A11S 026 DEFENSE MAPPING AGENCY WASHINGTON DC FG1/GLOBAL POSITIONING SYSTEM COPS) GEODETIC RECEIVERS,(J)FEB 82 W J GENUS

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GLOBAL POSITIONING SYSTEM4 (GPS) GEODETiC RECEIVERS

W.J. Senus

Defense Mapping Agency

Washington, D.C.

February 1982

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AD/A 111 026

GLOBAL POSITIONING SYSTEM (CPS) GEODETIC RECEIVERS

W.J. Senus

Defense Mapping AgencyWashington, D.C.

FEB 82

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Global Positioning System (GPS) Geodetic N/AReceivers S. PERFORMING OrG. REPORT NUMBER

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Dr. Walter J. Senus

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Global Positioning System, Geodesy, Positioning, NavigationREPRODUCED Sy

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,M A@STRACr (CAina -toe bb i N meueaed idautfy b block nmAr)The NAVSTAR Global Positioning System (GPS) when fully developed will pro-

vide world-wide, all weather, continuous, highly accurate radio navigationsupport to the full spectrum of military uses. In addition it has the potentiato revolutionize the navigational capability of the civil sector as well. TheDefense Mapping Agency, in conjunction with other government agencies in spon-soring the development of GPS user equipment which will benefit greatly theMapping, Charting, and Geodesy (MC&G) Community. Among these developments areGPS receivers for geodetic survey applications.

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- sGPS geodetic receivers are being built around several operating modes, eachof which has its advantages. The GPS program as well as the various techniquesbeing pursued are briefly reviewed. Data collected to date is reported whichindicated excellent performance both in the point positioning mode and in thedistance difference mode of operation. Further, the anticipated improvement inmeasurement accuracy will be provided with a corresponding reduction in timerequired to occupy a measurement site and therefore offers operational costreductions.

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GLOBA" POSITIONING SYSTEM (GPS)GEODETIC RECEIVERS

Dr. Walter J. SenmsChief Scientist, Advanced Technology Division

Defense Mapping AgencyWashington, D.C. 20305

BIOGRAPHICAL SKETCHES

Walter J. Senus is the Chief. Scientist within the Advance Technology Divisionat the Defense Mapping Agency Headquarters in Washington, D.C. Dr. Senusreceived his Bachelor and Master of Science degrees in Physics from SyacruseUniversity in 1967 and 1971 respectively and his PhD in Geodesy and Geophysicsfrom the University of Hawaii in 1976. His responsibilities as Chief Scientistinclude the development of techniques to utilize the Global Positioning System(CPS) as a geodetic tool to be applied against the many diverse Mapping, Chartingand Geodesy (MC&G) requirements to which DMA must respond.

ABSTRACT

The NAVSTAR Global Positioning System (GPS) when fully developed will provideworld-wide, all weather, continuous, highly accurate radio navigation support tothe full spectrum of military uses. In addition it has the potential to revolu-tionize the navigational capability of the civil sector as ell. The DefenseHapping Agency, in conjunction with other goverment agencies in sponsoring thedevelopment of QPS user equipment which will benefit greatly the Happing, Charting,and Geodesy (MC&G) Commanity. Among these developments Gre GPS receivers forgeodetic survey applications.

CPS geodetic receivers are being built around several operating modes, each ofwhich has its advantages. The GPS program as well as the various techniquesbeing persued are briefly reviewed. Data collected to date is reported whichindicated excellent performance both in the point positioning mode and in thedistance difference mode of operation. Further the anticipated improvement inmeasurement accuracy will be provided with a corresponding reduction in timerequired to occupy a measurement site and therefore offers operational costreductions.

TIE NAVSTAR GLOBAL POSITIONING SYSTEM

The NAVSTAR Global Positioning System (GPS) is a satellite hased navigationsystem designed to provide continuous all weather three dimensional positionad velocity, and accurate timing to properly equipped users on a worldwidebasis1 . The operational system available In the late 1980's will consist of18 to 24 navigational (NAVSTAR) satellites, distributed n circular orbitswLth an incIlnatLon of 550 . The configurations and the high altitude of theorbits imre that at least four satellites are "in view" of the user.

N&VSTAR CPS is a joint service program with the Air Force as the executiveservice. The system evolved from Air Force and Navy programs which wereI&Ltlted in the mid-1960's. The development program for the system isdivided into three phases. The initial phase consisted of six satellites1aunched in 1978-1979, distributed in two orbit planes having an inclinationof 630 to provide system evaluation and testing over the southwestern UnitedStates. The system is currently in the second phase during which extensiveOperatLonml testing will occur. The third phase will result in the full upoperational system. The NAVSTAR GPS operates on two L-band frequencies,I$73.42MH (I), and 1227.6 M s (L2). The system is ade up of three majorsegMents which include space, control and uses equipment segments. Figure(1) shows the inter relationship of these three major segments.

"A Susee Soment when fully operatLonal will consist of a constellation ofat least L satellites in six orbital planes. with three satellites in each01". The satellites will operate in clrcttLar :O.200 kilometer orbits at an

41

ISacliaation of 550 and with a 12-hour period. The overall conscellaeioa viU,be configured to provide a minimum of four satellites in view to a user at alltimes.

The Control Segment is comprised of monitor station's (MS) which perform routinetracking of the NAVSTAR satellites and provide observational data to the MasterControl station (MCS). The MCS incorporates the observations from all MS andall KAVSTAR satellites to determine and predict the satellites' ephemerides andclock parameters.

The NAVSTAR User Segment consists of User Equipment which will measure theequipment utilizes the broadcast data to develop position and velocity

Information. Thr nominal navigation performance of a suitably equipped GPS user,when the complete NAVSTAR constellation is operational, will be 16 meters SEP(Spherical Error Probable) world-wide.

The operational application's of the GPS are extensive with Figure 3 offeringa sample compilation of these.

GPS Geodetic Receiver. Although the navigation performance of the GPS is trulyresolutionary the utility of GPS to the geodetic survey community certainlyrequires significant imprcvement in system's positioning capability 2 . Severalschemes for exploitation have been proposed. One group of investigators suggestthat the GPS signal be monitored from two locations which define the endpointsof a baseline3 . The simultaneous reception of signals at these locations, prop-erly tagged in time, can then be cross correlated using processing techniquessuitable to Very Long Baseline Interferometry Figure 4. Results which have beenreported to date indicate 2-5 cm residual error when measurements were conductedover a calibrated baselines4 . This technique offers substantial promise to thegeodetic community expecially to those elements of the community who may nothave full-access to the GPS precision signal. Certain limitations that exist withthis concept as it is currently proposed are the need for two monitoring receivers,the inability to establish a single point position as is currently done with transitbased technology and the apparent large computer requirement driven by the datacorrelation requirement.

A second school of development has opted to pursue a technical approach whichhas as its basis the measurement of the doppler shift of the GPS signals as thesatellites pass overhead. This later approach seems to require a more sophisticatedreceiving system in that the receiver must receive the GPS signal, reconstruct theGPS carrier frequency which will contain the doppler phase information. Observa-tions made over a baseline of 100 or more kilimeter can produce distance measure-ments with an accuracy of better Chan lOcm5 . Subsequent to these Evans et alconducted a test of the sensitivity of the concept using experimental geodeticreceiving equipment used to determine the relative positions of two receivingsystems 6 . In this test periodic movements of the receiving antenna were introducedover a triangular pattern approximately 2 meters on each side. The resultsindicated that the relative position's of the antennas can be determined to 5 ca.

IPROTOTYPE GEODETIC RECEIVER

Specifications were written for a prototype geodetic receiver which will meetthe geodetic and geophysical requirements of U.S. Government agencies. Thelasd agency for development of the system is the Naval Surface Weapons Centerunder a task assigned by the Defense Mapping Agency. The Applied PhysicsLaboratory of Johns Hopkins University played a key role in setting the detailedspecifications for the receiver (AFL,. 1980). The system will be developed underthe guidance of a working group chaired by the Defense Mapping Agency Hydrographic/lopographic Center in response to the Interagency Coordination Plan of NationalGeodetic Survey, The National Aeronautics and Space Administration, The DefenseDepartment, and United States Geological Survey, with the support and meting

e reqaLrmants of the Bureau of Land Management. The components of the systemare receiver, computer/recorder/display, antenna/preamplifier, and on option caninclude communications modem, weather stacLon, radiometer, and off-line coptor

3Figure 5. The first three items, the receiver, computer and antenna will be usedin all applications and will display real-time absolute positions, record datafor post-processing, and include ports to receive weather data and provide datato a control computer, if desired. Recently a contract vas awarded to the TexasInstruments Co. in Lewisville, Texas for the fabrication of prototype GPS geodcticreceivers. The first of these receivers will be delivered in early 1983 with thisinitial contract allowing for the fabrication of up to 10 units.

SUMMARY OF GEODETIC RECEIVER ACCURACY

The accuracy of absolute positioning is limited by the expected uncertainties inthe orbits and clocks of the NAVSTAR satellites. The expected accuracy improvesfrom 10 to 15 meters for an instantaneous fix to better than two meters aftera few hours of observation. A substantial portion of the orbit and clock errorson the relative position of two or more locations can be eliminated simply bydifferencing positions computed from near simultaneous tracking of the NAVSTARsatellites. Geometry and environmental differences will lead to actual observa-tional differences at the sites. These differences can be minimized and improvedby using techniques similar to those incorporated in very long baseline interfer-ometry.

CONCLUSION

The development of the GPS as a tool for geodetic applications offers substantialrewards. It offers the potential of doing precise geodetic measurements overa variety of distances as well as at point positions. Further it offers thepotential for doing these measurement in a time much faster than previouslypossible. Figure 6 summarizes some of the evolutionary steps which have markedthe application of satellite based measurements to geodetic problems. Asindicated GPS will be a significant improvement.

REFERENCES

1. Frey, P.E. "NAVSTAR Global Positioning System", American Society of Civil Engineers,San Franciso, CA. 1977

2. Senus, W. J. and Hill, R. W. "GPS Applications to Mapping, Charting and GeodesyNavigation", Vol.28,No.2, Summer 1981

3. MacDoran, P.F. "System for Near Real-time Crustal Deformation Monitoring", U.S.Patent 4170776 Oct. 9, 1979

4. Councilman, C.C. and Shapiro, I.I. "Miniature Interferometric Terminals forEarth Surveying", Bull. Geod 53 pp 139-163, 1979

5. Anderle, R. J. " Application of NAVSTAR GPS Geodetic Receiver to Geodesy andGeophysics", Naval Surface Weapons Center TR8O-282, September 1980

6. Evans, A. G., Boran, B. R. and Fell, P. J. "Global Positioning System SensitivityExperiment", presented at National Aerospace Meeting, Institute of Navigation,April 1981

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