7

Networks and Data Sources



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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications 7 Networks and Data Sources

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications Federal Aviation Administration GPS Augmentation Systems Loni Czekalski Office of the Associate Administrator for Research and Acquisition Federal Aviation Administration MISSION AND USERS The FAA has the statutory mission to establish, operate and maintain navigation systems for the safe operation of aircraft in the United States. Navigation capabilities with appropriate accuracy, integrity, and reliability allow airspace users to safely travel from one location to another and operate in the different phases of flight: approach/departure, terminal, enroute and landing. At the present time the FAA has a ground based air navigation system. The agency is beginning to transition to a space based system because it will likely provide equal or better service and be more cost effective. This is important for the users, because the FAA is currently in the process of being reinvented into an agency supported by user fees and off the federal budget. The cost of operating and maintaining the ground based system is believed to be approximately $200M per year, and a space based system will work well and cost less. The purpose of FAA GPS augmentation systems is to provide the navigation capabilities required in a cost effective manner. When they become operational they will replace existing radio navigation aids to support instrument navigation in the enroute environment and many instrument approach procedures. The WAAS signal is not secured or controlled and is available to anyone with a suitable receiver. To date, a number of antennas and aircraft receivers are under development by various companies. WIDE AREA AUGMENTATION SYSTEM (WAAS) The WAAS system augments the DoD provided GPS Standard Positioning System (SPS) signal in space. The WAAS will provide sufficient accuracy for precision approaches, availability and continuity of service for sole means navigation requirements as well as improvements in GPS integrity. Through a ground station network linked by FAA communications systems the WAAS will provide navigation corrections to airborne users by means of geostationary satellites. A contract was awarded in August 1995 to Wilcox Corp. with teammates Hughes and TRW to develop and furnish the WAAS. The system is currently under development and is intended to provide a signal by early 1998. An initial WAAS system will begin operations in late 1998 and the end state full system is planned to become operational in 2001. The WAAS acquisition strategy was to build an expandable system with a ground component that uses already developed software with new software for integrity, availability and accuracy in independent modules. The phased introduction of the system would start with a functional verification system for testing, and use the FAA's terrestrial communications network to link ground facilities. The space component of the system will use leased communications satellite service to allow for expansion of the coverage area, new technology and phased implementation of improvements to meet performance requirements. The standard broadcast format for the WAAS augmentation message will allow for global utilization. SYSTEM DESCRIPTION As shown in figure 1 the WAAS is made up of eight functional parts. These functions are performed at reference stations and master stations to generate the WAAS message for broadcast to users. Each WAAS reference station (WRS) collects independent sets of data including geosynchronous satellite observables, GPS satellite observables and local troposphere observables and transmits the data to each master station. Independence of data sets is ensured by gathering observable parameters through independent sets of hardware. Data is collected at a rate consistent with the expected level of variation. For example, slowly changing troposphere data is collected less frequently than GPS satellite data. Prior to transmitting data to the master station each reference station verifies the reasonableness of the data. All data is time tagged. WAAS master stations (WMSs) provide correction processing, satellite orbit determination, verification/validation and generate the WAAS messages. WMSs collect all data received from WRSs and process it once per second. After the 250 bit WAAS message is generated it is transmitted via an FAA private data network using T-1 lines at 56KB/sec to ground earth

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications FIGURE 1 WAAS functions. stations (GESs) for transmission to satellites for retransmission to users. The coverage area to the non-precision approach level of capability is the US domestic airspace except for the Alaskan peninsula west of 160 degrees longitude. This coverage is obtained by using INMARSAT geosynchronous satellites Atlantic Ocean Region East, Atlantic Ocean Region West and Pacific Ocean Region. Locations of WRSs, WMSs and satellite communications earth stations are shown in figure 2. WAAS MESSAGE DATA The WAAS message contains correction data for users to apply to data obtained from GPS satellites. Message data are: use/don't use verified satellite fast corrections verified satellite long-term clock and ephemeris corrections verified ionospheric grid point (IGP) locations The data verification function verifies the integrity of all data provided to WAAS users prior to transmission and validates that data. WAAS MESSAGE BROADCAST The WAAS message is computed at the reference stations and master stations and transmitted over existing FAA private data network communications systems. At each GES a geosynchronous uplink subsystem (GUS) selects one WMS as it's message source and encodes the received message using 1/2 rate forward error correcting convolutional code. The resultant 500 symbol per second message is modulated on a C band signal and uplinked to the geosynchronous communications satellite for broadcast to the WAAS users on the GPS L-1 channel (1575.42 MHz). The WAAS message contains the correction data for the users of GPS satellite data. The maximum latency for the fast correction data is 5.2 seconds.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications FIGURE 2 WAAS concept. PERFORMANCE REQUIREMENTS Performance requirements for the WAAS are described for initial system, enroute through nonprecision approach and precision approach environments. The following performance requirements are for the enroute through nonprecision approach regimes with the initial WAAS requirements noted if they are different. Availability. The total system has a required availability of .99999. Navigation system and GPS/WAAS signal in space subparts also require .99999 availability. Initial WAAS has a required availability of .999. Accuracy. Horizontal positioning accuracy for the navigation system is .054 NM (100m) at 95% confidence and .27 NM (500m) at 99.999 % confidence. Integrity. The probability of hazardously misleading information (HMI) coming from the GPS/WAAS signal in space is a maximum of 10 −7 per hour. Time to alarm for the total system will be no greater than 10 seconds. Continuity of Function. Continuity of navigation services will be at least (1 - 10−5) per hour for each of the total system, navigation system and airborne elements. Continuity for the GPS/WAAS signal in space is at least (1-10−8) per hour. Continuity of fault detection, excluding outages of less than 5 minutes, is 1−(2 x 10−5) per hour for each element and the total system. Maximum Latency of Fast Correction. The maximum latency is specified for the GPS/WAAS signal in space as 5.2 seconds. SYSTEM OPERATION AND MAINTENANCE DATA System monitor, control and maintenance collects all transmitted WAAS message outputs including the WAAS signal quality, ionospheric grid definition, list of GPS and WAAS satellites, UTC, and any additional manually collected data. This data will be collected and maintained for system maintenance and quality control. The system as currently specified does not include provisions for access by the general community to data for post processing corrections. Because the communications systems used for WAAS carry surveillance and other safety of flight and security data the FAA does not allow other parties access to the system. Data would be available through the Notices to Airmen (NOTAM) system and possibly manually through the servicing maintenance organization on a case by case basis. An agreement for providing data at a port for government agency users is in negotiation.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications Scientific activities with suggestions for making this data available, such as type, frequency, quantity, media/method are welcome to contact the FAA GPS/ Navigation Product Team, AND-500, at 800 Independence Ave SW, Washington, DC 20591. GEODETIC CONSIDERATIONS The WAAS contractor is required to provide the surveys for site location and has already begun this activity. Monuments for locating the GPS and geosynchronous communications satellite receive antennas will be US Geodetic Survey Federal Base Network Point or equivalent. The accuracy requirements specified are: Horizontal: 5 cm Ellipsoidal Height 10 cm Orthometric Height 10 cm NAVD 88 Antenna placement errors relative to the local monument are specified to be within 1 cm horizontal and 2 cm vertical. LOCAL AREA AUGMENTATION SYSTEM (LAAS) The Local Area Augmentation System (LAAS) is intended as a system to provide more precise positioning capability for precision approaches. Local area in this case means from 25 to 30 miles. The FAA is in the process of conducting the analysis necessary to make a decision on the LAAS system. Because of the capital investment required and the budget environment the FAA would build the system only if the system is cost beneficial over it's expected service life. Current work is devoted to integrity/continuity, pseudolite proof of concept and system architecture. Later, specifications will be prepared and more detailed modeling and analysis conducted and a contract awarded to build the system. If all activities area completed as planned and if resources are available system specifications could be complete by the end of 1998. REFERENCES FAA (Federal Aviation Administration). 1995. Federal Aviation Administration Specification Wide Area Augmentation System (WAAS) FAA-E-2892A.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications U.S. Coast Guard Differential GPS Navigation Service Gene Hall U. S. Coast Guard ABSTRACT The United States Coast Guard provides a Differential Global Positioning System (DGPS) service for the Harbor and Harbor Approach (HHA) phase of marine navigation. DGPS technology is the first to economically offer geodetic accuracy meeting the Federal planning requirement of sub10 meters for harbor and harbor approach navigation. The DGPS service coverage area includes the coastal United States, Great Lakes, Puerto Rico, and most of Alaska and Hawaii. This DGPS service is available to the public navigator as an all-weather navigation sensor to supplement traditional visual, radar, and depth sounding techniques. The design process for the United States Coast Guard's DGPS service began with efforts to define system operational requirements. The goal of these requirements was to ensure the same level of user integrity provided by present Coast Guard electronic navigation aids (Loran-C and Omega). Refinement of operational requirements by risk analysis of specific harbor navigation scenarios was then conducted. The final system architecture evolved to meet the defined requirements under traditional restraints of current technology, present and future economics, and the flexibility to adapt to future requirements. The operational doctrine to define DGPS service parameters and the service management infrastructure has been developed. The DGPS operations phase has begun. This paper provides a brief history on the evolution of DGPS and describes the operation of the DGPS service including technical information and broadcast site specifications. DISCLAIMER- The views expressed herein are those of the author and are not to be construed as official or reflecting the views of the Commandant or of the U.S. Coast Guard. BACKGROUND The U.S. Coast Guard is mandated by Federal law (14 USC 81) to implement, maintain, and operate electronic navigation aids that meet maritime needs of the U.S. armed forces and/or U.S. commerce. The U.S. Coast Guard's expertise in enhancing maritime safety through the utilization of radio (electronic) navigation services dates to 1921 with the first operational radiobeacons. In the last two decades, the U.S. Department of Defense (DOD) has led technology from terrestrial to space-based radionavigation systems, first with TRANSIT, and then the prototype NAVSTAR Global Position System (GPS). In 1987, the U.S. Coast Guard Research and Development Center in Groton, Connecticut, began conducting research and testing of differential techniques to enhance GPS accuracy. Simply stated, the differential technique involves installing navigation equipment at a precisely known location. The equipment receives the GPS signal and compares the position solution from the received signal to its known location. The result of this comparison is then generated in the form of a correction message and sent to local users via radiobeacon broadcast. The received correction is applied by the user's GPS equipment to reduce the system position error, thereby improving the user's absolute accuracy. This effort was coordinated through the Special Committee (SC) 104 created by the Radio Technical Commission for Maritime Services (RTCM). The differential effort was driven by the search for a system with the capability to meet the accuracy requirement for Harbor/Harbor Approach navigation as had been defined in the Federal Radionavigation Plan (FRP). The FRP identifies that accuracy on the order of less than 10 meters (2drms)1 is required for the HHA phase of navigation [FRP 94]. The FRP also states requirement for the Coastal and Ocean phases for maritime navigation which have respectively been satisfied with Loran-C and Omega services. In 1989, the U.S. Coast Guard modified the existing marine radiobeacon located at Montauk Point, New York to broadcast differential corrections in the RTCM SC-104 format. The Montauk Point field tests demonstrated that Minimum Shift Keying (MSK)2 modulation of an existing radiobeacon signal was effective in transmission 1   2drms means twice the distance of the root mean square error. In practice, any position fix obtained using the given system has a 95% probability of having a radial error equal to or less than the 2drms value expressed. 2   Minimum Shift Keying is a special form of frequency modulation. MSK involves utilizing the smallest possible frequency shift of the carrier frequency to relay digital information. A shift up in frequency from the carrier relays a digital “1” and down to “0”. The actual shift in frequency is 1/4th the data transmission rate.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications of RTCM SC-104 format corrections. The MSK modulation technique could be utilized with no adverse effect on the automatic direction finding receivers of traditional marine radiobeacon users. Important to both the U.S. Coast Guard and the public, MSK technology is economical to implement at existing radiobeacons and within user receivers. By January 1990, the RTCM published the SC-104 format version 2.0 document. With a formal U.S. industry differential GPS correction standard and the initial radiobeacon broadcast success, Montauk Point began the first continuous public U.S. DGPS broadcast on August 15, 1990. This transmission marks the beginning of the U.S. Coast Guard transition from DGPS research and development towards implementation of a U.S. maritime differential GPS service. DGPS ARCHITECTURE The DGPS service architecture is shown in Figure 1. The functional elements of the U.S. Coast Guard DGPS Navigation Service include: FIGURE 1 DGPS service architecture. Reference Station - Precisely located GPS receiving equipment which calculates satellite range corrections based on a comparison of the satellite navigation message to its known location. Integrity Monitor - Precisely located GPS receiver and MSK radiobeacon receiver which applies differential corrections. The corrected position is compared to its known location to determine if the correction broadcast from the Reference Station is in tolerance. Broadcast Site - A marine radiobeacon transmitting correction data in the 285 to 325 kHz band. Control Station - Site for human centralized control of the DGPS service elements. DGPS performance data processing and archiving is accomplished here. The East Coast Control Station is located at the USCG Navigation Center in Alexandria, Virginia. The West Coast Control Station is located at the Navigation Center Detachment in Petaluma, California. Both sites are manned 24 hours per day. Communicative Network - An X.25 packet-switched service providing connectivity between broadcast sites and control stations. DGPS User Equipment - Consists of two interfaced receivers with a display; a radiobeacon receiver for MSK demodulation and a GPS receiver capable of applying differential corrections. TECHNICAL CHARACTERISTICS GPS correction data based on NAD-83 coordinates is provided for both real-time and post processing applications. Real-time correction data is broadcast to the user via radiobeacon only for satellites at an elevation angle of 7.5 degrees or higher through use of the type 9-3 message in the RTCM SC-104 format. The official GPS coverage provided is based on elevation angles of ten degrees or higher. Satellites at elevation angles lower than 7.5 degrees are adversely affected by spatial decorrelation, multipath, and minimal processing time between acquisition and actual use. Corrections for a maximum of nine satellites will be broadcast. If more than nine satellites are above 7.5 degree elevation angle, a situation which occurs less than one percent of the time, then corrections are broadcast for the nine satellites with the highest elevation angles [USCG Broadcast Standard]. The latency of this information is determined by the baud rate at which it is transmitted. There are 210 bits in a type 9-3 message (three satellites corrected) including the message header. Therefore, at 100 baud the latency is 2.1 seconds. Naturally, this time is cut in half when transmitting at 200 baud. In reality, latencies on the order of 2-5 seconds are realized depending on the number of satellites in use. Other factors contributing to latency include partial decoding techniques, parity checking, and the receiver's internal processing. GPS satellites data consisting of CA code, P1 and P2 Range, and L1 and L2 Carrier Phase information is collected every 30 seconds by the National Geodetic Survey (NGS) from both Reference Stations at each broadcast site. NGS processes the data and makes it available to the public for post processing applications. A

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications benefit to this arrangement is that NGS provides monument stability for each DGPS site by continually checking and updating geodetically surveyed antenna positions and reporting their findings to the Coast Guard. The output interval of 30 seconds for this data is set by the Control Station watchstander. Because the X.25 network is used for control, monitoring, and remote data access, limits must be set on the mount of data, not the type of data, shared. Otherwise, remote user access could interfere with and delay control station alarms. Only authorized users are allowed access to the DGPS X.25 network. Each DGPS broadcast site houses dual Ashtech Z-12/R Reference Stations to provide redundancy. Geodetic GPS antennas are used with built in low noise amplifiers to provide the necessary RF signal gain (35 dB) for the receiver to work properly with an antenna cable up to 30 meters long [Ashtech]. Each DGPS broadcast site houses dual Trimble 4000IM MSK Integrity Monitors to provide redundancy. The Integrity Monitor MSK antenna is a near field passive loop antenna. The GPS antenna includes an omni-directional L1 GPS receiving antenna [Trimble]. SYSTEM PERFORMANCE [Broadcast Standard] -Accuracy- The position accuracy of the USCG DGPS Service is within 10 meters (2drms) in all specified coverage areas. A reasonable approximation for determining the achievable accuracy at a given point is to take the typical error at a short baseline from the reference station (approximately 0.5 meters), add an additional meter of error for each 150 kilometers of separation from the reference station broadcast site, and add an additional 1.5 meters for the user equipment. Some high-end user sets are achieving pseudorange measurement accuracies of less that 30 centimeters in the absence or the abatement of multipath. Hence, the user with high-end equipment who is within 300 kilometers from a given broadcast can achieve accuracy better than 3 meters (2drms). The continuous velocity accuracy of the system (i.e. the vessel's speed over ground) is better than 0.1 knots rms in VTS areas which utilize Dependent Surveillance.3 -Availability- This is defined as the percentage of time in a one month period during which a DGPS Broadcast site transmits healthy pseudorange corrections at its specified output level. The DGPS Navigation Service was designed for, and is operated to, maintain a broadcast availability level which exceeds 99.7%, assuming a complete and healthy satellite constellation is in place (i.e. HDOP<2.3). Any DGPS area of coverage that falls within a Vessel Traffic Service region which utilizes ‘dependent surveillance' for vessel tracking will maintain a signal availability in the coverage area of 99.9%. A signal availability will be higher than a broadcast availability if a coverage area receives more than one broadcast. -Integrity- System integrity is built upon the foundation of the monitor stations. The Integrity Monitors will ensure the correction broadcast and signal strengths are in tolerance. Users are alarmed within 10 seconds if an out-of tolerance condition exists. The user equipment suite plays a significant role in assuring that the integrity of the system is preserved. It should be capable of automatically selecting the appropriate radiobeacon. A satisfactory broadcast is one which is classified as healthy, is presently monitored, and the pseudorange time out limit of 30 seconds for at least four satellites has not been reached. The user need not be within the advertised range of the broadcast for it to be satisfactory. -Reliability- This is the probability that the service, if useable at the beginning of a mission segment (maneuver), will remain available over the course of the maneuver. Reliability is the frequency with which failures occur and is measured in the number of outages per million hours of operation as shown in Table 1. TABLE 1 MANEUVER CATEGORY RELIABILITY (Outages/Mhr) <140 sec 2000 140 to 280 sec 1000 280 to 560 sec 500 -Coverage- The USCG DGPS Navigation Service is designed to provide coverage at the specified levels for all “Harbor and Harbor Approach Areas” and other “Critical Waterways” for which the U.S. Coast Guard provides aids to navigation. Due to the omni-directions nature of the broadcasts, and that a high power radiobeacon may cover more than one harbor, coverage often extends into 3   Dependent Surveillance is any Technology which depends on active participation between the mariner and the Vessel Traffic Service to control the flow of traffic.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications additional areas. As a result, complete coverage of the coast line of the continental United States is provided out to 50 nautical miles. Coverage is also provided for the Great Lakes, most of Hawaii, Alaska, and Puerto Rico. SITE MAP Of the 51 sites shown transmitting corrections (Figure 2), all but for (Millers Ferry, Sallisaw, Rock Island, and Alma) are presently controlled and monitored by the Control Station. Site specific information is provided in Table 2. PRESENT STATUS AND FUTURE PLANS On November 1, 1995, the Coast Guard DGPS system began operation under a ‘Preoperational phase'. This phase was used to operationally test and evaluate system performance. As a result, much was learned and many improvements to the DGPS service will be made over the next few years. On January 30, 1996, DGPS entered a ‘Initial Operational Capability' (IOC) phase in which the service is available for positioning and navigation. During IOC, enhancements to Control Station software and hardware will be accomplished, radiobeacon antennas will be upgraded to meet mission goals, transmitters will be replaced with new state-of-the-art equipment which operate with battery backup, and the DGPS service will undergo validation. All the while, coverage will be provided throughout North America with high time availability. Upon completion of IOC, the DGPS service will be declared ‘Full Operational Capability ' (FOC) meeting all availability, accuracy, integrity, and reliability performance requirements. Discussions are ongoing with other Federal agencies for additional sites west of the Mississippi to provide coverage for navigable portions of the Missouri and Arkansas Rivers. The Walla Walla site is established in support of the Federal Railroad Administration's “Positive Train Control” study. Present needs and plans do not call for utilization of signals from GLONASS or a geostationary system such as WAAS. FIGURE 2 DGPS sites as of March 1, 1996.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications SUMMARY The primary mission of the DGPS service is to provide sub-10 meter accuracy for the harbor/harbor approach phase of marine navigation. This is the most important issue we face as DGPS service providers. However, other users have found innovative ways to utilize DGPS services and where feasible, the Coast Guard DGPS network has expanded to meet their needs. NOAA is locating GPS receiving equipment at some of our Broadcast sites to predict GPS signal delays caused by the neutral atmosphere. The Coast Guard encourages sharing its resources with other agencies, academia, and the scientific community as the overall cost is reduced and everyone benefits from the valuable lessons learned. The U.S. Coast Guard will continue to fully cooperate on international fronts with the International Association of Lighthouse Authority (IALA) and the International Maritime Organization (IMO) to achieve global DGPS commonality. Nationally, the U.S. Coast Guard is consulting with other agencies to adapt the DGPS service to meet their needs. Agencies active in DGPS include the National Geodetic Survey (NGS) for inland surveying, the National Oceanic and Atmospheric Administration (NOAA) and the National Fish and Wildlife Association for hydrographic surveying, the Army Corps of Engineers (ACE) for dredging and coastal construction, the Department of Interior for natural resource mapping, the Federal Highway and Federal Railroad Administrations to name just a few. REFERENCES Federal Radioavigation Plan 1994, U.S. Department of Defence, DOD-4650.5 and U.S. Department of Transportation, DOT-VNTSC-RSPA-95-1, National Technical Information Service, Springfield, VA, May 1995. Broadcast Standard For The USCG DGPS Navigation Service, COMDTIMST M16577.1, April 1993, Commandant (G-OPN), U.S. Coast Guard Headquarters, Washington, DC. Ashtech Z-12-R Differential GPS Reference Station Technical Reference Manual, 2nd Draft, 1 November 1994. Trimble Technical Reference Manual CDRL #A008, Final Version October 24, 1995.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications TABLE 2 United States Coast Guard DGPS Site Information Atlantic and Gulf Coasts Broadcast Site Frequency (kHz) Trans Rate (BPS) Latitude(N) Longitude(W) Range (NM) Radiobeacon ID NAS Brunswick, ME 316 100 43 53.70 69 56.28 115 800 Portsmouth Harbor, NH 288 100 43 04.26 70 42.59 100 801 Chatham, MA 325 200 41 40.27 69 57.00 95 802 Montauk Point, NY 293 100 41 04.03 71 51.63 130 803 Sandy Hook, NJ 286 200 40 28.29 74 00.71 100 804 Cape Henlopen, DE 298 200 38 46.61 75 05.26 180 805 Cape Henry, VA 289 100 36 55.58 76 00.45 130 806 Fort Macon, NC 294 100 34 41.84 76 40.99 130 807 Charleston, SC 298 100 32 45.45 79 50.57 150 808 Cape Canaveral, FL 289 100 28 27.60 80 32.60 200 809 Miami, FL 322 100 25 43.97 80 09.61 75 810 Key West, FL 286 100 TBD TBD 110 811 Egmont Key, FL 312 200 27 36.03 82 45.65 210 812 Puerto Rico 295 100 18 27.77 67 04.01 125 817 Mobile Point, AL 300 100 30 13.65 88 01.45 170 813 English Turn, LA 293 200 29 52.74 89 56.50 170 814 Galveston, TX 296 100 29 19.79 94 44.21 180 815 Aransas Pass, TX 304 100 27 50.30 97 03.53 180 816 Great Lakes Region Broadcast Site Frequency (kHz) Trans Rate (BPS) Latitude (N) Longitude (W) Range (SM*) Radiobeacon ID Wisconsin Point, WI 296 100 46 42.60 92 01.40 40 830 Upper Keweenaw, WI 298 100 47 13.70 88 37.50 130 831 Sturgeon Bay, WI 322 100 44 47.70 87 18.80 110 832 Milwaukee, WI 297 100 43 01.60 87 53.31 140 833 Whitefish Point, MI 318 100 46 46.28 84 57.48 80 834 Neebish Island, MI 309 200 46 19.28 84 09.04 60 835 Cheboygan, MI 292 200 45 39.21 84 27.94 110 836 Saginaw Bay, MI 301 100 43 37.72 83 50.27 85 837 Detroit, MI 319 200 42 17.84 83 05.72 100 838 Youngstown, NY 322 100 43 13.871 78 58.20 150 839

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications GPS Operations and Data Handling at JPL Garth Franklin, Byron Iijima, Peter Kroeger, Ulf Lindqwister, Thomas Lockhart, Anne Mikolajcik, Mark Smith Jet Propulsion Laboratory INTRODUCTION JPL/NASA has been installing and operating permanent GPS stations for 5+ years, starting with the deployment of the 6-station TOPEX/POSEIDON ground tracking network. This permanent Network was installed during the early 1990s in support of the Topex oceanographic mission in collaboration between JPL/NASA, CNES, CEE, and ISAS. Since then JPL/NASA has installed an additional 30+ stations globally in support of the IGS and the GPS Global tracking Network, and another 20+ stations for various regional and local Networks (for example, the SCIGN array in Southern California) and projects (for example, the permanent DOSE site at Mammoth Lakes). We are currently operating 55+ permanent GPS stations for global, regional, and local Networks and projects. Current plans call for implementing another 20-25 sites in the next 2-3 years. DATA HANDLING JPL/NASA uploads data via regular telephone lines, Internet, and NASCOM (direct NASA communications lines from the three DSN stations) in 24-hour or 1-hour file segments. All routine data uploading and handling operations at the JPL/NASA data center have been automated. The data transfers start immediately after UTC midnight, and under ideal conditions all the data is obtained within 12 hours. In practice, 95+% of the data is collected automatically every day, with the remaining data uploaded the next day by the automated upload system or manually. All global stations that are part of the IGS Network are forwarded daily to the CDDIS Global Data Center at the Goddard Space Flight Center. The data is uploaded automatically via telephone lines or direct serial connections using Microphone Pro scripts running on Macintosh computers. The networked Macintoshes at JPL use Telebit T2500 Trailblazer modems to dial up stations with standard telephone connections. Three parallel lines are currently in use to dial 35+ stations. The data files are usually uploaded in CONAN binary format to reduce data transmission time and save costs. Remote Macintoshes, which are connected to the Internet, use direct serial connections to the TurboRogue receiver to upload data from 20 stations. The resulting files are stored on the Macintoshes until a workstation at JPL completes a successful FTP transfer from the Macintoshes to the local workstation (after which the file is removed from the Macintosh). The data collection and handling computer at JPL is a DEC 3000/500 Alpha workstation which transfers the files from the Macintoshes and then imports them into the GPS Network Operating System (GNOS) which inventories, validates, formats, and distributes the data. GNOS is controlled by an INGRES database that stores information about each data set collected. This information is used to determine the status of the site which is displayed in an easily understood format to the Network Operator.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications Site Location Current Communications Transfer to JPL (bps) Planned Communications Offloads per day current (planned) gold Goldstone, CA Internet N/A   1 (24) madr Madrid, Spain Internet N/A   1 (24) tidb Canberra, Australia Internet N/A   1 (24) cro1 St. Croix, U.S. Virgin Islands Internet 11470   1 (24) eisl Easter Island Internet 6000   1 (24) gol2 Goldstone, CA Internet 64000   24 jplm Pasadena, CA Internet 983000   1 (24) mcm4 McMurdo, Antarctica 47500   24 sant Santiago, Chile Internet 7450   1 (24) nlib North Liberty, IA Internet 6917   1 (24) pie1 Pie Town, New Mexico Internet 6091   1 (24) cice Ensenada, Mexico Internet -> Radio Modem 26200   1 (24) iisc Bangalore, India Internet -> Short Haul Modem 6300   1 (24) fai2 Fairbanks, AK Internet -> Switch 40960   24 fair Fairbanks, AK Internet -> Switch 40960   1 (24) kokb Kokee, HI Internet -> Switch 16380   24 tid2 Canberra, Australia Internet -> Switch 54000   24 auck Auckland, New Zealand Internet -> Telephone 50790 / 6960   1 chat Chatsworth, New Zealand Internet -> Telephone 50790 / 5557   1 moin Limon, Costa Rica Internet -> Telephone 44530 / 5234   1 (24) braz Brasilia, Brazil Telephone N/A Internet -> Telephone 1 (24) gode Goddard, MD Telephone 6418 Internet 1 (24) guam Guam Telephone 3781 Internet -> Telephone 1 (24) mdo1 McDonald, TX Telephone 6187 Internet 1 (24) quin Sacramento, CA Telephone 6446 Internet 1 (24) sey1 Mahe, Seychelles Telephone 2535 DSN Phone lines 1 (24) thu1 Thule, Greenland Telephone 2703 Internet -> Telephone 1 (24) usud Usuda, Japan Telephone 5259 Internet 1 (24) yar1 Yaragadee, Australia Telephone 7824 Internet 1 bogt Bogota, Columbia Telephone (shared) N/A Internet 1 (24) shao Shanghai, China Telephone (shared) 2123 Internet -> Telephone 1 (24) areq Arequipa, Peru Telephone -> Fax Switch 2356 Internet 1 (24) kwaj Kwajalein Island Telephone N/A Internet Mail 1 (24) gal1 Galapagos Island Mail N/A Internet 1 SCIGN Southern California Telephone 7000   1 FIGURE 1 JPL operated permanent GPS reference stations.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications FIGURE 2 GPS data flow for JPL operated sites.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications JPL Data Arshive Anonymous FTP access: machine: bodhi.jpl.nasa.gov (128.149.70.66) Username: anonymous Password: Your E-Mail Address (for statistical purposes) Tree Structure: FIGURE 3 JPL data archive.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications National Geodetic Survey Continuously Operating Reference System (CORS) William. Strange and Neil Weston National Geodetic Survey, National Ocean Service, NOAA INTRODUCTION The National Geodetic Survey (NGS) Continuously Operating Reference System (CORS) makes available for multiple use GPS observational data from stations operated by a number of different organizations to meet their specific needs. Special emphasis is placed on providing pseudorange and carrier phase data from stations established to broadcast real time correctors in support of navigation. A major objective of the NGS CORS is to provide surveying, mapping, and other positioning users easy access to the National Spatial Reference System (NSRS). The CORS is also intended to support GPS applications in areas such as crustal deformation, atmospheric water vapor determination, and ionospheric studies. CORS interaction with the Earth and Atmospheric Sciences is two directional. GPS stations established to support navigation, surveying, and other positioning activities can provide science users data to support their analyses. The science users can support upgrading of the CORS stations and the feedback from their analyses can allow application users of GPS to increase the ease and reduce the cost of performing their positioning. THE CORS NETWORK A basic premise of the CORS is that NGS will focus on making available data from GPS reference stations established by other groups rather than establishing additional stations. The CORS stations can be divided into four groups as a function of the operating agency objectives. These four groups are: Real Time Navigation Stations Positioning Support Stations Atmospheric Analysis Stations Geodynamic Research Stations Currently data is being provided from 70 CORS stations (see Figure 1) these stations are divided among the four groups as described below. Real Time Navigation Stations Several Federal agencies have established or will establish GPS reference stations to support real time navigation. The differential GPS (DGPS) network furthest along in development is that of the United States Coast Guard (USCG) which was declared initially operational on 30 January 1996. By agreement between the USCG and NGS the data from this network is made available to the CORS for distribution to non-navigation users. The CORS is currently receiving data from 39 of the 46 USCG stations. Data from the remainder of these stations will become available over the next few months. The USCG is also supporting the U. S. Army Corps of Engineers (USACE) in establishing similar stations along navigable rivers. Currently CORS receives data from three USACE stations. Data from at least six additional stations are expected over the next six months. The USCG and USACE stations have two sets of receivers and antennas at each site (Ashtech ZXII receivers). With few exceptions the antennas are mounted on 10 to 30 ft. Rohn towers located 20 to 50 meters apart. Antenna height is controlled by the requirement to track satellites to a 7.5 degree elevation angle. CORS takes data continuously from only one system with the other system sampled intermittently and serving as a backup if the primary system goes down. The potential exists to sample both systems continuously. Data are transmitted to the CORS Central Data Facility (CDF) using the AT&T X.25 telephone packet service. Raw GPS data is taken directly from the receiver to a Packet Assembler/Disassembler (PAD), placed in packets, and transmitted to the CDF within milliseconds after it is observed. The receivers can sample at a 0.5 sec sample rate, but transmission line capacity limits CORS access to a maximum sample rate of five seconds. These stations are now sampled at 30 sec rate because of cost considerations.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications FIGURE 1 CORS network as of April 1996.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications The possibility exists that USCG type DGPS stations may be extended nationwide. During 1996 tests are planned to evaluate the use of such stations for positive train control for railroads. Studies are also underway to evaluate the use of such stations to support real time automobile positioning. It is estimated that about 40 additional stations would be required to cover the rest of the U.S. Another real time navigation program that is well underway is the Federal Aviation Administration (FAA) program to provide GPS air navigation. This program has two components -- a Wide Area Augmentation System (WAAS) and a Local Area Differential GPS (LADGPS) System. The Contract to develop the WAAS has been awarded. This contract specifications reflects the requirements to provide data to CORS. The approximately 26 WAAS stations will have multiple systems and will provide data real time to the CORS. Data from these stations is expected to become available to CORS over the next two to three years. Positioning Support Stations Numerous GPS reference stations are being established by Federal, state and local governmental agencies to support surveying, mapping, engineering and Geographical Information System (GIS) requirements. A number of stations of this type which employ dual frequency receivers with full wavelength L2 are being incorporated into the CORS. Thirteen stations of this type are now providing data. Ten of these stations are operated by the Texas Department of Transportation. These stations have Trimble SSE receivers with a single system at each site. The antennas are mounted either on buildings or on towers located above monumented points. Data from all stations is aggregated in Austin, Texas in raw Trimble format and is downloaded daily over INTERNET. The other three positioning support stations include a station operated by the Riverside Water and Flood Control District at Blythe, California. This station has an Ashtech ZXII receiver. Data from this station is downloaded by the Scripps Institute of Oceanography (SIO). The data is downloaded from SIO to CORS daily over INTERNET. Another station is operated by the Harris-Galveston Flood Control District in Houston, Texas to support local subsidence monitoring. The antenna is mounted on top of a compaction meter rod which is anchored below the compacting layer. This station has a Trimble SSE receiver. Data is downloaded daily over INTERNET. The final station of this type is the station established by NGS at Gaithersburg, Maryland. This is the only station now providing data at a five second sampling rate. Data is downloaded over INTERNET hourly. The station has a Trimble SSE receiver. Atmospheric Analysis Stations The NOAA Forecast Systems Laboratory (FSL) is establishing stations to investigate the use of GPS to monitor precipitable water vapor in support of weather forecasting and climatology. Seven of these stations are currently providing data to CORS. These stations also provide data from surface meteorological sensors. The data from these stations is downloaded to FSL in Boulder at 30 minute intervals and converted to the RINEX format. At present the data is downloaded to the CORS CDF once daily over INTERNET. Within the next few months it is planned to begin downloading the data hourly over INTERNET. Additional stations of this type are planed by FSL over the next year. As this technology is developed to support operational weather forecasting several hundred stations of this type can be expected to be established across the United States. All of these stations are occupied By Trimble 4000 SSE receivers. Geodynamic Stations Nine stations now providing data to CORS are stations established to support reference system establishment, orbit determination, and crustal motion monitoring. Four of these stations are operated by NGS. The data from these stations is obtained once daily directly from the stations over INTERNET. The other five are NASA supported stations operated by the Jet Propulsion Laboratory (JPL). The data from these stations is downloaded daily over INTERNET from the CDDIS data center located at NASA Goddard Space Flight Center. The receivers at these sites are some version of Rogue/TurboRogue receivers. THE CENTRAL DATA FACILITY The CORS Central Data Facility (CDF) performs five primary functions: acquisition; formatting; quality control; dissemination; and archiving. Equipment Compliment The CORS CDF is in the process of being upgraded and will be operating in the upgraded mode shortly. This

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications discussion describes the upgraded facility. Figure 2 shows the hardware configuration. A data acquisition computer (HP745) will take in data from all stations, both over a 56K bps X.25 telephone line and over the INTERNET. This computer will format the data into a standard RINEX, version 2, format (if it is not already in RINEX), place the data in files in a compressed format, run the data through the UNAVCO Quality Control program and transfer the data to a data distribution computer (HP 715/33). The data distribution computer keeps the data for 31 days for direct INTERNET access. INTERNET access can be obtained using the World Wide Web (WWW), through the NGS home page either by way of the NOAA home page or by typing in the Uniform Resource Locator (URL) which is http://www.ngs.noaa.gov when using a browsing tool such as Mosaic. The CORS data can also be accessed using the ftp command as follows: ftp proton.ngs.noaa.gov Login: anonymous Password: your complete E-mail address cd cors Data from the USCG and USACE stations and the NGS Gaithersburg station are placed in hourly files, with the data available within minutes after the end of each hour. Data for other stations are placed in daily files and are available the following day. After 31 days data is migrated to a CD ROM mastering machine which operates in a UNIX environment and produces CD ROM masters. The number of days of data on each CD ROM is determined automatically and changes with time as more stations are brought on line. Data requests for data more than 31 days old are filled in one of two ways. If a small amount of data is needed the CD ROM is mounted and the data copied to the CORS computer in a special file for INTERNET access by the user. If a large amount of data is required the relevant CD ROMs are copied. Because data from the USCG and USACE stations are not stored on site, any disruption at the CDF could result in permanent loss of data from a large number of stations. To minimize this possibility a parallel computer setup is being implemented as shown in Figure 2. With this parallel setup the data files transferred from the primary acquisition computer to the primary distribution computer are immediately sent to the secondary distribution computer. Thus, all files are maintained on both distribution computers and users can be automatically transferred to the secondary distribution computer if the primary computer goes down. The secondary acquisition computer continually queries the primary acquisition computer to determine if it is functioning. If it finds the primary acquisition computer has failed to function, the secondary computer will automatically take over the data acquisition function. File Structure The information contained in the CORS directory is placed in a README file, which contains general information on CORS and on using the data and program subdirectories of the CORS directory. There are five sub-directories containing data, utility programs, and information (Figure 3). These are: rinex files containing the observational data in RINEX, version 2, format; coord files containing the ITRF and NAD 83 coordinates of station antenna L1 phase centers; station_log files containing site information for the stations similar to that contained in the IGS log files; utilities programs that can be used to manipulate the RINEX files; itrf files with information about the ITRF reference system. Under the rinex subdirectory the sub-directory structure is as indicated in figure 3. The actual RINEX data files have the following data structure: [ SSSS ][ DDD ][ H ].[ YY ][ T ] ; where SSSS is a four character site antenna identifier, DDD is the day of the year, H for stations with hourly files (this is a letter to identify the UTC hour of the day), YY is the year, and T is the file type. The four character site identifier is coordinated with IGS to prevent duplication. Many of the CORS sites have more than one receiver and antenna present. In such situations the last character of the identifier is a number which identifies the antenna being referenced. The letter assigned to H represents the UTC hour of the data in the file according to the convention: a = hour beginning at 00 UTC; b = hour beginning at 01 UTC; etc. The file type identifier, T, can be one of four characters: o - observational data file; n - navigation file; s - summary file; m - meteorological data file.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications CORS Data Collection and Distribution (Proposed Configuration, Phase 1.1) FIGURE 2 CORS data collection and distribution.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications FIGURE 3 Directory structure. The utility programs available in the utilities subdirectory are DOS based. There are also versions that work with Silicon Graphics, Sun Microsystem, and Hewlett Packard environments. The programs available are: gzip386.exe - program to compress and uncompress files; join24pc.exe - program to join RINEX observation or navigation hourly files; cato.exe - program to join RINEX observation files; decimate.exe - program to eliminate data to produce a reduced data rate (e.g. to go from a 5 sec data set to a 30 sec data set). Note: program join24 will not join files across midnight. Operational Considerations Surveying as well as scientific users of GPS are increasingly interested in centimeter (or better) accuracy in the vertical coordinate. To support this accuracy it is necessary to model antenna phase center variations to allow mixing of antenna types. To make such models available to surveying users they must be incorporated into commercial software. NOAA has underway an extensive program to develop and test antenna phase center models. A test range has been established at the NGS facility at Corbin, Virginia and a continuing program of determination of antenna phase center models based on field measurements is underway. Evaluation of the models based on testing at Corbin and at other sites in the Washington, D.C., area where there are precise ground connections between CORS sites and adjacent ground monuments is also underway.

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The Global Positioning System for the Geosciences: Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications Mitigation of multipath is important to a wide variety of users of CORS data. It would be optimum to have all CORS stations equipped with antennas having maximum resistance to multipath and to have these antennas mounted in such a way as to minimize multipath. However, it is unrealistic to expect that operators of stations of the CORS network will fund new antennas that are not required for their application. Scientific users of CORS data should consider funding the implementation of choke ring antennas at existing sites as an inexpensive way of gaining additional stations for use in scientific applications. In so far as mounting of antennas are concerned there are often operational constraints. For example, the USCG has placed its stations, whenever possible, at existing facilities where there is access to support and broadcast antennas. The USCG antennas are placed on 3 to 10 meters above the ground to allow tracking down to 7.5 degrees above the horizon. NGS is working with the USCG to determine the magnitude of multipath at the USCG sites, to determine the effectiveness of improved antenna types in decreasing multipath and to develop multipath models on a sit specific basis. Antenna stability is of great interest to scientific users of CORS data. Many scientific users of CORS data would like antenna stability at the millimeter level. Practical considerations may make this difficult if not impossible to achieve at many sites. The large distance above the ground required by the USCG for their antenna locations would make millimeter stability extremely expensive, if not impossible. Certainly the scientific community could be expected to fund the incremental cost of achieving millimeter stability. At the USCG sites there are ways of evaluating antenna stability. Because there are two antennas at each site the differential position between the two antennas can be monitored and used as a measure of antenna stability. Also, NGS has established two ground monuments at the USCG sites and positioned them relative to the CORS antennas. This provides another means of monitoring antenna stability.