Two experiments were performed in April and September 1976 to determine precise positions of radio sources using conventional Mark 2 VLBI techniques. Four stations in the continental United States observed at a wavelength of 18 cm. The recording bandwidth was 2 MHz. The preliminary results using analyses of fringe rate and delay are discussed and the source positions compared with the results of other measurements

Backer, D. C.

Johnston, K. J.

Spencer, J. H.

Waltman, W. B.

English

NASA Technical Reports Server

ASTROMETRY USING BASIC MARK II 
VERY LONG BASELINE INTERFEROMETRY 
J. H. Spencer, E. B. Waltman, K. J. Johnston 
E. 0. Hulburt Center for Space Research 
Naval Research Laboratory 
D. C. Backer 
Radio Astronomy Laboratory 
University of California 
ABSTRACT 
Two experiments have been performed in April and September 1976 to determine precise positions 
of radio sources using conventional Mark II very long baseline interferometry (VLBI) techniques. 
Four stations in the continental United States observed at a wavelength of 18 cm. The recording 
bandwidth was 2 MHz. The preliminary results using analyses of fringe rate and delay are discussed 
and the source positions compared with the results of other measurements. 
181 
https://ntrs.nasa.gov/search.jsp?R=19800020312 2020-03-21T17:49:51+00:00Z
182 RADIO INTEWEROMETRY 
In early 1976, the Naval Research Laboratory (NRL) undertook a program to use the North Ameri- 
can Very Long Baseline Network to investigate its astrometric potential. New techniques for deter- 
mining source positions and baselines were to be investigated. Little was known at that time about 
the astrometric properties of the Mark II processor in Charlottesville, Virginia. Objectives included 
(I ) developing a grid of self-consistent baselines that could be used in this and other experiments, 
(2) testing ideas of how to produce a consistent source catalog of a large number of sources in a 
short time, and (3) developing a method for increased position accuracy by using phase comparison. 
The method of using relative phase between sources previously had been limited to separations of 
10m2 rad (Wittels, 1975).* If an adequate source catalog could be used as a starting point with well- 
known baselines, a low observing frequency might provide a long enough coherence time to switch 
between distant sources enough times to track the phase and determine lobe ambiguities. In prac- 
tice, we compared closely-spaced pairs with other closely-spaced pairs. The 2 MHz bandwidth of 
the Mark II system would be needed to obtain a good signal-to-noise ratio even for extremely short 
scans. The concept leads to accuracies at the level of absolute phase when relative phase is main- 
tained for a day. 
By using an array of baselines, fringe rates would be used to determine initial source positions to 
-0i’OS using the longest baselines. Relative phase from the shortest baseline, Maryland Point to 
Green Bank, would then establish an improved position catalog using its 0: 15 spacing. Then the 
longest baselines, spacing -O!‘Ol, would again be used to determine the final catalog, one with 
errors at the few milliarcsecond level. 
Two observing sessions of 48 hours each were used for this program. In April 1976, data were ob- 
tained from the Owens Valley and Maryland Point observatories. In September 1976, all baselines 
between Green Bank, Haystack, Maryland Point, and Owens Valley have produced valuable data. 
The observations were taken at 18 cm, the lowest standard frequency of the network, to provide a 
long coherence time. Hydrogen maser frequency standards were used at all sites. The data were 
correlated on the National Radio Astronomy Observatory (NRAO) processor in Charlottesville and 
reduced using programs developed at NRL. In this paper, we will discuss the fringe rate source cat- 
alog based on our September data set. 
Application of basic Mark II techniques, as was done here, could investigate large numbers of survey 
sources to eliminate from further study those that show too much structure to be of astrometric in- 
terest. These techniques should be used at several frequencies to study the frequency dependency 
of the centroids of the source emission. As has been shown by Perley and Johnston (1979) from 
their VLA maps, the size and spectral index of 3C371 depends on frequency. At low frequencies, 
an extended nonthermal component 3 arcseconds northwest of the compact component biases the 
position found for this source. Measurements of 3C371 by Elsmore and Ryle (1976) made at 5 GHz 
*J. J. Wittels; Ph.D. thesis: Mass. Inst. of Technol., Cambridge, 1975. 
MOVEMENTS TERRESTRIAL AND CELESTIAL 183 
with a resolution of 3” and by Wade and Johnston (1977) at 2.7 GHz with a resolution of 01’6 have 
a discrepancy of OS 1 in right ascension. Readhead et al. (1978) discuss systematic changes in struc- 
ture and position angle with frequency such as found in 3C273 and 3C345. Structural differences 
such as these can be expected to lead to discrepancies in positions that can be resolved by a more 
complete study of the spatial and spectral characteristics of radio sources. 
Both the delay and fringe rate properties of our data were used to determine source position cata- 
logs and baselines. Because only the 2 MHz bandwidth of the Mark II system was used (no syn- 
thetic bandwidth), delay solutions that were precise enough to be of interest could not be estab- 
lished. The period of the 4 MHz clock (250 nanoseconds) limits solutions to about 25 nanoseconds. 
For a 3500 km baseline, this is an error of almost 0!‘5 in source position or 5 to 8 meters in baseline. 
The fringe rate depends on source position, baseline and instrumental clock in the basic form 
qb = C! Be cos h cos 6 + R (1) 
where a is related to the earth rotation rate, Be is the equatorial component of the baseline, h is the 
source angle from the meridian of the baseline, 6 is the source declination, and R is a functional for 
the oscillator frequency offsets between the independent oscillators at each station. For our data, 
R was a constant to within the error of measurement. 
It is important to note that in equation (l), a small change in the magnitude of the baseline Be is 
inseparable from a change of opposite sign in cos 6. This is because in this analysis a B, cos 6 are 
each individually time independent, only their product is determined. Outside constraints (such as 
limiting cos 6 over the range 0 I cos S I 1) must be used to factor this measured product. Observ- 
ing a large number of sources over the sky will establish a source catalog with the same error in cos 
F, but will not remove it (Shapiro, 1976). Similarly, a grid of baselines can be established using 
many stations. Baseline direction vectors and relative lengths can be found, but the scale of length 
cannot be separated from cos 6 without outside information. Likewise, the zero of right ascension 
cannot be determined from the observable data alone, 
It is difficult for radio astronomers to establish a right ascension for a catalog. Clark et al. (1976) 
and Wade and Johnston (1977) established their zero point in right ascension by adopting the posi- 
tion of 3C273B determined by Hazard et al. (1971) from lunar occultation timings. Elsmore and 
Ryle (1976) established their zero point relative to the FK4 position of /3 Persei. Fanslow (1978)* 
chose the position of NRA0140 as the zero point. The position of NRA0140 was established such 
that it minimized the difference between the optical and radio positions for those sources in the 
catalog having optical counterparts. In this paper, 3C273B was used to set the zero point of right 
ascension. 
*J. Fanslow, private communication, 1978. 
184 RADIO INTERFEROMETRY 
A variety of techniques has been examined to establish the zero of declination. We have tried 
holding the Owens Valley to Haystack baseline fixed to the value provided by Knight (1979) and 
Thomas et al. (1979). This value should be good to a few centimeters. Fixing one baseline pro- 
vides a scale length and establishes the other baselines, thus the source positions. 
One could fix a single source position and thus force the others and the baseline scale to follow. 
This approach was considered but not used because the declination of no one source is known well 
enough to grant it this favored status. 
Instead, the fringe rate catalog of table 1 has its declinations fixed by a statistical average of the 10 
best sources that also appear in the catalog of Wade and Johnston (1977). We did not select a 
weighted mean catalog of all the above mentioned catalogs (see Johnston et al. 1979, and Elsmore 
1978), because a systematic error found later in one of the component catalogs could be too hard 
to remove. The root mean square of the residual of the solution is 0.4 MHz or Ol’O5. A comparison 
of the solution obtained by fixing a baseline to the solution of table 1 shows that there are no sig- 
nificant differences to the source catalog. 
In table 2, the average differences in position for sources common in the above catalogs versus Wade 
and Johnston (1977) are displayed. As stated above, our average declination difference is defined 
to be zero. Also, fringe rate positions must degrade near the equator because the error in declina- 
tion goes as 1 /cos 6. 
The conclusion is that fringe rate techniques and Mark II VLBI can be readily used at a large num- 
ber of stations to establish baselines to - 1 meter and source catalogs. Certain errors became known 
or cancelled because of the grid of baselines, and we were able to get a satisfactory solution using 
only 48 hours of data. This technique may become important in studying the spatial, spectral, and 
temporal characteristics of large numbers of sources. 
REFERENCES 
Clark, T. A., Hutton, L. K., Marandino, G. E., Counselman, III, C. C., Robertson, D. S., Shapiro, I. I., 
Wittels, J. J., Hinteregger, H. F., Knight, C. A., Rogers, A. E. E., Whitney, A. R., Niell, A. E., 
R&n-rang, B. O., and Rydbeck, 0. E. H.; Astron. J., 81, 599, 1976. - 
Elsmore, B. and Ryle, M.; M.N. R.A.S., 174, 111, 1976. 
Elsmore, B.; Proc. of IAU Colloquium #48 “Modem Astrometry,” 1978. 
Hazard, C., Sutton, J., Argue, A. N., Kenworthy, C. M., Morrison, L. V., and Murray, C. A.; Nat. 
Phys. Sci., 233, 89, 1971. 
Source 
0224+671 
3C84 
NRA0140 
CTA26 
3C120 
05287 
DA267 
3C274 
3C273 
3c279 
OQ208 
04172 
1546+027 
1548+056 
1555+001 
NRA0512 
3c345 
3C418 
2134+004 
BL Lac 
CTA102 
3c454.3 
MOVEMENTS TERRESTRIALANDCELESTIAL 
Table 1 
Fringe Rate Catalog 
a 
02h24m41:1695+.0022 
03 16 29.5637a.0005 
03 33 22.4055+.0008 
03 36 58.9568r.0030 
04 30 31.5945+.0018 
08 51 57.25442.0008 
09 23 55.3204+.0010 
12 28 17.5750+.0060 
12 36 33.2460 
12 53 35.9278k.0161 
14 04 45.61672.0043 
14 42 50.4260+.0130 
15 46 58.2900+.0080 
15 48 06.887Ok.0110 
15 55 17.68372.0054 
16 38 48.1733k.0023 
16 41 17.6053+.0009 
20 37 07.4628k.0022 
21 34 05.2353+.0075 
22 00 39.36352.007 
22 30 07.7833r.0091 
22 51 29.5102*.0023 
6 
67”07’39!‘690r.006 
41 19 51.8832.005 
32 08 36.6182.011 
-01 56 15.755k.445 
05 14 59.180+.089 
20 17 58.3462.014 
39 15 23.549k.007 
12 40 01.250+.200 
02 19 41.6672.380 
-05 31 05.406+.940 
28 41 29.205k.046 
10 11 12.000+.300 
02 46 05.300+.400 
05 36 11.800+.300 
00 05 08.544224.7 
39 52 30.116k.028 
39 54 10.797+.010 
51 08 35.727k.010 
00 28 00.795+3.27 
42 02 08.569+.006 
11 28 22.5312.155 
15 52 54.150+.040 
185 
186 RADIO INTERFEROMETRY 
Table 2 
Weighted Mean Differences. Wade and Johnston (1977) - Other Catalogs 
Catalog 
Sources in 
Common ACX A6 
Clark et al. (1976) 17 -0~0001~0.0006 0’.‘002ro. 009 
Elsmore & Ryle (1976) 17 -0.0060~0.0015 0.049~0.016 
Fanslow (1978) 10 -0.0032+0.0026 0.012+0.017 
This paper 10 0.0002~0.0020 O.OOOt0.016 
I I I I 
Johnston, K. J., Spencer, J. H., Kaplan, G. H., Klepczynski, W. J., and McCarthy, D. D.; Proc, of 
Dec. 1978, U.S.N.O. Symp. on Star Catalogs, 1979. 
Knight, C. A., NASA CP 2 115, Radio Interferometry Techniques for Geodesy, 1980. 
Perley, R. and Johnston, K. J.; submitted to Astron. J., 1979. 
Readhead, A. C. S., Cohen, M. H., Pearson, T. J., and Wilkinson, P. N.; Nature, 276, 768, 1978. 
Shapiro, I. I.; Methods of Experimental Physics, vol. 12C, N. L. Meeks editor, Academic Press, pp. 
264-266, 1976. 
Thomas, J. B., Fanslow, J. L., Cohen, E. J., Purcell, G. H., Rogstad, D. H., Sovers, 0. J., Skjerve, 
L. J., and Spitzmesser, D. J.; NASA CP 2115, Radio Interferometry Techniques for Geodesy, 
1980. 
Wade, C. M. and Johnston, K. J.; Astron. J., 82, 791, 1977. 


Astronomy using basic Mark 2 very long baseline interferometry

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