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This form is convenient. A mail from G3PLX (extracted from the Chirplist) explains the method a little more:
When I started studying chirps, I didn't know what method was used by the operators to describe the timing, and it seemed easiest to give the starttime as that at which the transmitter started it's sweep from zero MHz. This way, it's easy to calculate the time at other frequencies, and it's also much easier to relate sweeps of different rates, by just adding the frequency times sweeprate to the chirptime. I also decided to use the interval between each sweep (the period), because I could see that all the sounders I could hear were easiest to describe in this way.
Much later I discovered that BR, who invented the Chirpsounder (or rather Professor Barry invented it then formed Barry Reseach Inc), had chosen to describe sounders in terms of the 2MHz start time, and also they assumed that ALL sounders were 100kHz and worked in a basic 5 minute unit, so what we know as a 900: sounder would be described in BR terminology as a 00,15,30,45 (or 05,20,35,50 or 10,25,40,55) sounder. To convert from our chirptime notation to BR notation, add 20 secs then work out from the chirptime/period which of the twelve fiveminute slots contain transmissions.

One problem arises because there are a few BR sounders which (like Corrine), transmit 5 times per hour, four of them in what we would call a period 900 pattern, and the other transmission once per hour, 5 or 10 minutes after one of the 15minute transmissions. My notation cannot handle this, at least not as a single pair of numbers. The best I can do for Corrine is to describe it as 900:325 at 5 watts plus 3600:25 at 180 watts, but the chirp periodfinder process usually identifies it as 600:25, so we just have to keep a watch for different combinations like these. 
I still think my notation is more useful. For example it can handle the USN and Australian sounders with 12 minute and 7.5 minute periods, which the BR notation cannot (and the early BR receivers cannot receive them properly), and my notation is still useful for 50kHz/sec and 125kHz/sec sounders (and there are also 250kHz/sec sounders!)
Copyright Murray Greenman and Peter Martinez, 1999  2003 