A systematic very long baseline interferometry (VLBI) sky survey, undertaken to find a suitable set of compact celestial radio sources from which a more complete VLBI reference frame can be constructed, discussed. The survey was conducted by searching known celestial radio sources for compact components by means of VLBI observations. Baseline lengths were about 7 x 10 to the 7th power RF wavelengths (lambda = 13.1 cm), so the spatial wavelengths being sampled by the interferometer were generally on the order of a few milliarcseconds. Hence, the radio sources detected have a measurable portion of their total flux density contained in components that are no more than a few milliarcseconds in angular extent. Existing information of radio sources were used as clues to source size

Morabito, D. D.

Preston, R. A.

English

NASA Technical Reports Server

- 
A VERY LONG BASELINE INTERFEROMETRY SKY SURVEY 
R. A. Preston and D. D. Morabito 
Jet Propulsion Laboratory 
INTRODUCTION 
The development of new high-accuracy celestial reference frames composed of compact extragalac- 
tic radio sources will permit astrometric and geodetic studies of previously unobtainable accuracy 
to be performed (Shapiro and Knight, 1970; Counselman, 1975). These studies require the use of 
the observational technique of very long baseline interferometry (VLBI). Previous investigators 
have utilized only a relatively small set of detected compact sources in initial experimental develop- 
ment of the technique. In this paper, we discuss a systematic VLBI sky survey which has been un- 
dertaken to find a suitable set of compact celestial radio sources from which a more complete VLBI 
reference frame can be constructed. The results of this survey are presently being used to form a 
VLBI reference frame of about 100 to 200 sources by determining precise relative positions (Purcell 
et al., 1979). 
SELECTION OF CANDIDATE SOURCES 
The survey was conducted by searching known celestial radio sources for compact components by 
means of VLBI observations. Our baseline lengths were about 7 X lo7 RF wavelengths (X = 13.1 
cm), so the spatial wavelengths being sampled by the interferometer were generally on the order of 
a few milliarcseconds. Hence, the radio sources detected by this survey have a measurable portion 
of their total flux density contained in components that are no more than a few milliarcseconds in 
angular extent. Into this category of radio sources fall certain quasars, BL Lacertae type objects, 
and galactic cores. 
In order to keep from searching all known celestial radio sources for VLBI components, we have 
used existing information on radio sources as clues to source size. The following criteria proved 
useful in determining which sources might have structure confined to the milliarcsecond level: 
(1) Spectrum - Power does not fall off with increasing frequency as rapidly as more typical 
radio sources in the range 1 to 10 GHz. An even stronger indicator is if the spectrum is 
flat, sloped upward, or peaked in this range. Such behavior is typical of the synchrotron 
self-absorption characteristic of compact sources. Of all the indicators of milliarcsecond 
*This paper represents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Tech- 
nology, under Contract No. NAS7-100, sponsored by the National Aeronautics and Space Administration. 
173 
https://ntrs.nasa.gov/search.jsp?R=19800020311 2020-03-21T17:49:45+00:00Z
174 
(2) 
(3) 
(4) 
(5) 
(6) 
RADIO INTERFEROMETRY 
structure, spectral properties have proved the most useful. In the absence of any other 
information about a source, we selected sources whose spectral index between 2.7 and 
5.0 GHz was k-0.6 (S = Sofa). 
Variability - Identified as radio or optical variable. 
Optical Identification - Identified as extragalactic. 
Size Limits - A short baseline interferometer can determine if a source has components 
smaller than its resolution limit (resolution does not exceed - 1 arcsecond). 
Interplanetary Scintillation - Radio sources that show scintillations when viewed through 
the solar corona possess components with angular sizes 20.1 arcsecond. 
Detection in High Frequency Surveys - A larger percentage of sources detected in surveys 
between 1 and 10 GHz are VLBI sources than is the case at lower frequencies. 
In general, we chose as candidates only those sources whose total flux densities were 20.7 Jansky 
at our observing frequency (2.3 GHz). 
To date, about 1000 candidate sources have been observed, with 665 of these being detected with 
VLBI. 
DATA COLLECTION AND REDUCTION 
The observations were all performed with pairs of antennas within NASA’s Deep Space Network 
(DSN) which were separated by intercontinental distances. In practice, this meant observing on the 
Goldstone-Australia or Goldstone-Spain baselines. All the observations were performed at 2290 
MHz. The data were recorded on the Mark II VLBI recording system, which preserved a 1.8 MHz 
data bandwidth by digitally sampling at a 4-Mbs rate (Clark, 1973). 
Matching tapes from the two antennas were cross-correlated on a special hardware/software com- 
puter at the National Radio Astronomy Observatory in Charlottesville, Virginia. Computer manipu- 
lation of the output of this correlator yielded the correlation constant, p, for each observation, or 
the fraction of bits on the two tapes that were correlated. Correlation constants were then con- 
verted into correlated flux densities, S, (i.e., VLBI source strengths) by means of the expression: 
s, = WJT,T, ($), (g), 
MOVEMENTS TERRESTRIAL AND CELESTIAL 175 
where 
S, = correlated flux density in Jansky 
Ti = the measured system temperature at antenna i in K 
(dS/dT)i= the inverse sensitivity of antenna i in Jansky/K (i.e., how strong does a source have to 
be in total flux density (Jansky) to raise the system temperature 1K). 
Most sources were observed for at least 3 minutes, but, for data processing, the observations were 
broken into l-minute segments. The 50 detection limit for this search for VLBI sources was about 
0.1 Jansky. The random uncertainty in detected source strength was about 0.02 Jansky. However, 
in practice, systematic errors at the 5 or 10 percent level dominate the random contribution for 
most sources. 
If a priori source positions are in error, the tapes must be correlated over a range of relative tape de- 
lay and delay rate offsets in order to detect a VLBI source. Appropriate searches in these param- 
eters were performed so that the sky was completely searched within 0.5 arcminutes of all nominal 
source positions. Almost all a priori source position errors should be covered by this degree of posi- 
tion searching. In addition, once detections were achieved, the measured delay and delay rate off- 
sets allowed positional errors to be measured to about one arcsecond. 
RESULTS 
Figures 1 a, 1 b, and lc display the general sky distribution of detected VLBI sources as a function of 
VLBI source strength. These maps are equal area representations, so that local spatial densities in 
one area may be compared to local densities in another. Catalogs of source strengths and positions 
will be published elsewhere. 
It should be noted that the strengths of the sources are a function of time, baseline length, and base- 
line orientation. Although the plots may not always show the true strength of a particular source 
at a certain epoch and on a certain baseline, they do indicate the general sky densities of sources as 
a function of strength. 
Figure la shows the global distribution of all 665 detected sources. The lower limit of correlated 
flux strength is about 0.1 Jansky. Several things are evident from this plot: 
(1) In general, the distribution is dense and rather uniform. 
(2) Pairs of DSN antennas cannot commonly view the sky below z-45” declination. 
176, RADIO INTERFEROMETRY 
I? hr, 
(b) 
12hn 
-90 deg RECLINATION 
90 dcg 
-90 deg DECLINATION 
12 hrr 
-90 deg DECLINATION 
, ) .- ) RIGHT 
ASCENSION 
/ 
- / 
/ 
;- - 
7 
1’ 
_- 
Figure 1. Sky distribution of VLBI sources: (a) 665 sources 20.1 Jansky, 
(b) 210 sources 20.5 Jansky, (c) 52 sources 21 .O Jansky. 
MOVEMENTS TERRESTRIAL AND CELESTIAL 177 
(3) The north polar area is more sparsely populated. This is directly due to a lack of suffi- 
cient high frequency surveys in this region to easily identify candidate sources to observe. 
(4) The areas near the plane of the galaxy (not shown) are more sparsely populated. This is 
particularly evident between +30” declination at right ascensions of about -5 and +7 
hours. This is due to the fact that high frequency radio surveys often skip the galactic 
regions and that interstellar charged particles near the galactic plane can cause intrinsi- 
cally small sources to be scattered to large angular sizes. 
We should point out that this is not nearly a complete map of the VLBI sky at the level of 0.1 
Jansky. Since one of our criteria for choosing candidate sources was, in general, to keep only those 
sources with total flux densities greater than 0.7 Jansky, we have missed a large number of very 
compact sources with lower total flux densities. 
Figure 1 b shows the distribution of 210 sources with correlated flux densities greater than 0.5 
Jansky. This is also a rather dense and even distribution, and is a more complete representation of 
the VLBI sky than was the 0.1 Jansky map. 
The DSN Block 1 VLBI System is being developed to calibrate radio metric navigation data from 
deep space probes by measuring earth rotational position and clock epoch and rate offsets (Fanse- 
low et al., 1978). The sensitivity of this system to VLBI source strength is 0.5 Jansky. In order to 
conserve antenna time, this system is intended to produce the appropriate calibrations with only 1 
or 2 hours of observing on each of two intercontinental baselines. During that short time span, a 
single pair of DSN antennas will see only a small segment of sky in common. In order to properly 
estimate the desired parameters, there must be a good distribution of sources within this small com- 
mon area of sky. This must be true at any time of day. Hence, this system requires a source cata- 
log of about 100 sources. The number of sources we have found with strengths greater than 0.5 
Jansky is nicely matched to the number of required sources. 
Figure lc displays the distribution of 52 sources with correlated flux densities greater than 1.0 
Jansky. This is also quite a complete representation of the VLBI sky, but the distribution is no 
longer very dense. Note that less than 10 percent of the total number of detected VLBI sources 
have correlated flux strengths greater than 1 .O Jansky. 
The falloff in number of sources with increasing source strength is shown more clearly in the histo- 
gram of figure 2. The number of detected sources at source strengths less than 0.5 Jansky is smaller 
than the true sky distribution, due to our candidate selection criteria and our approximate sensitiv- 
ity limit of 0.1 Jansky. The actual number of existing VLBI sources with correlated flux densities 
greater than 0.1 Jansky is probably several thousand. At the high end of the source strength scale, 
we see that only six sources are stronger than 2.0 Jansky and none are stronger than 3.5 Jansky. 
80 
60 
RADIO INTERFEROMETRY 
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 
CORRELATED FLUX, Jy 
Figure 2. Histogram of number of sources vs correlated flux. 
Geodetic VLBI investigations will often involve baselines shorter in length than the intercontinental 
baselines of this survey. A similar 13 cm VLBI survey has been completed on the same portion of 
sky for baselines of about 200 km length. The results of this survey show that if a given number of 
sources are required to form a VLBI reference frame, the sensitivity of a 200 km baseline 13 cm 
VLBI system need only be about half that of a similar intercontinental system. This difference is 
due to the fact that the shorter baseline system is sensitive to sources of larger maximum angular 
size. Actually, because of the shorter periods of source mutual visibility on the longer baselines, the 
number of sources necessary to form a reference frame for an intercontinental VLBI system would 
probably be larger than that for a 200 km system. Hence, intercontinental systems might have to 
be several times as sensitive as 200 km systems. 
- 
MOVEMENTS TERRESTRIAL AND CELESTIAL 
REFERENCES 
Clark, B. G.; Proc. of the IEEE, 61, 9, 1242-l 248, 1973. - 
Counselman, C. C., III; Rev. Geophys. Space Phys., 13, 270-271, 1975. - 
Fanselow, J. L., et al.; Time and the Earth’s Rotation, IAU Symp. No. 82, Reidel, 1979. 
Purcell, G. H., et al.; NASA CP 2115, Radio Interferometry-Tech. for Geodesy, 1980. 
179 
Shapiro, I. I., and Knight, C. A.; Earthquake Displacement Fields and Rotation of the Earth, Reidel, 
284-301, 1970. 


A very long baseline interferometry sky survey

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server