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MEMORANDUM
On the optimization of the RadioAstron mission
by using advanced observing methods
at ground radio telescopes and tracking stations,
and the advantages of using on-board H-maser frequency standard
and on-board accelerometer
Astro Space Center, Moscow, June 2003
1. Objectives
One
of the main peculiarities of the RadioAstron mission is its high-apogee orbit
where the spacecraft is moving at distances much greater than the Earth
diameter during the significant portion of its rotation period. Except of more
higher angular resolution a such situation has a number of peculiarities in
comparison with low-orbit SVLBI missions: the spacecraft is moving smoothly
because of much weaker effects of Earth atmosphere and irregularities of Earth
gravitation field, high altitudes provide more stable thermal regime
(illumination by the Sun), more convenient condition to use reference sources;
easier access for tracking station at lower tracking velocities; slow pass
through the UV-cells.
All
these peculiarities of high-orbit SVLBI can be used to increase the sensitivity
(longer possible integration time for a given UV-point) and the accuracy of
measurements of position of radio sources. High accuracy measurements of
spacecraft position (or orbit parameters) are necessary to materialize these
possibilities.
Calculated
sensitivity of space-ground interferometer with the upgraded VLA at 18-25 GHz
( l = 12 – 17 mm) for
integration time of 300 s (no loss of coherence) is about s ~ 3 mJy with the fringe size at
maximum base line B = 350 000 km of
7.1 е 10 microarcseconds. At the SNR of about 30 it would
provide the accuracy of position measurements of about 0.2ч0.3 μas.
In
this Memo we consider the advanced methods of ground radio telescopes and
tracking stations (TS) operation, on-board H-maser frequency standard (SHM -
already planned for the mission) and high-accuracy on-board accelerometer
applications to solve this objective.
2.
Sensitivity
The
sensitivity of space-ground interferometer is defined by the parameters of
space (SRT) and ground (GRT) radio telescopes under the condition of high
precision time and frequency synchronization (see for ex. [1-3])
s = (Tsys1 Tsys2)0.5(C
hs)–1
(2 Dν t K1 K2)-0.5
Jy,
where
K1,2 = ( h1,.2 А1,2) (2e)-1, h1,2 – aperture efficiency of antennae, А1,2 – theirs geometric area, hs –
recorder/processor efficiencies (one/two-bit sampling, fringe rate rotation
sampling, etc.,), Dν – recorded bandwidth, t - coherent integration time (fringe detection), N –
efficiency of phase coherence. Under the assumption of Gauss noise spectrum of
phase fluctuations the loss of coherence can be expressed as:
N ≈ 1 – 0.5 so2,
where
so is
the RMS value of residual phase fluctuations during the integration time (N ≡ 1
for hundred-per-cent coherence).
The
value of so is determined by the phase instabilities in all
elements of space-ground interferometer: instabilities of local oscillators and
other circuits in space and ground receivers and data acquisition systems, mechanical
vibrations of SRT and GRT’s, on-board and ground tracking antennas; phase
fluctuations due to propagation of the signal through the atmosphere in data
transfer (down link) and synchronization (phase-locked loop PLL); inaccuracy of
the knowledge of spacecraft acceleration.
The
contribution from all electronic circuits to residual phase fluctuations
can be made lower than from phase instability of the H-maser frequency
standard. At the VLA, the residual phase fluctuations se ' ν (GHz)/4,
which corresponds to se = 5o.5 at ν = 22 GHz [4]. We expect that
for the SRT electronics residual phase fluctuations will be at a level of se ~
8?.
One
can evaluate the total impact from mechanical vibrations by taking into account
allowable vibrations of 0.05 mm (RMS) for SRT and GRT along theirs axes, this
will give us at 22 GHz sm ' 4o when using on-board H-maser frequency standard (SHM), and sm ' 10o when using up-down link synchronization with 0.1 mm RMS
vibration for the on-board VIRK antenna and 0.05 mm RMS vibration for the TS
antenna along theirs axis. The conclusion is that usage of SHM considerably
reduces phase fluctuations from mechanical vibrations.
From
the VSOP mission experience it is known that the main reason limiting the coherence
time was phase fluctuations due to propagation of the signal through the Earth
atmosphere. These fluctuations reduce the stability of reference frequency by
an order of magnitude at the up-down link synchronization [5,6].
Lastly,
there is one more reason in coherence loss for space-ground interferometer:
this is an uncertainty in the determination of the SRT acceleration. The
uncertainty can be caused by the inaccuracy of the reconstructed orbit and by
the non-gravitational acceleration, which are difficult to simulate and to take
into account.
The
atmospheric effects and the anomalies in the acceleration behavior of the SRT
will be considered later in this Memo together with suggestions on the possible
way of corrections for these effects.
3.
Sensitivity in fringe fitting by
selfcalibration,
and sensitivity in image reconstruction and in differential
astrometry
The
problem of self-calibration at very long baselines is connected with the a
priori uncertainty in the intrinsic structure of the source under the
investigation, which is the ultimate objective of the VLBI study. As a result
of VLBA study at 15 GHz [7,8] it was found that in 160 compact radio sources
with flat spectra and with total flux densities above 1.5 Jy there are 97, 117,
and 151 sources which have correlated flux densities ?0.5, ?0.3, and ?0.1 Jy at maximum baselines correspondingly. One can
make the conclusion that there are more than 6000 radio sources with the total
flux density ?300 mJy, and more than a half of
such sources will have correlated flux densities ?100 mJy at the maximum baselines of
the VLBA system.
Mean
angular distance between such sources on the sky will be about 3o.
Time necessary to re-point RadioAstron SRT to the sources at angular distance
of ~ 10o is about 10
minutes with the slewing rate of 0.3 degree/s and necessary stabilization time
(as specified by the Technical Task for the project); there are about 10
sources available for differential astrometry in such area.
Angular
diameter of a compact radio sources (or its component) with the flux density of
100 mJy and circular Gaussian brightness distribution is equal to 16 mas
for maximum brightness temperature of Tmax = 1012 K. For
the same brightness temperature a source component with elliptical Gaussian
brightness distribution (jet) and with the major axis of 100 mas
will have the minor axis of only 2.5 mas. Characteristic angular sizes in
these models correspond to maximum angular resolution in the RadioAstron
mission.
Conclusion:
self-calibration may be used for the simplest models of source structure if 100
mJy correlated flux is reliably detected during fringe search.
4.
Atmospheric effects and the
possibility to correct phase errors
Phase
fluctuations caused by the propagation of radio signals through the Earth
atmosphere are the main effects limited the coherent integration time in VLBI
measurements. This statement was confirmed also for the space-ground radio
interferometer HALCA [5,6]. The possibility to make corrections to atmospheric
phase fluctuation was studied by many researches, and there were developed some
approved technique [9-11]. For space-ground interferometer atmospheric effects
are relevant only to GRT’s and to TS’s.
Two methods can be used to reduce the phase errors caused by the
troposphere: radio monitoring of the atmosphere brightness in several
frequencies around of water vapor line at 22 GHz in the direction of the
observed radio source (WLM – Water Line Monitoring), and/or self-calibration
GRT’s using one reference radio telescope located at the high mountain (HMRT -
for example, VLBA antenna at the Hawaii) observing the same source. In the last
case the reference antenna will be nearly free from the atmospheric effects and
all ground baselines (small compared to the space-ground baselines) can be
corrected for atmospheric effects.
Test
WLM experiments and regular observations conducted at the Owens Valley Radio
Observatory (OVRO) demonstrated an accuracy in the measurements of the
atmospheric path delay at a level of 0.15 mm, and it is planned to improve the
accuracy down to the level of about 0.05 mm [9]. Such accuracy provides the
possibility to conduct regular interferometer observations even at the
millimeter wavelength with future extension to submillimeter wavelengths. WLM
method was planned to be used in VSOP mission [12], but it was not realized.
Conclusion:
observing techniques presented above provide potential capability to reduce
atmospheric effects to such level when they will not constitute the main reason
limiting the coherence time in VLBI at high frequencies. We propose to develop
both methods of phase corrections at 22 GHz with ground radio telescopes during
the pre-launch time.
5.
High accuracy orbit determination
and anomalous acceleration
Space-ground
radio interferometer HALCA has demonstrated in practice high accuracy of orbit
determination [13]. The same technique of orbit measurements and reconstruction
will provide for RadioAstron mission the accuracy in SRT position of about 5 m
and with the accuracy of about 0.1 mm/s in STR velocity for each 1ч5 minutes.
Even better accuracy will be achieved in orbit reconstruction based on
long-term measurements and in post-correlation analysis.
Spacecraft
in the RadioAstron mission will move along elliptical high-apogee orbit with
the major semi axis nearly 30 times larger than the Earth diameter. The
accuracy of the measurements of radial distance and radial velocity will be
sufficiently good, but the determination of the tangential components for SRT
position and velocity will request some special approaches. It seems that
phase-reference observations of two or more radio sources will be necessary for
accurate determination of full vectors for the position, velocity and
acceleration. Such measurements cannot be conducted continuously because of
relatively slow slew speed of the SRT and limited number of re-pointings per
day. Therefore, there is necessity for independent monitoring of small and
probably variable SRT accelerations especially between the reference source
observations.
The
estimations show that the accuracy of acceleration measurements which can be
achieved by all facilities available in the TS-SRT orbit measurements system
including on-board SHM and reference-sources observing technique will be
limited by ~ 1ч2ּ10-8 m/c2 in the time intervals
between reference-sources observation. The errors are connected with the
uncertainties in the model of the Earth gravitation field near the SRT perigee
(Rmin ~ 104 km) and with the inaccuracy of the
calculations of solar light pressure along the whole orbit. For the last effect
10% inaccuracy in the albedo and midship of the spacecraft with SRT at
different orientations relative to the Sun direction will cause uncertainty in
the SRT acceleration of about σa ~ 10-8
m/c2. Acceleration due to solar wind pressure is about ~ 1ч20ּ10-10 m/c2,
and it varies by several times in the magnitude (inside the magnetosphere even
in direction) during a few seconds. The same acceleration is expected due to
gas evaporating from the SRT.
SuperSTAR
accelerometer (AM) recently developed and tested by ONERA provides the accuracy
of 10-10 m/c2along all three axis of the spacecraft [7].
The evaluations presented above have shown that solar pressure, solar wind
(variable in strength and direction) especially inside the magnetosphere, and
evaporation of gas from the spacecraft will cause SRT acceleration in the range
of 10-10 - 10-8
m/c2.
The
following Table 1 shows the magnitudes of uncontrolled SRT displacement due to
anomalous acceleration acting for some period of time Δt.
Table 1
τ (c) |
102 |
103 |
104 |
105 |
∆l = 0,5a(Δt)2 (mm) for a (a= 3σa =
3ּ10-8 m/c2) (a= 10-10 m/c2) |
0.15 5ּ10-4 |
15 5ּ10-2 |
1.5ּ103 5 |
1.5ּ105 5ּ102 |
At
λ = 13.5 mm uncontrolled
SRT displacement by Δl ~ 2 mm will cause 10% loss in coherence in
accordance with the expression σ = 2π(0.5Δl/λ) ~ 0.45.
Conclusion:
AM will provide a possibility to reduce considerably the effects of errors in SRT
acceleration thus increasing coherent integration time from several minutes to
several hours when new reference-sources observations could be done. AM will
also help to decrease time of fringe search at the correlator because of
smaller values in uncertainty of delay and fringe rate.
6.
Main parameters of the SuperSTAR
accelerometer
High
sensitive on-board AM developed in France by the IEA firm (Instrumentation
& Aerospace Equipments Research Unit) belonging to the Office National d'Etudes
et de Recherches Aerospatiales (ONERA) is successfully operating at the two
satellites launched by Russian rocket in 2002 from the cosmodrome Plesetsk
(project GRACE, Germany and USA) for the measurements of the Earth gravitation
field.
Main
parameters:
ћ
acceleration
range - +5
10-5 m/c2,
ћ
sensitivity - 10-10
m/(c2 Hz0.5),
ћ
size - 6-liter
cube,
ћ
mass - 6
kg,
ћ
power consumption
-
2 W,
ћ
analog output.
The
accelerometer may be installed at the RadioAstron spacecraft near the mass
center. The output information can be included into the science data headers
transmitted by the VIRK. The
possibility to use such device in VSOP-2 mission is also under discussion.
7.
The scheme of frequency
synchronization using on-board H-maser
The
scheme of frequency synchronization using on-board H-maser and up-down phase
loop is shown in Figure 1. On-board H-maser provides steady-going (without
interruptions connected, for example, with switching between tracking stations)
reference signal for local oscillators, sampling frequencies and the carrier of
15 GHz downlink data transmitter (VIRK). Phase loop produced by H-maser with
the receiving science data tracking station will permit necessary data for
orbit measurements and time alignment. The backup way of synchronization using
standard up-down phase loop providing reference frequency from the ground
H-maser at the tracking station is also possible.
The
development of the on-board H-maser (SHM) was started in 1996 according to the
international agreement between the ASC and ESA signed on January 15 1996 and
proved by RosAviaKosmos. Neuchatel Observatory in the Switzerland is the
institution responsible for the development, manufacturing, tests and delivery of the on-board
H-maser. All tasks necessary for the integration of the SHM at the RadioAstron
spacecraft (such as mounting, thermo stabilization, power supply, control, and
interfaces with VIRK) were completed in the ASC and in Lavochkin Association.
At present the development of SHM at the Neuchatel Observatory is continuing
but with the goal to install SHM at the ACES satellite.
Main
SHM specification requested by RadioAstron were conserved, they are presented
in the Table 2 below.
Table 2
Time interval ∆t (c) |
1 |
10 |
102 |
103 |
2 103 |
Allan deviation ∆ν/ν |
1,5 10-13 |
2,1 10-14 |
5,1 10-15 |
2,1 10-15 |
2,0 10-15 |
Coherence loss 1-N = 0,5(2π∆ν∆t)2 (at ν = 22 GHz) |
2 10-4 |
4,2 10-4 |
2,5 10-3 |
4,2 10-2 |
0,15 |
Conclusion:
the retention of SHM in RadioAstron science package will provide estimated
coherent integration time of 300 s, and it would provide possibility to achieve
longer integration time (up to 2000 s) under no other constraints.
8.
Conclusion
On-board
H-maser frequency standard and high accuracy on-board accelerometer included
into the scientific payload of RadioAstron mission will permit us to increase
the coherent integration time up to 5-30 minutes at the correlator before
fringe detection. This will be resulted in 2-5 times improvement in sensitivity
by increasing the coherence time up to 5-30 minutes. To reach these potential
figures we propose advanced observing methods using the measurements of the
atmospheric path delay variations by the monitoring of 22 GHz water vapor line
emission along a line of sight to the observing source (WLM) and/or by using reference radio
telescope located at high mountain. Additional gain in sensitivity can be
obtained by applying self-calibration in fringe-fitting procedure during image
reconstruction.
As
it is known from regular ground VLBI observations, maximum coherence time at 22
GHz is about 80 seconds. WLM observing technique or/and usage of reference
radio telescope on high mountain
(HMRT) will increase the integration time by 2-3 times. On-board H-maser
frequency standard will also provide the possibility to increase the
integration time by 2-3 times. On-board accelerometer will provide necessary
accuracy of orbit determination to realize potential maximum integration time
by 2-3 and to simplify fringe search at the correlator.
Table 3
Option |
Coherent
integration time (s) |
No
HMRT/WLM, SHM and AM |
80 |
HMRT
without SHM and AM |
160-240 |
HMRT/WLM
and SHM, but no AM |
320-720 |
HMRT/WLM,
SHM and AM |
720 |
Table 3 shows that necessary sensitivity of space-ground
radio interferometer could be achieved only with on-board H-maser frequency
standard and advanced observing techniques.
On-board
accelerometer and SHM will provide the possibility to obtain three-dimensional
distribution of the Earth gravitation field on the distance scales of Rmin
? 104 km with the high
accuracy for the first time.
We
propose to stimulate scientific and technical investigations for the
implementation of these approaches in the nearest future.
References
1.
Crane, P.G., & Napier, P.J. in
Synthesis Imaging in Radio Astronomy, ASP Conf. Series 6, Perley, R.A., Schwab,
F.R., & Bridle, A.H. (eds.), 139-165, 1989;
2.
Wrobel, J.M. & Walker R.C. in Syntheses imaging in Radio
Astronomy II ASP Series 30, Taylor, G.B., Carilli, C.L., and Perley, B.A.
(eds.), p…, 1998.
3.
Ulvestad, J.S., Space Very Long Baseline
Interferometry, p…,
1998.
4.
Sramek R.A., Atmospheric phase stability
at the VLA, VLA Test Memo N 175, 1989.
5.
Kobayashi, H. et al., Halca On Board VLBI
Observing System, PASJ, 52, 967-973, 2000.
6.
Suzuki, K., Kawaguchi, N. & Kasuga,
T., Up-Link Frequency Control Using Closed-Loop Mode in "Astrophysical
Phenomena Reveald by Space VLBI, Hirabayashi, H., Edwards, P.G. & Murphy,
D.W. (eds.), p.309-312, 2000/
7.
Kellerman, K.I., Vermeulen, R.C., Zensus,
J.A., & Cohen, M.H., AJ, 115, 1295-1318, 1998.
8.
Kovalev Yu.Yu., Kardashev, N.S.,
Kellermann, K.I. et al, in preparation.
9.
Woody, D., Carpenter, J., & Scoville,
N., Phase Correction at OVRO using 22 GHz Water Line Monitors, in Imaging at
Radio through Submillimeter Wavelengths, Maugum, J.G. & Radford, eds., ASP
Conf. Ser., V.217, 317-326, 2000.
10. Butler B., Some Issues for Water Vapor Radiometry at the VLA, VLA
Scientific Memo N 177, 1999.
11. Carilli, C.L. & Holdaway, M.A., Tropospheric phase calibration in
millimeter interferometry, Radio Sci., 34, 817-840, 1999.
12. Asaki, Y., Kobayashi, H., Hagiwara, N., & Ishiguro, M., A 22 GHz
Line Radiometer for Usuda Tracking Station, in Astrophysical Phenomena Revealed
by Space VLBI, Hirabayashi, H., Edwards, P.G. & Murphy, D.W. (eds.),
281-284, 2000.
13. Porcas, R.W., Rioja, M.J.,
Machalski, J., and Hirabayashi, H., Phase-Reference Observations with VSOP,
in Astrophysical Phenomena Revealed by Space VLBI, Hirabayashi, H.,
Edwards, P.G. & Murphy, D.W. (eds.), 245-252, 2000.