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  Lowrance Chirp, Side-Scan, and 360-degree-scan SonarHub

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Author Topic:   Lowrance Chirp, Side-Scan, and 360-degree-scan SonarHub
jimh posted 02-12-2014 03:52 PM ET (US)   Profile for jimh   Send Email to jimh  
According to some previous announcements, we should now, in February 2014, be seeing availability of the Lowrance SonarHub. This device is a new product from Lowrance that is a black box SONAR, that is, the unit has no display or control interface, and operates only when connected to a multi-function display.

First, let's understand what a sort of SONAR is being used by the SonarHub. Much depends on what transducers are connected. Four general types of transducers can be connected:

--downward aiming single-frequency transducers, which operate at the traditional frequencies of 50-kHz, 83-kHz, and 200-kHz and are used with conventional pulse SONAR;

--side-aiming transducers, which operate at much higher frequencies, typically at 455-kHz or 800-kHz;

--wider bandwidth down-aiming transducer for continuous-wave frequency moduated or chirp SONAR, which operates in what are described by Lowrance as three ranges low, medium, and high, but no frequencies are actually specified; and,

--a rotating high-frequency side-aiming transducer than can be manipulated in a 360-degree arc to produce an all-round sweep of echoes. The echo presentation is somewhat like a RADAR plan-position-indicator or PPI display. Lowrance refers to this function with the trade name SpotlightScan.

The single frequency transducers are the same transducers that have been used for many years with the Lowrance pulse SONAR products. The popular Skimmer model offers 83-kHz or 200-kHz frequencies. The larger and not so skimmer-like 50-kHz transducer also offers 200-kHz. These transducers when used with the SonarHub apparently produce the same performance as a Lowrance HDS unit would when using those transducers. This SONAR output is available on the remote display. I don't think this option will be used very much. In most installations, it would seem to be better to just connect these conventional transducers to the Lowrance HDS unit directly, and let the internal SONAR in the HDS perform this function.

StructureScan HD is Lowrance's marketing or brand name for a side-scan SONAR using higher than usual frequencies. The StructureScan HD transducer is a rather long and large transducer. I believe this device now contains three transducer elements: two are aimed outward to the sides of the boat to perform the side scan function, and a third is aimed downward to provide a high-frequency and high-resolution from a SONAR aimed directly down, under the boat, as is the normal orientation of a SONAR transducer. When a StrucureScan HD transducer is connected to the SonarHub, the StructureScan HD SONAR is available on the remote display. The frequency can be selected to be ither 455-kHz or 800-kHz, as desired. These transducers are called the LSS HD XDCR for Lowrance StructureScan HD transducer. (I don't know exactly what the HD stands for, but I am assuming it is high definition. although exactly what that means is not defined by Lowrance, at least as far as I can find.)

The big news, perhaps, is that the SonarHub can now provide continuous-wave frequency-modulated SONAR. A continuous-wave frequency-modulated SONAR is quite different from the SONAR that most recreational boaters are familiar with, which is a pulsed SONAR. A continuous-wave frequency-modulated SONAR uses a longer duration, continuous wave transmit signal which is frequency modulated or swept through a range of frequencies. The detection or demodulation of the return signal is performed by very sophisticated digital signal processors. I think the simplest way to look at this method is to consider the domains in which the signal may be evaluated. In conventional pulsed SONAR, there are only two domains: amplitude and time. In continuous-wave frequency-modulated SONAR, there are three domains: amplitude, time, and frequency.

The mathematical description of continuous-wave frequency-modulated SONAR signal processing is rather challenging to understand, even for engineers and mathematicians. The essential result of the process is to achieve an effective compression of the pulse time, providing better target resolution than can be obtained in the traditional pulse SONAR method. Because the signal is frequency modulated, its acoustic characteristic sounds like a bird chirping. This led to the method being called "chirp" SONAR. Later, probably by back formation, and because this method was also used with RADAR echo-ranging systems, an acronym was fitted to "chirp", with the notion that chirp stood for Compressed HIgh-energy RADAR Pulse. While chirp is a good description of continuous-wave frequency-modulated signals, I don't think the invented acronym is very good. There are several problems: it is not a RADAR when used in SONAR; there is no high-energy pulse, and, in fact, quite the opposite, as the energy is lower and there is not really a single pulse but more like a burst transmission over many frequencies. For this reason, when I refer to the method as a chirp SONAR, I am referring to its acoustic signature, and not to the invented acronym that tends to misrepresent it.

Because a chirp SONAR will be transmitting and receiving over a range of frequencies, the transducer used with it should be able to accommodate efficient transmitting and receiving of the whole range. This usually means the transducer will have to have a wider bandwidth. This characteristic is often expressed as a quality factor or figure of merit, called Q. In this case, we want a low-Q transducer so its response does not vary too much with frequency. Conventional transducers tend to have very narrow bandwidth or very sharp resonance and are high-Q.

The implementation of chirp SONAR so far has seen specialized transducers in use. This is true with the Lowrance SonarHub. It becomes a chirp SONAR when you connect an appropriate transducer. The transducer of choice seems to be an AIRMAR TM-150, which is, by comparison to the popular Lowrance Skimmer transducer, a giant. It is also expensive, about $300.

The SpotlightScan transducers are called a SPOTLIGHT XDCR and are intended to be clamped to an electric trolling motor and to be used when the boat is stationary or moving very slowly. There is also a nice integration of a heading sensor for the trolling motor or transducer that sends the orientation via NMEA-2000 to the display so it can provide the presentation with some sense of orientation to the boat. Exactly how this is accomplished is not particularly clear to me, but there is apparently some method available using what Lowrance calls a SPOTLIGHT heading sensor.

The black box SonarHub unit appears to only be workable when connected by Ethernet to a companion display, which must be one of the Lowrance HDS, HDS Gen2, or HDS Gen2 Touch models. However, if you have an HDS display, you will not get the chirp SONAR or the SpotlightScan SONAR, so it does not seem like this unit is a very good mate with the older HDS devices. Using it with an HDS Gen2 or HDS Gen2 Touch seems much more appropriate.

The signals from the black box SONAR are sent to the display using the very high-speed Ethernet network. The SonarHub has three Ethernet connections, so it acts like a small network hub, too.

The SonarHub is sold in various tiers. If you get the SonarHub with the LSS-2 HD transducer and the TM150 chirp transducer the MSRP is $1,000. The SonarHub with just one of those two transducer is $800. If you want to later buy just the TM-150, it is $300. (It makes more sense to buy the TM-150 in the bundle, as you save $100.)

jimh posted 02-12-2014 03:56 PM ET (US)     Profile for jimh  Send Email to jimh     
I have not found prices for the SPOTLIGHT XDCR or SPOTLIGHT heading sensor products. Maybe they are still in the pipeline.
Hoosier posted 02-13-2014 06:37 PM ET (US)     Profile for Hoosier  Send Email to Hoosier     
It sounds like the SPOTLIGHT is intended to tell a fisherman where to cast. What I would like to see is a forward looking sonar for navigation in hazardous waters.
jimh posted 02-13-2014 08:18 PM ET (US)     Profile for jimh  Send Email to jimh     
David--you'd need some sort of in-hull transducer at the bow stem, so that you could still operate at planing speed without ripping off the transducer. It would be better if the beam from such a transducer could be steered electronically instead of mechanically rotating the transducer.
Chuck Tribolet posted 02-13-2014 11:31 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The 2014 West Marine catalog shows a similar 360 degree
transducer from Humminbird called 360 Imaging. $1499
bow-mount, $1999 transom-mount.

Also in that catalog, the Lowrance SpotlightScan Trolling
Motor transducer is $499.

Eating fish can be expensive.

Chuck

Hoosier posted 02-15-2014 10:56 AM ET (US)     Profile for Hoosier  Send Email to Hoosier     
Looks like SIMRAD just announced this product.
jimh posted 02-15-2014 11:54 AM ET (US)     Profile for jimh  Send Email to jimh     
Dave--The Lowrance SonarHub is different from the just-announced Simrad ForwardScan. You should move to the thread discussing the Simrad ForwardScan to talk about that product. This thread is discussing the Lowrance SonarHub.
Hoosier posted 02-15-2014 12:35 PM ET (US)     Profile for Hoosier  Send Email to Hoosier     
I was referring back to my comment that I'd like to see a forward looking sonar.

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