Ships & Ocean Engineering (KRISO).

RRC unnecessary for DGPS messages

The range rate correction (RRC) was useful in reducing the latency error of pseudo-range correction (PRC), and the data baud rate. With selective availability (SA) removed, the temporal variations in the corrections are much smaller, so the issue has been raised as to whether the RRC term is still necessary. We provide results on PRC and RRC variation that account for seasonal, diurnal, and regional differences in the atmosphere and receiver noise statistics. We concluded that setting RRC to zero would help to reduce latency error, and supported this conclusion by static and dynamic tests using commercial receivers and the Radio Technical Commission for Marine Services (RTCM) correction message. The results will be used for defining differential correction messages in the new version 3 standard. The U.S. Coast Guard is conducting tests to address these issues based on results to date because of the potential benefits of reducing the data requirements for differential corrections and thus making available new services over the existing radio-beacon broadcast links. The results of this paper are expected to provide a thorough understanding of the factors affecting the temporal variations of the corrections, and will support the development of standards worldwide

Integrated waypoint path generation and following of an unmanned aerial vehicle

Purpose – The purpose of this paper is to present an efficient method to integrate path generation and following for an unmanned aerial vehicle.Design/methodology/approach – The shortest path is briefly reviewed using a straight line and a circular arc in a horizontal plane. Based on shortest paths, various path generation algorithms using oriented waypoints are described. Path design unit, which is structured concatenations of line segments and circular arcs, is proposed to represent different paths as one structure. Simple path following controller to follow a straight line and a circle was also implemented with linear‐quadratic regulator control laws. Some flight tests were conducted to verify the efficiency of proposed algorithm.Findings – Proposed method represents various paths between given waypoints efficiently by a small number of parameters. It does not need a large amount of memory storage and computation time to run in real time on a low‐cost microprocessor.Originality/value – This paper provi...

Flight Test of Attitude Determination System using Multiple GPS Antennae

Small Unmanned Aerial Vehicles (UAVs) or inexpensive airplanes, such as a Cessna single engine aircraft, require a navigation system with a cheap, compact and precise sensor. Over the past ten years, GPS receivers have begun to be used as primary or alternative navigation sensors, because their use can significantly reduce the overall system cost. This paper describes a navigation system incorporating a velocity-based attitude estimation system with an attitude determination system using multiple antennae, which was implemented and tested using a UAV. The main objective was to obtain precise attitude information using low cost GPS OEM boards and antennae. Attitude boundaries are derived from the relationship between the body frame and the wind coordinates, which are used to validate the resolved cycle ambiguity in an Euler angle domain. Angular rate based on Doppler measurements was used to exclude the degenerate pseudo-roll angle information during severe uncoordinated flight. Searching for cycle ambiguity at every epoch of the flight showed that the developed system gave reliable cycle integer solutions, although the carrier phase measurement was subject to additional errors, such as multipath, external interference, and phase centre variation. A flight test was performed using a 1/4-scale Piper J3 Cub model, CMC Allstar OEM boards, OEM AT575-70 antennae, and 700 MHz PC104 board.