Next Generation Differential GPS Architecture

The United States Coast Guard is engaged in a project to re-capitalize Reference Station (RS) and Integrity Monitor (IM) equipment used in the Nationwide Differential Global Position System (NDGPS). The Coast Guard in partnership with industry is developing a new software application to run on an open architecture platform as a replacement for legacy equipment. Present commercially available off-the-shelf Differential Global Positioning System (DGPS) RS and IM equipment lacks the open architecture required to support long term goals and future system improvements. The utility of the proposed new hardware architecture and software application is impressive - nearly every aspect of performance and supportability significantly exceeds that of the legacy architecture. The flexibility of the new hardware and software architectures complement each other to offer promising possibilities for the future. For example, the new hardware architecture uses Ethernet for internal and external site equipment communications. Each Local Area Network (LAN) will be equipped with a router and two 24 port switches. Various levels of password protection are provided to manage security both locally and remotely. While the new software application directly supports the legacy RS-232/422 interfaces to devices such as GPS receivers, a system design goal includes the ability to directly address each device from NCS. With the use of TCP/IP to RS-232/422 port server devices, the system can meet these forward reaching goals while supporting legacy equipment. New system capabilities include remote software management, remote hardware configuration management, and flexible options for management of licenses. The new configurable RS and IM architecture is a PCbased emulation of legacy reference station and integrity monitor equipment. It supports fluid growth and exploitation of new signals, formats, and technology as they become available, while remaining backward compatible with legacy architecture and user equipment. Examples of new capabilities include enhanced data management & anomaly analysis, universal On Change Reference Station Integrity Monitor (RSIM) message scheduling, improved satellite clock handling, additional observation interval modes, and Range Rate Correction monitoring in the IM. Engineering initiatives under development such as implementation of pre-broadcast integrity are also presented. This paper details challenges and goals that drove software and hardware design approaches destined to become the backbone of the Next Generation Differential GPS Architecture. Functional differences between legacy and next generation operation are explored. The new DGPS system architecture will allow the USCG radiobeacon system to continue to deliver and improve navigation and positioning services to our nation and its territories. Reprinted with permission from The Institute of Navigation (http://ion.org/) and The Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation, (pp. 816-826). Fairfax, VA: The Institute of Navigation

Analysis of NOAA-Generated Tropospheric Refraction Corrections for the Next Generation Nationwide DGPS Service

The U.S. Coast Guard has begun the modernization of its Nationwide Differential GPS (NDGPS) beacon network. One potential component of modernization is to provide the information necessary for long baseline, centimetrelevel, differential carrier phase processing. In order to achieve these results, improved handling of atmospheric refraction of the incoming GPS signals must be achieved. The utility of the NOAA tropospheric delays in position determination was accomplished by supplying the NOAA zenith delay estimates to an in-house ionospheric-free relative GPS processor. Results indicate that the most significant improvement is observed in upcomponent bias reduction of a few centimeters to more than four decimeters.La "U.S. Coast Guard” ha iniciado la modernizaciôn de su red de balizas del GPS Diferencial a nivel nacional (NDGPS). Una componente potencial de esta modernizaciôn es proporcionar la informaciôn necesaria para el procesado de lîneas base largas, de la fase portadora diferencial, a nivel de centfmetro. Para lograr estos resultados se debe alcanzar un mejoramiento en la manipulaciôn de la refracciôn atmosférica de las sehales GPS entrantes. La utilidad de los retrasos troposféricos de la NOAA en la determinaciôn de las posiciones fue llevada a cabo proporcionando a la NOAA estimaciones de retrasos cenitales para un procesador GPS interno relativamente no ionosférico. Los resultados indican que la mejora mâs significativa se observa en la reducciôn de bias (distorsiones) de componentes ascendentes, que van desde algunos centimetros hasta mâs de cuatro decimetros.L"'U.S. Coast Guard” a entrepris de moderniser son réseau de balises NDGPS (GPS différentiel au niveau national). Une composante potentielle de cette modernisation consiste à fournir les informations nécessaires au traitement de longues lignes de base, de la phase porteuse différentielle, au centimètre près. Pour obtenir ces résultats, il est indispensable de parvenir à améliorer la gestion de la réfraction atmosphérique des signaux GPS entrant. L’utilité des retards de la NOAA dans la détermination de la position troposphérique a été menée à bien en fournissant à la NOAA des estimations de retard, au zénith, pour un processeur GPS interne relativement non ionosphérique. Les résultats montrent que l ’amélioration la plus significative est observée dans la réduction des erreurs des composantes en amont, qui vont de l ’ordre de quelques centimètres à plus de quatre décimètres

Loran-C performance assurance assessment program

The Federal Aviation Administration (FAA) has accepted the Loran-C navigation system as a supplemental navigation aid for enroute use. Extension of Loran-C utilization to instrument approaches requires establishment of a process by which the current level of performance of the system is always known by the pilot. This system 'integrity' translates into confidence that, if the system is made available to the pilot, the guidance will be correct. Early in the consideration of Loran-C for instrument approaches, the Loran-C Planning Work-Group (LPW) was formed with membership from the FAA, the US Coast Guard, various state governments, aviation users, equipment manufacturers and technical experts. The group was hosted and co-chaired by the National Association of State Aviation Officials (NASAO). This forum was ideal for identification of system integrity issues and for finding the correct process for their resolution. Additionally, the Wild Goose Association (WGA), which is the international Loran-C technical and user forum, regularly brings together members of the FAA, Coast Guard, and the scientific community. Papers and discussions from WGA meetings have been helpful. Given here is a collection of the issues in which Ohio University became involved. Issues definition and resolution are included along with the recommendations in those areas where resolution is not yet complete

Issue 9: Contributors

List of issue 9 contributors

Performance of Multi-Beacon DGPS

Historically, maritime organizations seeking accurate shipboard positioning have relied upon some form of differential GNSS, such as DGPS, WAAS, or EGNOS, to improve the accuracy and integrity of the GPS. Groundbased augmentation systems, such as DGPS, broadcast corrections to the GPS signal from geographically distributed terrestrial stations, often called beacons. Specifically, pseudorange corrections for the GPS L1 C/A signal are computed at each reference site, then broadcast in the nearby geographic area using a medium frequency (approximately 300 kHz) communications link. The user then adds these corrections onto their measured pseudoranges before implementing a position solution algorithm. Within the United States, the U.S. Coast Guard operates 86 DGPS reference beacons. Similar DGPS systems are operated in Europe and elsewhere around the globe. While current DGPS receiver algorithms typically use one set of pseudorange corrections from one DGPS reference site (often the one with the “strongest” signal), many user locations can successfully receive two or more different DGPS broadcasts. This brings to mind obvious questions: “If available, how does one select the corrections to use from multiple sets of corrections?” and “Is it advantageous to combine corrections in some way?” We note that a number of factors might influence the effectiveness of any particular station’s corrections. Some of these refer to the effectiveness of the communications link itself, including concerns about interference from other beacons (skywave interference from far-away beacons on similar frequencies, a notable problem in Europe) and self-interference (skywave fading). Other factors refer to the accuracies of pseudorange corrections. For example, ionospheric storm-enhanced plasma density (SED) events can cause the corrections to have large spatial variation, making them poor choices even for users close to a beacon. Earlier work in the area of DGPS beacon selection has identified several options including choosing the beacon closest to the user or the beacon with the least skywave interference. There have also been suggestions on how to combine corrections when multiple beacons are available. The most common of these is a weighted sum of the corrections, where the weights are typically inversely proportional to the distance from the user to the individual beacon. This paper reexamines the concept of multi-beacon DGPS by evaluating methods of combining beacon corrections based on spatial relativity. Of relevance to this topic is our recent observation that DGPS accuracy performance is biased. The mean of the error scatter with DGPS corrections does not fall on the actual receiver position. We established this both by processing GPS L1 C/A observables from hundreds of CORS (Continuously Operating References Station) sites around the U.S.A. and via simulation using a Spirent GSS8000 GPS simulator. Specifically, we found that the position solution computed using DGPS beacon corrections is typically biased in a direction away from the beacon, and that the size of the bias depends upon the distance from the beacon. This bias grows with a slope of approximately one-third of a meter per 100 km of user-to-beacon distance. This paper compares the performance of several multibeacon algorithms assessed using GPS simulator data. These algorithms include the nearest beacon, a weighted sum based on distances, and a spatial linearly-interpolated correction using the actual locations of the transmitters (distance and angle). We note that as part of this research effort we developed a DGPS receiver using software-defined radio (USRP). A complete description of this system is included in the paper

Selecting the Proper GPS Guidance System for Your Operation

Computers and other electronics have become commonplace on most newer agricultural equipment. Producers can now collect more information about their operation easier than ever before. Most of this information is commonly tied to the location where it was collected. GPS coordinates are the most common way producers determine this location, so selecting the proper GPS system for the job is critical. Whether a producer has not used a GPS systems and wants to find an affordable entry level guidance system or wants to upgrade to the latest model with all the bells and whistles, there are many different things that should be considered when selecting a new system

GPS Buoy Campaigns for Vertical Datum Improvement and Radar Altimeter Calibration

The report was prepared by Kai-chien Cheng, a graduate research associate in the Laboratory for Space Geodesy and Remote Sensing, at the Ohio State University, under the supervision of Professor C. K. Shum. This report was supported by the National Oceanographic Partnership Program Grant (Dynalysis of Princeton #865618), National Aeronautics and Space Administration TOPEX/POSEIDON Extended Mission Grant (NAG 5-6910/JPL961462), National Aeronautics and Space Administration Earth Science Information Partnership CAN Grant (CIT #12024478), National Aeronautics and Space Administration Interdisplanary Science Project (NAG5-9335), National Science Foundation Digital Government Grant (EIA- 0091494, and the Ohio Sea Grant Program (R/CE-5).This report summarized three Global Positioning System (GPS) buoy campaigns in the Great Lakes from 1999 to 2003 that were carried out by the Laboratory of Space Geodesy and Remote Sensing Research in the Department of Civil and Environmental Engineering and Geodetic Science (CEEGS), at the Ohio State University. The report focuses on the field work procedure of GPS buoy operation in these past campaigns and is intended to provide experience for similar applications in the future. The campaigns in this report include the Holland Campaign in Lake Michigan in 1999, the Marblehead Campaign in Lake Erie in 2001, and the Cleveland Campaign in Lake Erie in 2003. The major objective of these campaigns is to establish a calibration site for multiple satellite altimeters by using the GPS buoy to and the existing tide gauges provided by the Center for Operational Oceanographic Products and Services (CO-OPS) in the National Oceanic and Atmospheric Administration (NOAA). The campaigns provide useful information to the applications including radar altimeter absolute calibration, the establishment of the safe navigation in the Great Lakes, and the development of an integrated shoreline information in a spatial information database for coastal management and decision making. Since the report focuses primarily on the field work procedure, only limited results are presented. The published calibration results using the data from these campaigns are cited in this report. Generally, the GPS buoy is defined by putting GPS equipments on a floating object, which includes different types of buoys and could even be a moving vessel. The use of GPS buoys is a relatively new technique for the marine applications and its designs and operations vary from one application to another. For example, its platform could range from a small lifesaver type to an autonomous ruggedized type buoy. However only the OSU waverider GPS buoy, a life-saver type buoy that was used in these campaigns, is stressed in this report. The OSU waverider GPS buoy is a fairly simple design: it is built by attaching a Dorne/Margolin Element with Choke Ring antenna on top of a 2- feet (diameter) life-saver buoy covered with a transparent radome. The buoy is tethered to a boat where the receiver, power supply and the operators reside. Marks are made on four sides of buoy and their offsets to the antenna reference point (ARP) are carefully measured in the laboratory. The operator needs to observe the water surface with respect to these marks in order to accurately refer ARP to the water surface. The buoy data is post-processed with differential GPS (DGPS) in kinematic mode after the field work. The campaign-related documents, including National Geodetic Survey (NGS) data sheets, GPS Station Observation Log, Visibility Obstruction Diagram, campaign proposal, and field work log, are attached in the Appendices

Introduction to GPS and Other Global Navigation Satellite Systems

No abstract availabl

Selecting the proper GPS guidance system for your operation

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