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More Details on Fast‐Scanning Data I have done a little more analysis on these data sets: in particular I looked for spikes in the time domain, plotted the Allan variance and looked at the power spectra of the variations in the total power. I did not see any spikes and most of what is seen in the Allan variance is as expected. There are some significant narrow features in the power spectra, and curiously these seem strongest when looking at the ambient load.
Spikes
As a first test I took the individual times series data streams (there are 8 of these for the 8 total power outputs from the IF processor and in this case they were sampled at the full 2kHz rate) and formed the first difference, i.e. (Pn+1 – Pn). Taking differences over this short time (0.5 msec) removes the rapid fluctuations. I found the rms of these differences and the largest absolute value as well as the number with absolute values above 4 times the rms. Typically the largest values were 4.5 to 5.5 times the rms, while the numbers of differences over 4‐sigma were between 30 and 40. These compare well with what is expected for Gaussian noise: for roughly 550,000 data points we expect 1.1 points outside 4.75‐sigma and 35 points outside 4‐sigma.
Strictly speaking this test looks for steps in the data rather than spikes. I therefore repeated it using the second difference, i.e. (Pn+2 – 2Pn+1 + Pn). The results were essentially the same.
This was done on all the data taken on blank sky and on the ambient load and I didn’t see any evidence of non‐Gaussian behaviour. Not surprisingly the data taken on Venus fail this test – the strong signals varying on quite short timescales produce a quite different distribution of differences.
I conclude that there is no evidence of any spikes and that there is therefore no need to include any de‐spiking in the time domain in the standard data processing chain. It may nevertheless be worth including a routine check of this type since spikes could still show up with a different combination of receiver bands and antennas.
Allan Variance
These are the fractional variances (i.e. normalized by mean power) for the differences of between adjacent integrations of time t (in seconds on x‐axis). With logarithmic scales, the data taken on the ambient load typically look like this (this is PM01 scanning around Az = ‐120, El = 70):
The last trace is what is expected for white noise with a bandwidth of 2GHz. We see that at the raw sample rate of 0.5msec the traces for the individual channels match this. For longer times they
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move up a little and then the trace flattens out for times between about 0.1 and 10 seconds. This is the typical behaviour that we expect with our SIS receivers. There are significant differences between the 4 channels, especially between 0.1 and 1 second, with TD_P1 being the worst. The lower traces are the averages for all the channels with each polarization and then for both polarizations. These curves remain lower than the individual traces, which means that most of the excess noise is not correlated between channels. For the longest timescales the variance turns up, implying that there is a drift and this is apparently correlated between the channels.
For the PM04 data taken at the same time there no obvious drift on longer timescales, but there is a quite prominent “feature” at about 16 msec, i.e. log10(t) =~‐1.8, in some channels.
If we now look at the same antenna but tracking in RA – Ralph described this as RA/Dec (‐120, 70) – this is much less prominent.
This suggests that the “feature” depends on the pointing direction or possibly on whether or not the antenna is tracking (but it was scanning in both cases).
Turning to the data taken looking at blank sky, we immediately see that there are far stronger fluctuations due to the atmosphere than those in the receivers. The atmospheric noise is of course
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highly correlated between the different channels, which is why they, and the averages of the channels, all converge to single trace at longer times.
This is PM04 scanning at a fixed Az and El. This was Band 7 and the conditions were quite poor. In this case the atmospheric noise starts to dominate at about 0.1 seconds and rises rapidly for longer times. Note that if we can make Lissajous patterns with say 1.25 Hz as the drive frequency, the beam goes from one edge of the map to the other in 0.4 seconds and so the relevant time for forming the difference between the centre of the map and the edge is 0.2 secs, i.e. log10(t) =~‐0.7. Under these (poor) conditions we would be getting errors of about 2x10‐4 of system temperature from each pass over the middle of the map. This, together with the number of passes, would set the errors for measurement of the large scale structures, e.g. for 100 passes and Tsys of 400K we might expect brightness temperature errors of order 8mK rms, which doesn’t sound too bad. Obviously this needs to be checked for the complete mapping process with simulations and/or real data.
For these on‐sky data the plots for the two antennas and for the different pointing directions are very similar so I won’t show more of these here. (I’ll put the spread‐sheets on the ticket.) The Venus plots look like this, but these are not very relevant because of the enormous signal to noise ratio.
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Power Spectra
The power spectra provide an alternative way of examining the noise to the Allan variance plots. I took the Fourier transforms of the raw data streams (actually after subtracting the mean from each to reduce round‐off errors) and calculated the amplitudes of the components [sqrt(real2 +imag2)]. To limit the number of points in the output, I binned the results in such a way so at frequencies above 0.8Hz the fractional spectral resolution remains fixed at about 0.5%. Here is a plot of the data on the ambient load for PM01 using log scales.
This is the average over all the channels. The red line is a power law with a slope of ‐0.75. Since these are the amplitudes, this means the power is going as about (frequency)‐1.5, i.e. a little faster than 1/f.
The observations of blank sky show the much larger and steeper contribution from the atmosphere.
Here the illustrative slope is ‐1.2, i.e. power proportional to (frequency)‐2.4. I haven’t checked what the slope expected for Kolmogorov turbulence is, but this seems plausible.
The plots for tracking a given RA and Dec are quite similar to those above (which are for fixed Az/El) although the noise level on the sky was noticeable larger, presumably because of the lower elevation. As with the Allan variance, the power spectrum plots for Venus show lots of structure but it is not very meaningful to look at that data in this statistical way. The plots for PM04 are qualitatively similar – the noise on the ambient load is significantly lower and flatter but the results on the sky are almost identical, as might be hoped.
The most important quality of the power spectrum is of course that it picks out periodic functions much better than the Allan variance does. It can be seen that there are features at roughly 50 Hz and 150 Hz in the plots above.
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If we use linear scales it is rather easier to see such spikes. Here is a plot for just one of the channels (TD‐P0) from the same data set as the top one on the previous page (PM01, amb load, fixed Az/El).
We see that there are numerous narrow features in the range 50 to 200 Hz. The normalization is such that a sinusoidal component with an amplitude of 1 has a power of 0.5 and hence a value on the plots of 0.707. This is the channel TD_P1 that showed the largest excess in the Allan plot. The mean level in these raw units is about 30,000 so these are quite small fluctuations but they are contributing significantly to the noise. They are also in a range of frequencies which would correspond to interesting angular scales on the sky: at 600 arcsec/sec, 50 Hz corresponds to 12 arcsec.
To see a little more of these here is a plot of the region from 110 to 190 Hz showing all the channels and the averages, with the vertical axis on a log scale to increase the separation of the traces.
This shows a curious result: most of the features that are present on the individual channels are not seen of the spectra of the averages, which are the lowest plots. (Look at the spike on the left, at ~118 Hz, as an example.) In fact the only features that make it into the overall average are the group around 151 Hz (which is only present on polarisation 1) and the one at around 169 Hz (which is on both polarizations). The other features cancel‐out on averaging, which implies that these fluctuations in the power levels must be occurring with opposite signs on different channels.
Without going into a lot of detail I note that:
1) The level of these features, especially the ones that cancel, is lower in the observations of the sky than on the ambient load. Since the gain must have been lower on the load than the sky, this is not one would perhaps expect for an additive interference. It is I suppose possible that these features are to do with standing waves between the receiver and the load.
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2) Similar features, but with a completely different pattern of frequencies, occur on the PM04 data, although there are not quite as many. There is again a mixture of features which cancel and some which do not. Here is the overall view, for the same case as the top of the previous page, i.e. channel TD_P1 from PM04, amb load, fixed Az/El:
The biggest problem here is the strong spike at 25 Hz. This is what creates the feature noted in the Allan variance plots (top plot of page 2), which immediately implies that it is making a very significant contribution to the noise. It does cancel moderately well in the overall average and it is lower, but still present, in the data on sky. We may nevertheless need to track this one down and remove it since it is at a frequency that would show up strongly in the maps. Alternatively we might be able to apply a “notch” filter in the frequency domain to get rid of so long assuming that the frequency is stable. The main feature does appear to be at exactly 25 Hz, i.e. half the mains frequency.
3) Another possible explanation for the features that cancel on averaging is that they are a result of something like phase switching or LO phase rotation going on in the system. We should perhaps investigate whether we need to make some special settings for the single‐dish observing modes to turn things off.
4) I did not see any evidence for spectral features at frequencies higher than 200 Hz (although obviously we need to look at each antenna and receiver individually before assuming that this is always the case). If it is then this suggests that we can routinely use an integration time of 2 msec for collecting the data, since that implies a Nyquist frequency of 250 Hz and this means that all the features are properly measured. An integration time of 4 msec or longer, which would in almost all cases be enough for sampling the map, might lead to the aliasing of some of these features to troublesome places in the band. If we use 2 msec sampling and we want to compress the data further, we should apply a low‐pass filter before resampling it. The obvious thing to do is to take a Fourier transform of the raw data, check it for spectral spikes, which could if necessary be suppressed, apply a suitable filter and then throw away the high frequencies so as to transform back onto more widely separated points in the time domain.
When there is an opportunity to take more data we should try things like switching bands to see at what stage in the system these effects are occurring.
REH 9th Dec 2013
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