A proposed satellite laser ranging station in Taiwan
Cheinway Hwang , Tingjung Lin, and Yishan Lee
Dept of Civil Engineering, National Chiao Tung University
1001 University Road, Hsinchu 300, Taiwan, ROC
1Department of Civil Engineering, NCTU
Benjamin Fong Chao, Institute of Earth Sciences,
Academia Sinica, Taipei, Taiwan, ROC
Ming Yang, Dept of Geomatics, National Cheng Kung University
No. 1, University Road, Tainan 701, Taiwan, ROC
TP Tseng, SPACE Research Centre, RMIT University, Australia
5th FORMOSAT-3 / COSMIC Data Users Workshop
And the International Conference on GPS Radio Occultation
April 13-15, 2011, Taipei, Taiwan
Introduction
• The primary objective :
(1) track current and future Taiwan satellites with corner-cube
reflectors for orbit improvement.
(2) Define the geocenter coordinate and join the international terrestrial
reference frame, serve as a reference station for plate tectonics of
Taiwan
(3) provide data for time-varying gravity/climate change research
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(3) provide data for time-varying gravity/climate change research
• SLR and FORMOSAT-7 (COSMIC follow on)
potentially equipped with SLR reflectors, to be launched in late 2014.
The accuracy of SLR-derived ranges, is better than 1 cm, and can be
used to validate the accuracy of GPS-derived orbit, and calibrate
antenna offset and center-of-mass
SLR ground station- Argentina
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Seeber (2003)
Example of corner-cube reflector: The CHAMP laser
retroreflector array
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Telescope and Laser: can we reduce the size and
time-delay?Mobile and automated
SLR
•Mobile
•Small or no telescope
•Automated, real-time
Conventional SLR:
•Fixed
•Large telescope
•Manual operation
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Validation of LEO GPS-derived Orbit:
Examples from GRACE and Jason-1
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Examples from GRACE and Jason-1
Difference between GPS-derived orbit (one-D) and SLR
range for satellite GRACE A over 300 days, since DOY 100,
2007
(average RMS : 3.5 cm)
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Difference between GPS-derived orbit and SLR ranges for
satellite (altimeter) Jason-1 over 170 days
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SLR data processing: normal point reduction
• Subdivide the accepted fit residuals FR into fixed intervals (bins) starting from
0h UTC. The bin sizes for various satellites are listed at the end of this note.
• Compute the mean value and the mean epoch of the accepted fit residuals
within each bin i. Let ni be the number of accepted fit residuals within this bin.
• Locate the particular observation Oi with its fit residual FRi , whose
observation epoch ti is nearest to the mean epoch of the accepted fit residuals
in bin i.
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in bin i.
• The normal point is computed as
• (10) Compute the RMS of the accepted fit residuals in each bin i from their
mean value :
where the summation over j is over the accepted points within the bin.
RMS of raw and filtered (normal-point) orbits for
COSMIC satellite FM5
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Improving regional orbit of FORMASAT-7 using
SLR
11Department of Civil Engineering, NCTU XI ,YI ,ZI:Inertial Coordinate System
Near-real time orbit improvement for COSMIC2
satellites using SLR
Orbit error in radial, along-track and cross-track direction
(projection in the station-Leo direction)
• One-cycle-per-orbital-revolution (1CPR) empirical cofficients
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:argument of latitude
, , :empirical coefficients for the orbit errors in the
radial, along-track and cross-track direction
:time-dependent
• Adjustment model
:error vector in the distance between station and Leo due to orbit error
:real-time LEO state vector (based on real-time orbit)
:true LEO state vector
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:true LEO state vector
:SLR observable
:SLR state vector
V :residual vector of
Establishment of a reference frame of Taiwan
using SLR observations
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Global and near-Taiwan ITRF stations (GPS, SLR,
VLBI, DORIS)
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• SLR can be used to define geocenter coordinates and the
motion of geocenter, which can be expressed as a time
series of degree 1 harmonic coefficients in the Earth’s
gravity field model
1111 3 CRRCdx ==
==
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R : the Earth’s radius
and :fully normalized potential coefficients
1111 3 SRRSdy ==
1010 3 CRRCdz ==
C S
Example of using SLR to solve for geopotential
coefficients and station coordinates by GEODYN II
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Differences between the geocenter motions estimated
from SLR data of CHAMP and the result from ITRF2005
18Department of Civil Engineering, NCTU(Guo et al., 2008)
G P S
L E O
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G P S G ro u n d S ta tio n L aser S ta tio n
Scenario 1: Ground Station to GPS (ground – high)
Scenario 2: LEO (Receiver) to GPS (low – high)
Combined scenario: Form three Layers (ground – LEO – GPS)(Zhu and Shih, 2003)
Scenario 1. Ground stations to GPS
- Usefulness: reference frame (site positions), EOP, trop., GPS orbits, ...
- weakness: scale, geocenter (especially, z-direction)
Scenario 2. LEO to GPS
- Usefulness: orbit, gravity, occultation, ....
CHAMP (GRACE、FORMOSAT-3) is a (low altitude) satellite, its dynamic orbit is
sensitive to gravity, including 1st. degree term (geocenter). And at the same time is a
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sensitive to gravity, including 1st. degree term (geocenter). And at the same time is a
flying receiver (station).
Combined together to form 3 layers(at different height)
Ground--LEO--GPS
Solving orbits of GPS and LEO, ground station coordinates, EOP, etc.(incl. gravity)
together simultaneously. Overcome the weakness of each individual scenario.
Solution is more homogeneous and consistent.
SLR and GPS Data from CHAMP and GRACE strengthen
the reference frame
Estimated geocenter variation in Z-directions
DAY 020502 030502 040502 050502
ground only 16.2 ±±±±13.0 3.8 ±±±±11.7 -39.4 ±±±±13.1 19.4 ±±±±12.1
unit: mm
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ground+LEO -7.7 ±±±±4.2 7.3 ±±±±4.7 -0.5 ±±±±4.7 0.9 ±±±±4.7
(Zhu and Shih, 2003)
Time-varying gravity recovery using
combined GPS and SLR observations
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1. Restitution of the orbits of GPS satellites from GPS ground tracking data.
2. Determination of the orbits of CHAMP and GRACE satellites from space
receiver GPS data based on fixed GPS ephemerides and clocks from step 1.
3. Simultaneous determination of orbits of the GPS and LEO satellites and
recovery of gravity field model coefficients, EOPs, and station positions from
GPS ground and LEO space-borne data in the same solution. Attitude,
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GPS ground and LEO space-borne data in the same solution. Attitude,
accelerometer, thruster, and K-band data can also be added if required. This is
the one-step or integrated procedure.
4. SLR data can be added with some realistic weighting.
)()( 332211
1
332211 UcUcUcNcNcNcX ++++=−
Time series of delta C20 from CSR RL04 and SLR solutions from
September 2006 to December 2007
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The possible locations of the Taiwan SLR station
• Low cloud cover rate
• Avoid aviation safety
• Accessibility to station
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• Power supply
Comparison of possible locations of Taiwan SLR
station
Ali Mountain
weather station((((阿里山氣象站阿里山氣象站阿里山氣象站阿里山氣象站))))
Kunyang at
Provincial high
14((((昆陽昆陽昆陽昆陽))))
Mt. Hehuan
(合歡山主峰合歡山主峰合歡山主峰合歡山主峰))))
Mt. Small-
Snowy((((小雪山雷達站小雪山雷達站小雪山雷達站小雪山雷達站))))
Height 2413m 3070m 3390m 2997m
Administrative Chiayi Nantou Nantou Taichung
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Administrative
division
Chiayi Nantou Nantou Taichung
Accessibility
(transportation)
Good Good Good Bad
Power supply Good Average Average Unknown
Difficulty in site
construction
Unknown Easy Easy Hard
man-made
interference
Low High Average Very Low
Ali Mountain weather station
((((阿里山氣象站阿里山氣象站阿里山氣象站阿里山氣象站))))
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Advantage:Good maintenance and stable power supply
Disadvantage:1. The weather-induced disasters may interrupts road access
2. Less suitable space for site construction
Kunyang
((((昆陽昆陽昆陽昆陽))))
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Advantage: Entrance limitation in snowing season
Disadvantage: High man-made interference
Mt. Hehuan
(合歡山主峰合歡山主峰合歡山主峰合歡山主峰))))
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Advantage: Vehicle control
Disadvantage:Too many tourists
Mt. Small-Snowy
((((小雪山雷達站小雪山雷達站小雪山雷達站小雪山雷達站))))
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Advantage:Very low man-made interference
Disadvantage:High construction difficulty
Conclusions and suggestions
• Data from the Taiwan SLR station can be applied to precise orbitdetermination/validation, determinations of geocenter coordinate andtemporal gravity (primary). Other applications are fundamentalphysics, solid/ocean tides, time-transfer and alternative trackingwhen GPS fails (secondary).
• An NSC proposal (2011-2014) was submitted to develop theories and
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• An NSC proposal (2011-2014) was submitted to develop theories andcomputer programs for SLR applications.
• The success of the proposed Taiwan SLR station will depend onmany parties in Taiwan, including NSPO, NSC, MOI and academicinstitutions.