How does a radar unit work?
The word radar stands for „Radio Direction Finding and Ranging“", meaning it is used for determining the position and distance of objects through the transmission and reflection of radio waves. These short, electromagnetic waves, also known as pulses are emitted by your radar antenna on board. When these pulses collide with other vessels, buoys, or a surrounding port facility etc., they are reflected and subsequently picked up by your radar antenna. Your connected radar equipment or radar-capable multifunction display evaluate the information sent, provide a reading for potential obstacles and objects, calculate your distance from them, and also determine their direction/bearing as well. Through the continuous rotation of the radar antenna, a uniform dispatch of pulses is generated in all directions, ensuring that not just one side of your vessel is monitored.
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Radar-capable multifunction displays
What is the big advantage of radar technology? - Radar vs. AIS
With a radar system on board you will be able to see ALL boats and obstacles in the area. When using AIS systems, you only see a ship or obstacle that sends its own specific AIS data. Although the use of AIS in commercial shipping is mandatory, this is still unfortunately not the case in regards to recreational marine vessels. So, in order to minimise the risk of a possible collision in extreme fog, in the dark, or in heavy rain, we recommend our SVB water sports enthusiasts to purchase and use a radar unit in addition to their AIS transponder.
What is a pulse radar?
Radars differ in the way the high frequency signal is generated and processed. A pulse radar unit, also known as an impulse radar, is the classic type of radar. It works with a magnetron, i.e. an electron tube, which provides high frequency with a high power (over 2000 W). In order to do this, it must warm up before operation, which is done by electrical heating. Depending on the size of the radar, this can take several minutes. Pulse radars emit high-power, short pulses at a rate of a few tenths of a microsecond. These short pulses improve resolution at far range, but weaken the echo. As a result, a high transmission power of several kilowatts is required. When transmitting and shortly afterwards, the receiver cannot evaluate echoes. This means that an area around the antenna cannot be used and is called the dead zone. Depending on the technology and the range set, this zone can be several meters or even several miles in size. Over the years, pulse radar technology has been constantly developed, so that today there are considerably more sophisticated radars with clear advantages available to purchase.
What is a pulse compression radar?
Unlike pulse radar, pulse compression radar achieves long-range resolution not by the speed of pulses, but by a constantly changing frequency of the transmitted signal, allowing the receiver to determine within one echo from which part of the transmitted signal it originates. Pulses of different lengths are sent one after the other, short pulses with low power for close range and longer pulses with more power for longer distances. To receive a reliable echo, pulse compression radars require considerably less transmitting power than pulse radars (20-40 watts). The receiver mathematically codes the radio frequency module carrier to increase the transmitted waveform’s bandwidth and then compresses the received echo waveform. This technique is known as pulse compression. Interference caused by rain or swell is automatically reduced by these radars and the sensitivity of the receiver is increased.
Pulse compression radars
What is a broadband radar and what are the advantages of broadband radar technology?
Broadband radar works with FMCW technology (Frequency Modulated Continuous Wave) and is therefore also called continuous wave radar. Broadband systems feature hardly any radial deformations and distortions. This means that close-range targets are displayed especially clearly without distortion, but also allows long-range targets to be easily identified. The latest generation of broadband radar antennas can even map objects at a distance of 32 nm clearly and distinctly. Broadband radar doesn’t require a warm-up period, which is very useful in certain conditions, such as sudden fog. They also emit much less radiation, reducing health hazards caused by radiation. Thus, you can mount your new broadband radar antenna on to any point on board, without having to search for the antenna’s blind zone. Modern broadband radar antennas can display rain fronts, storms and other weather conditions and distinguish them from other signals. The only disadvantage of broadband radar technology is that so-called radar beacons (RACON), SART signals, and RTEs cannot be identified and displayed.
What is a Doppler radar?
Doppler radars, also called Doppler pulse compression radars, are the very latest in radar technology. Doppler radars are significantly higher in resolution, emit less radiation, are lighter and more efficient than older, analogue magnetron radars. At close range, they offer better detection of objects such as buoys or kayakers. They require no warming-up, are immediately ready for use and have ranges of up to 48 nautical miles. Doppler radars can detect the smallest changes in echo delay using motion detection. Shorter echo delays identify nearing targets and longer delays detect distant targets. Furthermore, moving targets can be differentiated from stationary targets. A colour-coded display (approaching targets red, moving targets green) helps to interpret the radar data more quickly. The MARPA function (Mini Automatic Radar Plotting Aid) detects other ships including their speed and bearing and calculates the nearest point of approach (CPA - Closest Point of Approach) as well as the time to closest approach (TCPA - Time to Closest Point of Approach). The danger alert triggers an audible alarm as soon as a tracked target enters the pre-set danger zone. Doppler technology must be fully supported by your multifunction display. It is generally not possible to network radar antennas from different manufacturers. To use MARPA displays, we recommend an external compass which is connected to the radar unit and your multifunction display via NMEA 2000.
Interpreting radar echoes.
Reading and interpreting a radar display of the surroundings and objects requires some practice. In this respect, it is important to bear a few points in mind.
1 The size of displayed objects
The size of displayed objects is not necessarily proportional to their true size. In addition to the size of objects and the surface area the object has, the displayed size depends on the material composition of the target. Metal reflects very well, wood reflects poorly. The shape, position and height of the object is also important for the size displayed on the radar screen. The perfect target for your radar device to pick up would be positioned vertically to the water's surface, rectangular shaped with no round corners. It would be metallic with a large surface area and located at the same height as the radar antenna.
2 Relative motion
Always remember that your radar antenna is at the centre of the radar display and therefore does not move. The movement of your own ship is included in the movements of the displayed echoes. As a result, even fixed objects such as buoys or sections of land can appear to move on your screen.
3 Radial expansion of radar pulses
When objects on the same bearing are very close together, magnetron radar antennas often detect them as one large object. To ensure that two objects are detected & displayed separately, they must be more than one pulse width (half the actual duration of the pulse from the antenna to the object and back again) apart. To ensure a clear target definition even at close range, you should reduce the pulse width of your radar antenna. This also affects nautical mile ranges. In the case of the latest radar antennas with broadband, pulse compression and Doppler technology, this problem no longer exists, since they are generally more high-resolution in close range than older magnetron devices. The display can also be further improved on your multifunctional display by using appropriate filters.
We strongly recommend that you read the operating instructions for the antenna and your display monitor carefully before using your new radar system on the water. Familiarise yourself with the operating features of your system and if necessary, try it out in port. Whenever you install a new radar system, make sure that you check the correct bearing first!
Which faults and errors can be expected in radar equipment?
1 Reception gaps and losses
Radar antenna waves move at the speed of light. The time lapse between transmission and reception of pulses is generally so short that the radar antenna moves only minimally in the meantime. Nevertheless, during the brief "changeover" from transmission to reception, gaps may occur in the reception which may result in losses, especially if the object to be picked up is located at a long range and consequently the waves have to travel a longer distance. This is why radar systems from Garmin, Raymarine, Simrad, B&G and Furuno have functions that make it possible to switch to longer ranges. The system then automatically transmits extended pulses to keep any losses to a minimum.
2 False echoes
Another fault and misrepresentation can occur when several ships are using radar units on board within the same range. These glitches appear on your radar in the form of helically arranged points in the centre of your screen. Most manufacturers offer radar technology with interference suppression settings, which can soften and filter out these errors.
A different type of interference and erroneous display may occur if several vessels with a radar installation on board operate within the same area. These interference pulses are displayed on your radar as spiral points in the centre of the screen. Most manufacturers of radar equipment offer you interference suppression settings which can reduce and filter out the false and interfering echoes on the radar display.
How can I avoid collisions at sea and take evasive action?
If an object is approaching the centre of your screen, i.e. you, then you should be alert and, if necessary, take countermeasures and evasive action. An evasive manoeuvre should normally be carried out to starboard (see Rule 19 of the International Regulations for Preventing Collisions at Sea)
What is a Radar Overlay?
To identify unclear radar signals, detect approaching objects in time and avoid collisions, we recommend that you compare your radar image on board against other displays such as AIS, GPS and an electronic sea chart. A radar overlay is when radar images and electronic charts are superimposed on each other with the same alignment.
We recommend using an external compass sensor to ensure smooth radar overlay on your multifunction display. Some autopilots come with a sensor already installed.
The optimal mounting location for radar antennas
Radar antennas should be mounted as high as possible on board, which improves their range. In the case of older radar antennas, a high installation height is also important so that the deck area of your vessel is within the blind zone of the antenna, outside the (vertical) beam angle of the main lobe. This is because the radiation from impulse radar antennas can be harmful to your health if you are in close, regular contact. The higher the antenna is mounted, the larger the dead zone. However, if the radar antenna is mounted too high, the pulses will hit the water surface late and close range will be reduced. With new radar technologies such as broadband or pulse compression, radiation exposure is comparable to that of a mobile phone and echoes are displayed at close range from as little as 10 m, making the dead zone irrelevant here.
On sailing yachts, round random radar antennas are usually mounted on the mast. This saves space, in addition to the advantages already mentioned. Various mast mounts are available for this purpose. We especially recommend installing on the mast with a gimbal mast suspension. This mount is self-levelling, ensuring that your radar antenna always remains in a horizontal position even in heavy swell. This increases the antenna power many times over and thus provides better radar images.
How is a radar antenna wired on board?
Radar units are usually connected directly to the plotter via an Ethernet or RayNet interface. Some radar antennas require a power cable in addition to the data cable. Radar data cannot be transmitted via the NMEA protocol. Another alternative is a radar antenna with WiFi. In this case no wiring to the plotter is necessary. If possible, we recommend a wired radar antenna, as the WiFi signal may be affected by storms or other external influences.
Written by our SVB (technical) experts
Our team of SVB technical experts is there to give our customers professional advice on all matters relating to technology on board. With specialist know-how, extensive training, and a technical flair for electrical connections, fittings etc, our crew are more than qualified to advise our customers.