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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
◆
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
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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
←
Laser Ranging System
at Coudé Room
Right optical table (3.5m1.8m)
is for ASTROD I mission
→
Servo-Control, Adaptive Optics &
Laser →Ranging System
LAGEOS-1, 5860 km
11h UT, Jan. 20, 2001 Echo Points:>1700
2000
LAGEOS-1
1500
O-C(ns)
1000
500
0
-500
-1000
-1500
11.55
11.6
11.65
11.7
11.75
11.8
11.85
11.9
11.95
12
12.05
GPS 36, 20030 km
15h UT, Jan. 22, 2001 Echo Points:>1130
1400
GPS36
1200
1000
O-C(ns)
800
600
400
200
0
-200
15.1
15.2
15.3
15.4
15.5
15.6
15.7
15.8
5000
AJISAI
4000
3000
O-C(ns)
2000
1000
0
-1000
-2000
-3000
23.28
23.3
23.32
23.34
23.36
23.38
23.4
23.42
23.44
23.46
23.48
23.5
UT
Day Time SLR: 23h16m UT, Fab.17, 2003 Echo Points: 377 AJISAI
3500
TOPEX
3000
2500
2000
O-C(ns)
1500
1000
500
0
-500
-1000
-1500
3.96
3.97
3.98
3.99
4
4.01
4.02
4.03
UT
Day Time SLR: 3h56m UT, Mar.6, 2003 Echo Points: 77 TOPEX
4.04
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)
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Telescope tracking and pointing accuracy:  1
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Laser beam divergence: adjustable, better than 1
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Timing: GPS
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Receiver: SPAD or Avalanche Photodiode Array
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Timing counter: Event Timer, resolution: 3ps
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Coronagraph, Filtering ( spectral, spatial, temporal )
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Ground target calibration
Laser Requirements
for Pulse LR
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If ground station & S/C have specifications:
Diode-pumped Nd:YAG laser, 532nm, 200mJ/p,
100ps, 100Hz, 1 laser beam divergence
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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
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Pulse laser ranging accuracy can be less 3cm
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Key Requirements of Ground LR
Station for ASTROD I (CW LR)
Diode-pumped CW Nd:YAG laser for
interferometric laser ranging
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100 fW Laser Phase Locking
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Optical comb
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FADOF Filter
Laser Requirements
for CW LR
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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
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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
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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
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The travel time of laser beam is more than 500
seconds for one AU distance from ground station
to the ASTROD I S/C.
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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
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Calculating the orbit of spacecraft
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Using Newtonian Law
r
r   
r
i
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j i
i
ij
3
ij
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
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Day-Time Laser Ranging
Technique
The mean photoelectron ratio NB caused by the sky
background light on the detector is:
NB 

4
N  r2 Ar qTt
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
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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
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Coronagraph- FADOF
The sunlight shield system consists of a narrow-
band interference filter, a FADOF (Faraday
Anomalous Dispersion Optical Filter) filter, and a
shutter
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The Sun light should be less than 1 % of the laser
light at the photo-detector
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Optical Layout of LR for
ASTROD I
←
1.2m telescope
Optical layout
Pulse Laser Ranging Optical Layout
Reflector
Imaging
Guiding
Pointing
Counter
GPS
To Telescope
Detector
Shutter
Beam
Expander
Pin-hole
Reflector
Filter
Discriminator
FADOF
PIN
Rotating Transmission
Disk
Film
Diode-Pumped
Nd: YAG Laser
CW Laser Ranging
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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
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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