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.
An analysis of current transmitters has been done using the SDR technology.
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/