1. Technical Considerations on VLBI Astrometry with FAST Zhihan Qian（钱志瀚） and Bo Zhang（张波） Shanghai Astronomical Observatory, CAS
Outline
Existing VLBI stations in China (CVN)
The sensitivity of CVN with FAST at L—band
Requirements of accuracy of FAST station positions for VLBI astrometry
Determination of reference point of FAST
Phase—referencing for relative astrometry with FAST
Suggestions on FAST VLBI Astrometry in early stage
2015/7/29-31
FAST2015

2. Exisiting VLBI Stations in China
Diameter (m)
Coordinates
Band
Tianma 65
31 06N
121 08 E
L, S/X, C, X/Ka, Ku, K, Q
Sheshan 25
121 12 E
L, S/X, C, K
Miyun 50
40 33N
116 09 E
S/X
Kunming 40
25 02N
102 48 E
Urumqi
43 28N
87 11 E
L, S/X, C, K, Q
* S/X is common band
2015/7/29-31
FAST2015

3. Exisiting VLBI stations and FAST
in China
北京密云
上海天马
2015/7/29-31
FAST2015
FAST

4. L-band only SH(25) UR(25) 3249km 2645km TM(65) 1525km FAST(300) 北京 密云上海 佘山
2645km
TM(65)
1525km 昆明
FAST(300)
FAST2015

5. Sensitivities of baselines of CVN with FAST at L-band
Antenna parameters
Sensitivities of baselines
Antenna
Diameter
(m)
Tsys
(K) 
SEFD
(Jy)
FAST
300m
25K
0.5
1.95 TM
65m
0.6
34.7 UR
25m
234
Baseline
Bandwidth
Smin (t=10min)
Smin (t=4hr)
FAST——TM
300MHz
14Jy
2.8Jy
FAST——UR
36Jy
7.3Jy
TM——UR
150Jy
31Jy
* Smin=1
2015/7/29-31
FAST2015

6. Comparison of baseline sensitivity and observed source number
Baseline Length
Relative Smin
Relative Observed Source Number
TMUR
3249km 1
FASTUR
2645km
0.23
8.9
FASTTM
1525km
0.09 35
2015/7/29-31
FAST2015

7. Baseline resolution(=18cm)
Baseline Length
SNR
, (as)
TMUR
3249km 10
182
FASTUR
2645km 43 52
FASTTM
1525km
111 35
Sc=0.3 mJy ; t=4hr
2015/7/29-31
FAST2015

8. Using HSA: Arecibo, VLBA, GBT, Effelsberg.
Importance of high sensitivity & high precision astrometry A sample: VLBI Resolution of Pleiades Distance Controversy
Using HSA:
Arecibo, VLBA,
GBT, Effelsberg.
From: Carl Melis et al, 2014
2015/7/29-31
FAST2015

9. Astrometric errors caused by baseline errors
Absolute astrometry:
Let: B=1.0 cm, B=1500 km; =0.0014(1.4 mas)
Relative astrometry:
Let: B=1.0 cm, Δ=3; = (73 as)
Δ=1; = (24 as)
2015/7/29-31
FAST2015

10. Requirements of baseline victor accuracy of TM-FAST for relative astrometry
FAST is suitable for relative astrometry
Sources of baseline errors
Station position errors
EOP (Earth Orientation Parameter) errors
Model errors of Earth solid tides and ocean tides
Model errors of water and atmosphere loading, and so on
Sources of station position errors
Stability of reference point of antenna
Measurement errors of reference point
Requirements of TM—FAST baseline accuracy
If Δ3, 1.00 mas; B137 cm
0.10 mas; B13.7 cm
0.01 mas; B1.37 cm
2015/7/29-31
FAST2015

11. Reference Point of FAST
Definition:
“O” is the reference point of FAST.
Measurement:
Firstly, to measure the positions
of feed and illuminated reflector
in a local geodetic reference frame;
Secondly, to derive the positions
of reference point “O” in a local
geodetic reference frame;
Finally, to transfer positions of “O”
into International VLBI reference frame.
From: FAST Project Documents
2015/7/29-31
FAST2015

12. VLBI PhaseReferencing
Observing methods:
Simultaneously observing reference and target
sources
In-beam (2.5 for FAST at =18cm)
Using a nearby small antenna to observe reference sources
Alternately observing reference and target sources (It is not suitable for FAST)
2015/7/29-31
FAST2015

13. Using two antennas in FAST site for phase-referencing observations
To construct a new antenna (12m or larger) nearby FAST for the observations of the reference sources (as well for the geodetic VLBI observations);
To observe the target sources by FAST;
Recently, there are a new 65m antenna and a old 25m antenna separated about 6km in Sheshan area, Shanghai. Therefore, it is possible to do VLBI phase-referencing observations using 4-antenna mode if there is an another antenna nearby FAST.
2015/7/29-31
FAST2015

14. 4Antenna Interferometer (FASTTM baseline)
for VLBI phase-referencing observations 北京 密云 上海 佘山
1525 昆明
FAST2015

15. Conclusions
The VLBI astrometry plays an important role for the astronomical research.
The CVN including FAST will become a very powerful VLBI network around the world. The sensitivity of CVN will reach dozens Jy with enough SNR for VLBI astrometry.
The requirements of the accuracy for the FAST reference point should be less than 10cm in order to obtain better than 0.1 mas accuracy of the positions of target sources in the early stage.
It is very useful to construct a small antenna nearby the FAST for VLBI phase-referencing observations. (See Arecibo Online News, June 28, 2010)
The suggestions on the FAST VLBI astrometry in the early stage:
Pulsar VLBI astrometry, especially, for the new pulsars discovered by FAST pulsar survey.
Detection of extra-solar planets by VLBI astrometry.
2015/7/29-31
FAST2015

16. To detect the wobble of the parent star for estimation of the orbit and the mass of extra-solar planet
2015/7/29-31
FAST2015

17. Thank you for your attention
2015/7/29-31
FAST2015