An ionosonde, or chirpsounder, is a special radar for the examination of the ionosphere. The basic ionosonde technology was invented in 1925 by Gregory Breit and Merle A. Tuve  and further developed in the late 1920s by a number of prominent physicists, including Edward Victor Appleton. The term ionosphere and hence, the etymology of its derivatives, was proposed by Robert Watson-Watt.
An ionosonde consists of:
- A high frequency (HF) radio transmitter, automatically tunable over a wide range. Typically the frequency coverage is 0.5–23 MHz or 1–40 MHz, though normally sweeps are confined to approximately 1.6–12 MHz.
- A tracking HF receiver which can automatically track the frequency of the transmitter.
- An antenna with a suitable radiation pattern, which transmits well vertically upwards and is efficient over the whole frequency range used.
- Digital control and data analysis circuits.
The transmitter sweeps all or part of the HF frequency range, transmitting short pulses. These pulses are reflected at various layers of the ionosphere, at heights of 100–400 km, and their echos are received by the receiver and analyzed by the control system. The result is displayed in the form of an ionogram, a graph of reflection height (actually time between transmission and reception of pulse) versus carrier frequency.
An ionosonde is used for finding the optimum operation frequencies for broadcasts or two-way communications in the high frequency range.
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. In addition to their use in probing ionospheric properties, these transmitters are also used for over-the-horizon radar systems.
An analysis of current transmitters has been done using 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
- F.C. Judd, G2BCX (1987). Radio Wave Propagation (HF Bands). London: Heinemann. pp. 12–20, 27–37. ISBN 0-434-90926-2.
- Peter Martinez, G3PLX: Chirps and HF Propagation http://jcoppens.com/radio/prop/g3plx/index.en.php
- Radar Handbook (M. Skolnik) http://www.helitavia.com/skolnik/Skolnik_chapter_24.pdf
- Pieter-Tjerk de Boer, PA3FWM: Chirp Signals analyzed using SDR http://websdr.ewi.utwente.nl:8901/chirps/
- Davies, Kenneth (1990). Ionospheric Radio. IEE Electromagnetic Waves Series #31. London, UK: Peter Peregrinus Ltd/The Institution of Electrical Engineers. pp. 93–111. ISBN 0-86341-186-X.
- Gwyn Williams, G4FKH (May 2009). "Interpreting Digital Ionograms". RadCom. RSGB. 85 (05): 44–46.
- Breit, G.; Tuve, M.A. (1926). "A Test of the Existence of the Conducting Layer". Physical Review. 28 (3): 554–575. Bibcode:1926PhRv...28..554B. doi:10.1103/PhysRev.28.554.
- Appleton, E. V. (January 1931). "The Timing of Wireless Echoes, the use of television and picture transmission". Wireless World (14): 43–44.
- http://www.ngdc.noaa.gov/stp/iono/ionogram.html : NOAA Ionosonde portal to information and data services
- http://www.ngdc.noaa.gov/stp/IONO/Dynasonde/ : NOAA Dynasonde: Real-time Ionospheric Explorer by advanced and prototype analysis methods.
- http://ulcar.uml.edu/digisonde.html : Lowell Center for Atmospheric Research, MA, USA
- http://ulcar.uml.edu/stationlist.html : Ionosonde (partial) station list by location
- http://car.uml.edu/common/DIDBFastStationList : DIDBase Fast Station list
- http://www.iono.noa.gr : National Observatoy of Athens, Greece
- http://www.sil.sk.ca/content/cadi : Canadian Advanced Digital Ionosonde (CADI)
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