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 Fenggan Yunnan Observatory, Chinese Academy of Sciences Beijing 15/07/2006

One of suggested ground stations for ASTROD1 mission: ◆ 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

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

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

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

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

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

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

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

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

◆ 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

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

◆ 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

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

◆ 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).

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

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.

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°

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.

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

The Distance between Spacecraft and Earth

1.2m Telescope Pointing Ahead Angle

◆ Day-Time Laser Ranging Technique The mean photoelectron ratio NB caused 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

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

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

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

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

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.

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

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