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Comparison between the polar coordinates determined by different observational techniques
Pergamon Press. Printed in Great Britain 0275-1062/83$3.00+.00
Chin.Astron.Astrophys.7 (1983) 322-330 Act.Astron.Sin.24 (1983) 47-57
COMPARISON DIFFERENT
BETWEEN
THE POLAR
OBSERVATIONAL
COORDINATES
DETERMINED
BY
TECHNIQUES
GU Zhen-nian, WANG Shu-he, ZHENG Shanghai Observatory, Academia Sinica
Da-wei
Received 1982 August 21
ABSTRACT The main characteristics of several polar coordinate systems in present use are compared. The systematic differences between the classical technique (BIH 68) and the new technique (DMA and IASOM) are analysed in depth and their external errors estimated.
1.
INTRODUCTION
At present, organizations that provide polar motion parameters using classical astronomical observations are BIH, IPMS and ILS, the instruments used include VZT, PZT and ASTR, and there is no marked difference in the measuring accuracy of the various optical instruments, [l]. Since the sixties, new techniques using doppler and laser ranging of satellites and radio interferometershave appeared and been applied to the determination of the polar parameters and more and more used in astronomy and geodesy. It was predicted by some [2]that, compared to the classical methods, the new methods will raise the accuracy in the determination of the parameters of Earth's rotation by one order of magnitude, and this will provide most valuable data for the study of the fine details in the Earth's rotation and hence for the establishment of an accurate, adjusted Earth reference system. With the accumulation of data by both the classical and new techniques, we can now make valid comparisons between them and derive more realistic conclusions. In this paper, we compare the capabilities of the different systems in the determination of the polar drift, analyse the differences between the techniques and estimate their external precisions. This is important for the 1983 MERIT Main Program and for the future transition from the classical to the new techniques.
2.
DATA
The data used in this paper are the x,yvalues at 0.05 y given in the BIH and IPMS annals [3,4]. A summary of the data is shown in TABLE 1. BIH 68 is a purely astronomical solution, IPMSL is a pure
latitude solution, IPMSL+T is a combined latitude and time solution without the r-terms, and ILS is taken from [S]. All the series are homogeneized. There were some abnormal values in the IASOM series and all x- or y- values with errors greater than 3 times the mean measuring error were discarded.
3.
POLAR CHARACTERISTICSOBTAINED BY DIFFERENT TECHNI@lES
For the six series of TABLE 1, we made AR spectral estimates over the frequency range above one half-year. The results are shown in Fig. 1. For accuracy, we kept a constant data-span of 6 years, and moved year by year. The precise periods of the spectral peaks were estimated using the Househodev method, together with the amplitude A and phase 8. The results are shown in TABLES 2 - 5. There is good agreement among the four series BIH68, IPMSL, DMA and IASOM in the period, amplitude and phase of the Chandler wobble within each six-year interval. In the x-component, the four series give a mean period of 432.0 d, and a mean amplitude or 0.143", and in the y-component, 430.7 d and 0.140". AS TABLE 2 shows, the ILS result differs somewhat from the other series, particularly in the x-component, the Chandler period and amplitude are clearly smaller. This may have something to do with the error of determination of the polar coordinates of the ILS system consisting of only five stations. TABLE 3 gives the parameters of forced polar motion. Apart from the x-component in the BIH68, the annual terms given by the other series agree well with one another. In the x-component, the mean period of the four series is 364.5 d, and the mean amplitude is 0.116"; in the y-component, the\7
323
Polar Coordinates
T(d)
‘i”(d) Fig. la
TABLE 1
Fig. lb
Summary of Data
Technique Classical
System BIH68 IPMSL Ip=L+T
Doppler Laser
ILS DMA IASOM
Span of Data 1972.00-1981.95 1972.00- 1981.95 1972.00-1978.95 1972.00-1978.95 1972.00- 1981.95 1976.35- 1981.95 x
W.0.Y KU6
HIH,.-DMA
T’(d)
Fig. 2a
Note unsmoothed values smoothed values smoothed values smoothed values unsmoothed values unsmoothed values
TABLE
2
-
-
-
-
I35.G
1X2.2
0
137.6
143.0
106.3
A
9
--
__
-i
-
142.0
31i.v
5 321.8
li6.2
iiy.
152.1
1
317.
124.6
,122.
424.5
---
410.6
P
-~
--
114.1
412.1
406.3
l3Y.d
A
4V.6
50.0
$41.1
435.2
1OS.Y
140.6
431.11
nean
--
431.6
Q
-
IASO
--
434.8
.-
I’
1_-
52.7 44.%
143.3
137.6
A
Q
432.7
110.3
141.1
131.6
436.9
.-
-
32 component
Term
P
110.0
--
431.5
430.0
P
--
lK7.I
-_
435.3
137.8
.-
,-
--
I1
WhlS,,
433.8
IIIH,,
-
-
P
I-
A
-
- -
The Chandler
-.“-
77.5
123.6
417.1
16G.n
111,l
426.8
--
11s
108.i
139.7
4311.5
184.2
li9.2
433.7
IPMS‘+,
-
--
.-
._
I
12.1
151.3
426.i
86.4
141.
431.4
l5S.i)
I39,O
433.1
219.5
137.2
^I
__
__
II
__
-
-
--
--
--
-1
--
- --
-426.0
2SY. I
134.0
16O.C
160.2
153.9 12.8
1W.B Id.4
$27.7
64.6
140.6
429.3
13.1
lil,2
426.0
-.- __-.--
84.4
140.2
430.3
~_
1%. I
13Y.6
I__~
133.6
433.0
221.6
135.9
219.0
II3
I ii
--
1_
--
.-
212.5
132.2
427-s
ZM.1
126.1
_.,_^_. -. 431.‘5 4M.5
138.2
--
.-_
_-
Imean
451.6
-__-
-
IASOM
430.9
--
28S.O
i3i.j
432.0
InI*
y component
630 Phase in days.
430.9
T-
TABLES 2-5. P = Period in days. A = Amplitude in 0XxX. Asterisks mark CBS~S of phase ffuctuatioIu3
--_
220.2
130.2
433.9
291.1
i31.J
432.3
PMsr+l
I
l981.95
1
1976.00
.-__----~
I9SO.95
197f.00
--p
1979.95
I
1914.nn
1979.93
1973.00
---
1977.05
I
1972.no
TABLE
I
I
17O.'J
-
Ill.6
e
362.4
183.4
I29.!J
366.5
153.4
12.9
.I
P
B
.I
'
0
.f
365.1
366.1
f’
178.0
95.3
360.5
-
179.6
q7.4
-
36il.5
11~1.1 Ifii.8
_-
115.1
361.6
--_
--
_-
lXG.i
_-
--
IS.6
liti.
364.2
I', 1.s
18h.3
8
-_
118.1
132.6
.I
-_
197.4
366.0
-_
19'j.O
191.0
106.9
X6.0
367. j
P
" 0
110.8
_-
-
178.1
‘I3.i
35R.3
IASOM
-
.-
_-
_-
--
-
176.6
99.4
iGO.
185.9
119.7
365.4
l8Y.v
123.1
366.8
195.8
113.8
367.1
mean
x component f>hlA
Term
-_
3Gi.S
--
IPLIS‘
-
X7.3
-
Annual
123.7
f
I
I’
The
3
-
_
_-
-
183.2
iOG.7
3Gi.1
192.3
95.2
3G6.2
11.5
209.3
IO2.5
3G'J.5
258.4
84.1
35s.2
273.7
96.9
364.1
~_
280.2
103.5
365.9
2SO.I
105.4
365.9
--
284.3
1111. i
166.4
-.-
illtl,,
280.1
97.9
.363.8
x3.3
107.2
364.7
2s3.s
106.6
3G5.2
287.1
106.5
363.6
il’MS#.
-
271.2
85.1
359.2
- .__ --
--
--
-
285.7
to4.3
366.4
2S2.5
103.9
363.5
.-
260.9
80.3
i59.l
280.7
97.4
365.5
.- -.-
.-
.-
286.2
101.6
262.1
al.0
359.2
--
I
.
I
282.B
105.3
363.5
283.9
103.
~66 b
mean
98.1
263.2
04.6
iU.9
278.2
-
-.
_-
2i.J.9
-
-_.
---
--
287.i
102.4
3b5.9
- -.
INl.4
1R8.7
101.8
366.4
~PM&.*r
I--
366.9
276.0
94.1
1
-1 36G.f
iI.s
-.-__-- --. _-
-i-
I 36i.7 I 1 :;;:: __-’ 364.5
_.__~__
~~-
---
ASOM
y component
._
366.0
_---
-
1976.00 / 1981.95
*9*d.Y5
1975.00
1979.95
I
lY74.00
1973.00 I 1978.95
1472.00 I 1977.95
SPan
TABLE
120.8
-
_-
123.4
e
250. I
10.1
_-
239.5
11.1
P
224.9
242.3
9
A
-
10.3
10.3
A
-
1. 9
7.9
A
e -
Z6)c.h
P
-
221.9
6.1
X6.9
251.9
-
0.1
S.6
275.7
216.6
10.9
10.9
e -
283.9
292.9
Y2.8
12.2
275. I
248.8
12.6
7.7u.u
167.8
16.3
270.0
DMA
-
-
_--
-
-- -
-
i
,51.2
8.8
2’34.5
__-
,A SOM
- -
263.3
16.7
26Y.3
168.3
21.0
270.0
1,s
t
_-
-
231.5
10.4
266.3
145.4
Il.5
266.6
IPMS,,
_-
T
I i
254.7
i.G
?%.*I
265.5
6.8
286.6
154.9
9.7
277.9
24.2
7.0
268.2
212.2
6.0
269.6
IiIH,,
-
-_
_-
-_
--
20.x
5.7
2GG.U
2ll.i
6.6
280.3
132.7
6.S
274.3
21.9
9.1
2i2.3
206.4
9.7
270.3
Il’MSL
-- ---
---l-71.3*
7.s
273.%
236.i
10.4
288.7
112.3
11.1
278.2
238.7
11.1
268.0
---
161.5
14.1
269.1
mean
1
2 component
P
.-
.-
--
-
Term
A
_- -
--
266.6
267.3
P
-- --.
150.9
166.1
--
12.6
13.3
e -- -
268.0
269.3
lPt&,
P
_-
-
A
-- -
I-
The 0.75-year
- -
4
--
DSfA
i
~
_-
_._
--
--
--
-
!
__
I ASOX
143.8
9.2
276.1
--
24.6
8.0
270.2
209.3
8.9
270.0
mean
137.x*
5.2
261.4
253.4
b.i
2s3.i
.- --
.-
.-
- --
y component I LS
--
-.
X.1
8.1
274.3
221.4
7.9
272.L
$ -
I
I.
-_
-_--
Il’hlSLtT
-___
Polar
Coordinates
327
< i 3 /
7
I
L
L -
I _i-
(
I’
I‘rnq ”
GU
328
o’0,rDMrl
et al.
1450MI
T(d)
Fig. 2b
are 364.2 d and 0.098". TABLE 5 shows that, for the period, amplitude and phase of the semi-annual term, only the x-component of the BIH68 series gives stable values, the others, give rather dispersed values. We should point out here that the x-component of the BIH68 series gives consistently larger annual and semi-annual amplitudes over different six-year intervals than the other series. For this, we also show the result from IPMSL+T in the table, and we note the BIH68 result is again the larger one. This shows that it is the BIH68 that led to the deviations in the x-component. TABLE 4 gives some results on the 0.75 y term. In the x-component, its mean amplitude may reach 0.011". Graber made maximumentropy estimates from the IPMS, BIH68 and DMA data [6], and found all three series to contain a periodic motion with frequency 1.3 cycles per year and amplitude one-tenth of the Chandler term. Graber believes this period to be real. The results over the various six-year intervals in TABLE 4 give rather unstable values for its period and phase, and in the y-component in the DMA and IASOM, this term is not found at all. Hence we think that the question whether there exists a 0.75-y period in the polar motion remains to be investigated.
4.
DIFFERENCES BETWEEN THE VARIOUS SYSTEMS
Fig. 2 shows the differences in the polar coordinates in the years 1976.35- 1981.95 among the three systems based on different techniques, BIH68, DMA and IASOM. The yearly mean differences are given in TABLE 6, the last row gives the overall average and the
degree of relative long-term stability. The numbers show that all three series have a stability better than O.Oln, and that, in the x-component, BIH68 and IASOM both differ from DMA by the same constant amount of about 0.017". Curves of fit calculated from the AR spectral estimates of the three different series are added in Fig. 2 (solid curves). Results for components with amplitudes greater than 0.005" are shown in TABLE 7. Here, B is reckoned from 1976.0 and the last row shows the results BIH68- BIH79 given in the BIH Annals. From TABLE 7 and Fig. 2, we see that, in the x-component, periods of a year and of half a year exist in the two difference series BIH68- DMA and BIH68- IASOM. Their amplitudes are both slightly larger than the values given in the BIH Annals, the phase of the annual term also differs by over half a month. In addition, in the x-component, there exists a systematic periodic difference between the BIH68 and the new techniques with a period slightly longer than the Chandler period and an amplitude of 0.008". Clearly, these periodic systematic errors originate in the BIH68 system. One particular feature in Fig. 2 should be particularly pointed out: the two difference series BIH68- DMA and BIH68- IASOM (and their fitting curves) both show a peak at around 1979.0 with an amplitude of about 0.02" in both x and y, whereas no feature is seen at this point in the DMA- IASOM series. Apparently, the discrepancy comes from the BIH68 series. This shows that, the classical and the new techniques may give discordant results regarding detailed local variations in the polar motion and the
329
Polar Coordinates
TABLE 6
Yearly Averages of the Differences Among Three Systems
-I----5 co~onent
y component
Year
IIIH,,
1978 1977 1976
-
27.6 21.; 10.5
llY‘\
61H,s -
DHA
IASOM
’
.z
l.%SOhi
-5.2 3.h 1.6
I%,,\
,
-23.3 -21.3 -21.1
-0.3 -10.0 3.B
-9.2 -1O.f -1I.T
-
1.cxm
-0.7 14.3 1.:
--~
mean
'
17.6*6.9
TABLE 7
l.l&-i.3
- I6.4&lj.4
-5.l*t”.7
2.c+9.3
Periodic Systematic Differences Between Three Systems
3: component i
DNA
-
I'
.1
345.3
I
6.2
:!component
I’
0
7l.V
NM,,
TABLE 8
--
365.2
24.0
182.6
7.0
~
I
7.7
389. I -~
-
,I
605.7
iAScni ~-. NH.,
6.Rk6.4
fl
225.4
a.3
$4.8
.-
-___
124.9 -14.3
Measuring Errors of Three Pclar Coordinate Systems 3: component (O!'OOl) y component (O!'OOl) iIU.6
+7.:
46.4
+I).0
discordance is related to the differing measuring accuracy and reduction methods.
5.
+9.0
+i.7
ESTIMATES OF ACCURACY
Since BIH68, DMA and IASOM are three independent polar coordinate systems, their measuring errors are also independent. Let EMB, EMD, EMI be the mean errors of the three systems and let EMB,D, EMB_I, EMD_I be the mean errors in the respective difference
series after deducting Clearly, we have
the
systematic
errors.
EM; - (EM%+ -IEML, -EM&I)/? EM: - (EML + EM:_,- EM;&~ EM:- (EML c EM:_,- Eht;J/z The results calculated accordingly are given in TABLE 8. They show that, after
development over lo-odd years, the systems based on the new techniques now have a
330
GU et
better measuring accuracy than the classical technique systems with numerous instruments. When setting up a more accurate Earth coordinate reference and measuring and ivestigating fine details in the physics of the polar motion we think that the new technique system should be given a weight not less than the classical technique system.
6.
CONCLUSIONS
Intercomparison of the polar parameters determined by different techniques is an important means for the study of the dynamics of the Earth and provides some useful data for perfecting the international service on the Earth’s rotation in future. From the foregoing analysis and comparison, we have arrived at the following conclusions. There are no marked differences 1. in the determination of the main features of the polar motion between the classical and the new techniques. But in the determination of the fine details, then we must pay attention to the differing degrees of agreement achieved. Clear systematic differences exist 2. in the r-component between BIH68, DMA and from IASOM. Both BIH68 and IASOM differ DMA by an approximately constant amount of Between BIH68 and DMA, and between 0.017”.
al.
BIH68 and IASOH, systematic differences with annual and semi-annual periods exist. The amplitudes are 0.025” and 0.010” for the first pair, and 0.030” and 0.010” for the second pair. We also discovered a period slightly longer than the Chandler period, having an amplitude of 0.008” in the differences. These differences come from BIH68 system. In the determination of the polar 3. coordinates, the new techniques have a better external precision than the classical technique. It is very necessary to develop further and to perfect the new techniques. We suggest that in the MERIT Main Period of Joint Observations beginning in 1983 and in the future international polar service, besides taking into consideration the systematic differences between the various systems, we should increase the weight of the new technique system. According to our analysis of external precision, the weight for the new system should not be less than the total weight given to the classical system. ACKNOWLEDGEMENT We sincerely thank Director YE Shu-hua for her concern and guidance in We also thank Colleagues JIN this work. Wen- jing and ZHAO Ming for many helpful discussions,
REFERENCES
(continued
from
p.3211
smooth fluctuations. This is because the oversimplified assumptions of a sharp inner boundary at ra and of an absolutely homogeneous core. If the correctness of the present 6. non-homogeneity mechanism can be further confirmed, then we should be able to
determine a set of optimum values for the parameters R/q, y2/y1, Z, m, n when fitting the calculated curves to the observed spectra. Such parameter values will no doubt contribute to our understanding of the physics of the sources.
Chin.Astron.Astrophys.7 (1983) 322-330 Act.Astron.Sin.24 (1983) 47-57
COMPARISON DIFFERENT
BETWEEN
THE POLAR
OBSERVATIONAL
COORDINATES
DETERMINED
BY
TECHNIQUES
GU Zhen-nian, WANG Shu-he, ZHENG Shanghai Observatory, Academia Sinica
Da-wei
Received 1982 August 21
ABSTRACT The main characteristics of several polar coordinate systems in present use are compared. The systematic differences between the classical technique (BIH 68) and the new technique (DMA and IASOM) are analysed in depth and their external errors estimated.
1.
INTRODUCTION
At present, organizations that provide polar motion parameters using classical astronomical observations are BIH, IPMS and ILS, the instruments used include VZT, PZT and ASTR, and there is no marked difference in the measuring accuracy of the various optical instruments, [l]. Since the sixties, new techniques using doppler and laser ranging of satellites and radio interferometershave appeared and been applied to the determination of the polar parameters and more and more used in astronomy and geodesy. It was predicted by some [2]that, compared to the classical methods, the new methods will raise the accuracy in the determination of the parameters of Earth's rotation by one order of magnitude, and this will provide most valuable data for the study of the fine details in the Earth's rotation and hence for the establishment of an accurate, adjusted Earth reference system. With the accumulation of data by both the classical and new techniques, we can now make valid comparisons between them and derive more realistic conclusions. In this paper, we compare the capabilities of the different systems in the determination of the polar drift, analyse the differences between the techniques and estimate their external precisions. This is important for the 1983 MERIT Main Program and for the future transition from the classical to the new techniques.
2.
DATA
The data used in this paper are the x,yvalues at 0.05 y given in the BIH and IPMS annals [3,4]. A summary of the data is shown in TABLE 1. BIH 68 is a purely astronomical solution, IPMSL is a pure
latitude solution, IPMSL+T is a combined latitude and time solution without the r-terms, and ILS is taken from [S]. All the series are homogeneized. There were some abnormal values in the IASOM series and all x- or y- values with errors greater than 3 times the mean measuring error were discarded.
3.
POLAR CHARACTERISTICSOBTAINED BY DIFFERENT TECHNI@lES
For the six series of TABLE 1, we made AR spectral estimates over the frequency range above one half-year. The results are shown in Fig. 1. For accuracy, we kept a constant data-span of 6 years, and moved year by year. The precise periods of the spectral peaks were estimated using the Househodev method, together with the amplitude A and phase 8. The results are shown in TABLES 2 - 5. There is good agreement among the four series BIH68, IPMSL, DMA and IASOM in the period, amplitude and phase of the Chandler wobble within each six-year interval. In the x-component, the four series give a mean period of 432.0 d, and a mean amplitude or 0.143", and in the y-component, 430.7 d and 0.140". AS TABLE 2 shows, the ILS result differs somewhat from the other series, particularly in the x-component, the Chandler period and amplitude are clearly smaller. This may have something to do with the error of determination of the polar coordinates of the ILS system consisting of only five stations. TABLE 3 gives the parameters of forced polar motion. Apart from the x-component in the BIH68, the annual terms given by the other series agree well with one another. In the x-component, the mean period of the four series is 364.5 d, and the mean amplitude is 0.116"; in the y-component, the\7
323
Polar Coordinates
T(d)
‘i”(d) Fig. la
TABLE 1
Fig. lb
Summary of Data
Technique Classical
System BIH68 IPMSL Ip=L+T
Doppler Laser
ILS DMA IASOM
Span of Data 1972.00-1981.95 1972.00- 1981.95 1972.00-1978.95 1972.00-1978.95 1972.00- 1981.95 1976.35- 1981.95 x
W.0.Y KU6
HIH,.-DMA
T’(d)
Fig. 2a
Note unsmoothed values smoothed values smoothed values smoothed values unsmoothed values unsmoothed values
TABLE
2
-
-
-
-
I35.G
1X2.2
0
137.6
143.0
106.3
A
9
--
__
-i
-
142.0
31i.v
5 321.8
li6.2
iiy.
152.1
1
317.
124.6
,122.
424.5
---
410.6
P
-~
--
114.1
412.1
406.3
l3Y.d
A
4V.6
50.0
$41.1
435.2
1OS.Y
140.6
431.11
nean
--
431.6
Q
-
IASO
--
434.8
.-
I’
1_-
52.7 44.%
143.3
137.6
A
Q
432.7
110.3
141.1
131.6
436.9
.-
-
32 component
Term
P
110.0
--
431.5
430.0
P
--
lK7.I
-_
435.3
137.8
.-
,-
--
I1
WhlS,,
433.8
IIIH,,
-
-
P
I-
A
-
- -
The Chandler
-.“-
77.5
123.6
417.1
16G.n
111,l
426.8
--
11s
108.i
139.7
4311.5
184.2
li9.2
433.7
IPMS‘+,
-
--
.-
._
I
12.1
151.3
426.i
86.4
141.
431.4
l5S.i)
I39,O
433.1
219.5
137.2
^I
__
__
II
__
-
-
--
--
--
-1
--
- --
-426.0
2SY. I
134.0
16O.C
160.2
153.9 12.8
1W.B Id.4
$27.7
64.6
140.6
429.3
13.1
lil,2
426.0
-.- __-.--
84.4
140.2
430.3
~_
1%. I
13Y.6
I__~
133.6
433.0
221.6
135.9
219.0
II3
I ii
--
1_
--
.-
212.5
132.2
427-s
ZM.1
126.1
_.,_^_. -. 431.‘5 4M.5
138.2
--
.-_
_-
Imean
451.6
-__-
-
IASOM
430.9
--
28S.O
i3i.j
432.0
InI*
y component
630 Phase in days.
430.9
T-
TABLES 2-5. P = Period in days. A = Amplitude in 0XxX. Asterisks mark CBS~S of phase ffuctuatioIu3
--_
220.2
130.2
433.9
291.1
i31.J
432.3
PMsr+l
I
l981.95
1
1976.00
.-__----~
I9SO.95
197f.00
--p
1979.95
I
1914.nn
1979.93
1973.00
---
1977.05
I
1972.no
TABLE
I
I
17O.'J
-
Ill.6
e
362.4
183.4
I29.!J
366.5
153.4
12.9
.I
P
B
.I
'
0
.f
365.1
366.1
f’
178.0
95.3
360.5
-
179.6
q7.4
-
36il.5
11~1.1 Ifii.8
_-
115.1
361.6
--_
--
_-
lXG.i
_-
--
IS.6
liti.
364.2
I', 1.s
18h.3
8
-_
118.1
132.6
.I
-_
197.4
366.0
-_
19'j.O
191.0
106.9
X6.0
367. j
P
" 0
110.8
_-
-
178.1
‘I3.i
35R.3
IASOM
-
.-
_-
_-
--
-
176.6
99.4
iGO.
185.9
119.7
365.4
l8Y.v
123.1
366.8
195.8
113.8
367.1
mean
x component f>hlA
Term
-_
3Gi.S
--
IPLIS‘
-
X7.3
-
Annual
123.7
f
I
I’
The
3
-
_
_-
-
183.2
iOG.7
3Gi.1
192.3
95.2
3G6.2
11.5
209.3
IO2.5
3G'J.5
258.4
84.1
35s.2
273.7
96.9
364.1
~_
280.2
103.5
365.9
2SO.I
105.4
365.9
--
284.3
1111. i
166.4
-.-
illtl,,
280.1
97.9
.363.8
x3.3
107.2
364.7
2s3.s
106.6
3G5.2
287.1
106.5
363.6
il’MS#.
-
271.2
85.1
359.2
- .__ --
--
--
-
285.7
to4.3
366.4
2S2.5
103.9
363.5
.-
260.9
80.3
i59.l
280.7
97.4
365.5
.- -.-
.-
.-
286.2
101.6
262.1
al.0
359.2
--
I
.
I
282.B
105.3
363.5
283.9
103.
~66 b
mean
98.1
263.2
04.6
iU.9
278.2
-
-.
_-
2i.J.9
-
-_.
---
--
287.i
102.4
3b5.9
- -.
INl.4
1R8.7
101.8
366.4
~PM&.*r
I--
366.9
276.0
94.1
1
-1 36G.f
iI.s
-.-__-- --. _-
-i-
I 36i.7 I 1 :;;:: __-’ 364.5
_.__~__
~~-
---
ASOM
y component
._
366.0
_---
-
1976.00 / 1981.95
*9*d.Y5
1975.00
1979.95
I
lY74.00
1973.00 I 1978.95
1472.00 I 1977.95
SPan
TABLE
120.8
-
_-
123.4
e
250. I
10.1
_-
239.5
11.1
P
224.9
242.3
9
A
-
10.3
10.3
A
-
1. 9
7.9
A
e -
Z6)c.h
P
-
221.9
6.1
X6.9
251.9
-
0.1
S.6
275.7
216.6
10.9
10.9
e -
283.9
292.9
Y2.8
12.2
275. I
248.8
12.6
7.7u.u
167.8
16.3
270.0
DMA
-
-
_--
-
-- -
-
i
,51.2
8.8
2’34.5
__-
,A SOM
- -
263.3
16.7
26Y.3
168.3
21.0
270.0
1,s
t
_-
-
231.5
10.4
266.3
145.4
Il.5
266.6
IPMS,,
_-
T
I i
254.7
i.G
?%.*I
265.5
6.8
286.6
154.9
9.7
277.9
24.2
7.0
268.2
212.2
6.0
269.6
IiIH,,
-
-_
_-
-_
--
20.x
5.7
2GG.U
2ll.i
6.6
280.3
132.7
6.S
274.3
21.9
9.1
2i2.3
206.4
9.7
270.3
Il’MSL
-- ---
---l-71.3*
7.s
273.%
236.i
10.4
288.7
112.3
11.1
278.2
238.7
11.1
268.0
---
161.5
14.1
269.1
mean
1
2 component
P
.-
.-
--
-
Term
A
_- -
--
266.6
267.3
P
-- --.
150.9
166.1
--
12.6
13.3
e -- -
268.0
269.3
lPt&,
P
_-
-
A
-- -
I-
The 0.75-year
- -
4
--
DSfA
i
~
_-
_._
--
--
--
-
!
__
I ASOX
143.8
9.2
276.1
--
24.6
8.0
270.2
209.3
8.9
270.0
mean
137.x*
5.2
261.4
253.4
b.i
2s3.i
.- --
.-
.-
- --
y component I LS
--
-.
X.1
8.1
274.3
221.4
7.9
272.L
$ -
I
I.
-_
-_--
Il’hlSLtT
-___
Polar
Coordinates
327
< i 3 /
7
I
L
L -
I _i-
(
I’
I‘rnq ”
GU
328
o’0,rDMrl
et al.
1450MI
T(d)
Fig. 2b
are 364.2 d and 0.098". TABLE 5 shows that, for the period, amplitude and phase of the semi-annual term, only the x-component of the BIH68 series gives stable values, the others, give rather dispersed values. We should point out here that the x-component of the BIH68 series gives consistently larger annual and semi-annual amplitudes over different six-year intervals than the other series. For this, we also show the result from IPMSL+T in the table, and we note the BIH68 result is again the larger one. This shows that it is the BIH68 that led to the deviations in the x-component. TABLE 4 gives some results on the 0.75 y term. In the x-component, its mean amplitude may reach 0.011". Graber made maximumentropy estimates from the IPMS, BIH68 and DMA data [6], and found all three series to contain a periodic motion with frequency 1.3 cycles per year and amplitude one-tenth of the Chandler term. Graber believes this period to be real. The results over the various six-year intervals in TABLE 4 give rather unstable values for its period and phase, and in the y-component in the DMA and IASOM, this term is not found at all. Hence we think that the question whether there exists a 0.75-y period in the polar motion remains to be investigated.
4.
DIFFERENCES BETWEEN THE VARIOUS SYSTEMS
Fig. 2 shows the differences in the polar coordinates in the years 1976.35- 1981.95 among the three systems based on different techniques, BIH68, DMA and IASOM. The yearly mean differences are given in TABLE 6, the last row gives the overall average and the
degree of relative long-term stability. The numbers show that all three series have a stability better than O.Oln, and that, in the x-component, BIH68 and IASOM both differ from DMA by the same constant amount of about 0.017". Curves of fit calculated from the AR spectral estimates of the three different series are added in Fig. 2 (solid curves). Results for components with amplitudes greater than 0.005" are shown in TABLE 7. Here, B is reckoned from 1976.0 and the last row shows the results BIH68- BIH79 given in the BIH Annals. From TABLE 7 and Fig. 2, we see that, in the x-component, periods of a year and of half a year exist in the two difference series BIH68- DMA and BIH68- IASOM. Their amplitudes are both slightly larger than the values given in the BIH Annals, the phase of the annual term also differs by over half a month. In addition, in the x-component, there exists a systematic periodic difference between the BIH68 and the new techniques with a period slightly longer than the Chandler period and an amplitude of 0.008". Clearly, these periodic systematic errors originate in the BIH68 system. One particular feature in Fig. 2 should be particularly pointed out: the two difference series BIH68- DMA and BIH68- IASOM (and their fitting curves) both show a peak at around 1979.0 with an amplitude of about 0.02" in both x and y, whereas no feature is seen at this point in the DMA- IASOM series. Apparently, the discrepancy comes from the BIH68 series. This shows that, the classical and the new techniques may give discordant results regarding detailed local variations in the polar motion and the
329
Polar Coordinates
TABLE 6
Yearly Averages of the Differences Among Three Systems
-I----5 co~onent
y component
Year
IIIH,,
1978 1977 1976
-
27.6 21.; 10.5
llY‘\
61H,s -
DHA
IASOM
’
.z
l.%SOhi
-5.2 3.h 1.6
I%,,\
,
-23.3 -21.3 -21.1
-0.3 -10.0 3.B
-9.2 -1O.f -1I.T
-
1.cxm
-0.7 14.3 1.:
--~
mean
'
17.6*6.9
TABLE 7
l.l&-i.3
- I6.4&lj.4
-5.l*t”.7
2.c+9.3
Periodic Systematic Differences Between Three Systems
3: component i
DNA
-
I'
.1
345.3
I
6.2
:!component
I’
0
7l.V
NM,,
TABLE 8
--
365.2
24.0
182.6
7.0
~
I
7.7
389. I -~
-
,I
605.7
iAScni ~-. NH.,
6.Rk6.4
fl
225.4
a.3
$4.8
.-
-___
124.9 -14.3
Measuring Errors of Three Pclar Coordinate Systems 3: component (O!'OOl) y component (O!'OOl) iIU.6
+7.:
46.4
+I).0
discordance is related to the differing measuring accuracy and reduction methods.
5.
+9.0
+i.7
ESTIMATES OF ACCURACY
Since BIH68, DMA and IASOM are three independent polar coordinate systems, their measuring errors are also independent. Let EMB, EMD, EMI be the mean errors of the three systems and let EMB,D, EMB_I, EMD_I be the mean errors in the respective difference
series after deducting Clearly, we have
the
systematic
errors.
EM; - (EM%+ -IEML, -EM&I)/? EM: - (EML + EM:_,- EM;&~ EM:- (EML c EM:_,- Eht;J/z The results calculated accordingly are given in TABLE 8. They show that, after
development over lo-odd years, the systems based on the new techniques now have a
330
GU et
better measuring accuracy than the classical technique systems with numerous instruments. When setting up a more accurate Earth coordinate reference and measuring and ivestigating fine details in the physics of the polar motion we think that the new technique system should be given a weight not less than the classical technique system.
6.
CONCLUSIONS
Intercomparison of the polar parameters determined by different techniques is an important means for the study of the dynamics of the Earth and provides some useful data for perfecting the international service on the Earth’s rotation in future. From the foregoing analysis and comparison, we have arrived at the following conclusions. There are no marked differences 1. in the determination of the main features of the polar motion between the classical and the new techniques. But in the determination of the fine details, then we must pay attention to the differing degrees of agreement achieved. Clear systematic differences exist 2. in the r-component between BIH68, DMA and from IASOM. Both BIH68 and IASOM differ DMA by an approximately constant amount of Between BIH68 and DMA, and between 0.017”.
al.
BIH68 and IASOH, systematic differences with annual and semi-annual periods exist. The amplitudes are 0.025” and 0.010” for the first pair, and 0.030” and 0.010” for the second pair. We also discovered a period slightly longer than the Chandler period, having an amplitude of 0.008” in the differences. These differences come from BIH68 system. In the determination of the polar 3. coordinates, the new techniques have a better external precision than the classical technique. It is very necessary to develop further and to perfect the new techniques. We suggest that in the MERIT Main Period of Joint Observations beginning in 1983 and in the future international polar service, besides taking into consideration the systematic differences between the various systems, we should increase the weight of the new technique system. According to our analysis of external precision, the weight for the new system should not be less than the total weight given to the classical system. ACKNOWLEDGEMENT We sincerely thank Director YE Shu-hua for her concern and guidance in We also thank Colleagues JIN this work. Wen- jing and ZHAO Ming for many helpful discussions,
REFERENCES
(continued
from
p.3211
smooth fluctuations. This is because the oversimplified assumptions of a sharp inner boundary at ra and of an absolutely homogeneous core. If the correctness of the present 6. non-homogeneity mechanism can be further confirmed, then we should be able to
determine a set of optimum values for the parameters R/q, y2/y1, Z, m, n when fitting the calculated curves to the observed spectra. Such parameter values will no doubt contribute to our understanding of the physics of the sources.