Technics of the Lights

RAdar beaCON - RACON

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A RAdar beaCON (English: RACON) is a radar response beacon (transponder) commonly used for marking hazards, especially for shipping. They are also often installed on buoys and other ship navigation aids. The radar beacon responds to the signal received from a radar.

Operation

Radar beacons respond to a radar signal with their own signal on the same frequency. As a result, navigation beacons that are not large enough to reflect the radar signals can also be detected with the radar. The response signal is in the form of a Morse code with a specific characteristic of its own, usually a single letter or number, to identify the navigational beacon. To avoid confusion with other radar reflections, only Morse codes starting with a dash (not starting with a dot) are used. To prevent the Morse code from being obscured when the image on the radar screen is zoomed in or out, the response signal must be proportionally scalable.

The radar beacon's signature is displayed on the radar screen as Morse code, which begins at the beacon's location and radiates in a short line from that point. By means of this characteristic, a beacon can be identified on nautical charts. In order to also be able to show objects behind the beacon on the screen, the beacon reacts with interruptions in the transmission of the response signal.

A common misunderstanding is that AIS transmitters are a replacement for radar beacons. This is not the case. Radar beacons are not dependent on communication with satellites and therefore offer more security than AIS systems. In addition, the purpose of AIS systems is substantially different from the purpose of a radar beacon. AIS transmitters and radar beacons are complementary, but not substitutes.

Usage

In Scotland, the Northern Lighthouse Board operates 29 racons, 22 on Lighthouses and 7 on buoys, but they are also used by other organizations such as rig owners. The use of these aids other than for navigation purposes is prohibited.

They are used as markers for:
 ⛆ Lighthouses and navigation buoys
 ⛆ The position of inconspicuous coastlines
 ⛆ Navigable waters under bridges
 ⛆ Offshore drilling platforms and other structures
 ⛆ Ecologically sensitive areas, such as coral reefs

They are also used for marking:
 ⛆ Temporary, new and not (yet) mapped hazards (with Morse code "D" per IALA guideline O-139)
 ⛆ To identify center lines and turning points
 ⛆ As a line of guiding beacons

Characteristics

The characteristics of radar beacons are defined in ITU-R Recommendation M.824 Technical Parameters of Radar Beacons (RACONS) and IALA Guideline R-101. Radar beacons typically operate in the 9320 MHz to 9500 MHz marine radar bandwidth (X band) and typically also operate in the 2920 MHz to 3100 MHz marine radar bandwidth (S band). Modern radar beacons are frequency agile, meaning they have a wide-bandwidth receiver that detects an incoming radar pulse, matches the beacon's transmitter to the received bandwidth, and responds with a 25 microsecond long signal within 700 nanoseconds.

Older radar beacons operate in a slow "sweep" mode, with the transponder moving across the X band every one to two minutes. It only responds when the radar beacon is tuned to an incoming radar signal at the time the radar pulse arrives, meaning it only responds 5% of the time.

To prevent a radar beacon's response from masking important radar targets that lie behind the beacon, radar beacons only operate part of the time. In the UK the time limit for the radar beacon is 30%, which usually means that the beacon will respond to an incoming signal in a 20 second period, and not for the next 40 seconds. Sometimes 9 seconds on and 21 seconds off is used.

Leading line

Radar beacons can also be used for a leading line of beacons by a succession of two or more radar beacons. The beacons must then be placed within range of each other. This is used, for example, for an obstacle-free waterway with sufficient water depth.

Ramarks

A variant of the radar beacons are the so-called ramarks. These are radar beacons that continuously transmit a signal over a wide bandwidth, without having to be activated by an incoming radar signal. The signal forms a line of Morse characters on the radar screen from the vessel's position to the edge of the radar screen, and thus gives no indication of distance in nautical miles. Ramarks are used relatively little.