Application of the connected-element radio interferometer of the National Radio Astronomy Observation in Green Bank, West Virginia to the determination of improved source coordinates, astronomical constants, and variations in Earth rotation parameters is discussed. It is concluded that because of the brevity and discontinuity of the data so far no reliable conclusions regarding the accuracy of the data can be drawn

Branham, R. L.

Johnston, K. J.

Josties, F. J.

Kaplan, G. H.

Klepczynski, W. J.

Mccarthy, D. D.

Spencer, J. S.

English

NASA Technical Reports Server

-- 
PROGRESS REPORT ON THE USNO/NRL GREEN BANK 
INTERFEROMETER PROGRAM 
W. J. Klepczynski, G. H. Kaplan, D. D. McCarthy, 
F. J. Josties, and R. L. Branham 
U.S. Naval Observatory 
K. J. Johnston and J. S. Spencer 
Naval Research Laboratory 
INTRODUCTION 
The connected-element radio interferometer of the National Radio Astronomy Observatory (NRAO) 
in Green Bank, West Virginia, is being used by the U.S. Naval Observatory (USNO) and the Naval 
Research Laboratory (NRL) in a joint program to apply radio interferometric techniques to the de- 
termination of improved source coordinates, astronomical constants, and variations in Earth rota- 
tion parameters. This is the first time that a radio interferometer has been applied to astrometric 
work on a full-time, continuous basis. 
Radio interferometry was first used for the determination of Earth rotation parameters by Elsmore 
(1973) and by Shapiro, et al. (1974) with accuracies approaching 3 ms. Observations by Wade and 
Johnston (1977) indicated that the Green Bank interferometer also could be used effectively in this 
pursuit. With the construction of the very large array (VLA) by NRAO, the Green Bank interfer- 
ometer became available for continuous use for astrometric work in October 1978. It was decided 
to initiate a series of observations to evaluate connected-element radio interferometry for the above 
purposes. 
The primary observables and operational aspects of this program are described elsewhere (Johnston, 
et al., 1978). 
The Green Bank Interferometer 
The Green Bank instrument has been described by Hogg, et al. (1969). This instrument consists of 
three 26-meter antennas which are located along an azimuth of 242”. In the present operational 
mode, the two movable antennas are separated by 1.5 and 2.4 km from the fixed antenna. A 14- 
meter antenna is located approximately 35 km from the fixed antenna at an azimuth of 204”. The 
operating frequencies are 2695 and 8085 MHz with an IF bandwidth of 30 MHz and a total system 
temperature of about 125 K. The IF signals from the remote antenna are transmitted to the Green 
Bank site via a phasestable microwave link. 
63 
https://ntrs.nasa.gov/search.jsp?R=19800020303 2020-03-21T17:50:50+00:00Z
64 RADIO INTERFEROMETRY 
In the initial phase of the program, two radio sources which pass close to the zenith at Green Bank 
were observed extensively over a large range of hour angles. These two sources (NRA0 150 and 
3C345) were the primary sources for the determination of UT0 until July 1979. Another 19 sources 
distributed over a declination range of -30” to +70” were used primarily to determine polar motion. 
These sources were observed at least live times daily at various hour angles, with each observation 
being at least 5 minutes long. From October 1978 through mid-March 1979, the observations were 
made only at 2695 MHz. After this time, all observations were made at both observing frequencies 
in order to eliminate the ionospheric contribution to the interferometer phase noise. 
The data, in the form of 30-second averages of fringe amplitude and phase, are recorded on magnetic 
tape and sent to the USN0 daily. They are reduced using a set of programs which apply a correc- 
tion for differential path delay in the atmosphere that is based upon a standard model atmosphere 
and ground-level weather data, along with corrections for relativistic effects, and drifts in the local 
oscillator. The phases also are rotated to correspond to the apparent places of the sources observed, 
computed using the adopted IAU values for precession and the nutation series. No corrections were 
applied for ionospheric effects, differential solid Earth tides, source structure effects, or for errors 
in the nutation series. 
Source Positions 
The list of sources observed in the Universal Time/Polar Motion (UT/PM) program is derived from 
the catalog of Wade and Johnston (1977), with a few additional sources at high and low declina- 
tions. Updated positions were obtained for most of the sources from S-band (2695 MHz) observa- 
tions which were made during several weeks of excellent observing weather in October and Novem- 
ber 1978. At this point, several sources which showed exceptionally large postfit residuals were 
eliminated from the observing schedule. (See table 1.) 
Because of a degradation in the quality of the observations during the winter of 1978-79, several 
tests on equipment and of observing procedures were begun in February 1979. It was found that 
ionospheric effects contributed to the short-term (-0.5 hour to -4 hours) phase drifts at S-band. 
Dual frequency observations were begun in mid-March 1979. A set of observations, obtained by 
using the S- and X-band observations to eliminate the effect of the ionosphere, produced interfer- 
ometer phases with about half the intrinsic scatter of the pure S-band phases. This result implied, 
that, at the current solar maximum, the differential ionosphere over our 35km baseline is impor- 
tant at S-band at the 0.5-cm level, with significant variations occurring over time spans as short as 
several minutes. 
Therefore, it seems apparent that dual-frequency observations are necessary in order to achieve 
angular measurements with accuracies approaching the O!Ol level. However, the data reduction 
procedure for combining the S- and X-band data is laborious because of the many lobe ambiguities 
in the X-band data which must be resolved. Because of current manpower constraints, less than a 
month of dual-frequency data has been fully reduced at this time, but this relatively small data set 
has permitted further position improvements for a number of sources. 
Source 
1519-273 
0237-234 
1245-l 97 
0336-019 
1226+023 
0742+103 
1502+106 
2251+158 
0851+203 
1328+254 
1901+319 
0923+392 
1641+399 
2200+420 
0355+508 
2037+5 11 
0954+658 
0224+67 1 
1749+701 
SURVEYS OF THE SEVENTIES 65 
- 
Table 1 
Source Position Catalog 
1 5h 19m37S230 
02 37 52.780 
12 45 45.225 
03 36 58.957 
12 26 33.246 
07 42 48.470 
15 02 00.159 
22 51 29.521 
08 51 57.255 
13 28 15.924 
19 01 02.312 
09 23 55.322 
16 41 17.607 
22 00 39.365 
03 55 45.261 
20 37 07.459 
09 54 57.855 
02 24 41.166 
17 49 03.395 
kO.020 
0.020 
010 
008 
005 
004 
005 
003 
003 
004 
006 
003 
005 
004 
003 
007 
007 
008 - . 
a(1 950.0) 
- 
6( 1950.0) 
-27” 19’ 29!‘60 ?0[‘30 
-23 22 06.15 30 
-19 42 57.80 20 
-01 56 16.94 07 
02 19 43.26 03 
10 18 32.60 05 
10 41 17.64 04 
15 52 54.29 03 
20 17 58.39 03 
25 24 37.46 03 
31 55 13.88 03 
39 15 23.54 03 
39 54 10.78 02 
42 02 08.57 03 
50 49 20.28 02 
51 08 35.75 02 
65 48 15.52 03 
67 07 39.70 02 
70 06 39.61 02 
Figure 1 gives an indication of the sensitivity of the 35-km baseline to errors in source position. 
The figure shows the error in source position (either E-W or N-S) that is needed to produce a 30” 
phase error at S-band at some time during the observing day, as a function of source declination. 
The limits of the observing day are determined by two conditions: (1) the source must be at an 
altitude of 20” or more and (2) the source must be less than 6 hours from the local meridian. It can 
be seen that the interferometer’s sensitivity to source position errors deteriorates rapidly below the 
equator, especially in the N-S direction (declination). The situation below the equator is a little 
worse that the figure indicates since right ascension and declination errors become increasingly cor- 
related as the hour-angle coverage decreases. 
Earth Rotation Parameters 
It is to be emphasized that the results and data presented below on Earth rotation parameters have 
not yet been corrected for the effects of the ionosphere, source structure, or solid Earth tides. Fig- 
ure 2 shows the components (Bx, By, and Bz) of the baseline vector as determined from 2-day 
averages of interferometer data for the representative period from April 11 to May 30, 1979. The 
width of each curve represents twice the probable error of its determination, which ranges from 4 
66 RADIO INTERFEROMETRY 
PCSITION ERROR FOR A+ = 300 
0. 16 
0. 16 
Ei 
6 0.14 
Y 
u” E 0. 12 
cr’ E 0. 10 
6 
6 0.06 
7 
7-J 0.06 
k? 
0.04 
0. 02 
-4c 3. I 
I I -T--T--T-- 
Cl = E-W 
A = N-S 
I I I I 1 I 
J -20.0 0. 0 20. 0 40. 0 60. 0 60. 0 
DECLINRTION 
Figure 1. Sensitivity of the 35-km baseline to errors 
in source position. Source position error (ordinate) 
needed to produce a 30” phase error at S-band as a 
function of source declination (abscissa). 
to 24 ps, corresponding to 1.2 to 7.2 mm. The top curve represents the length of the baseline which 
should be invariant under a rotation of the baseline vector. To achieve a goal of a precision of 2 ms 
in time, the baseline must be determined with a precision of approximately 5 ps which corresponds 
to about 2 mm in baseline length. 
The components of the baseline vector (Bx, By, and Bz) are expected to vary due to the motion of 
the instantaneous axis of rotation with respect to some reference point and to variations in the 
Earth’s rate of rotation. In order to insure the accuracy of the delay tracking of the interferome- 
ter, the sidereal time used in controlling the telescope is derived by using values of UT1 -UTC, pro- 
vided by the Rapid Service of the Bureau International de 1’Heure (BIH). Thus, the baseline com- 
ponents shown in figure 2 have the effects of a first approximation to variations in the Earth’s rota- 
tion rate removed from them but are not corrected for the effects of polar motion. 
SURVEYS OF THE SEVENTIES 67 
1. ,(I t 
1 
BP==--~-- - 
1 
Figure 2. Normalized changes in the baseline components (Bx, By, Bz) for the 
period April 11 to May 30. Width of the curves is twice the formal probable error. 
The top curve represents the change in baseline length (IBl). 
For the current configuration of the Green Bank interferometer, the equations relating changes in 
baseline components to the motion of the pole and changes in the Earth’s rate of rotation are: 
-B, cos h -B, sin h 
B, sin h -B, cos h 
(B, cos X - B, sin X) (Bx sin X + By cos X) 
where 
Bx, By, Bz are the components of the reference baseline in a local left-handed system; 
AB,, AB,, AB, are observed changes in the baseline components; 
Ax, Ay are changes in the polar coordinates from those of the reference data; 
68 RADIO INTERFEROMETRY 
T is the change in UTl-UTC from the value assumed in the observations; 
X is the adopted longitude of the interferometer (5h 19m 2OSOOO). 
Since the determinant of the matrix formed by the coefficients of the right-hand side of these equa- 
tions is zero, the matrix cannot be inverted. In other words, the single baseline can only be used to 
determine two combinations of the three quantities of interest. In the future, it is hoped, with the 
cooperation of the National Science Foundation (NSF) and NRAO, to establish a second long base- 
line nearly orthogonal to the current one to overcome this difficulty. Until that time, it has been 
decided to publish values of UTO-UTC and variations in the component of polar motion determined 
from variations in the component of the interferometer baseline (McCarthy, et al., 1979). Table 2 
lists the values of UTO-UTC obtained for the more recent period of observations preceding this con- 
ference (April 11 to May 30) and their probable errors. The formal probable errors for the quan- 
tity UTO-UTC are about 2 ms for 2-day averages and, in some instances, approach 1 ms. 
Table 2 
Values of UTO-UTC as Determined 
by the 35-km Interferometer 
Date 
79 412 43975.000 0”2819 0.0015 
79 414 43977.000 0.2791 0.0016 
79 416 43979.000 0.2687 0.0019 
79 418 43981 .OOO 0.2662 0.0017 
79 420 43983.000 0.2560 0.00 17 
79 422 43985.000 0.2492 0.00 18 
79 425 43987.500 0.2449 0.0026 
79 427 43990.000 0.2302 0.0023 
79 429 43992.000 0.2234 0.0018 
79 5 1 43994.000 0.2419 0.0017 
79 5 4 43997.000 0.2085 0.0019 
79 5 6 43999.000 0.1991 0.0018 
79 5 8 4400 1 .ooo 0.1905 0.0021 
79 510 44003.000 0.1874 0.0022 
79 512 44005.000 0.1888 0.0024 
79 514 44007.000 0.1805 0.0020 
79 516 44009.000 0.1712 0.0013 
79 518 440 11 .ooo 0.1664 0.0020 
79 520 440 13.000 0.1629 0.0008 
79 522 44015.000 0.1571 0.0021 
79 524 44017.000 0.1495 0.0024 
79 526 44019.000 0.1450 0.0017 
79 526 4402 1 .OOO 0.1396 0.0020 
79 530 44023 .OOO 0.1334 0.0018 
MJD I UTO-UTC 
- 
SURVEYS OF THE SEVENTIES 69 
In order to test the accuracy of our UTO-UTC data, they may be compared with the values derived 
from the Earth rotation parameters of the BIH Rapid Service. Using the values for Ax, Ay, and 
UTl-UTC from the BIH Rapid Service, values for UTO-UTC are computed for the dates listed in 
table 2. The differences between the computed values for UTO-UTC and those determined by the 
interferometer are shown in figure 3. The values are given in the BIH 1979 System and have been 
adjusted by a constant so that there is no difference between the observed and computed values on 
the first date. Figure 4 shows the same comparison but for a period covering the initial months of 
operation from October 19 to December 12, 1978. These data were computed in the BIH 1968 
System and also adjusted to make the difference between observed and computed values on the 
first day equal to zero. Because of the brevity and discontinuity of the data presented here, no re- 
liable conclusions regarding the accuracy of the data can be drawn. 
-10. I I I I I I I I I I I I I I I I I I 
. . . . . 
E : : 
tsi 
zl : 0-l 6l El !I 
MJD - 43000 Cl2 MU';1 TO 30 MA; 1978> 
3-l s-4 
Figure 3. The difference in UTO-UTC between that observed and that computed from the BIH Rapid Service 
data (referred to the BIH 1979 System), for the period April 12 to May 30. 
70 RADIO JNTERFEROMETRY 
10. 
Z 
H 
P 
0. 
3 
I 
P 
3 
-10. 
0 
x 
r 
x 
I 
x 
x 
x 
I I I I I I I 1 1 
x 
x x 
II 
T 
I 
-! 3 
I 
x 
: 
d 
2 
: F : I? Fifl 
al m m al 03 al 
JD - 43000 (19 OCTOBER TO 12 DECEMBER 1978> 
Figure 4. The difference in UTOUTC between that observed and that computed from the BIH Rapid 
Service data (referred to the BIH 1968 System), for the period October 19 to December 12. 
REFERENCES 
Elsmore, B., Nature, 244,423 (1973). -- 
Hogg, D. E., Macdonald, G. H., Conway, R. G., and Wade, C. M., Astron. J., 74, 1206 (1969). - 
Johnston, K. J., Spencer, J., Mayer, C., Klepczynski, W. J., Kaplan, G. H., McCarthy, D. D., and 
Westerhout, G., Modern Astrometry (IAU Coll. 48), 1978. 
McCarthy, D. D., Klepczynski, W. J., Kaplan, G. H., Josties, F. J., Westerhout, G., Johnston, K. J., 
and Spencer, J. H., Annual Report for 1978 (Bureau International de 1’Heure). 
Shapiro, I. I., Robertson, D. S., Knight, C. A., Counselman, C. C., Rogers, A. E. E., Hinteregger, H. 
F., Lippincott, S., Whitney, A. R., Clark, T. A., Niell, A. E., and Spitzmesser, D. J., Science, 
186,920 (1974). 
Wade, C. M. and Johnston, K. J., Astron. J., 82, 791 (1977). - 
._. --_ . ._ 


The USNO/NRL Green Bank interferometer program

The USNO/NRL Green Bank interferometer program

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server