A chirp transmitter is a shortwave radio transmitter that sweeps the HF radio spectrum on a regular schedule. If one is monitoring a specific frequency, then a chirp is heard (in CW or SSB mode) when the signal passes through. These transmitters are mainly used for over-the-horizon radar systems  and for probing ionospheric properties.
For better identification of chirp transmitters the following notation is used: <repetition rate (s)>:<chirp offset (s)>, where the repetition rate is the time between two sweeps in seconds and the chirp offset is the time of the first sweep from 0 MHz after a full hour in seconds. If the initial frequency is greater than 0 MHz, the offset time can be linearly extrapolated to 0 MHz.
List of chirp transmitters
- Radar Handbook (M. Skolnik) http://www.helitavia.com/skolnik/Skolnik_chapter_24.pdf
- Peter Martinez, G3PLX: Chirps and HF Propagation http://jcoppens.com/radio/prop/g3plx/index.en.php
- Pieter-Tjerk de Boer, PA3FWM: Chirp Signals analyzed using SDR http://websdr.ewi.utwente.nl:8901/chirps/