Radar
Radar is an indispensable navigational tool due to its ability to “see” through the night and inclement weather. Charts offer a dated view of the boat's surroundings. Charts coupled with GNSS positioning can give a false sense of security. Radar provides real time perspective. Because it is so important, we want two independent radar systems.
magnetron based radars
Traditional radars are based around a magnetron that generates a powerful microwave signal. The magnetron uses a combination of electricity and magnets to excite electrons and aim the signal at a particular direction. Traditional radars produce pulses of microwaves that bounce off objects and create an echo read and interpreted by the radar unit. Magnetron based marine radars use either X-Band or S-Band frequencies. X-Band is better for collision avoidance because it tends to reach further.
One important measure of traditional radar is power, generally listed in KW. The higher the power rating, the stronger the pulse and return signal will be. Small radars start around 4KW. Large yachts and workboats are 25KW or more.
Solid State Radars
A relatively new type of radar is based around solid state technology (aka Broadband). Not simply an upgrade in components, solid state radar does more then broadcast a pulse. Instead, it broadcasts a constant signal where the frequency changes as the system sweeps.
Solid state radar has several advantages; The ability to "see" objects within 50 feet where traditional radar is blind for 200 feet. It uses very little electricity allowing us to stay online in the event of a general power failure. In addition, solid state radar responds immediately without warm-up or period of adjustment. It also tends to be combined with Doppler, as described below.
Radome vs. Open
Both bands of radar are offered as radome or open array. Radome is radar where the antenna is covered by a plastic shield. These are often used by sail boats because they do not get hung up in sails. Open arrays have bar shaped antennas that rotate and are available in higher power ranges.
One measurement of a traditional radar is the width of its beam. A tighter beam is more precise with the ability to locate a particular object and/or distinguish between two objects. With a wide beam system a piling may appear as a medium sized blob. In a narrow beam system that same piling appears as a well defined dot.
Mounting
Many of the measurements described above are meant to convey how far a radar can "see." Many manufacturers claim a figure in miles for each unit. The truth is that no factor determines the range of a radar more than the height it is mounted above the water. Radar is a line of sight system, incapable of detecting objects over the horizon. Therefore, even though a radar is rated at 96 nautical miles it is unlikely to see an object at that range.
The general formula for radar height and range: 1.22 times the square root of the elevation of your radar plus 1.2 times the square root of the elevation of the target. Antennas on Fir will be mounted 56 feet above the water. Regardless of the power of our radar, we will be able to see large ships at about 15 nautical miles (1.22 times 7.48) plus (1.2 times 5). A 10 foot buoy will be seen at 13 nautical miles (1.22 times 7.48) plus (1.2 times 3.16)
Software
A large part of radar performance is software. High and ultra high definition radar have superior graphic rendering software.
Radars have a large number of operating parameters. A good radar technician can alter these operating parameters to adjust for the exact conditions facing the boat. Furuno radars are particularly noted for offering these options to a skilled operator. We do not plan to have a dedicated radar technician however most systems offer modes or filters that allow a less trained person to achieve similar results. One filter may adjust the signal to account for rain and heavy seas that would otherwise "white out" the screen. Other filters increase the sensitivity to identify weather or flocks of birds for fishing. Good filters do not produce the same result as a good radar technician but it is the best answer in our case.
MRPA is a software feature that allows the system to lock on to a contact. Obviously, every radar will see a contact and display it on the screen. A MRPA system tracks the contact and determines its relative speed and heading. More advanced systems calculate the point of nearest contact and other data designed to indicate the importance of that contact. MRPA systems require the user to manually mark a contact for tracking while ARPA systems automatically commence tracking contacts. Furuno seem to be at the forefront of this technology at the moment.
In 1842 Christian Doppler discovered that sound waves from a moving object modulate as the object passes by a stationary object. Newer radars (nearly all broadband radars) use this principle to determine the speed and relative direction of an object. This combined with MRPA and AIS provide multiple ways to track the speed and direction of other boats.
As indicated under chartplotters, an advantage of modern software driven systems is that information from various devices can be integrated. Radar can be overlaid across charts and satellite images such that we can confirm our distance to physical landmarks (which may have shifted). Radar imagery can be combined with AIS information as yet another check on the location, speed and heading of other vessels.
CHOICE
Based on all of this, we have decided to purchase two radar systems: One is a traditional X-Band radar. The other is a broadband radar.
We have decided on a 25KW traditional system with 4 or 6 foot open array offering a beam width of less than 2 degrees. We also want a broadband satellite with a 6 foot open array that offers Doppler tracking.
All the big four manufacturers make radars that meet our general requirements. Furuno has been the industry leader for many years. They clearly have the software advantage.