The Slough Ionosonde

Since 1931, hourly soundings (at least) have been made at Slough. Various instruments have been used during this time, but the instrument that is specific to this comparison is the KEL. The KEL IPS42 is a simple sounder in comparison with modern equipment. It was specifically designed to carry out regular routine sounding of the ionosphere. The transmitter generates a 5 kw pulse using four valves in a parallel push/pull distributed amplifier. This is broadcast using a single rhombic antenna, which also doubles as the receiver. Since there is only one receive aerial, the information in the return signal is limited to frequency and time of flight (known as virtual height). The return analogue information is converted to a digital signal and processed by a Commodore 64 6502 chip. The results are then recorded onto magnetic tape.

The Chilton ionosonde

In contrast to the KEL, the Digital Portable Sounder (DPS) at Chilton is a much more versatile instrument. Although it is being used at RAL for regular monitoring (following the schedule of the Slough site) it was originally designed as a research instrument, and is therefore much more flexible. Improvements in the receiver sensitivity mean that quality data can be obtained from a pulse of just 300 W, generated by a solid state MOSFET. The transmitter antenna consists of a 'crossed delta'; two orthogonal triangles with a vertical apex at a height of 27m and with a base 50m across. The receiver is a phased array of four crossed loop antennas five feet in diameter. Three of the aerials are positioned on the vertices of an equilateral triangle with 60m sides, the fourth is in the centre of the triangle.

As the receivers form an array, it is possible to infer much more information from the return echo. As well as frequency and virtual height, the DPS can determine the polarisation, Doppler shift, and direction of arrival of each echo in a sounding. This is invaluable when trying to separate oblique signals from those of interest in regular soundings, i.e. those from directly above the sounder. The DPS analyses the return data using a 486 PC and writes the results to magnetic tape.

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13/02/97 Chris Davis