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Solid state radar

Jun 1, 2009

The Navico group recently introduced a new radar: the 24-mile Simrad BR24. This isn’t an unusual event in the fast-moving world of marine electronics. This particular radar model, however, is different from any other marine radar previously released. The BR24 is a completely solid-state, low-power design that can display targets as close as a boathook’s length away. And the BR24 provides this close-in information using a transmitter that is a bit less powerful than the one in your cell phone. It’s likely the BR24 will set a new level of performance for yacht radars.

The technical advances of the BR24 don’t suddenly make conventional magnetron-based radars obsolete, of course. In fact, radar units in general have been getting better and more capable with each model. The BR24, however, does point the way toward exceptional performance for mariners such asccurately detect and display targets at very short range can be critical for navigation safety. We need to “see” the daymarks, isolated pilings and unofficial aids we casually refer to for navigation guidance in good weather. Conventional marine radars are usually unable to display the images of targets closer than about 180 feet, a distance that can be well beyond visual range. The BR24 avoids this limitation, clearly showing even the closest radar reflective objects (including, on our sea trial, small wood pilings).

The BR24’s exceptional target imaging capability is primarily the result of the use of a continuous wave radio transmitter in place of the pulse transmitter in the conventional radar, plus the use of separate transmitting and receiving antennas. (This technology, a continuous wave transmitter and separate transmitting and receiving antennas is used in aircraft radar altimeters).

Measuring distance
Standard pulse radars find the distance to a target by measuring the time delay from transmission of a pulse to the arrival of the reflected return signal. A non-modulated, continuous wave radar can determine the speed of an object that is reflecting energy back to the receiving antenna by measuring the Doppler shift frequency of the returning energy (the technique used in speed measuring radars employed by police and LIDARS), however, it cannot determine the distance to a target. To obtain distance information, the BR24 frequency modulates the continuous wave carrier. The change in frequency is at a rate chosen to match each selected maximum viewing range.

The frequency of energy reflected from a target can therefore be related to the time that has elapsed since the beginning of each frequency sweep of the modulating signal. This data comparison provides an accurate measure of the time that has elapsed since the signal was transmitted, thereby providing the distance to the target. For example, detection of a frequency that was equal to the frequency of the modulating signal at the mid point between the start and end of the frequency sweep would define the target’s position to be at 50 percent of the maximum selected range. Detection of even weak return signals is enhanced by the fact that the receiver circuits can analyze the incoming data by comparing its modulated frequency component with the modulation impressed on the transmitted signal.

Similar to an aircraft radar altimeter, the BR24 uses separate transmitting and receiving antennas, eliminating the need for the switching circuits used in a conventional radar to alternately connect the antenna to first the transmitter and then the receiver. The two printed circuit board antennas are mounted to the radar’s antenna rotation mechanism with the receiving antenna above, the transmitting antenna below. Stacking of the two antennas results in a slightly higher radome than those used to enclose a conventional shared antenna set. A close examination of the antennas discloses a small but significant difference in the pattern of the conductors on the circuit board. The difference is visual evidence of the fact that the antennas have opposing polarity, a technique that reduces the overall sidelobe response of the antenna system. Sidelobes are undesirable but generally unavoidable characteristics of directional antennas that cause energy to be radiated in unwanted directions away from the pattern of the main beam, simultaneously creating areas of undesirable increased sensitivity to incoming signals.

Milliwatt power
The transmitter in the BR24 radiates only 100 milliwatts (1/10 of a watt) of radiofrequency (RF) energy, less than many cell phones. While 100 mw is a very small fraction (1/20,000) of the typical 2 kW peak pulse power of a magnetron radar transmitter, it is more than sufficient to provide excellent results out to the radar’s maximum 24 nautical mile range. The BR24s ability to image distant targets at long range, with very low power can be understood when its average power, 100 mw, is compared with the average power of the 2 kW radar. A reasonable approximation of the difference between the two radars is evidenced by their DC power consumption, 18 watts for the BR24, 28 watts for a typical 2 kW magnetron transmitter radar. Both sets use a part of the power for the motor that rotates the antenna. The filament in the vacuum tube in the conventional radars magnetron also consumes a fair amount of power.

There is nothing new in the ability to achieve reliable radio communication over vast distances using minute amounts of RF power, including radar mapping of the moon and ham radio communication over continental distances. The performance of the BR24 is aided by the improved differentiation of target echo signals from the background noise provided by the frequency modulation of the carrier.

Easy installation
Installation of the BR24 radome is simplified by its relatively low, 16.3-lb weight and the use of Ethernet cable for connection to the associated chartplotters (various models from Lowrance, Northstar and Simrad). Power for the radome is supplied via four small wires that, with the Ethernet cable, result in an overall cable diameter of only 1/2 inch. Anyone who has “enjoyed” cutting and splicing the typical 22 conductor plus coax cable used with conventional radars will particularly appreciate this aspect of the installation of the radome. The antenna specifications for the BR24 are typical for any similar sized radome, horizontal beam angle of 5.4 degrees, vertical beam angle of 25 degrees. The very low emitted power of the BR24 eliminates concern about exposure of personnel close to the radar to RF energy; the intensity at the surface of the radome is significantly below the level considered safe for human exposure.

The BR24 is the radar sensor for Navico’s Lowrance, Northstar and Simrad chartplotters. Operation of the BR24 radar with any of the compatible Lowrance (HDS - High Definition System), Northstar (8000i, 12” & 15”, M121 an M84) or Simrad (NX40, NX45 and GB40) chartplotters is simplified by radar’s “instant-on” capability. With no magnetron filament to heat before activating the transmitter, there is no need for the usual 90 second (or longer) time delay. Sailors whose boats have limited electrical power will appreciate being able to operate the radar in a “watchman” or intermittent mode when the radar is used as a general surveillance device when voyaging in open water.

Although the BR24 can be used successfully without much attention to the various radar controls they are all there, accessed through the menus on the associated chartplotters. The list includes gain and STC (automatic and manually selected), rain, EBL/VRM, choice of display, heading up, north up, course up, plus relative and true motion (provided the necessary heading sensing equipment is installed). Learning to use the BR24 will be exciting, especially for experienced radar users.

Navico claims that the use of FM modulation improves the detection of targets in rain. The clear skies during the sea trial in Miami prevented us from verifying this claim.

For bigger vessels that are already equipped with conventional 48 to 72-mile range equipment, the BR24 can be used in a specialty role: it can provide the close-in images that the conventional radar sets may not be able to deliver.

Contributing editor Chuck Husick is a sailor, pilot and writer who rides his bicycle in Tierra Verde, Fla.

Using FM continuous wave to measure range

A conventional radar transmits a stream of short duration pulses of radiofrequency energy. The receiver then measures the time that has elapsed since the transmission of a pulse and the detection of a returning echo signal from a target. Since the propagation speed of the radar signal in air is known (and is reasonably constant), the time delay from pulse transmission to detection of a return signal can be used to determine the target’s radar range.

The range measurement system employed in the Navico broadband radar determines the distance to a radar target in a different fashion. It compares the frequency of a target echo to the known and linearly increasing frequency used to modulate the transmitter.

For example: if the modulating frequency varied from 1 Hz to 100 Hz during the time it takes the transmitter’s signal to travel 1,000 feet and the measured frequency of the signal reflected from a target was 50 Hz, we would know that the target was located 500 feet from the transmitter. In actuality, the modulating frequency is swept over its designed frequency range very rapidly since it takes only 6.3 microseconds for the radar signal to travel one nautical mile.

For the technically inquisitive, the published specifications for the BR24 state that the available range settings are from 1/32 to 24 nautical miles in 16 range steps, the sweep repetition frequency is 200 Hz and the sweep time 1 ms with a sweep bandwidth of 65 MHz max. Antenna horizontal beam width is 5.2 degrees. The BR24 radar automatically adjusts the modulating frequency sweep rate (rate of change) to match the user-selected maximum range.

Accurate measurement of distance, especially to close-in targets, achieved by this radar requires very rapid and precise comparison of the frequency in the echo signal with the transmitted signal. The signal processing achieved in this radar has only very recently become affordable with the development of new solid state high-frequency devices.

Chuck Husick

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