1. Analysis of an interference affecting the data of the Siberian Radioheliograph
Veronika S. Kobets
Institute of Solar-Terrestrial Physics
Russia, Irkutsk

2. Location of the SSRT/SRH
SSRT is located in the Tunka valley near south coast of the Baikal lake.
Solar observations are made during daylight hours from
00 till 10 UT in summer and from 02 till 08 UT in winter.

3. Siberian Solar Radio Telescope
South North
West East
256 antennas
5.7 GHz
Longest baseline 622 m
Spatial resolution 21′′(15’’)
The period of polarization modulation is 14 millisecond. The SSRT observes the Sun in the two-dimensional mode with a spatial resolution of up to 21arcsecond, and in the one-dimensional mode with a spatial resolution of up to 15 arcsecond. In the two dimensional mode, images are formed as a result of the change of the Sun’s position relative to interference maxima, firstly, because of the simultaneous receiving at different frequencies in the 5.67–5.79 GHz band and, secondly, because of Earth’s diurnal rotation. The latter determines the transit time of the interference maximum across the Sun; therefore SSRT can produce images every 2–3 minutes at most. 

4. Siberian Radioheliograph
The SSRT is now being upgraded.
The upgrading is aimed at providing the aperture synthesis imaging in the frequency range 4–8 GHz [Lesovoi et al., 2011, 2014] instead of the single-frequency direct imaging due to the Earth rotation.
The SRH uses an essentially different imaging principle, Fourier synthesis (Lesovoi et al., 2014).

5. Siberian Radioheliograph
48 antennas
5 frequencies in the 4–8 GHz range
Longest baseline 107 m
Spatial resolution 1’-2‘
West East
South
The time to switch from one frequency to another is currently 2s, and the accumulation time at each frequency is 0.28s for each circularly polarized component.
The sensitivity to compact sources reaches 10−4 of total solar flux.

6. Parameters of instruments
Siberian Solar Radio Telescope
Siberian Radioheliograph
Number of antennas
256 48
Frequency
5.7 GHz
5 freq in the 4–8 GHz range
Baseline
622 m
107 m
Spatial resolution
21′′(2d), 15’’(1d)
1’2’
Time resolution
0.014s
0.56s (2s to change freq)
Imaging principle
Frequency scanning
Fourier synthesis
Image formation time s
0.56s

7. Data of the Siberian Radioheliograph
Correlation plots

8. Correlation plots
Correlation plots (badary.iszf.irk.ru/srhCorrPlot.php)represent:
Solar activity variations
State of SRH systems
These plots are sums of complex correlations obtained for different antenna pairs.
Each value of the plot is an integral over a spatial spectrum. Limits of the integration are determined by the task.

9. Correlation plots
Frequencies
4.5 GHz
5.2 GHz
6.0 GHz
6.8 GHz
7.5 GHz

10. Causes of the interference
Radio observations are affected by different kinds of interference:
radio-frequency interference (RFI) from man-made equipment – can be several orders of magnitude stronger than weak signals of interest:
Ground-based transmitters
Satellites
Local disturbing factors.

11. Interferences on SRH
SRH works at the wide frequency range so its data including correlation plots are subjected to a number of radio-frequency interference.
Frequencies
4.5 GHz
5.2 GHz
6.0 GHz
6.8 GHz
7.5 GHz

12. Annual correlation plots
7.5 GHz
Jul 16
Sept 16
Oct 16
Nov 16
Dec 16
Jan 17
Feb 17
Mar 17
Apr 17
00: : : : : : : : : : :00 UT

13. Averages annual correlation plots
7.5 GHz
Jul 16
Sept 16
Oct 16
Nov 16
Dec 16
Jan 17
Feb 17
Mar 17
Apr 17
Interference
Interference
00: : : : : : : : : : :00 UT

14. Averages annual correlation plots after noon alignment
7.5 GHz
Jul 16
Sept 16
Oct 16
Nov 16
Dec 16
Jan 17
Feb 17
Mar 17
Apr 17
Interference
Interference
00: : : : : : : : : : :00 UT

15. Shadowing of antennas
One of the reasons of interference is the shadowing of one antenna the other on short bases and low elevation angles. This may be an explanation of the light strips on averaged annual correlation plots. The dark areas on the right picture show the moments of shadowing of the antenna pairs (dark area at the center), (dark area on the left) and dark area on the right).
7.5 GHz
Jul 16
Sept 16
Oct 16
Nov 16
Dec 16
Jan 17
Feb 17
Mar 17
Apr 17
00: : : : : :00 UT
00: : : : : :00 UT

16. Summary
Data of the SRH are subjected to a number of radio-frequency interference.
Due to the high sensitivity of the correlation plots, one can determine the dependence of the interference on the time of year (declination) and time of day (hour angle).
One of the reasons for this kind of interference is the shadowing at the short antenna bases, when one of the antennas gets into the area of the geometric shadow of the neighboring antenna.

17. Thank for your attention!