Fmin, the minimum observable frequency.

Although both instruments are able to measure fmin for over 90% each month between December 1993 and April 1994, the correlation between the sites is not good. The scatter of points is so great that no real trend is visible in much of the data. It has since come to light that a bug existed in the ADEP package, whereby a strong sporadic E layer confused the system, and the foEs value was attributed to fmin. This accounts for those points with a low Slough fmin that have anomalously high Chilton fmin. Despite this erroneous group of points, the correlation is still not good, and indeed the data from January and February 1994 seems to indicate that fmin measured at Chilton is approximately one half of that at Slough. That this should be so is not surprising. The minimum observable frequency is strongly instrument dependent, since it is a function of Signal to Noise Ratio (SNR) and receiver sensitivity. It has already been stated that the two instruments differ in both these respects.


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15/05/97 Chris Davis