NASA TECHNICAL NASA TM X-73072 MEMORANDUM (NASA-TB-X-73072) liif LAGECS SYSTEM (NASA) E76-13179 68 p BC $4.5~ CSCI 22E Thls Informal documentation medium is used to provide accelerated or speclal release of technical information to selected users. The contents may not meet NASA formal editing and publication standards, my be re- vised, or may be incorporated in another publication. THE LAGEOS SYSTEM Joseph W< Siry NASA Headquarters Washington, D. C. 20546 NATIONAL AERONAUTICS AND SPACE ADMlNlSTRATlCN WASHINGTON, 0. C. DECEMBER 1975 1. i~~1Yp HASA TW X-73072 4. Titrd~rt. 5.RlpDltDM December 1975 m UG~SSYSEM 6.-0-cad8 . 7. A#umrtsI ahr(onninlOlyceoa -* Joseph w. Siry . to. work Uld IYa n--w- WnraCdAdbar I(ASA Headquarters Office of Applications . 11. Caoa oc <irr* 16. i+ashingtcat, D. C. 20546 12TmdRlponrrd~~ 12!3mnm&@~nsnendAddrs Technical Memorandum 1Sati-1 Aeronautics and Space Adninistxation Washington, D. C. 20546 14. sponprip ~gmcvu 15. WDPa 18. The LAGEOS system is defined and its rationale is daveloped. This report was prepared in February 1974 and served as the basis for the LAGMS Satellite Program development. Key features of the baseline system specified then included a circular orbit at 5900 km altitude and an inclination of lloO, and a satellite 60 cm in diameter weighing same 385 kg and mounting 440 retro- reflectors, each having a diameter of 3.8 cm, leaving 30% of the spherical surface available for reflecting sunlight diffusely to facilitate tracking by Baker-Nunn cameras, The satellite weight was increased to 411 kg in the actual design thr~aghthe addition of a 4th-stage apogee-kick motor. The number and diameter of the retroreflectors are now 426 and 3.81 a'.resvectively. The net effect of these partially compensating changes is not significant. The diffusely reflecting area was increased to 47% of the satellite surface. 17. IW Word, 6u@mstdby AumalS)) 18. Dimiknia, Strranent Unclassified - UnlimSted Cat. I8 10. SIEvitv clad. (01 this mport) 20. Sruity Mf.(of thh m) 21. ruo. of P- a. Ria' Unclassi fLed Unclassified 67 $4.25 ; The General LhfSOS Review ................................ 3 A . The Orbital Altitude ................................ 34 Bo The Orutal Intanat- ............................. 39 The I,AGHlS Retmmflectar MW........................ W A . The Retmreflectar .................................. 1. The Retrareflector Diameter ................... .. 41 2 . Coat~s........................................ 43 3 . The Dihedral Angle .............................. 43 B . The Retroreflector Array ............................ 45 1. The ,cess kpth ................................ 45 2. The Satellite Mameter And The Number of Retroreflectors ............... 45 3 . Wra Tracking ................................. 47 C . Test4,ng and Handling A.ocedures ..................... 5C A. The Orbit ........................................... 51 1. The Eccent~ricity................................ 51 2 . The Inclination ................................. 92 iii 3 . The Altitude .................................... 53 a . Resollance Effects Associated Kth a Specific Altitude Region .................. 53 b . General Considerations ................... ... 53 . 8. The Satellite Mass .................................. 55 C . Attitude Determination .............................. 55 V . Ac-1-nts ........................................ 59 VI . Rgerences .............................................. 60 . 1. U04tX Qgstem Nominal Baseline Parameters ................... 2 2. The General LACa310S Review. October ll. 1973. Atseadance ..... 4 3. HDPAP ObJectives ........................................... 5 4. Earth and Ocean Physics Application Program ................. 6 5 • h~re~mntRequi~~m~t~ Sv mmm....emmemme....m*mm*memm. 7 6 . Barth Qnamics Experiments .................................. 8 7 . System Capabilities an3 Wlestoines ......................... 9 8 . LAGXIS Program Objectives .................................. 10 9 . LAGms ............................**...*.*..*.**.....*.***.11 10. Orbit Selection Cansiderations ..............................12 ll. Planned and Possible Laser Sites ......................... 13 l.2. Possible Laser Sites ....................................... U, 13. San Andreas Fault Bxperintent ............................... 15 l4 . LAGS8 Mission Objectives .................................. 16 15. Mission Objectives ......................................... 17 16. Fact.ars Influencing 3Qhemeris Accuracg ..................... 18 17. Geopotential Model &mrs .................................. 19 18. Surface Force Model Bmrs ................................. 20 19. Radiation Perturbations vs Orbit Altitude .................. 21 20 . Hagnitudes of Peurbing Forces ............................ 22 2l Orbit Parameters ........................................... 23 22 . Error Budget For Range Measurements ................... .... 24 23 . Satellite PBFameters ...................................... 25 24 . Received Signal Levels ............................. 26 25 . Spherical Segment Retrmeflection Area ................... 27 Time History of The Echo Signal .......................... 28 Reflectivity Versus Angle of Incidence ................... 29 Beam Pattern at No& Incidence ......................... 30 Effective Reflecting of Segments of LA- ............... 31 Computed Probability Distribution of Retroreflected Pulses.32 Retroreflectcr Configuration ............................. 33 Estimated Orbit Perturbation Uncertainties For Certain LA- Alternatives .............................. 36 Estimated Relative Effectiveness of Alternative LkGEDS Orbits for Earth Dynamics Measurements ............ 37 The LAGIDS Retroreflector Review. October 29. 1973. Attendance ............................. 42 Retroreflector hay Gain as a Function of Dihedral Angle Offset ............................................. 44 Effect of Recessing Cube Corners ......................... 46 Signal Strengths ........................ ... ...... 48 LAGHIS Retroreflector Array Parameters ................... 49 ~IksonanceEffects Associated With A Specific Attitude Recion ................................. 54 Delta Launch Vehicle Capabilities ........................ 56 FOREWORD The LAGEOS system is defined and its rationale is developed. his report was prepared in February 1974 and served as the basis for the LAGEOS Satellite Program development. Baseline parameter values specified then and those actually selected for the design are as follows: Value Baseline Actual Parameter Feb. 1974 Design Altitude (km) 5900 5900 Eccentricity 0 0 Diameter of Satellite (an) 60 60 Weight of Satellite (kg) 385 411 Number of Retroreflectors 440 426 Retroreflector Diameter (cm) 3.8 3.81 Diffusely Reflecting Surface ($1 30 47 Dihedral Angle Offset (arc sec) 1.5 1.25 Recess Depth (rain) 1 1 Ratio of Moments of Inertia 1.1 1.03 The satellite weight was increased in the actual design as a result of launch vehicle modifications, which included the addi- tion of a 4th-stage apogee-kick motor. The smaller number of retro- reflectors is compensated for to a limited extent by the slightly larger retroreflector diameter. The net effect on performance is not significant. Covering the retroreflector mounting rings with aluminum, which detailed analysis of thermal and other factors showed to be feasible, increased by more than half the portion of the spherical surface area available for diffusely reflecting sunlight for Baker-Nunn camera tracking. Measuremt ~tsof a number of retroreflection patterns showed that a dihedral angle of 90° + 1225 gave an energy maximum in the aberration annulus, hence this value was used instead of a theoretical estimate which was the basis for the earlier figure for the offset. The smaller moment of inertia ratio resulted in suitable stability character- istics for the satellite, as well as a considerable simplification in the fabrication process. The LA- program, centered around the first new spacecraft in the NASA Earth and Ocean F'hysics Applications Program (BOPAP), is entering an implementation phase as various aspects of the Phase B Definition Study get underway at the Marshall Space Flight Center. (1) A review cf the LAGEXIS F?rogramls objectives and scientific and technical features is in progress. The initial aims have been to review the study entitled "Use of a Passive Stable Satellite for Earth Physics Applicationsn which had been conducted by the Srnithsonian Astrophysical Observatory (SAO), and to consider other views related to the orbit altitude and inclination and the satellite size and mass in order to provide a basis for the specification of the LAGEXIS System as a basis for the Phase B Definition Study. (2) These processes were begun at meetings on October 11, 1973, at which these latter points were discussed in considerable detail. Aspects of the SAO Study having to do with the retroreflectors themselves were considered at a meeting or. October 29, 1973. (2) Both meetings revealed the need for further imatigation cf a number of specific points. Various studies have been conducted or initiated in response to the needs indicated during these meetings and in subsequent discussions. Ter;F,ai;ive conclusions reached at these meetings and later on the basis of a number of the studies and additional discussions are described here, and the corresponding guantities are listed in Figure 1. These values LAGEos msm NOMINAL BASELINE PARAMETP;RS Altitude Inclination 110 deg Eccentricity 0 Diameter of Satellite 60 cm Weight of Satellite