To measure differences between short and long paths around the world requires the ability to measure delay times of about 1 to 140 ms with a resolution of about ±1 ms. To measure with sufficient resolution to resolve individual reflection paths requires rather higher resolution.
However, resolution is only part of the story. The project involves measuring the delay times of signals for hours on end, and if the clock was to wander off in that time, the accuracy of the result would be lost. For the delay to be measured with an accuracy to match the resolution would require an accumulated clock error of less than 125 ±us per day, or nearly one part in 10 7. Only the most expensive rubidium or caesium standards can achieve this low order of drift over long periods, so the decision was made to utilise the timing provided by the GPS (Global Positioning Satellite) system. We have discovered that many of the sounder tansmissions are also controlled in this way. The one second references pulses generated by a good GPS receiver are accurate to ±1 us on a continuous basis.
In addition to the precision second pulse, the system makes use of the GPS NMEA messages, which allow the equipment to recognise which UTC second each pulse refers to. The NMEA information on its own is not sufficiently accurate, since it suffers unreliable delays in serial transmission and reception, and is also the timing is affected by the actual data transmitted.
|Copyright Murray Greenman and Peter Martinez, 1999 - 2003|