DE605 RFI

There is evidence for interfering equipment on a nearby cell tower that affects the low band. To investigate details, we use one TBB dump from 2023-10-26 13:34 UTC, and 16 dumps taken during a Jupiter observation (rcu mode 3) on 2023-12-27 20:35–22:26 UTC. Each dump used here has about 1 sec duration. In the standard analysis they get correlated, calibrated and imaged with the standard LOFAR frequency resolution (subbands of ca. 200 kHz) and with 0.1 sec time resolution. Visualisation of such a data set is not easy. My standard approach is to select a frequency (and maybe time) range, show the corresponding averaged allsky image, and potentially select a point on the image to plot the spectrum and time series.

Here is an example plot for a dump taken 2023-12-27 21:19 UTC:

(click on the image for full-size version)

The panels on the top right show the total dynamic spectrum and the integral over time and frequency. The red-shaded region is used for the imaging on the left. The maps show four polarisation products. Note that these are not exactly the Stokes parameters, but combinations of X and Y. The sky is shown in an orthographic projection, which compresses the elevation range near the horizon. Outside of the horizon circle the plots show nearfield imaging of the ground level, where the distance goes with $1/(r-1)$, it thus starts at infinity on the circle (consistent with the sky image at the horizon) and then decreases further away from the circle.

The lower right shows the spectrum in the direction of CasA (relative to the mean sky spectrum).

In this example the signal is dominated by the sky, but some RFI is seen near the horizon.

Here we use the same dump, but image the frequency range 54–60 MHz. The RFI source is seen at the horizon near azimuth -33 deg (north towards east). The lower right panels are for this direction. Note that the spectrum looks very different from CasA, but is stable with time.

This source seems to be there at least most of the time, with a pretty stable spectrum.

To get an overview, we have to reduce the number of dimensions. Because many RFI sources appear at the horizon at large distance, it seems appropriate to only plot the signal around the horizon.

This PDF shows such plots for all dumps from 2023-12-27, subdivided into 0.1-sec chunks (11 per dump, the last one is always shorter): tbb_image_horizon_spec_de605_2023-12-27_jupiter.pdf Values for each frequency are normalised by the median around the horizon for this frequency. The dump shown above is on pages 67–77.

Let us show integration 5 of the same snapshot as before:

The RFI source shown above is the bright blob below the centre of the image. It is quite prominent, but not the only obvious RFI. When going through the PDF pages, we see that some of the features are very variable with time.

To investigate the time dependence better, we plot the same dump with now much shorter subintegrations of 2 msec on 511 pages (note large file): tbb_image_horizon_spec_de605_2023-12-27_jupiter_2msec.pdf The celltower RFI is constant at this time resolution, but many other sources are extremely variable, sometimes periodic.

To study the time variability better, we plot the same data differently as one dynamic spectrum per azimuth and page: tbb_image_horizon_spec_de605_2023-12-27_jupiter_2msec_dynspecs_fft.pdf The dynamic spectra are in the left panels, the right panels shows a Fourier transform along the time axis to show periodic power variations. The celltower azimuth is strongest on page 74: