Accurate Navigation Solution Research Articles

지구저궤도 GPS 수신기의 시험 및 성능 분석 방법

우주 공간에서 GPS 수신기의 사용은 지구저궤도에서 일반화가 되었다. 최근 대부분의 위성은 위성 위치를 찾기 위한 항법 해로써 GPS 수신기를 사용한다. 그러나, GPS 수신기로부터 직접 획득한 항법 해의 정확도는 지도 제작과 같은 위성 활용에서 충분하지가 않다. 정밀궤도결정과 같은 후처리 개념이 위성 위치 정확도를 향상시키기 위해서 위성 자료 처리에 최근 적용되고 있다. 정밀궤도결정은 GPS 수신기의 항법 해가 아닌 원시 측정 자료를 사용한다. 원시 측정 자료의 성능은 GPS 수신기의 원시 측정 자료 정확도 및 추적 루프 알고리듬에 의해서 결정된다. 이 논문에서는 원시 측정 자료의 성능을 평가할 수 있는 기법을 제안하였다. GPS 수신기의 항법 해와 정밀궤도결정의 항법 해를 얻기 위한 지구저궤도위성의 시험 환경 및 절차를 기술하였다. 추가로, GPS수신기의 항법 해, 원시 측정 자료, 정밀궤도결정의 항법 해에 대한 정확도를 분석하였다. 제안된 기법은 일반적인 저궤도 위성에 적용 가능하다. The use of GPS receiver at outer space becomes common in low earth orbit. Recently most of satellites use GPS receiver as navigation solution for finding satellite position. However, the accuracy of navigation solution acquiring directly from GPS receiver is not enough in satellite application such as map generation. Post-processing concepts such as Precise Orbit Determination (POD) are recently applied to satellite data processing to improve satellite position accuracy. The POD uses raw measurement data instead of navigation solution of GPS receiver. The performance of raw measurement data depends on raw measurement data accuracy and tracking loop algorithm of GPS receiver. In this paper, a method for evaluating performance of raw measurement data is suggested. Test environment and procedure of the low earth orbit satellite acquiring for navigation solution of GPS receiver and navigation solution of POD are described. In addition, accuracy on navigation solution of GPS receiver, raw measurement data, and navigation solution of POD are analyzed. The proposed method can be applicable to general low earth orbit satellite.

Recursive Estimation of Spacecraft Position and Velocity Using X-ray Pulsar Time of Arrival Measurements

The use of pulsars for spacecraft position determination has been considered since their discovery. These celestial sources provide unique signals that can be detected by sensors placed onboard spacecraft. Upon sufficient detection and processing, these signals can be used to generate range measurements with respect to an inertial ref- erence location. Multiple measurements can be used to maintain an accurate navigation solution and enhance autonomous vehicle operation. This paper provides a description of blending pulsar-derived range measurements within a Kalman filter for continuous determination of Earth-orbiting spacecraft position and velocity. Several examples at different orbital altitudes are presented to establish the expected performance using recursive meas- urements obtained from models of pulsed X-ray signals.

6DoF SLAM aided GNSS/INS Navigation in GNSS Denied and Unknown Environments

This paper presents the results of augmenting 6DoF Simultaneous Localisation and Mapping (SLAM) with GNSS/INS navigation system. SLAM algorithm is a feature based terrain aided navigation system that has the capability for online map building, and simultaneously utilising the generated map to constrain the errors in the on-board Inertial Navigation System (INS). In this paper, indirect SLAM is developed based on error analysis and then is integrated to GNSS/INS fusion filter. If GNSS information is available, the system performs feature- based mapping using the GNSS/INS solution. If GNSS is not available, the previously and/or newly generated map is now used to estimate the INS errors. Simulation results will be presented which shows that the system can provide reliable and accurate navigation solutions in GNSS denied environments for an extended period of time.

Concurrent mapping and localization using sidescan sonar

This paper describes and evaluates a concurrent mapping and localization (CML) algorithm suitable for localizing an autonomous underwater vehicle. The proposed CML algorithm uses a sidescan sonar to sense the environment. The returns from the sonar are used to detect landmarks in the vehicle's vicinity. These landmarks are used, in conjunction with a vehicle model, by the CML algorithm to concurrently build an absolute map of the environment and to localize the vehicle in absolute coordinates. As the vehicle moves forward, the areas covered by a forward-look sonar overlap, whereas little or no overlap occurs when using sidescan sonar. It has been demonstrated that numerous reobservations by a forward-look sonar of the landmarks can be used to perform CML. Multipass missions, such as sets of parallel and regularly spaced linear tracks, allow a few reobservations of each landmark with sidescan sonar. An evaluation of the CML algorithm using sidescan sonar is made on this type of trajectory. The estimated trajectory provided by the CML algorithm shows significant jerks in the positions and heading brought about by the corrections that occur when a landmark is reobserved. Thus, this trajectory is not useful to mosaic the sea bed. This paper proposes the implementation of an optimal smoother on the CML solution. A forward stochastic map is used in conjunction with a backward Rauch-Tung-Striebel filter to provide the smoothed trajectory. This paper presents simulation and real results and shows that the smoothed CML solution helps to produce a more accurate navigation solution and a smooth navigation trajectory. This paper also shows that the qualitative value of the mosaics produced using CML is far superior to those that do not use it.

A Multiple Filter Approach for Precise Kinematic DGPS Positioning and Carrier-Phase Ambiguity Resolution

: The most precise relative positioning obtained using differential GPS depends on an accurate determination of carrier-phase integer ambiguities. To achieve high precision, many current static and kinematic surveying algorithms use a floating-point solution until enough information becomes available to fix the carrier-phase ambiguities accurately. A new method for kinematic carrier-phase ambiguity resolution, based upon a multiple-model adaptive Kalman filter, is presented in this paper. This method starts with the floating-point results, yet attains the precision of the correct fixed-integer solution smoothly and rapidly, eliminating the need to decide when to switch from the floating to the fixed-integer solution. This method is based on a blending of solutions from multiple filters, each of which hypothesizes a different ambiguity set. The new method was evaluated during ground and flight tests. Initial results indicate that the method is capable of quickly resolving carrier-phase ambiguities and smoothly transitioning to a highly accurate (centimeter-level) navigation solution.

Ridge Regression based EKF for GPS Navigation under Bad GDOP Conditions

In this paper, ridge regression based extended Kalman filter (EKF) for accurate GPS navigation solution under bad GDOP conditions (ie high GDOP values) is obtained. The effectiveness of the ridge regression based EKF for GPS navigation solution under bad GDOP conditions is illustrated in respect of a typical simulated trajectory.

Stereographic Observations from Geosynchronous Satellites: An Important New Tool for the Atmospheric Sciences

The capability of making stereographic observations of clouds and their temporal changes from two simultaneously scanning geosynchronous satellites is a new basic meteorological analysis tool with a broad spectrum of applications. Stereo height measurements, because of their higher horizontal resolution and dependence on only straightforward geometric relationships, represent a big improvement over previously used infrared-based techniques. Verification using high altitude mountain lakes indicates that stereo cloud height accuracies of ± 0.1–0.2 km are possible near reference points of known elevation. The smallest cloud features observed by the present operational geosynchronous satellites (GOES), which have spatial and temporal resolution of 1.0 km and 3 min, can be measured with height accuracies of ± 0.5 km. Absolute stereo height accuracy, far from landmarks, is also about ± 0.5 km if accurate navigation solutions are available for both satellites. Computer remapping of digital image pairs allows the display and height measurement on time sequences of stereo images using an interactive information processing system. Rapid time sequencing of the series of stereo image pairs gives an effective 4-dimensional representation of cloud dynamics. Several interactive techniques have been developed for point measurements of cloud height and cloud height contouring. The stereo observations have been applied to meteorological problems, including: cloud top height contours of intense convective clouds in hurricanes and severe thunderstorms; cloud top and cloud base height estimates for satellite cloud-wind altitude assignment; atmospheric temperature profiles from the combination of stereo heights and infrared cloud top temperatures; determination of cloud emissivity; and finally, comparisons are made between stereo-observed, tropopause-penetrating thunderstorm tops with altitudes up to 17 km and severe weather observations from radar and surface reports. Present stereo coverage using the operational GOES satellites for research demonstrations includes large areas of North and South America and the surrounding oceans. Recommendations are given for operational stereo observations with greater coverage and improved performance employing a third GOES satellite at 105°W.

Navstar/Global Positioning System 18-Satellite Constellations

The original baseline constellation for the Navstar/Global Positioning System consisted of 24 satellites. This would provide continuous world-wide three-dimensional navigation, and it would insure an accurate navigation solution 100 percent of the time at any location on earth. As a cost-saving measure, the Air Force has now been directed to develop Navstar/GPS with an 18-satellite constellation scheduled to be operational in 1987. This results in a constellation which will cause Navstar users to experience occasional outages of the system. Studies have been performed of various alternative 18-satellite constellations with the goal being to minimize the impact of outages on the Navstar user. Until recently, it has been generally accepted that with 6 satellites in each of the 3 orbit planes, the best arrangement would be to have the satellites spaced uniformly 60 deg apart. Studies by The Aerospace Corporation have shown that this is not true. A three-plane non-uniform arrangement has been found to provide far superior performance from the point of view of minimizing outages. Still another approach is to have 18 separate orbit planes, one for each satellite, which also provides superior performance. This paper discusses the difficulties inherent with the uniform 3-orbit-plane constellation and how these difficulties are in large measure overcome by the adoption of an alternate arrangement of the 18 satellites.