The term “spread spectrum technology” (SST, in Garmin parlance), comes from the fact that these units send out a pulse wave that blasts through a spread-out spectrum of frequencies all at the same time, instead of pinging through the depths with a single or perhaps double frequency. The term “CHIRP” comes from “compressed high intensity radar pulse”.


FM/CHIRP Fishfinder. Photo courtesy of Airmar.

Why mix radar terminology in with a discussion about fishfinders? CHIRP was developed for military radar many years ago, and this technology also uses a multiple-frequency format. The acronym is catchy and has been around for a long time, so as the multiple-frequency format became applicable to fishfinders, the term came into use and stuck. Confusing? You betcha. Luckily, we can set aside the terminology because when you boil it all down to the bottom line, these technologies are one in the same.

Most serious mariners will already understand that the frequency of a fishfinder’s pulse, broadcast through the water by its transducer, determines how it sees the world below the surface. The easiest way to visualize a transducer in operation is to think of it as both a speaker, and a microphone. The speaker shouts, then the mic listens. Micro-seconds later, your fishfinder interprets and displays the results.

As a speaker, the transducer produces an acoustic wave—the “ping”—which travels through the water at about 4,800’ per second. It has a specific wave form which can be a long and penetrating at a low frequency (commonly around 50kHz), or it can be a shorter wave form at a higher frequency, commonly around 200kHz.

Photos courtesy of Airmar.

Traditional fishfinder. Photo courtesy of Airmar.

Low-frequency waves travel farther, but they provide less detail; think of them like the large waves created when a boulder falls into the water. These waves travel on for a long, long time, but roll right over small objects without being affected. Think of high-frequency waves, on the other hand, like the ripples created when a pebble falls into the water. They’re smaller and they don’t travel as far, but as a result, when they hit small items there’s a more significant affect. Thus, they provide higher detail.

Whatever you call this new technology, be it CHIRP or SST, the wave pulses created by the transducer are blasted out in a series of frequencies, throughout the spectrum, all at the same time. As a result your fishfinder can see both deep and shallow, with low detail and high detail, all in the same instant. Net result? Target definition is enhanced radically, while depth range is increased dramatically—these units can tell the difference between a shrimp and a sardine, clear down to 10,000’ in some cases. New fishfinders of this kind came from Simrad and Garmin first, and Raymarine quickly followed with one of their own. So the question remains: which is the best one for you? Stay tuned…

-Lenny Rudow