Power pulses and frequency variation
Broadband and conventional radar examined
A year ago, the introduction of the Navico 24-mile broadband radar (BR24) scanner, marketed under the names Simrad, Lowrance, and Northstar, caused quite a stir in the boating community. It has been extolled for its virtues of low power, no “main-bang blanking,” very good performance in rain and sea clutter, and very sharp detailed displays. Some observers have claimed that broadband radar will overtake conventional radar for recreational boaters, but Navico itself says that, while broadband is the better choice in many situations, conventional radar is superior in certain applications. My goal here is to provide some insight into the performance differences between conventional and broadband radar by comparing the major engineering details.
I used a 24-mile Raymarine Pathfinder conventional radar for the comparisons. One of my sources for broadband radar was online documentation for the BR24. Another source was Don Korte, senior product manager and principle engineer for Navico Americas, whose help in answering my technical questions I greatly appreciate.
Pulses. Broadband and conventional radars both emit pulses and determine the time-of-flight interval between when the system emits the pulse and when the system receives the pulse after it reflects off a target. Time-of-flight directly determines range to the target. We’ll look at internal pulse characteristics, and how the two radar systems measure time-of-flight, but first let’s discuss pulse power, pulse duration, and pulse repetition interval (the time between pulses). The most significant performance differences between broadband and conventional radar systems, detection performance and average emitted power, result from these external characteristics.
Conventional radar emits short-duration, high-power pulses; broadband radar emits long-duration, low-power pulse. Conventional radar uses different pulse durations at different ranges: shorter pulses at short range for improved range resolution and longer pulses at long range for improved detection. Similar details of broadband pulses are proprietary, so I have used the nominal values in online technical specification documents in my analysis.
Pulse energy and detection performance. The energy in each pulse, which one calculates by multiplying pulse power times pulse duration, directly determines target detection performance. Compared to broadband radar, conventional radar pulses have one-third more energy at short range and 20 times the energy at long range. Consequently, at all range settings conventional radar detects smaller targets at the same range, similar targets at longer range, and similar targets at similar ranges in heavier fog and drizzle. The difference is significant at long range, in which case conventional radar detects similar targets at twice the range and targets .05 the size at similar range. Moreover, a target that broadband detects in clear weather, conventional can detect in 10-meter visibility fog. Conventional provides better target detection.
Average power and safety. Simple calculations show that broadband emits 20 to 80 times less average power than does conventional depending on range setting. Many people claim that the much lower average emitted power makes broadband radar much safer than conventional radar, especially when mounted on smaller vessels where there is more opportunity for passengers or crew to be in the beam. Keep in mind, however, that conventional 24-mile marine radar systems meet accepted safety standards, so even though conventional systems emit much more power than broadband systems there is no reason to question their safety. Following the common-sense guidance to mount the antenna several feet above the level of any crew or passenger’s head should remove any lingering concerns about safety. Moreover, people are opaque to radar so mounting any radar antenna, conventional or broadband, where someone might block the beam is a bad idea because such blockage would produce a radar-blind sector. Even though it might be safe to mount a broadband radar so low that it could irradiate passengers or crew, doing so would compromise radar performance.
Power consumption. Power consumption, not just emitted power, is important for voyagers on a limited electrical budget. Power consumption specifications for the broadband scanner are 1.6 W in standby mode and 17 W when transmitting. Corresponding specifications for the conventional scanner are 9 W in standby and 28 W when transmitting. That is, when active, broadband draws 11 W less power than conventional, roughly 1.0 amp from a 12-volt system. Power saving in standby mode is less, 7.4 W, but still significant for cruisers. Broadband definitely consumes less power.
Internal pulse characteristics and main-bang blanking. Turning from the pulse’s external characteristics to internal characteristics, pulse structure has far-reaching consequences. The conventional pulse waveform is a sinusoid of constant frequency for the duration of the pulse. The broadband pulse waveform is also a sinusoid, but the pulse sweeps over a range of frequencies instead of being constant. The reason for the difference lies in how time-of-flight is measured. Conventional radar measures the time between when it emits the pulse and when it detects the reflected pulse. In order for this to work, the emitted pulse and received reflected pulse must not overlap in the receiver. The system accomplishes this by not turning on the receiver until some time after it transmits the pulse. The delay in turning on the receiver causes the system to be blind to close-in targets because pulses reflected from nearby targets arrive at the receiver while the receiver is off. This no-recieve effect is called “main-bang blanking.”
The narrowest pulse, which is used at the shortest range setting, requires at least 10 meters (32.8 feet) blanking. Range blanking is about 22 meters (72.1 feet) on my conventional system, so there is a comfortable margin. Broadband’s much longer pulse would require 80-mile main-bang blanking if overlapping pulses were not allowed. Clearly, this is not practical and broadband has to work with overlapping emitted and reflected pulses. In order to do so, broadband separates emitted and received pulses using frequency rather than time.
The broadband pulse frequency varies steadily with time rather than remaining constant. Consequently, the instantaneous frequencies of the transmitted pulse and any reflected pulse are different, providing the desired pulse separation. The difference between the two frequencies directly relates to time-of-flight and hence range.
Mounting height and main-bang range blanking. Since broadband can handle overlapping emitted and received pulses, there is no need for close-in blanking, and so broadband detects and displays targets as close as a couple feet. In contrast, a typical, conventional system does not display any targets closer than about 73 feet. It is not correct, however, to think that broadband displays everything on the water within 73 feet of the vessel or that conventional does not display anything within 73 feet. The reason is that the blanking range is slant range from the scanner rather than horizontal range from the vessel. Slant range and horizontal range may be quite different if the scanner is mounted high above the water as it would be on a sailboat’s mast.
On the one hand, if I mounted a broadband scanner 30 feet above the deck of my sailboat, the slant range to the bow would be 37 feet. Any target displayed at 37 feet or closer would be own-ship clutter and broadband’s capability to detect and display targets closer than 37 feet would be useless to me.
On the other hand, if I mounted a conventional scanner at 30 feet, the edge of the main-bang blanking zone (73-foot slant range) would be 66 feet horizontal range from the mast, roughly one boat length from the bow. Main-bang blanking makes a one-boat-length deep doughnut-shaped annulus at the bow unavailable to me.
With the scanner mounted low to the waterline as on a small powerboat, slant range and horizontal range are roughly the same, and so the benefit in coverage provided by no main-bang blanking is significant, a 73-foot radius circle about the boat. However, the coverage benefit is much less significant as vessel size and scanner mounting height increase.
Range resolution at short range. Image sharpness depends on resolution, both range resolution and cross-range (side-to-side) resolution. Conventional and broadband systems use antennas with similar characteristics and so they have similar cross-range performance. Range resolution is a very different situation. Range resolution in a conventional system depends on how well pulse arrival time is measured. This is roughly the pulse duration. At short-range settings with 0.065-microsecond pulse durations, arrival time resolution is about 0.065 microseconds, which corresponds to a range resolution of 10 meters (32.8 feet). On the other hand, the broadband system measures frequency and calculates range from the frequency. Frequency resolution is roughly the reciprocal of pulse duration. The 1.0-microsecond pulse provides 1-kHz resolution in frequency, which corresponds to time of flight of 0.014 microseconds, and 2.14-meter (7 feet) range. Broadband range resolution is about five times better than conventional at the shortest range setting.
Range resolution at long range. While range resolution generally deteriorates as the range setting increases in both types of radar, the details and reasons are different. The determining factor for conventional radar is pulse duration. In my system, pulse duration increases as the range setting increases from the minimum to six nautical miles and remains at the six-mile value for all longer-range settings, and so conventional-system range resolution deteriorates with increasing range up to six miles and remains constant thereafter.
The determining factors for broadband radar are the signal processing algorithms and the unit’s computer. While details are proprietary, general results can be deduced from standard signal processing techniques. Simply, the computer would have to process more data points every time the range setting increases in order to maintain the theoretically ideal resolution. Since the computer has limited processing capability, broadband range resolution deteriorates with every increase in range setting beyond the point at which the computer capability maxes out, requiring that more data be discarded with each increase in range setting. According to Navico, broadband and conventional provide similar range resolution at 16 nautical miles. Consequently, broadband range resolution, which is up to five times better than conventional at the shortest range settings, steadily deteriorates with increasing range settings. Broadband is better than conventional at short range. They are equal at 16 miles. Conventional is better at longer ranges.
Rain and wave clutter. Range resolution determines performance in clutter from rain and waves. Every radar set processes all returns from each resolution cell (the volume defined by the antenna beamwidth and range resolution) as a single object. In order for a target to be detected in clutter, the energy reflected by the target must be greater than the energy reflected by all the non-target clutter from rain and waves in the resolution cell. Because broadband’s range resolution is roughly one-fifth of the conventional scanner’s at short-range settings, its resolution cell is one-fifth the size, and it experiences one-fifth the clutter at short-range settings.
Rain clutter limits the maximum range at which a scanner can detect a given target. Heavier rain, larger-range resolution cell, and greater range increase clutter in the resolution cell and decrease the maximum range at which a target may be detected. The two systems exhibit similar rain clutter performance at 16 nautical miles because their resolution cells are the same size at that range setting. Broadband is better at shorter ranges, as much as five times better, because range resolution is better, but worse at longer ranges because range resolution is poorer.
Wave clutter similarly limits maximum detection range. However, whereas rain clutter steadily increases with distance because the resolution cell increases, wave clutter reaches a peak where the front surface of the wave is perpendicular to the scanner. Wave clutter decreases at shorter and longer ranges. The point of maximum wave clutter increases as wave height increases and as the scanner’s mounting height increases. Overall, wave clutter is a short-range problem, and broadband’s superior range resolution at short range gives it superior performance in wave clutter.
Minimum range. Range resolution controls how well the scanner separates targets. Displaying the information to the user is another matter. Using a photography analogy, you need enough pixels on the display to give good visual detail. The minimum range setting on a typical conventional scanner, 760 feet (0.125 nautical miles) matches the 33-foot range resolution. Broadband’s four-fold to five-fold better range resolution nicely matches the four-fold better minimum range setting, 200 feet. That is, broadband provides coverage in the sub-800 foot range that conventional is incapable of providing.
Invisible targets. The different internal pulse characteristics have other consequences. Since the pulses are quite different, broadband radar does not detect conventional radar and vice versa, which means that the two systems do not interfere with each other. It also means that broadband will not trigger RACONs (radar beacons), SARTs (search and rescue transponders), active radar target enhancers such as the Sea-Me and EchoMax, and radar detectors such as the CARD (collision avoidance radar detector) system. Owners of broadband systems should be aware that they cannot detect many of the modern electronic navigation aids. In addition, captains who have mounted active radar target enhancers and CARD systems should be aware that they are not effective against broadband systems.
Conclusion. The distinctive features of broadband radar stem from the desire for low power. Low power requires longer pulses to get adequate energy on target. Pulses so long that emitted and received pulses overlap require frequency modulated pulses. This, in turn, allows no main-bang blanking and very high-range resolution at short range. Superior range resolution at short range means superior performance in wave and rain clutter. Superior range resolution at short range also facilitates very sharp detailed displays of close-in vessels and structures, and makes possible much shorter range settings than found on conventional scanners.
On the other hand, low power leads to poorer target detection performance. If you are interested in short-range performance, large targets, operations in clutter, and crisp displays, select broadband. If you are interested in long-range performance, small targets, and operation in heavy rain and thick fog, pick conventional. If triggering RACONs, SARTs, CARD systems, and active radar target enhancers are important, pick conventional.
Philip Gallman is a sailor and an electrical engineer who has done radar research for the Defense Department.
As this issue of Ocean Navigator was going to press, Simrad Yachting announced an upgraded version of the BR24 radar, the Broadband 3G, providing 30 percent better range and target performance. Examining the performance improvement with the help of engineers at Navico Americas, it is clear that some of the improvement comes from tweaking the electronics and some comes from increasing the transmit power.
On the one hand, increased transmit power decreases safety slightly and increases power consumption slightly, but in neither case is the difference significant when compared to comparable conventional pulse radar. On the other hand, increased transmit power increases pulse energy and hence improves target detection performance (target size vs. range). Indeed, at short-range settings for which conventional radar uses very short duration pulses, Broadband 3G pulses have more energy than conventional radar pulses, meaning that broadband detection performance should be better than conventional at short-range settings.
On balance, the Broadband 3G provides improved target detection with unchanged safety, power consumption, and display sharpness compared with the BR24. Moreover, target detection is at least as good as comparable conventional radar for range settings shorter than about one nautical mile.