The results of approximately 1300 observations of 67 radio sources are presented. Most of the measurements were made at the stations of the Deep Space Network in California, Spain, and Australia at wavelengths of 13.1 and 3.6 cm, between 1971 and 1978. The formal errors in the derived source positions are generally in the neighborhood of 0.01 seconds of arc and the positions agree fairly well with those previously published

Cohen, E. J.

Fanselow, J. L.

Purcell, G. H., Jr.

Rogstad, D. H.

Skjerve, L. J.

Sovers, O. J.

Spitzmesser, D. J.

Thomas, J. B.

English

NASA Technical Reports Server

VLBI MEASUREMENTS OF RADIO SOURCE POSITIONS 
AT THE JET PROPULSION LABORATORY 
G. H. Purcell, Jr., J. L. Fanselow, J. B. Thomas, E. J. Cohen, 
D. H. Rogstad, 0. J. Sovers, L. J. Skjerve, and D. J. Spitzmesser 
Jet Propulsion Laboratory 
ABSTRACT 
We present the results of approximately 1300 observations of 67 radio sources. Most of our meas- 
urements were made at the stations of the Deep Space Network in California, Spain, and Australia, 
at wavelengths of 13.1 and 3.6 cm, between 1971 and 1978. The formal errors in the derived 
source positions are generally in the neighborhood of 0.01 seconds of arc, and the positions agree 
fairly well with those published by other groups. 
165 
https://ntrs.nasa.gov/search.jsp?R=19800020310 2020-03-21T17:49:47+00:00Z
166 RADIO INTERFEROMETRY 
INTRODUCTION 
The observational material I am going to discuss this afternoon is identical with what Brooks Thomas 
discussed this morning in connection with the geophysical parameters - namely baselines, UT1 , and 
polar motion. However, I am going to consider it now from the standpoint of the source positions. 
Historically, JPL has been interested in radio source positions as an adjunct to its VLBI measure- 
ments of parameters required for spacecraft navigation, in particular, UTl, polar motion, and sta- 
tion locations. When our system becomes fully operational, we expect to make measurements of 
station locations and polar motion accurate to 30 cm or better, and UT1 at a comparable level. In 
order to achieve this goal, we require a source catalog with positions accurate to something like 
O!‘Ol. Furthermore, we expect to observe at arbitrarily assigned times, with only about 2 hours of 
data per week on each baseline. Our catalog therefore needs to be fairly large, say a hundred 
sources; and we want the objects well distributed over the accessible sky. 
JPL is also interested in using extragalactic sources more directly for spacecraft navigation by mak- 
ing differential measurements between spacecraft and nearby natural sources. For that reason, we 
have a special interest in sources near the ecliptic - particularly those parts of the ecliptic where 
spacecraft are likely to turn up. 
Finally, we have recently started a project to test the stability of the celestial reference frame by 
measuring differential proper motion in a few close pairs of sources. 
EQUIPMENT, OBSERVATIONS, AND REDUCTION PROCEDURES 
I am not going to dwell on our equipment and reduction procedures, because Brooks has already 
discussed those in some detail, and they have been described elsewhere (Purcell et al., 1978); but I 
would like to remind you of some of the salient points. Our data consist of about 1300 observa- 
tions of 67 sources made in the course of 30 observing sessions between 197 1 and 1974 and in 1977 
and 1978. Most of these sessions involve the baselines between the Deep Space Network stations in 
California and Spain, and California and Australia; but there is also one session between Haystack 
and Owens Valley. During the earlier years, we observed only at a wavelength of 13.1 cm, and we 
measured only fringe rates; more recently, we have been using bandwidth synthesis methods to 
measure delays as well as rates; now, we generally observe simultaneously at 13.1 and 3.6 cm. In 
several of these sessions, we have combined data at the two frequencies to reduce the corrupting 
influence of the ionosphere on the observations. 
In the final step of the reduction procedure, we fitted all the data simultaneously with an analytic 
model containing 384 adjustable parameters. Of these parameters, 133 describe the source posi- 
tions, the others are divided among the clock model (129), the troposphere model (64), UT1 and 
polar motion (43), and station locations (14). 
MOVEMENTS TERRESTRIAL AND CELESTIAL 167 
RESULTS 
Figure 1 shows the distribution on the sky of our 67 sources, and, on a different scale, the formal 
errors on the estimated right ascensions and declinations. Very large errors, those greater than 
OI’O3, have not been plotted. In addition, one source, NRA0 140, was used as the reference point 
for right ascension and therefore has no horizontal error bar. The figure shows that the sources are, 
in fact, fairly well distributed over the visible sky, and that the errors in the positions are mostly in 
the neighborhood of O[‘Ol. You can see that the positions of southern sources - ones that we can 
observe only on the California-to-Australia baseline - are generally less well determined than those 
of the northern sources. I attribute this result largely to the fact that these southern sources are 
visible for only about 4 hours. This circumstance leads to correlations among the errors in some of 
the parameters, and thus to a general weakening of that part of the solution. Until recently, there 
were very large errors in the declinations of some sources near the equator that had been,observed 
only on the California-to-Spain baseline, which has a small polar projection. However, we have re- 
lieved much of that problem now by adding some observations of these sources on the other 
baseline. 
6c 
40 
20 
0 
-20 
-40 
+ ERROR 
SCALE + + 
+ 
Ll-l 
++ 
l . + ‘=c 
+ -I- 
+ 
+ + 
++ 
+ I 
+ 
-r . - 
I . 
++-t 
. 
+# + 
24 20 16 12 8 4 0 
RIGHTASCENSION (HOURS) 
Figure 1. Distribution of sources. 
168 RADIO INTERFEROMETRY 
I would like to emphasize today the progress our group has made in the last year or so. Figures 2 
and 3 are‘histograms that show the distributions of the formal errors in our source positions as they 
were 9 months ago (the white blocks) and as they are now (the black blocks). Notice that each 
error bin contains a white column and a black column, with the shorter column always placed in 
front so that both can be seen. Figure 2 shows that the mean formal error in right ascension has 
decreased from about 0!‘032 (for 52 sources) in September of last year to 01’014 (for 63 sources) 
now - an improvement of better than a factor of two. The improvement in the declinations is even 
more dramatic (figure 3), from 01’030 (for 45 sources) to OI’Ol 1 (for 63 sources). 
We are, of course, continuing to compare our results with those of other groups. To this end, we 
have constructed a composite reference catalog from the lists of Clark et al. (1976), Elsmore and 
Ryle (1976), and Wade and Johnston (1977). This catalog is a weighted average of the three lists, 
with slight adjustments in the origin of right ascension of two of them, to produce the best overall 
agreement. In a few cases of serious discrepancy, we deleted individual values from the average. 
Finally, we readjusted the origin of right ascension of the composite list to produce the best overall 
agreement with our own list, and computed the residual differences between the two. 
Figure 4 shows the differences between the right ascensions in the reference catalog and those in 
our list, for 28 sources. I think you can see that the scatter in the points is entirely consistent with 
the error bars; in fact, &i-square per degree of freedom is 0.99. However, the agreement in the dec- 
linations, shown in figure 5, is quite poor, owing mainly to a mean difference between the reference 
list and the JPL list of about -01’02. We have as yet no satisfactory explanation for this difference. 
q SEPTEMBER, 1978 
n JUNE, 1979 
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 (LO8 0.09 0.10 
ERROR (ARC SECONDS) 
Figure 2. Distribution of errors in right ascension. 
MOVEMENTS TERRESTRIAL AND CELESTIAL 169 
0. 1c 
2 0.05 
s 
-0. 1c 
, - 
i - 
I - 
0.01 0.02 a03 
q SEPTEMBER, 1978 
a JUNE, 1979 
a04 a05 a06 a07 a08 a09 0.10 
ERROR (ARC SECONDS) 
Figure 3. Distribution of errors in declination. 
I I I I I 
ti I 
tt I’ 
I T- i 
0 4 8 12 16 20 24 
RIGHT ASCENSION (HOURS) 
Figure 4. Comparison of right ascensions. 
170 RADIO JNTERFEROMETRY 
a 15 
H 
is 
2 
0.10 
! 
- 1 J 1 I- I 
0 4 8 12 16 20 
RIGHT ASCENSION (HOURS) 
24 
Figure 5. Comparison of declinations. 
The general problem of comparing two sets of source positions is an awkward one just now. The 
fact that different groups are reducing their data using different nutation series and different rates 
of precession is certain to lead to systematic differences in results at a level of O:Ol , especially when 
the epochs of the observations differ. Fortunately, it seems likely that we can resolve these differ- 
ences in the near future. 
Table 1 presents the source positions themselves. Note that these positions are barycentric. To ob- 
tain positions consistent with normal practice in optical catalogs, you must add the elliptic aberra- 
tion terms that are provided. Keep in mind also that we reduced the positions to 1950.0 coordi- 
nates using the conventional (Woolard) nutation series and Newcomb’s value of the precession 
constant. 
FUTURE PLANS 
We have made substantial progress in the past year, and we expect this progress to continue in the 
near future. Even at our present level of sensitivity, we can easily reach 50 or 60 more sources. We 
are improving our calibration techniques to account better for fluctuations in instrumental and 
propagation delays. We are also continuing to improve our delay model in anticipation of accu- 
racies substantially better than OYOl. 
SOURCE 
NAME 
P 0104-408 
P 0106+01 
P 0113-118 
DA 55 
DW 0224+67 
OE 400 
3C 84 
P 0332-403 
NRA0 140 
CTA 26 
NRA0 150 
P 0402-362 
VRO 41.04.01 
P 0420-01 
3c 120 
P 0438-43 
NRA0 190 
P 0537-441 
DA 193 
P 0605-08 
P 0607-15 
P 0727-11 
P 0735+17 
01 363 
DW 0742+10 
OJ 425 
4C 55.16 
4c 71.07 
OJ 287 
OJ 499 
P 0859-14 
r 
MOVEMENTS TERRESTRIAL AND CELESTIAL 
Table 1 
Radio Source Positions 
171 
SOURCE POSITION (SOLAR SYSTEM BARYCENTER, 1950.0) 
RIGHT ASCENSION ERROR DECLINATION ERROR 
Olh04m27s57081 
01 06 04:51419 
01 13 43.21450 
01 33 55.09459 
02 24 41.14105 
03 00 10.09103 
03 16 29.54517 
03 32 25.20880 
03 33 22.38543 
03 36 58.91001 
03 55 45.23306 
04 02 02.56611 
04 20 27.96800 
04 20 43.51973 
04 30 31.58204 
04 38 43.14982 
04 40 05.27072 
05 37 21.03832 
05 52 01.37726 
06 05 36.00444 
06 07 25.95624 
07 27 58.07365 
07 35 14.10211 
07 38 00.15161 
07 42 48.44097 
08 14 51.64006 
08 31 04.34169 
08 36 21.49599 
08 51 57.22910 
08 59 39.95510 
08 59 54.92583 
osooo92 -40°5U'21:3385 01.10106 Of00263 -0112442 
0.00054 +Ol 19 01.1607 0.0080 0.00215 -0.0207 
0.00074 -11 52 04.4395 0.0118 0.00297 -0.0977 
0.00066 +47 36 12.6398 0.0053 0.00724 0.2273 
0.00083 +67 07 39.4256 0.0049 0.02483 0.2741 
0.00060 i-47 04 33.5013 0.0055 0.01868 0.1882 
0.00593 +41 19 51.7340 0.0310 0.01869 0.1564 
0.00091 -40 18 23.8807 0.0099 0.01999 -0.1861 
* +32 08 36.5561 0.0058 0.01809 0.1104 
0.01674 -01 56 16.8612 1.1855 0.01559 -0.0370 
0.00058 +sO 4Y 20.1587 0.0038 0.02673 0.1598 
0.00094 -36 13 11.7154 0.0107 0.02144 -0.1545 
0.04666 +41 43 06.7997 0.6590 0.02470 0.1123 
0.00059 -01 27 28.6683 0.0085 0.01846 -0.0337 
0.00064 +05 14 59.6152 0.0091 0.01908 -0.0112 
0.00092 -43 38 53.4225 0.0098 0.02687 -0.1437 
0.00218 -00 23 20.5738 0.0327 0.01951 -0.0298 
0.00088 -44 06 44.6030 0.0097 0.03038 -0.0891 
0.00058 +39 48 21.9245 0.0048 0.02889 0.0277 
0.00164 -08 34 20.2899 0.0216 0.02271 -0.0368 
0.00074 -15 42 03.2823 0.0103 0.02336 -0.0424 
0.00074 -11 34 52.5831 0.0106 0.02284 -0.0150 
0.00061 +17 49 09.3064 0.0082 0.02335 -0.0502 
0.00077 +31 19 02.1255 0.0087 0.02594 -0.0655 
0.00056 +lO 18 32.6957 0.0062 0.02241 -0.0435 
0.00057 +42 32 07.8557 0.0043 0.02854 -0.1099 
0.00i47 +55 44 41.4885 0.0287 0.03617 -0.1428 
0.00291 +71 04 22.6217 0.0107 0.06203 -0.1609 
0.00048 +20 17 58.5040 0.0046 0.02063 -0.0894 
0.00093 +47 02 56.9833 0.0139 0.02778 -0.1586 
0.00100 -14 03 38.8984 0.0143 0.01950 0.0186 
r ELL. ABERRATION R.A. DEC. 
* 
NRA0 140 right ascension reference 
The research described in this paper was carried out at the Jet Propulsion Laboratory, California 
Institute of Technology, under NASA contract NAS7-100. 
REFERENCES 
Clark, T. A., Hutton, L. K., Marandino, G. E., Counselman, 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., 
Ronnang, B. O., and Rydbeck, 0. E. H.; Astron. J., 81, 599, 1976. 
Elsmore, B., and Ryle, M.; Monthly Notices Roy. Astron. Sot., 174,411, 1976. 
172 RADIO INTERFEROMETRY 
Table 1 (continued) 
SOURCE SOURCE POSITION (SOLAR SYSTEM BARYCENTER, 1950.0) ELL. ABERRATION 
NAME RIGHT ASCENSION ERROR DECLINATION ERROR R.A. DEC. 
4C 39.25 04h23m55s29607 Of00048 +39"15'23:7306 0:10037 Of02258 -01.1160; 
P 1055+01 LO 55 55.29831 0.01438 +Ol 50 04.5763 1.4224 0.01039 -0.038: 
P 1104-445 11 04 50.35273 0.00091 -44 32 53.1388 0.0093 0.01346 0.1971 
P 1127-14 11 27 35.65773 0.00061 -14 32 54.5078 U.0082 0.00775 0.053r 
P 1144-379 11 44 30.84975 0.00073 -37 55 30.8424 0.0083 0.00747 0.1804 
P 1148-00 11 48 10.11818 0.00065 -00 07 13.1473 0.0089 0.00554 -0.0275 
3C 273 12 26 33.24224 0.00052 +02 19 43.3448 0.0065 0.00178 -0.0424 
3c 279 12 53 35.83076 0.00080 -05 31 08.0123 0.0119 -0.00091 0.0044 
OP-322 13 13 20.u4317 0.00076 -33 23 09.8640 0.0~87 -0.00343 0.1627 
DW 1335-12 13 34 59.80762 O.OOUO5 -12 42 09.8032 0.0091 -0.00511 0.0455 
P 1349-439 13 49 50.30438 0.00484 -43 57 48.3349 0.0224 -0.00891 0.2071 
OR 103 15 02 00.17015 0.00085 +lO 41 17.8086 0.0117 -0.01310 -0.0804 
P 1510-08 15 10 08.91346 0.00072 .-08 54 47.6550 0.0103 -0.01369 0.0144 
DW 1555+00 15 55 17.70879 0.00061 +OO 06 43.5522 0.0087 -0.01686 -0.029c 
DA 406 16 11 47.93564 0.00057 +34 20 19.9820 0.0060 -0.02170 -0.1427 
NRA0 512 lb 38 48.19450 0.00070 +39 52 30.2209 0.0137 -0.02S34 -0.1361 
3c 345 16 41 17.62954 U.00054 +39 54 10.9214 0.0046 -0.02551 -0.1341 
DW 1656+05 16 56 05.63938 0.00513 +05 19 46.8476 U.2463 -0.~2037 -0.0429 
NRA0 530 17 30 13.55606 0.00085 -13 02 45.8376 O.Oi23 -0.02217 -0.0033 
3T 465 17 38 36.34191 0.00118 +47 39 28.8555 0.0083 -0.03244 -0.0908 
P 1741-038 17 41 20.63771 o.OuO67 -03 48 48.8899 0.0100 -0. 02 i c.97 -0.0223 
1749+701 17 49 03.45582 0.00185 +7O 06 39.7025 0.0087 -0.06503 -0.0866 
3c 371 18 07 18.61013 0.00128 +65 48 57.1599 0.0050 -0.06517 -0.U616 
w-236 19 21 42.25904 0.00121 -29 .!D 26.3129 0.0132 -0.02579 -0.0520 
P 1933-400 19 33 51.15233 0.00709 -40 04 47.4184 0.0518 -0.02909 -0.0689 
Iv-198 19 58 04.62966 o.oou92 -17 57 16.9055 O.0142 -0.02272 -0.0604 
JW 637 20 21 13.34445 0.00104 -1-61 27 18.0444 0.0071 -0.04349 0.1094 
? 212'1-004 21 34 05.22134 0.00052 4-00 28 25.1264 0.0084 -0.01682 -0.0267 
? 2145-1'16 21 45 36.09253 0.0005ti +06 43 40.9133 0.0085 -0.01614 o.oooi 
IX-192 21 55 23.25580 0.ouo8i -15 15 30.0017 0.tJ130 -0.01588 -0.0941 
JRO 42.22.01 22 00 39.37976 u.00052 +42 02 08.4480 0.0046 -0.02010 0.1516 
:TA 102 22h3~m07:81548 usouo79 +11"28'22':7831 0': 0102 -0"01286 n'io2as 
IY-172.6 22 43 39.80282 0.00063 -12 22 40.2035 il.0097 -0.01113 -2.0912 
) 2245-328 22 43 51.51756 u.ouo94 -32 5i 44.2844 0.0127 -0.01342 -0.1852 
ic 454.3 22 51 29.52926 0.00047 +15 52 54.2989 0.0061 -0.01121 u.0549 
2345-16 23 45 27.686Yl O.OOU68 -16 47 52.4ti98 0.0102 -0.UO6U6 -0.1229 
Purcell, G. H., Cohen, E. J., Fanselow, J. L., Rogstad, D. H., Skjerve, L. J., Spitzmesser, D. J., and 
Thomas, J. B.; in F. V. Prochazka and R. H. Tucker (eds.), IAU Colloquium 48, University 
Observatory Vienna, Vienna, p. 185, 1978. 
Wade, C. M., and Johnston, K. J.; Astron. J., 82, 791, 1977. 


VLBI measurements of radio source positions at the Jet Propulsion Laboratory

VLBI measurements of radio source positions at the Jet Propulsion Laboratory

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