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

2. 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  N
Longitude  E
Elevation m

3. Specifications of 1.2m Telescope
Telescope Mounting: Alt-Az
Focus: Coudé focus
Focal Length: afocal, + a imaging lens
Field of View: 
Encoder resolution: 
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
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. 15h UT, Jan. 22, 2001 Echo Points:>1130
GPS 36, km
15h UT, Jan. 22, Echo Points:>1130

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

20. 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°

21. 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.

22. 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

23. The Distance between Spacecraft and Earth

24. 1.2m Telescope Pointing Ahead Angle

25. ◆ 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

26. Spatial filter
a pinhole shutter of 20-30 in receiving optical path
Spectral filter
the narrow band filter of 0.1nm for 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

27. 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

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

29. 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

30. 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.

31. 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

32. Thanks