A series of experiments was conducted during the last decade to explore the capability of very long baseline interferometry (VLBI) to measure the crustal and rotational motions of the Earth with accuracies at the centimeter level. The observing stations are those of NASA's Deep Space Network in California, Spain and Australia. A multiparameter fit to the observed values of delay and delay rate yields radio source positions, polar motion, universal time, the precession constant, baseline vectors, and solid Earth tides. Source positions are obtained with formal errors of the order of 0''.01. UT1-UTC and polar motion are determined at 49 epochs, with formal error estimates for the more recent data of 0.5 msec for UT1-UTC and 2 to 6 mas for polar motion. Intercontinental baseline lengths are determined with formal errors of 5 to 10 cm. The Love numbers and Earth tide phase lag agree with the commonly accepted values

Fanselow, J. L.

Purcell, G. H., Jr.

Rogstad, D. H.

Sovers, O. J.

Thomas, J. B.

English

NASA Technical Reports Server

14324
TDA Progress Report 42-71 July - September 1982
Determination of Intercontinental Baselines and
Earth Orientation Using VLBI
O. J. Sovers, J. L. Fanselow, G. H. Purcell, Jr.,
D. H. Rogstad, and J. B. Thomas
Tracking Systems and Applications Section
A series of experiments has been conducted during the last decade to explore the
capability of very long baseline interferometry (VLBI) to measure the crustal and rota-
tional motions of the earth with accuracies at the centimeter level. The observing sta-
tions are those of NASA's Deep Space Network in California, Spain and Australia. A
multiparameter fit to the observed values of delay and delay rate yields radio source posi-
tions, polar motion, universal time; the precession constant, baseline vectors, and solid
earth tides. Source positions are obtained with formal errors of the order of 0".01. UT1-
UTC and polar motion are determined at 49 epochs, with formal error estimates (la) for
the more recent data of 0.5 msec for UT1-UTC and 2 to 6 mas for polar motion. Inter-
continental baseline lengths are determined with formal errors of 5 to 10 cm. The Love
numbers and earth tide phase lag agree with the commonly accepted values.
I. Introduction
Over the last few years, considerable progress has been
made toward realizing the potential capability of radio inter-
ferometry to measure the crustal and rotational motions of
the earth at the centimeter level (e.g., Refs. 1-5). Toward this
goal, a series of experiments with NASA's Deep Space Net-
work (DSN) antennas has been conducted over the last decade.
In all, 48 sessions have been carried out using 8 different an-
tennas on three continents. Delay and/or delay rate observ-
ables have been measured on two local baselines (at Goldstone,
California and at Madrid, Spain), on a transcontinental base-
line (California to Massachusetts, USA), and on two inter-
continental baselines (Goldstone to Madrid and to Canberra,
Australia). A multiparameter fit has been applied to these ob-
servables to extract astrometric and geophysical parameters.
These parameters include source positions, polar motion,
universal time, the precession constant, baseline vectors and
solid earth tides. This article summarizes the geophysical re-
sults obtained from the intercontinental measurements.
II. Interferometry Technique
In the present experiments, two separate interferometry
systems were employed. The prototype system used in the
early measurements recorded a single narrowband (24 kHz)
channel at S-band (2.3 GHz) and therefore could only measure
delay rate accurately (Ref. 6). To obtain measurements of de-
lay as well, a new system was developed and implemented in
1977 (Ref. 7). It records six time-multiplexed frequency chan-
nels to permit calculation of delay by bandwidth synthesis
https://ntrs.nasa.gov/search.jsp?R=19830006053 2020-03-21T04:54:42+00:00Z
(BWS), a technique pioneered by Rogers (Ref. 8). Three
2-MHz-wide channels are placed at S-band and three at X-band
(8.4 GHz) to allow dual-frequency calibration of charged-
particle delays. This new system can measure delay with a
precision (system noise error) of approximately 100 psec,
given a correlated source strength of 0.5 Jy, an integration
time of 3 minutes, a spanned bandwidth of 40 MHz, and two
64-m DSN antennas with system temperatures of 35 K.
III. Summary of Experiments
The need to optimize determination of each class of astro-
metric and geophysical parameters imposes stringent and
sometimes conflicting requirements on the design of a VLBI
observing schedule. Maximum sensitivity to polar motion and
UT1 requires concurrent or consecutive sessions (within 24-
48 hours) on the California/Spain baseline (essentially east/
west) and the California/Australia baseline (with a large north/
south component). Development of a reference frame requires
complete coverage of the sky and several observations of each
source during the period of mutual visibility for each pair of
stations. Establishing accurate positions of enough radio
objects to provide "nearby" reference points for navigating
interplanetary spacecraft, as well as for measurements of earth
orientation with only 3 hours of VLBI data in a subsequent
operational mode, requires approximately 100 sources. Obser-
vations which meet the above requirements have been carried
out between August 1971 and February 1980. In all, 117 ex-
tragalactic radio sources were observed in 48 sessions, which
ranged in length from 2 to 24 hours. Of the 2414 individual
observations, 692 were made at S-band only, 366 at X-band
only, and 1356 were dual-frequency. The observations in-
cluded 2399 measurements of delay rate and 2152 of delay.
IV. Model and Fit to Experimental Data
In considering the multitude of effects contributing to the
delay model for VLBI, it is helpful to group them as either
modeled or unmodeled effects, with the models containing
parameters that may be either adjusted or fixed at their
a priori values. Modeled quantities that were fixed at their
a priori values include nutation (Wahr series, Ref. 9) and the
effects of the ionosphere. Effects of ocean loading and plate
motion are unmodeled.
There are eight categories of modeled and adjusted param-
eters: station locations, tropospheric delays, clock offsets and
rates, polar motion and UT1, source positions, the precession
constant, the gamma factor of general relativity, and solid
earth tides. The solution provided a catalog of approximately
100 positions of radio sources with declinations ranging be-
tween -^40° and +70° (with formal error estimates of the order
of 0".01). A number of discontinuities and nonlinearities in
the station clocks were detected and modeled in many of the
sessions. There were 114 parameters describing delay due to
the troposphere; these were constrained to agree with the
Chao monthly mean model (Ref. 10) to within 3%. The long
span of data enables us to solve for the luni-solar precession
constant, in addition to UT1 and polar motion at 49 epochs.
A rotation about the polar axis of 2.37 milliarcsecond/year
(mas/yr) is included in the model to correct UT1 for the new
IAU expressions for Greenwich mean sidereal time and pre-
cession (Ref. 9). The BIH Circular D provides a reference point
for earth orientation: the values of both components of polar
motion and two values of UT1 (one for each intercontinental
baseline) are constrained on December 20-21, 1979, during
which period there were two intercontinental observing ses-
sions. In total, the fit to the 1971-1980 data includes 744 ad-
justed parameters.
The observables in the fit were weighted in inverse propor-
tion to the sum of squares of known random error sources,
plus an additional noise term which is adjusted to make chi-
square per degree of freedom equal to 1 for each session. The
software used to perform parameter estimation is based on the
square-root-information-filter method (Ref. 11) implemented
for the VAX 11/780 computer. Approximately 8 CPU hours
are required for a solution for the 1971-80 data. RMS residu-
als for the grand fit are 0.5 nsec for delay and 0.3 psec/sec for
delay rate. The resulting geophysical parameters are discussed
in some detail in the next section.
V. Results
A. Earth Orientation
The solution for universal time and polar motion produced
the average formal error estimates given in Table 1 as functions
of the date of observation. Clearly, the quality of the data im-
proved substantially as the interferometric system evolved,
with the 1980 data exhibiting formal uncertainties of 6 to
20cm.
Figures 1 and 2 show comparisons of our x and y polar
motion values for the latter part of the data (1977-80) with
BIH Circular D. Taking into account the 4 mas error of the
BIH data (Ref. 12), there are no outstanding discrepancies
with the possible exception of three points that differ by ~ 1.5
to 2a. For the UT1 results similarly plotted in Fig. 3, short-
period tidal fluctuations have been removed from the VLBI
data in order to permit comparison with the heavily smoothed
BIH values. The solid curve represents lunar laser ranging
(LLR) data as smoothed over a 15-day interval by Fliegel et al.
(Ref. 13). These data originally consisted of several hundred
points in the range of the plot.
Figure 3 shows that the UT1 values measured by VLBI,
LLR and BIH generally agree with one another if the BIH
values are assigned errors of approximately 2 msec, and the
LLR values errors of 1 msec or less. Both the VLBI and LLR
results suggest- the same oscillation of—2 msec amplitude
about the BIH values. The three points of large discrepancy
(~2 to 3a) between VLBI and LLR in February 1977 and
February 1980 require further investigation.
The value obtained for the luni-solar precession constant
is smaller than the 1976 IAU value by 3.7 ± 0.9 mas/yr. This
discrepancy may indicate the need for a revision of either the
precession constant or of the long-period (18.6-year) term in
the nutation series. Such an inference is supported by calcu-
lations in which the fit was repeated with a modified nutation
constant, and the precession constant was solved for. The rms
delay residual for the 1971-80 VLBI data reaches a minimum
when the amplitude of the 18.6-year term in the Wahr nuta-
tion series is increased by ~7 mas (to 9".210 in obliquity), at
which point the precession constant equals the 1976 IAU
value within its error estimate.
B. Baselines and Earth Tides
On the premise that our data, especially the 1971-1977 por-
tion, lack quality sufficient to detect plate motion, each sta-
tion was initially assumed to have a single location for the
entire span of data. Table 2 shows the lengths of the two inter-
continental baselines obtained from such fits in 1978 and at
present. Comparison of baseline lengths in this table is compli-
cated by the substantially different modeling used in the two
calculations (e.g., the 1978 fit did not use the Wahr nutation
series). In spite of the lack of separation of the effects of im-
proved data quality and improved modeling, Table 2 shows
the qualitative enhancement in system performance over the
past decade. The decrease by nearly a factor of 3 in the formal
baseline errors between 1978 and the most recent solution sug-
gests that future improved experiments will permit detection
of plate motion within the next few years
A useful test of internal consistency is to divide the data
into parts and to determine if fits to the parts lead to consis-
tent values for theoretically constant parameters. To test
baseline repeatability, the span of data was divided into two
portions containing nearly equal numbers of observations:
1971-78 (average epoch 1978.2) and 1979-80 (average epoch
1979.9). All of the data were again simultaneously fit, but the
Spanish and Australian 64-m antennas were each allowed to
have different locations in the two spans of data. As shown in
Table 3, the resulting baseline lengths for the two parts are in
good agreement.
The Spanish baseline errors in Table 3 again show the im-
provement of data quality with time. It is of interest to note
that, even though our results are consistent with no motion,
the differences for both the Spanish and Australian baselines
in Table 3 are more consistent with the motion inferred from
paleomagnetic data (Refs. 14, 15). For a 1.7-year span,
Morabito's application (Ref. 15) of Minster and Jordan's
model (Ref. 14) indicates changes of +4 and -6 cm respec-
tively for the Spanish and Australian baselines.
Assuming that the vertical and horizontal Love numbers
and the solid-earth-tide-phase lag values are identical at all sta-
tions we obtain 0.63 ± 0.03, 0.063 ± 0.017, and -1.7 ± 1.6
degrees respectively. These agree well with the commonly
accepted values of 0.603 - 0.611, 0.0832 - 0.0842, and
0 degrees (Refs. 16, 17).
VI. Conclusions
The radio intefferometric system under development at JPL
during the past decade has improved continually in quality.
By 1-980 it had reached the point where earth orientation
parameters could be determined with formal uncertainties of
6 to 20 cm, and source positions with formal uncertainties of
the order of 0".01. Formal uncertainties of 5 to 10 cm in
intercontinental baseline lengths promise detection of plate-
tectonic motion in the near future.
/
The present set of results represents very nearly our final
fit to the 1971-80 data. Minor changes in the model, such as
improved ephemerides and earth tide models, as well as adop-
tion of the J2000 system, will probably not have significant
repercussions.
References
1. Thomas, J. B., et al., "A Demonstration of an Independent-Station Radio Inter-
ferometry System with 4-cm Precision on a 16-km Baseline," /. Geophys. Res., 81,
pp. 995-1005, 1976.
2. Rogers, A. E. E., et al., "Geodesy by Radio Interferometry: Determination of a
1.24-km Base Line with 5-mm Repeatability," /. Geophys. Res., 83, pp. 325-334,
1978.
3. Ryan, J. W., et al., "Precision Surveying Using Radio Interferometry," / Surveying
and Mapping Division, Proc. ASCE, Vol. 104, No. SU1, pp. 25-34, 1978.
4. Niell, A. E., et al., "Comparison of a Radio Interferometric Differential Baseline
Measurement with Conventional Geodesy," Tectonophysics, 52, pp. 49-58,1979.
5. Herring, T. A., et al., "Geodesy by Radio Interferometry: Intercontinental Distance
Determinations with Subdecimeter Precision," /. Geophys. Res., 86, pp. 1647-1651,
1981.
6. Thomas, J. B., An Analysis of Long Baseline Radio Interferometry, Technical Report
32-1526, Volume VII, 37-50, Volume VIII, 29-38, Volume XVI, 47-64, Jet Propul-
sion Laboratory, Pasadena, Calif., 1972.
7. Thomas, J. B., An Analysis of Radio Interferometry with the Block 0 System, Publi-
cation 81-49, Jet Propulsion Laboratory, Pasadena, Calif., 1981.
8. Rogers, A. E. E., "Very Long Baseline Interferometry with Large-Effective Bandwidth
for Phase-Delay Measurements," Radio Science, 5, p. 1239,1970.
9. Kaplan, G. H., "The IAU Resolutions on Astronomical Constants, Time Scales, and
the Fundamental Reference Frame," USNO Circular No. 163, United States Naval
Observatory, Washington, D.C., 1981.
10. Chao, C. C., The Tropospheric Calibration Model for Mariner Mars 1971, Technical
Report 32-1587, pp. 61-76, Jet Propulsion Laboratory, Pasadena, Calif., 1974.
11. Bierman, G. J., Factorization Methods for Discrete Sequential Estimation, New
York, Academic Press, 1977.
12. Dickey, J. 0., "Analysis of Lageos Polar Motion Results Using Lunar Laser Ranging,"
EOS 62, p. 841, 1981.
13. Fliegel, H. F., Dickey, J.O., and Williams, J.G., "Intercomparison of Lunar Laser and
Traditional Determinations of Earth Rotation," IAU Colloquium Proc. No. 63,
O. Calame, ed., Grasse, France, 1981.
14. Minster, J,. B., and Jordan, T. H., "Present-Day Plate Motions,/. Geophys. Res., 83,
pp. 5331-5354, 1978.
15. Morabito, D. D., Claflin, E. S., and Steinberg, C. J., "VLBI Detection of Crustal Plate
Motion Using DSN Antennas as Base Stations," DSN Progress Report 42-56, pp.
59-75, Jet Propulsion Laboratory, Pasadena, Calif., 1980.
16. Wahr, J. M., "The Tidal Motions of a Rotating, Elastic and Oceanless Earth," pp.
162-171, Thesis, University of Colorado, 1977.
17. Lambeck, K., The Earth's Variable Rotation: Geophysical Causes and Consequences,
pp. 13, 111, Cambridge University Press, 1980.
ORIGINAL
Table 1. Earth orientation parameter formal errors
in fit to 1971-1980 VLB! data
1971-4
1977
1978
1979
1980
Polar motion,
X
...
10
10
6
6
mas
y
...
4
3
2
2
2.1
0.9
0.7
0.5
0.5
Table 2. Baseline length results from VLBI data
Date California toSpain Australia
1971-78 8 390 429.66 ± 0.16m
1971-80 8 390 429.84 ± 0.05m
10588965.85 ± 0.26 m
10588966.32 ± 0.11 m
Table 3. Baseline consistency test results
Average
date
California to
Spain Australia
1978.2 8 390 429.78 ± 0.09m
1979.9 8390429.85 ± 0.05m
10588966.36 ±0.11 m
10588966.27 ± O . l l m
Difference 7 ± 10 cm -9 ± 15 cm
ORIGINAL FA
OF POOR QUALITY
40
1 20
O
X 0
2 -20
x
1 ' T
CALIF - SPAIN
CALIF - SPAIN, AUSTRALIA
REFERENCE
DAY
n
-40 I I
1977 1978 1979 1980
YEAR
Fig. 1. Polar motion x-component results from 1977-1980 VLB! data
20
,0
o
> o
2 -10
I I
O CALIF - AUSTRALIA
T CALIF - SPAIN, AUSTRALIA
REFERENCE
±
-20L_L
1977 1978 1979 1980
YEAR
Fig. 2. Polar motion y-component results from 1977-1980 VLB! data
ORIGINAL
OF POOR QUAL5TV
6i r
• CALIF - SPAIN
O CALIF - AUSTRALIA
= -2
-4
-6
1977 1978 1979
YEAR
1980
Fig. 3. UT1 results from 1977-1980 VLB! data. Short-period tidal terms have been removed
to permit comparison with BIH values


Determination of intercontinental baselines and Earth orientation using VLBI

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server