Noise variations

In beamformed observations of Ter5 (mattermost link) peaks and variations of the noise level of coherent and incoherent beams were found. Investigations using the interferometric data confirmed their presence, albeit with lower time resolution and therefore less detail.

It became clear that most antennas are behaving normally, and only a few are causing these problems. No strange effects are found in the cross-correlations, the only parameter that seems to vary is the system noise per antenna.

Going through all existing data manually would be too time consuming, so we have to find a way to detect problems with some automatic statistics. Here is an attempt in that direction.

I take the autocorrelations of all antennas. Gaps in the data (between scans) are filled but masked. Data are then normalised (per scan, because different targets can have very different noise levels) as follows: The median of the XX and YY levels are taken (per frequency), and XX and YY are normalised by this. XY is normalised with the geometric mean of the two. This brings the median noise in X and Y to a value of unity. The XY correlation can have arbitrary phase due to delay differences. Because of this, it is rotated using the median of XY (real and imaginary). This means XY will generally be close to positive real. The amplitude can differ from antenna to antenna, but it should be relatively constant. The imaginary part should be constant close to zero.

For an overview of all antennas, I take the median of these normalised data over all frequencies and plot them per antenna and time. An example for the Ter5 data set 1648081798, in which the variations were found originally, is shown here: