signal-to-noise

This option plots the signal-to-noise ratio as a function of time on each space-earth baseline, and may also be enabled by pressing the `n' key. As described in Section 8.1, uvplot will write the baseline coordinates to a uvplot.dat file if the appropriate uvplot.par parameter is set. The signal-to-noise option reads information from this file and uses it to generate SNR plots. No distinction is made, however, of when the uvplot.dat file was created, and so the data used for generation of SNR plots may be from either the current or a previous run of uvplot. This ability to read in previously created uvplot.dat files is useful for calculation of SNR plots for the same observing parameters but different source models. When using previously generated uvplot.dat files, the user must be sure to examine the uvplot.dat header information to note the exact date and stations which were used, as well as the location of the source. Confusion can easily arise when several data sets have been saved and are being used to re-display information. The program will not accept alternate names for - data files, and when a new - track solution is generated, any previously created uvplot.dat file is overwritten. In order to preserve the results of a given run, the file uvplot.dat should be copied or renamed to another file using DOS copy or rename commands. To later display SNR plots using this file it must first be copied back to uvplot.dat. (The same is true for the radius.dat file: in order to preserve this file against being overwritten by a new execution of uvplot, it must be copied to a new file name.) The same method will apply to viewing polarization signal to noise plots. The user can simply generate a uvplot.dat file then toggle the polarization mode on and off to view the different signal to noise plots for differing models. Or the modes can be switched between total intensity and polarization mode without rerunning uvplot.

The source correlated flux is calculated using a model consisting of up to 20 circular or elliptical gaussian components, for which the positions, fluxes, polarizations, and sizes are specified in uvplot.par. The screen is split into three boxes. The upper left box displays the observation parameters: the source name, and the observing frequency, bandwidth, and integration time. On top of the main screen is the same altitude vs time plot discussed in Section 5.2.2 above.

The main screen consists of the signal-to-noise plots.The plots are arranged in stacked horizontal boxes, the number of boxes being equal to the number of ground VLBI stations. This display shows only the SNR ratios for the satellite-earth baselines, not the earth-earth baselines. The ground VLBI station is indicated to the left of the box. Each box has a scale on the left side indicating the amplitude of the SNR. The user may specify the maximum scale in uvplot.par; if the relevant parameter is set to 0, the scale on the side is determined automatically, i.e., the maximum SNR will be slightly larger then the maximum SNR value for that baseline. If a number other then 0 is entered, this number will be taken to be the maximum value of SNR to be plotted for all baselines.

Depending on the scale, a horizontal dashed line may or may not be visible on the plots. By default this line is plotted at a SNR threshold set in the uvplot.par file. An option in the uvplot.par file allows the user to specify this value. This is meant to denote a threshold below which the SNR is considered unacceptable, and is typically 5-7 for VLBI observations.

The SNR curves are colour coded in a way similar to the bf uptime plots. Green indicates that the VLBI data which would be obtained have a SNR above the threshold, that both the ground VLBI station and the satellite can observe the source, and that a telemetry link is possible. Red indicates that, although the SNR would exceed the threshold, the data from that baseline are not available due either to lack of source visibility, violation of a pointing constrain, or lack of a telemetry link. If the SNR bar colour is magenta, the data have a SNR below the threshold. See Figure on p. .