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Author Topic:   CHIRP SONAR
Jessielove posted 05-04-2012 07:09 PM ET (US)   Profile for Jessielove   Send Email to Jessielove  
I read an article in the current Great Lakes Angler magazine about Airmar's new chirp or pulse-compression transducer technology currently being offered by Simrad, and Garmin. Is this the next big advance in SONAR technology, or just a minor improvement being hyped up?

Simrad (BM-2)

Garmin (GSD 26)

Your thoughts?

jimh posted 05-04-2012 08:19 PM ET (US)     Profile for jimh  Send Email to jimh     
Pulse-compression is already the state of the art--at least the publicly known state of the art--in echo-ranging systems. Whether or not recreational fishermen will adopt it is really a question of cost:benefit ratio. Pulse-compression is not a "minor improvement being hyped". It is a significant improvement in the method and requires very sophisticated signal processing. Pulse-compression was not dreamed up in the marketing department to put a name to a minor improvement. I am sure at one time the technique was considered a military secret.

The article (linked above and attributing pulse-compresion to AIRMAR) sounds like a press release from AIRMAR. The pulse-compression technology is not in the transducer--it is in the sophisticated mathematics of the digital signal processing. The transducer for a wide-band SONAR system needs to be wide band. Most traditional SONAR transducers were made to be narrow-band and high-Q devices. A pulse-compression SONAR system needs a wide-band and low-Q transducer. AIRMAR's role in pulse-compression SONAR is just to make the proper transducer. AIRMAR is not the source of the magic. Their transducers help the process to work better, but they are not the process itself.

GARMIN's literature (linked above) includes the term spread spectrum. The term "spread-spectrum" has been around for a long while, but I do not think it is appropriate to pulse-compression SONAR. Spread spectrum was invented by a rather famous actress--I am not kidding--named Hedy Lamarr. The literature I have read about pulse-compression SONAR seems to suggest it uses a linear frequency modulation technique, not a random frequency hopping technique. Spread spectrum is used in carrier division multiple access systems like cellular telephony. As far as I know a pulse-compression SONAR is not a frequency hopping system, particularly a chirp system which uses sweep frequency modulation

Visit the Wikipedia page on chirp and listen to the audio file examples. The audio files will explain the source of the name chirp. I think that rather odd acronym CHIRP used in that article was made up long after the term chirp was used with these signals. I had to laugh about the terms mentioned for the acronym. I never head of those terms. I think chirp was the name for this technology long before anyone came up with those terms in the acronym. In radio the term chirp has been used for about 80-years (or longer) to describe an on-off keyed signal that had some frequency drift or frequency modulation. Although the ultrasonic signals of SONAR are not audible, they are of the same sort--a continuous wave keyed on-off signal with some frequency modulation--ergo called chirp.

Also, the terms proposed for the acronym CHIRP make very little sense. There is no Compression involved that I can comprehend in the transmitted pulse. If anything, the transmitted pulse is much longer, not compressed or shorter, than conventional SONAR. It is the mathematics of the signal processing that perform a pulse compression. The actual pulse in a chirp SONAR is longer than the pulse in conventional SONAR. The pulse compression is really virtual pulse compression, not literal pulse compression, and occurs in the signal processing, not in the actual pulse being transmitted. See the Wikipedia article on pulse compression.

There is nothing particularly High-Intensity about the technique. As a matter of fact, the peak power or intensity is much lower than in short-pulse duration methods. The continuous wave power is much lower than the peak power used in short-duration pulse methods. So this term makes no sense.

There is no RADAR involved in SONAR. RADAR uses radio waves, and SONAR uses sound waves. So this make no sense, either. I think the name chirpcame from the sound of the signals; they sound like birds chirping.

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