By Tom Burden
If you haven’t paid attention lately, there’s surprising new technology that is transforming both radar and sonar for the recreational boater. Old-school devices (the style we’ve been used to over the past 30 years and much more) have operated using similar principles, both above and below the water’s surface. Radar and sonar both transmit a short, powerful, single-frequency burst of energy. In radar, units since World War II utilize a microwave-emitting device called a klystron or magnetron; sonar transducers vibrate a hunk of ceramic material at high frequencies. Both blast out a “main bang” or “tone burst” and wait for a return echo, then the software inside the display translates this reflected energy into a picture.
Meanwhile, over several decades, military radar and sonar has evolved down new pathways, moving away from fixed-frequencies and tone bursts. Finally, some of this previously secret and exotic technology is now filtering down to commercial fisherman and weekend boaters. Get ready for frequency-sweeping pulse compression technology, commonly called by the acronym CHIRP (Compressed High Intensity Radar Pulse). Instead of transmitting only 200 or 50kHz, for example, CHIRPing devices transmit a signal that sweeps linearly upward (from 40 to 75kHz, 130 to 210kHz, or other frequency ranges).
Traditional “tone burst” fishfinders have always demanded a trade-off between pulse length and target resolution, and no targets can be resolved unless they’re larger than the physical wavelength of the pulse. If your fishfinder transmits a pulse that is 500 microseconds in duration, it has a pulse length of about 27". So fish less than 27" apart will look like a single large mass, and fish less than that distance from the bottom will be hard to detect, appearing to be attached to the sea bed. Longer pulse lengths (like 50kHz) transmit more energy into the water and improve deep-water penetration, except doubling the pulse length reduces target discrimination by half, likewise reducing detail and discrimination. So lower frequency delivers more power and depth, but less resolution.
With CHIRP fishfinders, this trade-off disappears. CHIRP fishfinders transmit less peak power than a conventional fishfinder, but their wide-band, frequency modulated pulses (130-210kHz, for example) can be very long in duration and put 10-50 times more energy into the water. Using digital pattern matching and signal processing, CHIRP devices achieve unprecedented resolution and target detection. Your ability to resolve individual fish, or separate fish from bottom structure, is now a matter of inches, instead of several feet with traditional fishfinders. See individual fish in groups, instead of a single mass.
Depth ranges of 10,000 feet are standard with these broadband devices, which include black box sounder modules from Garmin (GSD 26), Simrad (BSM-2) and Raymarine (CP450C). These sounders feature dual-transceivers that allow for simultaneous and independent dual transducer operation, which allows complete customization, as you can CHIRP or dial each transducer into specific frequencies.
CHIRP devices can transmit simultaneously on high and low frequencies. The lower frequency gives greater depth penetration, and it requires less power than a higher frequency signal so it generates less noise. The result is a “whisper into the water” that locates the fish without disturbing them. The higher frequency signal gives even finer detail at shallow to mid-water depths. Simrad claims that the BSM-2 delivers five times better resolution than their fixed-frequency BSM-1 sounder at a depth of 500'. You can select from a range of frequencies to best suit the conditions in which you are fishing.
As we mentioned above, CHIRP technology is old stuff in the military. We first encountered an affordable spin-off of CHIRP for recreational boaters a couple years ago with a new, lower-powered but very sharp-imaged Navico BR-24 Broadband Radar (Navico is the parent company of Lowrance, Simrad and B&G, and sells versions of their radar under all three brands).
Navico’s solid-state X-band radar technology is similar to CHIRP fishfinders, except that the “chirping” happens in the air instead of under water. The BR-24 and its successors, the Broadband 3G and 4G, utilize what Navico calls FMCW (Frequency Modulated Continuous Wave) Technique.
Like CHIRP sonars, Broadband Radar sends a continuous transmission wave with linear increasing frequency (hence the term Broadband). The wave retains its frequency as it travels out and reflects back from any objects. Meanwhile, the transmitter continues to output an increasing frequency. The difference between the currently transmitted and received frequencies, coupled with the known rate of frequency increase, is how the radar precisely calculates a “time of flight” and target distance. Since FMCW constantly builds up radar return energy (vs. transmitting a single pulse), this system provides target detection superior to pulse radars while transmitting at far lower energy levels.
Navico’s radars offer the following advantages over traditional radar:
Lowest RF transmission for safe, flexible installation: Broadband Radar 3G transmits at 1/10,000 the power of typical pulse radars (emitting about 1/5 the energy of an average cell phone), so the radome is safe to mount in locations never before possible. In addition, the lowest DC power draw of any X-band marine radar makes this system well suited for sailboats and other vessels with limited power.
Improved short-range target discrimination: Broadband Radar provides better target resolution, even at an amazingly close 1/32 nm range. Docks, channel markers, moored vessels and other critical targets are displayed with clarity and separation, for added confidence in close quarters. Broadband technology also eliminates the “main bang” of a pulse radar—the obscured “dead zone” immediately around the vessel—which interferes with close target detection.
No warm-up time: no more waiting 2-3 minutes for a magnetron to warm up. When darkness falls or the fog rolls in, you are always ready. Ideal for sailboats wishing to save power or boats at anchor not wishing to run the radar continuously.
Now for the bad news: much lower maximum range than conventional radar. Although they excel in close-range navigation, broadband radar is inferior to the old-school high-powered technology outside the three-mile range or so, but they’re closing the gap. Navico has mitigated this problem with their 3G radar, doubling the transmitting power of the BR-24, and increasing the range by 30 percent. Their 4G device narrows the gap even more, with maximum range of 36 nautical miles.