NDGPS NETWORK ENHANCEMENTS IN FY01

L. W. Allen
(United States Coast Guard Navigation Center, Alexandria, Virginia, USA)

D. B. Wolfe, C. L. Judy, E. J. Haukkala, R. W. James
(United States Coast Guard Command and Control Engineering Center, Portsmouth, VA, USA)

BIOGRAPHY

CDR Len Allen is the Chief of the Operations Planning Division at the U.S. Coast Guard’s Navigation Center. He has been in the Coast Guard for 24 years and has had a variety of Electronics Engineering, Project Management and Major Acquisition assignments.

Mr. Wolfe is the Project Manager for the Maritime Differential GPS, Nationwide DGPS and Short Range Aids to Navigation projects at the USCG Command and Control Engineering Center (C2CEN), Portsmouth VA. He earned his B.S.E.E. from Drexel University in 1990.

Mrs. Judy is the Senior Software Engineer at the USCG C2CEN for Differential GPS and Short Range Aids to Navigation. She holds her B.S.B.A. from Old Dominion University, in Norfolk, Virginia.

LT Haukkala is a member of C2CEN’s Nationwide DGPS implementation team. LT Haukkala graduated with a B.S.E.E. from the USCG Academy, in New London, Connecticut in 1990 and in 1998 received his M.S.E.E. from the University of Rhode Island in Kingston, RI.

ENS James is involved with C2CEN’s DPGS and Vessel Traffic Service (VTS) projects. He graduated with a B.S. in Physics from New Mexico State University in Las Cruces, NM in 1999 and received his commission from the Coast Guard’s Officer Candidate School in 2000.

ABSTRACT

While implementing the world’s largest ground-based GPS augmentation service, Nationwide DGPS (NDGPS), the U.S. Coast Guard (USCG) has implemented numerous enhancements designed to operate in concert to create a more accurate, robust and appealing NDGPS network. NDGPS is the descendant of Maritime DGPS designed to fulfil the USCG’s original responsibility, i.e. provide DGPS coverage to all harbors and harbor approaches of the United States. The Maritime DGPS Broadcast Site network continues to provide differential corrections along the Atlantic, Pacific and Gulf Coasts, the Great Lakes and the coastal waterways of Hawaii, Puerto Rico and southern Alaska. As the number of users and uses of DGPS increased, the need to expand the DGPS coverage area has also increased. An agreement with the U.S. Army Corps of Engineers expanded the USCG’s DGPS coverage to the inland rivers of the US. The NDGPS expansion effort now includes over 70 Broadcast Sites and 56 more scheduled. When completed, it will provide double DGPS coverage across the continental United States and along the transportation corridor in Alaska and single coverage over the rest of Alaska, Hawaii and Puerto Rico.

In 1997, the Department of Transportation (DOT) to assemble an Executive Steering Group and a Policy and Implementation team comprised Federal agencies to determine the cost effective way to provide DGPS corrections to terrestrial users throughout the United States. The system had to meet a high level of accuracy, integrity and availability; and be flexible enough to be easily improved to meet future user needs. The Executive Steering Group and team looked at several existing systems including commercial FM subcarrier and satellite provided systems, the Federal Aviation Administration’s Wide Area Augmentation System and Local Area Augmentation System and the Coast Guard’s DGPS service. This Executive Steering Group decided the most efficient and cost effective way to create a nationwide system was to expand the USCG’s Maritime DGPS network. Based upon their experience with providing DGPS coverage, USCG was designated as lead agency responsible for the design, construction and implementation of the Broadcast Sites and Control Stations in the NDGPS expansion.

Integrity, availability and accuracy continue to drive enhancements to the NDGPS network. This paper details several projects that work in tandem to propel NDGPS into a versatile network that only a high degree of accuracy can bring. Topics include: specific aspects of the High Accuracy NDGPS project sponsored by the USCG Navigation Center (NAVCEN) and TASC; the Federal Highway’s (FHWA) high accuracy project technologies; current high accuracy engineering implementations including Long-Range Aids to Navigation (LORAN) diplexing; RF studies; and future NDGPS system upgrades like the Nationwide Control Station (NCS) and RSIM Transmitter Control Interface (RTCI). From software and hardware enhancements to engineering ground stations, NDGPS is continuously improving to meet the needs of all maritime and terrestrial users.

INTRODUCTION

Differential Global Positioning Service (DGPS) is a land-based augmentation system that receives and processes signals from orbiting GPS satellites, calculates corrections from known positions and broadcasts these corrections via a Medium Frequency (MF) Transmitter to DGPS users in the Broadcast Site’s coverage area.

The United States Department of Transportation (DOT) is coordinating the implementation of a network of DGPS broadcast sites across the continental United States, Alaska, Hawaii and Puerto Rico. Seven Federal agencies including the Federal Railroad Administration (FRA), Federal Highway Administration (FHWA), U.S. Army Coups of Engineers, U.S. Air Force, National Geodetic Service, Office of the Secretary of Transportation and United States Coast Guard (USCG). These federal agencies are receiving enormous assistance from many state agencies, which have provided land and local technical assistant to find, select and in some cases purchase sites for the project. When completed, this nationwide broadcast network will consist of over 126 sites and provide a standardized signal for DGPS service throughout the United States [1].

Originally designed for use in harbor/harbor approach navigation, vessel tracking, and buoy positioning, expanded use of the NDGPS network includes positive train control, precision farming, smart vehicles, snow plow management, accurate waterway dredging and improved emergency response [2].

The passage of Public Law 105-66, Section 346 granted authority and funding to install the system. Thus far the NDGPS program has spent about $9 million in construction and engineering. It will cost about $27.5 million to complete coverage in the continental US and about $22.5 million to complete single coverage in Alaska. Subsequent software and hardware enhancements to existing and developing ground stations have become important aspects of the project. These enhancements will rely heavily on accuracy improvement and utilization of emerging technologies. In cooperation with other federal agencies, USCG engineers are exploring use of these emerging technologies in a number of engineering projects including FHWA’s High Accuracy NDGPS project, diplexing onto Long-Range Aids to Navigation (LORAN) broadcast towers, and improved Radio Frequency (RF) performance. Research in DGPS antenna modeling and DGPS signal coverage software has been ongoing since project conception. Additionally, implementation of the Nationwide Control Station (NCS) and RSIM Transmitter Control Interface (RTCI) will contribute to a seamless nationwide system.

DGPS TRANSITION TO NDGPS

The primary purpose of the maritime DGPS network is to provide the position accuracy and integrity needed to meet the navigation requirements of inland rivers, harbors and harbor approach areas up to 20 miles offshore of the continental US, Puerto Rico and selected portions of Alaska and Hawaii [3].

In 1996, the President assigned DOT as the responsible agency for all civilian GPS matters and to implement a national GPS augmentation for terrestrial transportation [4]. In January 1997, DOT formed the DGPS Executive Steering Group and Policy and Implementation Team (PIT) to develop a nationwide differential system. After reviewing several options, the conversion of U.S. Air Force Ground Wave Emergency Network (GWEN) sites into DGPS sites based upon USCG standard DGPS design was determined to be the most efficient and cost effective method of providing nationwide differential coverage. After a successful prototype GWEN-to-DGPS conversion site was established and tested at Appleton, Washington [5], the Executive Steering Group decided to expand USCG’s DGPS network into a nationwide system.

NDGPS SYSTEM REQUIREMENTS

NDGPS is required to provide dual coverage throughout the continental US and along the transportation corridor between Anchorage and Fairbanks Alaska. The rest of Alaska, Hawaii and Puerto Rico will have single coverage. Each Broadcast Site is required have automated integrity monitoring which provides the user an indication if either a GPS satellite or the DGPS reference station is out of tolerance within 5 seconds. Each broadcast site must have an operational availability of 99.7 percent of the time and controlled by either the East or West Coast Control Stations. The double coverage throughout the continental U.S. and along the transportation corridor in Alaska will provide a signal availability of better than the required 99.9%. Availability represents the percentage of time the DGPS signal is usable. The USCG DGPS system provides users with broadcast messages as defined by the Radio Technical Commission for Maritime Services (RTCM) [6] and utilizes Reference Station Integrity Monitor (RSIM) [7] messages for intra-system communication.

NDGPS SYSTEM IMPLEMENTATION

The NDGPS design is based on the USCG’s DGPS maritime service that began initial operation in 1996 with service coverage of major harbors and the nation’s coasts. Achieving full operational capability (FOC) in 1999, the Maritime system uses two (2) Ashtech Reference Stations to calculate differential corrections and two (2) Trimble Integrity Monitors to meet monitor/integrity requirements. This system utilizes a Southern Avionics transmitter to amplify the reference station correction through a MF antenna at the authorized Maritime Radiobeacon band of 285-325kHz. The unmanned broadcast sites are monitored continuously by one of two remote Control Stations over a wide area network (WAN). The USCG presently uses an X.25 format WAN. Each Control Station is capable of controlling all of the broadcast sites if there is a major failure of the other control station.

A major component of the NDGPS implementation plan is the reuse of USAF GWEN facilities as DGPS Broadcast Sites. GWEN is a highly redundant network of sites hardened to withstand electromagnetic pulse and operated via Low Frequency (LF) ground wave signals. USAF transferred 50 GWEN sites and all GWEN spare parts to USCG, significantly reducing the cost of implementing NDGPS coverage and saving the USAF decommissioning costs.

The reused antenna at the Nationwide sites is 299 feet tall and has twelve Top Loading Elements (TLEs), which yields an efficiency of approximately 55%. Bandwidth is between 30 and 80 kHz, significantly better than any other antenna currently in the USCG’s DGPS inventory. The most efficient maritime DGPS antenna has an efficiency of only 15%. The Nationwide antenna efficiency translates into an expanded coverage area at the same power, reducing the number of required sites – an important factor in designing a system to ensure nationwide coverage.

SITE SELECTION

The initial determination of NDGPS site placement was based on the signal strength predictions using Millington’s method. This is a prediction model for ground wave signal strengths over smooth surface with varying ground conductivities. As a result of the poor resolution of the US ground conductivity data, skywave signal cancellation, and terrain effect errors in the expected coverage area exist. Thus, more refined tools and coverage verification are needed.

As a cost saving measure, many NDGPS sites were left in their original GWEN positions with the remaining sites selected to fill in the predicted coverage gaps. As more sites are installed, the coverage model will be updated using actual measurements to ensure that complete double coverage of the US is achieved.

Figure 1. End of FY 01 2001 Predicted Dual Coverage (Courtesy USCG NAVCEN)

Fifty GWEN sites have been or will soon be converted for NDGPS use at their current locations. The higher-powered RCA transmitter coupled to a much more efficient antenna provides far greater coverage from the converted GWEN site when compared to the pre-existing maritime DGPS sites, requiring fewer sites to be built. Several of these sites are located in the vicinity of maritime sites and will replace these lower power sites. Many of the maritime sites that are being replace are also located in areas where more prone to maintenance problems due to salt spray, flooding or hurricane damage. Moving the site away from the coast to the more protected GWEN sites decreases maintenance and improves the operational availability.

The remaining sites required to complete the NDGPS system will be constructed using relocated GWEN equipment or entirely new antennas and transmitters based upon the GWEN design. In addition to the 50 GWEN site, it is estimated that it will take 15 to 16 sites to cover Alaska and 15 to 18 sites to cover the continental U.S.

The PIT decides on a location for a new NDGPS site, usually a specific town or city. A standard site selection guide is sent to a local State or Federal agency representative to assist in locating possible sites in that area. The local representative looks for potential sites, within a 30-50 mile radius of the location, which meet the criteria outlined in the selection guide. The selection guide includes criteria on the required property size (11.2 acres), environmental concerns, the availability of power and telephone service and the presence of any tall objects that could mask the GPS antennas. Upon completion of this stage of the review, the number of potential sites has usually been narrowed down to two or three and the results are sent to USCG Command and Control Engineering Center (C2CEN). C2CEN sends a representative to visit these sites for a final survey before deciding where the site will be installed.

Despite these procedures, the selected site occasionally has a unique problem. For example, at a planned site in Ohio a 600-foot metal bridge is located only a few hundred feet away from the broadcast antenna. Even using computer modeling, the effects to the coverage area will not be known until after installation is completed. At the Annapolis, Maryland site, a problem arose because of an 855-foot tall television tower located 980 feet to the southwest of the site. It is anticipated that the television tower will turn the broadcast antenna into a somewhat directional array. At certain frequencies in the DGPS spectrum, the radiation pattern has a significant gain to the northeast of the site and losses orthogonal to the main lobe. In California, the Essex NDGPS site is also the home to an endangered tortoise. Consequently, special training has to be given to the maintenance crews to limit the impact to the tortoise and its habitat.

The largest area requiring construction of new sites is in Alaska where there are no GWEN sites. The cost of construction in Alaska is also significantly higher that the cost to build a site in the lower 48. The estimated cost of installing an NDGPS site in Alaska is $1.3 million. With 15 sites planned there, the USCG is searching for ways to reduce this cost. One idea is to broadcast the DGPS signal using pre-existing broadcast antennas, such as those at LORAN-C stations positioned throughout the state. With an expected cost savings of over $600,000 per site, this implementation would drastically reduce the cost to achieve DGPS coverage in Alaska. The engineering aspects of this proposal are discussed later in the paper.

COMMUNICATIONS NETWORK

All communications between Broadcast Site equipment (Reference Station, Integrity Monitor and Transmitter) and the Control Station is performed via an X.25 Packet Switching Network, a Router and Data Servicing Unit (DSU). Communications is established from the Control Station application using dedicated virtual circuits with each piece of equipment. These virtual circuits are opened and remain open until communications with the Broadcast Site is explicitly terminated. Each Broadcast Site has a 9.6 kbps line providing service to the X.25 network. Each Control Station requires a single 56 kbps line; a second line is installed to ensure redundancy. The Control Station application initiates the calls after which two-way communication can occur. System implementation enforces the restriction that only one Control Station can establish and maintain a connection with a given Broadcast Site.

The Control Station router performs protocol translation making the application network-independent. The Control Station has a fast-Ethernet network, permitting interconnectivity between Server and Client machines. A 100 Base T connection provides fast updates between redundant server machines

CONTROL STATION

FIGURE 2. DGPS CONTROL STATION AND BROADCAST SITE CONFIGURATION

System requirements specify that NDGPS Broadcast Sites be monitored and controlled on a continual basis from a central location and that these Control Stations have the capability to simultaneously monitor and control at least 200 sites.

There are two operational Control Stations, USCG Navigation Center (NAVCEN) located in Alexandria, Virginia and USCG Navigation Center Detachment (NAVDET) located in Petaluma, California, and one Control Station Engineering mockup located at the Command and Control Center (C2CEN) in Portsmouth, Virginia. Each control suite is the capable of monitoring and controlling the entire system.

The control functions afford watchstanders the capability to change site parameters and disable sites, i.e., turn off corrections, as circumstances warrant. The monitor features alert watchstanders to DGPS site or DGPS and GPS system anomalies, which they in turn can respond to. Figure 2 shows the relationship between a Control Station and a Broadcast Site.

LATEST ENGINEERING INNOVATIONS

USCG C2CEN has several projects underway to improve Broadcast Site performance. The first is an in-depth analysis of the entire RF network to identify ways to improve system performance and decrease the amount of off-air time. The maritime DGPS network is experiencing systemic problems with power fluctuations and high levels of reflected power automatically shutting down the transmitters. These problems are most likely caused by variations in the weather changing the antenna characteristics beyond the tuning capabilities of the Automatic Tuning Unit (ATU).

The underlying problem is that antennas used in the maritime DGPS network are shorter electrically than ideal quarter wavelength antennas. These electrically short towers have higher currents and voltages and are more susceptible to corona effects, arc-overs and weather variations. The decision to use these antennas was an economical one, but they have created problems in meeting availability requirements. Numerous improvements to the system have already been made – improved ground planes and insulators and the addition of corona rings. An RF study will look at these and other areas in an effort to improve system-wide on-air availability

Another project will install an Autonomous Controller and Data Logger (ACDL) at each Broadcast Site. In the event of a communications loss with the Control Station, this commercial ACDL software program and computer will capture and log all USCG required RSIM messages, automatically respond to basic alarms and attempt to establish communications with the Control Station using an alternate network such as a dial-up modem. When the network connection is reestablished, the ACDL will send all data logged during the outage to the Control Station. Implementing ACDL will provide the capability to compute availability using the data captured during the communications loss. This will give a more accurate picture of Broadcast Site performance than is currently possible.

C2CEN is in the process of creating a portable DGPS site. There are several reasons why a broadcast site can be off air for an extended period of time. Damage from severe weather, an earthquake or even extended maintenance can cause a site to fail to meet the 99.7% availability requirement which equates to approximately 2 hours per month. In these cases, a portable DGPS site could be set up at or near the off-air site to provide a temporary signal until the main site is brought back on-line.

HIGH ACCURACY NDGPS

Many applications demanded better accuracy, integrity and availability then either the SPS or even the PPS services provide, even with SA turned off. The first augmentation system developed to address these shortfalls is the USCG’s DGPS. USCG needed a radionavigation system to provide better than 10 meters accuracy along navigable US waterways to improve maritime traffic safety. USCG also needed improved accuracy to more efficiently position the thousands of navigation buoys which line the nation’s rivers and harbors.

This graph shows the accuracy of the NDGPS site in Whitney, Nebraska. A position was plotted every half hour over a 24-hour period. This site is located in the farmland region of the US and is used by farmers for precision farming.

FIGURE 3. DGPS Accuracy – Whitney, Nebraska OPERATIONAL AVAILABILITY

The most stringent requirement for availability is 99.9 percent. A dual coverage system will easily meet that requirement and also provide the flexibility of taking a site off air for maintenance while still meeting the operational availability requirement as follows:

Ao = (RS1Ao+RS2Ao) - (RS1Ao x RS2Ao)

Ao = (.997 + .997) - (.997 x .997)

Ao = 99.999%

Where:

Ao = operational availability

RS1Ao = availability reference station 1

RS2Ao = availability reference station 2

Having reference stations spaced closely enough to provide dual coverage also enables other advanced applications such as carrier phase tracking, float solutions and regional area augmentation, which are discussed briefly below.

HIGH ACCURACY ENGINEERING IMPLEMENTATIONS

The USCG, FHWA, and NGS are currently involved in the High Accuracy DGPS Demonstration Project. The Interagency GPS Executive Board (IGEB) funds this project with the participating agencies contributing personnel resources. The ultimate goal of High Accuracy DGPS is to provide 20 cm, three dimensional accuracy, with a 90% confidence level throughout the completed NDGPS network. High Accuracy DGPS will accomplish this by using the characteristics of the GPS frequency to provide decimeter level corrections. In order to achieve this level of accuracy both carrier and code phase observables will be broadcast on a different frequency than the current NDGPS Service. The High Accuracy DGPS Broadcast will utilize a higher transmission rate (500-1000 BPS) and will achieve 1 – 2 second update rates, depending on the final data format. From a given site both the current (standard) DGPS Service and the High Accuracy DGPS Service will be simultaneously broadcast through the use of a diplexer. A diplexer allows the use of the same antenna to broadcast two or more signals at different frequencies without adversely affecting each other. Potential applications include waterway efficiency through reduced under-keel clearance requirements, snowplowing of highways, lane keeping in automobiles, automated machine control and land and marine surveys.

The project is divided into two phases, the first phase will evaluate single site positioning performance as well as evaluate several data formats, transmission rates, and various broadcast parameters. The second phase will assess multi-site positioning performance, evaluate integrity algorithms, develop navigation grade data link receivers, and explore application development. The actual broadcast portion of the test program is scheduled to begin on September 1, 2001 from an NDGPS site in Hagerstown, Maryland. A second test site in Annapolis, Maryland, is being considered and may come on line early next year. Test details, such as broadcast parameters and schedules will be posted on the USCG NAVCEN web site (www.navcen.uscg.gov) as soon as they become available.

LONG RANGE AIDS TO NAVIGATION (LORAN) CO-LOCATION

C2CEN RF engineers have reviewed the possibility of diplexing the DGPS signal onto the current LORAN broadcast towers. Diplexing NDGPS on the LORAN towers would provided outstanding efficiencies at the DGPS frequencies, providing extended coverage at reduced costs. Also, since the LORAN towers are located in areas where DGPS coverage is needed it would result in a significant cost savings.

After the initial study, it was determined that diplexing was feasible but not cost effective from an implementation standpoint because of the size and characteristics of the components necessary to withstand the high power of the LORAN signals.

However, because of cost savings with regard to site construction, USCG continues to evaluate other ways to use the LORAN infrastructure. The focus has been changed to evaluate the use of one of the tower’s guy wires, below the top loading element, as a slopping t- antenna or to radiate the LORAN tower through an alternate feedpoint. This would eliminate the power feedback problems and potential negative affects on the LORAN signal diplexing while allowing for an increase in antenna efficiency. Plans are in place to conduct a study later this year at the LORAN Support Unit (LSU) in Wildwood, New Jersey.

RF STUDIES

Several of the new Nationwide sites have pre-existing structures nearby such as cellular phone towers and television broadcast antennas. Questions were raised about how these structures would affect the DGPS signal and the site’s expected coverage area. Although the radiation pattern of a normal Nationwide tower is omni-directional, i.e., equal power in each direction for all frequencies, nearby metal structures could block or redirect the signal creating unexpected coverage gaps. [8]

C2CEN Radio Frequency (RF) engineers were asked to find answers to these questions. Since these structures may not have been installed when GWEN was operational or did not adversely affect the GWEN’s LF signal, the only way to determine the effect on the DGPS Medium Frequency (MF) signal prior to conversion was via computer modeling. Once C2CEN engineers are able to model the radiation pattern and see these effects, the implementation team may be able reevaluate the site location to one with less of an effect. If for whatever reason selecting another site is not feasible, the analysis will help in planning what additional sites might be required to meet the necessary coverage.

NATIONWIDE CONTROL STATION (NCS)

NCS has been designed adhering to Object Oriented design principles. To implement the greatest degree of flexibility, the application incorporates data-driven dynamic design, maximizing the use of a relational database to ensure data integrity and robust processing.

Implementing client-server architecture, NCS processing is easily allocated into the primary areas of its driving requirements to monitor and control. The Server portion of the application performs system monitoring including all network communications and data storage. The Client provides an interface for the control functions, i.e. watchstander-initiated changes and System Status and Information display.

The Control Station provides online data storage for at least one year with plans to add a data warehouse with capabilities for long-term system performance and trend analysis.

REMOTE TRANSMITTER CONTROL INTERFACE (RTCI)

DGPS Maritime sites use a SAC Transmitter which is controlled by means of an RSIM control drawer. The RCA transmitter, converted for NDGPS use, did not have the same integrated control and monitoring functionality.

An RSIM Transmitter Control Interface (RTCI) was developed to provide the required capability to monitor and control the transmitter. A working prototype has been constructed and is currently undergoing tests at the C2CEN engineering mockup.

A rack mountable Industrial Computer with a thin film transistor (TFT) flat panel display is used as a interface for the system. This computer utilizes a 600 MHz Celeron processor with 64MB of SDRAM. Microsoft Windows NT operating system is used on an INX9000 computer manufactured by Ann Arbor Technologies fitted with an Ethernet card to communicate with programmable logic controllers (PLC) hardware (Figure 4).

This module provides the Ethernet link between the PC and PLC hardware. The EBC processes the analog and digital input signals, formats the I/O signals to conform with the Ethernet standard, transmits the signals to the PC-based controller, receives and translates the output signals for the PC-based controller software, and distributes the output signals to control the transmitter.

Figure 4: RTCI Control Panel

ACADEMY RESEARCH AND DEVELOPMENT

The C2CEN and the US Coast Guard Academy (CGA) have fostered a mutually beneficial partnership in the development and augmentation of the Maritime and Nationwide DGPS projects. C2CEN engineers are able to utilize the expertise and knowledge of the CGA instructors while providing the prospective officers a chance to work on authentic USCG problems with experienced engineers and technicians. Current DGPS-sponsored projects at the Academy include DGPS Antenna Modeling, Directional Signal Strength Meter and DGPS Signal Coverage Software.

DGPS ANTENNA MODELING

The DGPS network has experienced problems with antenna insulator disintegration and transmitter outages during inclement weather since USCG declared the system fully operational in March of 1999. This CGA- based project is using Numerical Electro-magnetics Code for windows (NECWIN), a powerful software application, where modeling antenna characteristics of actual antennas can be predicted.

Predictions are compared to an installed antenna in order to confirm the theoretical measurements. Ground characteristics near antennas are then examined to determine the best site properties. Completion of the study should result in more efficient and supportable antennas, thereby providing better coverage and service to maritime and other user communities at lower cost. Data was compiled last year and is currently in the process of being analyzed while new ground composition is gathered apropos to how it affects antenna performance.

DIRECTIONAL SIGNAL STRENGTH METER

As the number of NDGPS sites increase there exists the potential for skywave interference on the DGPS signal from adjacent sites. Skywaves, i.e., radio waves that bounce off the earth’s ionosphere at night, can be stronger from sites several hundred miles away than from the groundwave of a nearby site. The change in phase of the sky wave can also cancel the desired groundwave. Currently the Coast Guard does not have a method of measuring the strength of these skywaves or the negative effect they may have on their own or other site’s ground wave..

CGA is building a directional multi-channel signal strength meter with the ability to null out unwanted signals in order to determine the signal strengths of various DGPS beacons. High-speed analog to digital converters in conjunction with a mini-loop antenna array, RF band pass filters and pre-amplifiers comprise the hardware component of the receiver. The remainder of the project is essentially software and DSP algorithm development. MATLAB© is used exclusively for the proof of concept, and the entire software package is GUI driven to allow for ease of system configuration and data display to the user. Concepts from Cadet projects, along with implementation of additional DSP algorithms make directional measurement of DGPS beacon signal strength possible.

Although the receiver is not complete, a large amount of the software and hardware framework has been completed and initially tested. Once the antennas are modeled within the software for directional gains and the array is coupled with software, extensive field-testing can be completed in order to validate the software package.

DGPS SIGNAL CONVERAGE SOFTWARE

In an on-going effort to improve the management of groundwave radionavigation systems such as LORAN and Differential GPS, CGA is working on creating an improved LF/MF propagation prediction model. Current ground wave propagation models rely largely on Millington’s Method and disregard other known error sources.

The initial determination of NDGPS site placement was based upon the signal strength predictions using Millington’s method. Comparisons between real data and predicted data reveal propagation patterns have a delta of several miles. Reasons for these differences could include the affects of terrain and the poor resolution of the US ground conductivity data. The goal is to find a solution for these errors while preserving the validity of the current Millington’s Method derived DGPS Coverage Software’s propagation model. Careful evaluation of real data versus predicted patterns will be used to improve the CGA model ensuring accurate coverage prediction and proper placement of the NDGPS sites to meet the dual coverage goals.

INTEGRATION INTO THREE FEDERAL SYSTEMS

NDGPS sites will be integrated into three Federal systems: USCG’s DGPS system for continuous integrity monitoring and control, NGS’s CORS system for high accuracy (2 to 5 centimeter) positioning applications, and the National Oceanic and Atmospheric Administration (NOAA) Integrated Precipitable Water Vapor System for real-time input of water vapor data into the national weather models.

OTHER APPLICATIONS

Other US government agencies use the equipment at DGPS Broadcast Sites [11]. NGS uses a connection to both Reference Stations to poll data as part of their CORS. This provides GPS carrier phase and code range measurements in support of 3-dimensional positioning activities throughout the United States. NGS posts the data on its Internet web-site (www.ngs.noaa.gov/CORS/) to enable post-processing of GPS data, providing positioning accuracies that approach a few centimeters both horizontally and vertically. This data is also used by USCG to adjust its own reference positions thereby improving its differential corrections accuracy.

NOAA also utilizes Broadcast Site resources to augment its weather forecasting capabilities by mounting the GPS Surface Observing System (GSOS) instrument package on one of the reference masts at each site. GSOS contains equipment to measure local pressure and temperature and a GPS antenna to measure the time delays between the L1 and L2 frequency reception to determine the water vapor present in the air. The data is transmitted to NOAA Forecast Laboratories where it is evaluated by the GPS Integrated Precipitable Water Vapor Observation System to improve weather forecasts.

FREE OF USER FEES

The format of the broadcast signal will be fully compliant with both RTCM SC-104 and ITU-R M.823. These non-proprietary, international standards are now used in over 36 other countries leading to a seamless international system. Broadcasting NDGPS corrections free of direct user fees, as required under Public Law 105-66, Section 346, will further encourage acceptance of the standard. An additional benefit of using an open, internationally accepted standard is that it creates a world market for GPS equipment manufacturers and creates lower equipment costs for users through economies of scale and competition. Thus, both the manufacturers and users benefit.

CONCLUSION

Several challenges remain before the NDGPS system can attain full operational capability. Aside from political issues dealing with US Congressional funding, these unique challenges include improvements in the existing GPS infrastructure, environmental issues associated with constructing new sites, and the sliding scale of technology. NDGPS continues to improve to meet the growing needs of federal and state agencies and the general public. The High Accuracy NDGPS project and the NOAA’s Forecast Systems Laboratory’s water vapor project will open the door to new applications that were not possible only a few years ago. As required under Public Law 105 section 346, we must continuously upgrade the system to meet the needs of the user.

Coast Guard engineers will continue to work with other agencies to develop the must useful tool possible and encourage the development of compatible systems worldwide.

1. Wolfe, D. B., C. L. Judy, E. J. Haukkala, D. J. Godfrey, Engineering the World’s Largest DGPS Network,. May 2000.

2. Allen, L. W., Paper: Nationwide DGPS.

3. U.S. Department of Transportation and U.S. Department of Defense, 1999 Federal Radionavigation Plan, December, 1999.

4. Presidential Decision Directive NSTC-6 of March 28, 1996

5. Allen, L. W., Nationwide DGPS, Proceedings of the 10th International Technical Meeting of The Satellite Division of The Institute of Navigation, ION GPS-97, pg. 675, 1997.

6. U.S. Department of Transportation and United Stated Coast Guard, Broadcast Standard for the USCG DGPS Navigation Service, Technical Report, USCG, COMDTINST M16577.1, 1993.

7. Radio Technical Commission for Maritime Services, RTCM Special Committee No. 104, RTCM Recommended Standards for Differential Navstar GPS Service, V2.2, RTCM Paper 11-98/SC104-STD, January, 1998.

8. Radio Technical Commission for Maritime Services, RTCM Special Committee No. 104, RTCM Recommended Standards for Differential NAVSTAR GPS Reference Stations and Integrity Monitors (RSIM), V1.1, RTCM Paper 12-2001/SC104-248, May, 2001.

9. Godfrey, D., D.W. Wolfe, J. Hartline, Worldwide Beacon DGPS Status and Operational Issues, May 2001.

10. Mangs, G., Mittal, S., Stansell, T., Worldwide Beacon DGPS Status and Operational Issues, Leica Geosystems, Torrance, May 1999.

11. Memorandum of Agreement of May 18, 1994 between the National Oceanic and Atmospheric Administration, Coast and Geodetic Survey and the USCG for cooperation in the establishment of the USCG DGPS Service.

12. Wolfe, D., D. Godfrey, R. James, USCG Tender Deployable Differential Reference Station, June 2001.