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 MissionXiong Yaoheng, Zheng Xiangming, Song Fenggan Yunnan Observatory, Chinese Academy of Sciences

Beijing 15/07/2006

◆◆ Introduction

Coordinates:

Latitude 25.0299 N

Longitude 102. 7972 E

Elevation 1991.83 m

One of suggested ground stations for ASTROD1 mission:

Yunnan Observatory 1.2mTelescope LR system

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: 0.1s• Timing Interval Counter:

SR620, Resolution: 20ps

• Detector: SPAD (single photon avalanche photodiode)

• Operated from 1998

Servo-Control, Adaptive Optics & Laser →Ranging System

Laser Ranging System at Coudé Room

Right optical table (3.5m1.8m) is for ASTROD I mission

←

→

LAGEOS- 1

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- 500

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11. 55 11. 6 11. 65 11. 7 11. 75 11. 8 11. 85 11. 9 11. 95 12 12. 05

UT

O-C(

ns)

LAGEOS-1, 5860 km

11h UT, Jan. 20, 2001 Echo Points:>1700

GPS36

- 200

0

200

400

600

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15. 1 15. 2 15. 3 15. 4 15. 5 15. 6 15. 7 15. 8

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GPS 36, 20030 km 15h UT, Jan. 22, 2001 Echo

Points:>1130

AJ I SAI

- 3000

- 2000

- 1000

0

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23. 28 23. 3 23. 32 23. 34 23. 36 23. 38 23. 4 23. 42 23. 44 23. 46 23. 48 23. 5

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TOPEX

- 1500

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3. 96 3. 97 3. 98 3. 99 4 4. 01 4. 02 4. 03 4. 04

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O-C

(ns)

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 AccuracyAccidental 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)(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, 10

0ps, 100Hz, 1 laser beam divergence If ASTROD I S/C is in 1AU, with a 30cm telescope: 1.2m telescope can receive 3.9105photons/per p

ulse from the S/C S/C can receive 2.4104photons/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)(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 5105photons/per second from the

S/C S/C can receive 3.1104photons/per second from 1.2m LR syst

em 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 ModelPointing 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 ModelLocal 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 t

he ASTROD I S/C that to be observed. 2. much less time will be needed to do the pointing model o

bservation.

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

rr

rij

ij

ijii 3

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:

trrB qTANN 2

4

For 1.2m laser ranging system on daytime:NB= 6.1106 photoelectrons/sec

To reduce above sky background light, we need: Spatial & Spectral filterTiming 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 f

or 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 telescopeOptical layout

←

Pulse Laser Ranging Optical Layout

Detector Counter GPS

Imaging

DiscriminatorFilter

FADOF PIN

Diode-PumpedNd: YAG Laser

Reflector

To Telescope

Beam Expander

GuidingPointing

Shutter

Pin-hole

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

Thanks