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Laser Ranging Technique for ASTROD I Mission

Laser Ranging Technique for ASTROD I Mission. ◆ Introduction ◆ Key Requirements of Ground LR Station for ASTROD I ◆ Telescope Pointing and Pointing Ahead ◆ Day-Time Laser Ranging Technique ◆ Optical Layout of LR for the ASTROD I Mission.

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Laser Ranging Technique for ASTROD I Mission

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  1. Laser Ranging Technique for ASTROD I Mission ◆Introduction ◆Key Requirements of Ground LR Station for ASTROD I ◆Telescope Pointing and Pointing Ahead◆Day-Time Laser Ranging Technique ◆Optical Layout of LR for the ASTROD I Mission Xiong Yaoheng, Zheng Xiangming, Song FengganYunnan Observatory, Chinese Academy of Sciences Beijing 15/07/2006

  2. ◆Introduction One of suggested ground stations for ASTROD1 mission: Yunnan Observatory 1.2mTelescope LR system • Coordinates: • Latitude 25.0299  N • Longitude 102. 7972  E • Elevation 1991.83 m

  3. Specifications of 1.2m Telescope • Telescope Mounting: Alt-Az • Focus: Coudé focus • Focal Length: afocal, + a imaging lens • Field of View: 3 • Encoder resolution: 0. 36 • Axis Accuracy: both Alt. and Az.  1 • Pointing Accuracy: after modification , 1 • Drive Mode: torque motor through friction disk for Az.; drive directly for Alt. • Tracking Accuracy: 1 for stars

  4. 1.2m Laser Ranging System • Range: 400 ~ 20,000km • Accuracy: ~ 3cm • Nd:YAG Laser • 100mj/p, 200ps, 4Hz • Timing: GPS • Resolution: 0.1s • Timing Interval Counter: SR620, Resolution: 20ps • Detector: SPAD (single photon avalanche photodiode) • Operated from 1998

  5. ← Servo-Control, Adaptive Optics & Laser →Ranging System Laser Ranging System at Coudé Room Right optical table (3.5m1.8m) is for ASTROD I mission →

  6. LAGEOS-1, 5860 km 11h UT, Jan. 20, 2001 Echo Points:>1700

  7. GPS 36, 20030 km 15h UT, Jan. 22, 2001 Echo Points:>1130

  8. Day Time SLR: 23h16m UT, Fab.17, 2003 Echo Points: 377 AJISAI Day Time SLR: 3h56m UT, Mar.6, 2003 Echo Points: 77 TOPEX

  9. Evaluation of Laser Ranging • Ranging Ability laser pulse energy, receiving telescope diameter, detector, telescope tracking and pointing accuracy • Ranging Accuracy Accidental errors: laser pulse width, time accuracy, time interval counter accuracy System error: correction of mass, system delay, ground target calibration, atmospheric parameters and correction

  10. New Trend of Laser Ranging •LR Accuracy: Toward Millimeter • LR Data: KHz Laser, Million Echo for a Single Pass • LR Model: Passive  Active (Transponder) Two-Way LR, Interplanetary LR (1~2AU) • Diffuse Reflective LR: Space Debris • Interferometric LR: Higher Ranging Accuracy • Chinese LLR: 2nd Phase of Chinese Lunar Mission

  11. ◆Key Requirements of Ground LR Station for ASTROD I(Pulse LR) • Telescope tracking and pointing accuracy:  1 • Laser beam divergence: adjustable, better than 1 • Timing: GPS • Receiver: SPAD or Avalanche Photodiode Array • Timing counter: Event Timer, resolution: 3ps • Coronagraph, Filtering ( spectral, spatial, temporal ) • Ground target calibration

  12. Laser Requirements for Pulse LR • If ground station & S/C have specifications: Diode-pumped Nd:YAG laser, 532nm, 200mJ/p, 100ps, 100Hz, 1 laser beam divergence • If ASTROD I S/C is in 1AU, with a 30cm telescope: 1.2m telescope can receive 3.9105photons/per pulse from the S/C S/C can receive 2.4104photons/per pulse from 1.2m LR system on the ground • Pulse laser ranging accuracy can be less 3cm

  13. ◆Key Requirements of Ground LR Station for ASTROD I (CW LR) • Diode-pumped CW Nd:YAG laser for interferometric laser ranging • 100 fW Laser Phase Locking • Optical comb • FADOF Filter

  14. Laser Requirements for CW LR • If ground station & S/C have following specifications: 2 diode-pumped CW Nd:YAG lasers, 1.064 m, 1w, with a Fabry-Perot reference cavity: 1 laser locked to the cavity, the other laser pre-stabilized by this laser and phase-locked to the incoming weak light, 1 laser beam divergence • If ASTROD I S/C is in 1AU, with a 30cm telescope: 1.2m telescope can receive 5105photons/per second from the S/C S/C can receive 3.1104photons/per second from 1.2m LR system on the ground • CW laser ranging accuracy will be several mm

  15. ◆Telescope Pointing and Pointing Ahead For laser divergence and long distance range, such as ASTROD I mission, ground telescope must have the pointing and tracking accuracy of one arcsecond according to spacecraft ephemeris. For a high tracking and pointing accuracy, telescope must have good axis, good encoders and a stable optical system. The system errors of telescope pointing can be moved using a mathematics model and through star observation & CCD image processing, to reach an accuracy of  1 (RMS).

  16. Global Pointing Model Using the Spherical Harmonic Function to 4th Terms: AsinZ = A0+A1cosZ+A2cosAsinZ+A3sinAsinZ+A4cos2Z+ A5cosAcosZsinZ+A6sinAcosZsinZ+A7cos3Z+ A8cosAsinZcos2Z+A9sinAsinZcos2Z+A10cos4+ A11cosAsinZcos3Z+A12sinAsinZcos3Z Z = B0 + B1cosZ +…… Through star observation in sky and image processing, to solve Ai , B1, i=0, 1, ……12. Then let A, Z be in all telescope pointing to reach its accuracy   1

  17. Local Pointing Model • Telescope pointing accuracy will change with time, such as temperature, sunshine, humidity, wind direction. Global pointing model can not be kept a long time. • Local Pointing Model: Around the S/C orbit, we can do a simplified observation and modification using Hipparcos Catalogue (accuracy:1 mas) before every ASTROD I LR. • Advantage: 1. to make sure the telescope pointing accuracy   1 for the ASTROD I S/C that to be observed. 2. much less time will be needed to do the pointing model observation.

  18. AsinZ = C0 + C1(Z-Z0) + C2(A-A0)sinZ0 + C3(Z-Z0)2 + C4(Z-Z0)(A-A0)sinZ0 + C5(A-A0)2sin2Z0 Z = D0 + D1(Z-Z0) + D2(A-A0)sinZ0 …… |A-A0 | 5° |Z-Z0| 5°

  19. Telescope Pointing Ahead • The travel time of laser beam is more than 500 seconds for one AU distance from ground station to the ASTROD I S/C. • Ground telescope must point ahead when emits a laser beam to the S/C according to its orbit ephemeris, and vice versa.

  20. Calculation Telescope Pointing Ahead Angle • Calculating the orbit of spacecraft • Using Newtonian Law • Physical model: When Calculating S/C orbit, following factors are considered: 9 large planets, Sun, Moon and 3 small planets: Ceres, Pallas, Vesta. Universal gravitation, post-Newtonian effect, and solar zonal harmonic term

  21. The Distance between Spacecraft and Earth

  22. 1.2m Telescope Pointing Ahead Angle

  23. ◆Day-Time Laser Ranging Technique • The mean photoelectron ratio NBcaused by the sky background light on the detector is: For 1.2m laser ranging system on daytime: NB= 6.1106 photoelectrons/sec To reduce above sky background light, we need: Spatial & Spectral filter Timing gate

  24. Spatial filter a pinhole shutter of 20-30 in receiving optical path • Spectral filter the narrow band filter of 0.1nm for 1.064 m or 532nm in receiving optical path Fabry-Perot filter →high transmission coefficient → 60% • Timing gate according to S/C ephemeris with a accuracy of 20ns for the detector in LR

  25. Sunlight Shield System • Coronagraph- FADOF The sunlight shield system consists of a narrow-band interference filter, a FADOF (Faraday Anomalous Dispersion Optical Filter) filter, and a shutter • The Sun light should be less than 1 % of the laser light at the photo-detector

  26. ◆Optical Layout of LR for ASTROD I ← 1.2m telescope Optical layout

  27. Pulse Laser Ranging Optical Layout Reflector Imaging Guiding Pointing To Telescope Detector Counter GPS Shutter Beam Expander Pin-hole Filter Discriminator FADOF PIN Diode-Pumped Nd: YAG Laser Reflector Rotating Disk Transmission Film

  28. CW Laser Ranging • Transmit/Receive sharing same optical path model can not be used for CW laser beam at the 1.2m telescope • Two possible methods for CW laser ranging: 1. Attaching the CW laser device on the 1.2m telescope, depend on its size and cooling system 2. Another small telescope (=50cm) that close to 1.2m telescope transmits CW laser beam, and the 1.2m telescope receives the return photo-electrons.

  29. Conclusion • Yunnan Observatory 1.2m Laser Ranging system in China is a ground station for the ASTROD1 mission It’s ready! • Requirements of LR for the ASTROD 1 mission are: Diode-pumped (Pulse or CW) Nd:YAG laser Detector (SPAD or avalanche photodiode array ) Event Timer Coronagraph, Filtering Weak Laser Phase lock and Optical comb

  30. Thanks

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