The Canadian Aids to Navigation System 2011
Radio Aids to Navigation
Radar Reflectors and Radar Beacons (RACONS)
The detection of a radar target is essentially dependent on the level of energy reflected back into the radar receiver from the target. When an aid to navigation gives a poor radar echo, equipment may be fitted to the aid to give an enhanced echo on a radar display. There are two main methods of producing a radar enhanced target. The first is by using a passive device such as a radar reflector to enlarge the apparent echoing area of the target and the second is by the use of a radar beacon (RACON), which is an active device. RACONS can also be used to avoid confusion between radar targets that may look similar, because they produce an easily identifiable coded trace on the radar display. This feature makes RACONS effective in the marking of important or radar inconspicuous features or structures such as gently sloping shorelines and bridge piers.
Certain fixed shore aids and most buoys are designed or equipped to enhance the aids’ ability to reflect radar signals. Radar reflectors may also be established as independent aids to navigation. Independent radar reflectors are symbolized on charts and those established on lighted aids are advertised in the appropriate List of Lights, Buoys and Fog Signals publication.
Radar Beacons (RACONS)
A RACONS is comprised of three main components: a receiver, a transmitter and an antenna common to both the receiver and the transmitter. A radar within the range of the RACONS interrogates the RACONS each time the radar antenna points towards the RACONS. The RACONS receiver detects the radar interrogating the signal and triggers the RACONS transmitter. The transmitter may reply with a single pulse for each trigger but normally the response consists of a series of coded pulses (Morse code) for RACONS identification. After triggering, a finite time must be allowed for the RACONS to respond. This results in a transmission that is delayed in time (and range) with respect to the passive echo of the structure on which the RACONS is mounted. The delay is generally equivalent to a range of less than 100 metres and, therefore, can often be disregarded at ranges greater than a few nautical miles. It is at short ranges that this error is significant. Normally, the station structure echo is visible and its range can be measured to full radar accuracy.
Radar operators may notice some broadening or spoking of the RACONS presentation when their vessel approaches closely to the RACONS. This effect can be minimized by adjusting the IF gain or sweep gain control of the radar (other targets will also be reduced in intensity).
Care must be exercised in the use of the radar display controls. The RACONS presentation can be virtually eliminated by operation of the Fast Time Constant controls of the radar. The RACONS replies may also be suppressed by the operation of the automatic video processor which is found in an increasing number of marine radars.
Frequency Agile RACONS
This RACONS is now the most commonly used RACONS in Canadian waters. The Frequency Agile RACONS (RACONS) measures the frequency and signal strength of the interrogating radar pulse, then tunes its transmitter to that frequency before responding. This RACONS provides service for X band marine radars and some installations also provide service for the marine radar band of 2920 to 3100 MHz (10 cm or S band). While it is possible for a response to be displayed on each antenna scan of every radar within range, in actual practice, these RACONS are programmed to turn off for a pre-selected period at regular intervals to prevent the masking of other echoes.
The locations, codes and operating frequencies (X, S or X and S) of RACONS are published in Notices to Mariners and listed in the appropriate marine publications, i.e. Radio Aids to Marine Navigation, Sailing Directions and List of Lights, Buoys and Fog Signals. The Canadian Hydrographic Service charts display the Morse code of RACONS only in areas where more than one RACONS can be interrogated simultaneously.
Global Positioning System (GPS)
The Global Positioning System is a worldwide satellite based radio navigation system developed and operated by the United States Department of Defence.
In the Global Positioning System, a transmission originates from satellites orbiting the earth. These transmissions contain information providing a receiver with precise, continuous, worldwide, all weather three dimensional position information for land, sea and air applications. Although the primary mission is to meet military requirements, the Global Positioning System is also being made available without cost for civil navigation. Two levels of service are being provided, one for civilian use and the second for military use.
Standard Positioning Service (SPS)
Standard Positioning Service, provided for civilian use, is a positioning and timing service made available to all Global Positioning System users on a continuous, worldwide basis with no direct charge. The Standard Positioning Service provides the capability to obtain horizontal positioning accuracy within 20 metres at 95% probability.
With the Global Positioning System operational, all of Canada’s navigable waters are effectively blanketed by a continuous, all-weather, accurate positioning signal. It is capable of meeting the need of civil and marine interests in all but the most restrictive navigation situations.
The system enables users equipped with suitable receivers, on land, at sea or in the air, to establish their position, speed and time at any time of the day or night and in any weather conditions. The system provides a level of accuracy equal to or better than any other radio navigation system available today.
Differential Global Positioning System (DGPS)
Figure 3: DGPS station coverage in Sandspit, Alert Bay, Richmond, Amphitrite point, Wiarton, Cardinal, Saint-Jean-sur-Richelieu, Lauzon, Rivière-du-Loup, Pt, Escuminac, Moisie, Rigolet, Cape Norman, Cape Race, Cape Ray, Fox Island, Hartlen Point, Western Head and Partridge Island.
A method of obtaining greater accuracy from GPS is through a technique called Differential GPS (DGPS). This technique corrects the inherent inaccuracies of the GPS signal by comparing the position calculated by GPS to a known geographic position.
Figure 4: Position information from Reference station is compared to position received from GPS satellite and DGPS receiver generates corrections to be broadcasted to the user through the MF Transmitter.
A reference station is established at a known geodetic location and position information received from the GPS satellites is compared to this known geodetic location. Based on this information, differential corrections are generated and broadcast to the user via Marine MF radiobeacons.
The Canadian Coast Guard implemented the differential service nationally in 1996 and declared the DGPS service as providing a Full Operational Service (FOS) on May 28, 2000. The Canadian DGPS service complements the United States Coast Guard system.
The system provides a positioning accuracy of 10 metres or better 95% of the time and provides integrity monitoring. In this sense, a warning signal advising the mariner that the service is unreliable is automatically transmitted in any case where the accuracy provided by a reference station falls below established limits. Should the differential broadcast be lost or unavailable, a DGPS receiver can continue to operate in GPS mode using the SPS signal.
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