||The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (September 2015)|
When a racon receives a radar pulse, it responds with a signal on the same frequency which puts an image on the radar display. This takes the form of a short line of dots and dashes forming a Morse character radiating away from the location of the beacon on the normal plan position indicator radar display. The length of the line usually corresponds to the equivalent of a few nautical miles on the display.
Within the United States, the United States Coast Guard operates about 80 racons, and other organisations also operate them, for example the owners of oil platforms. Their use for purposes other than aids to navigation is prohibited, and they are used to mark:
- lighthouses and navigation buoys
- navigable spans under bridges such as
- to identify centre lines and turning points
- offshore oil platforms and other structures
- including approximately 35 in the Gulf of Mexico 
In other parts of the World they are also used to indicate:
- temporary, new and uncharted hazards (with a Morse character "D")
- as leading line racons
Their characteristics are defined in the ITU-R Recommendation M.824, Technical Parameters of Radar Beacons (RACONS). Racons usually operate on the 9320 MHz to 9500 MHz marine radar band (X-band), and most also operate on the 2920 MHz to 3100 MHz marine radar band (S-band). Modern racons are frequency-agile; they have a wide-band receiver that detects the incoming radar pulse, tunes the transmitter and responds with a 25 microsecond long signal within 700 nanoseconds.
Older racons operate in a slow sweep mode, in which the transponder sweeps across the X-band over 1 or 2 minutes. It only responds if it happens to be tuned to the frequency of an incoming radar signal at the moment it arrives, which in practice means it responds only around 5% of the time.
To avoid the response masking important radar targets behind the beacon, racons only operate for part of the time. In the United Kingdom, a duty cycle of about 30% is used — usually 20 seconds in which the racon will respond to radar signals is followed by 40 seconds when it will not, or sometimes 9 seconds on and 21 seconds off (as in the case of the Sevenstones Lightship). In the United States a longer duty cycle is used, 50% for battery-powered buoys (20 seconds on, 20 seconds off) and 75% for on-shore beacons.
Ramarks are wide-band beacons which transmit continuously on the radar bands without having to be triggered by an incoming radar signal. The transmission forms a line of Morse characters on the display radiating from the centre of the display to its edge. They are not used in the United States.
|Look up racon in Wiktionary, the free dictionary.|
- Emergency Position-Indicating Radio Beacon (EPIRB)
- Marine VHF radio
- Search and Rescue Transponder
- Light List, Volumes 1-7. United States Coast Guard. 2009.
- Light List, Volume III, Atlantic Coast, Little River, South Carolina to Econfina River, Florida (PDF). Light List. United States Coast Guard. 2009.
- Light List, Volume VI, Pacific Coast and Pacific Islands (PDF). Light List. United States Coast Guard. 2009.
- Light List, Volume IV, Gulf of Mexico (PDF). Light List. United States Coast Guard. 2009.