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Hi guys. For the last few weeks I have been researching the latest in echo sounders/fish
finders. Having been away from the market for a few years since I got my first generation
digital sounder from Raymarine (DSM 250), I was pleasantly surprised to see breakthrough
technology such as CHIRP Sounders. Alas, I could find very little real technical
data on their performance. All I see are one-pager marketing brochures with very
little real technical data. The best source of info was actually this forum. I have
learned a lot from reading the posts here but the total picture still remained fuzzy.
So I set out to research the field on my own and have managed to gather useful data
on how well these systems may perform. I thought I share that with you all in this
post.
As a way of background, I am an electrical engineer
with a few decades of experience in audio/video/computing fields. Those skills don’t
necessary translate well to understanding sounder technologies but I also happen
to have a specialty in room acoustics and signal processing which do have a decent
overlap (see some of the articles I have written here:http://www.madronadigital.com/Library/Library.html). That familiarity allowed me to dig in and get
a far better grasp of CHIRP technology than reading the marketing brochures from
manufacturers.
That said, you don’t become an expert in a new field
by just reading things for a few weeks
 .
So while you hopefully see fair amount of explanation to follow on sounder technology,
I don’t put them forth as authoritative. There may be mistakes in my analysis and
certainly some holes as I will describe later. I look forward to feedback from you
all to complete the picture and update this post accordingly. Thanks in advance.
Background
Understanding the value of CHIRP requires first learning
how traditional echo sounders work and their limitations which CHIRP sets out to
solve. Fortunately pulse sounders as I call them are very simple devices as far
as theory is concerned. A short pulse of a single frequency is sent out using a
transducer and then we listen to see how long it takes to get a reflection back.
The returned data has timing which tells us how far away the “target” is from the
transducer and an amplitude number which tells us something about its characteristics.
These two pieces of data is plotted on an LCD screen and we get our echo sounder.
Those two pieces of data also set the limits of our
sounder performance:
1. Range Resolution
This is a metric that
says how well we can distinguish targets that are close to each other.
A range resolution of
say, 3 feet, would mean that multiple targets within 3 feet would be rendered as
one. Computing range resolution is pretty easy as it is completely proportional
to our pulse length. All we have to do is multiply the pulse lengthy by the speed
of sound in water, divide by two and we arrive at a distance. That distance is our
minimum range resolution. For example, if our pulse (ping) is 0.5 milliseconds (0.0005
seconds), our range resolution is 1.28 feet (0.0005 * 5118 / 2). Here is a good
visualization of what happens when the targets are separate or too close to be distinguished:
On the left the targets are separated enough to be
seen as two bumps. The two on the right are shown as a single return since they
are too close together (this is a radar image so shows targets horizontally but
the concept is the same).
Note that frequency
that we use within that pulse has nothing to do with this. Only the pulse length
determines the range resolution.
You might ask why we don’t make the pulse smaller
to improve our resolution. Problem there is power. Remember that the amplitude of
the return pulse is something of interest to us. That pulse needs to rise up above
the level of noise (see next section). If we shorten the pulse, we proportionally
reduce the amount of power we put in the water to detect target. If in the above
example we made our pulse 10 times smaller to have 0.128 feet/1.5 inch of range
resolution, we would wind up with just 10% of the power. That would sharply reduce
our range. We could compensate by boosting power but that increases the burden on
the transducer which now has to handle much higher peak power. See more in later
section.
One compromise may be to have adaptive pulse lengths.
Keep them short for good shallow range resolution but lengthen them in deep water.
The latter will give us the power to penetrate deep at the expense of losing resolution.
Note that once you use longer pulses, you lose resolution there at all water depths.
All targets suffer in that body of water relative to resolution.
So what is the pulse length of your sounder? Amazing
as it may sound, it is rarely documented in recreational sounders. And with some
exceptions (such as a Koden sounder I found), no control is given to the user to
adjust on his own. Fortunately this becomes a moot point with CHIRP sounders as
I explain later.
2. Target Resolution
This is my term to describe what size targets we
can resolve. A fundamental principal in acoustics says that to get a reflection,
the target needs to be larger than the wavelength of our sound wave. What is a wavelength?
Single frequency tones repeat on a cycle. The Wavelength is the distance at which
the wave repeats:
The wavelength is computed easily by taking the speed
of the wave through our medium and dividing it by frequency. As you know, one of
the common frequencies in sounders is 50 KHz. Sound travels through salt water at
around 5118 ft/sec which makes our wavelength 1.2 inches. When a target is smaller
than this, the sound refracts around it as if it is not there. Think of taking a
1 inch water hose and putting your finger in the middle of the water stream. You
will notice that little reflects back as the bulk travels around your finger. Now
put the palm of your hand in front of it and almost all the water bounces back.
Similar concept here. A closer example is to put your hand in front of the tweeter
on your speaker. You immediately hear the effect in how the high frequencies become
muffled. Now do the same on the woofer and you will not notice much reduction. The
longer sound waves of the frequencies from the woofer don’t “see” your hand as their
wavelength is many feet and therefore travel around your hand. The ones from twitter
are an inch or two and hence get reflected.
Higher frequencies are therefore available to increase
our resolution in this regard. At 200 KHz, our wavelength drops to 0.3 inches. At
455/800 KHz which are frequencies used for structure and side scan sounders, the
number drops yet again to 0.13 and 0.08 inches respectively.
So why not use the higher frequencies all the time?
Well, there is no free lunch here and our depth ability suffers as frequencies go
up. Salt water is a good sound transmission medium at lower frequencies but not
high. This is an insidious problem because our sound not only needs to make it down
to our target, but also return back to us to measure it, doubling any losses! Computing
exact losses are difficult but a lot of experimentation has given us approximations
which you can find in online calculators. Using common parameters (e.g. 20 degree
C temp and typical salinity of salt water), we get these numbers for signal loss
(in Decibels) per 1000 feet of water column (multiplied by two to account for roundtrip
loss):
50 KHz: 5 dB
200 KHz: 49 dB
455 KHz: 88 dB
800 KHz: 152 dB
As we see, the losses become staggering as frequencies
go up. What do you say? Boost the power to make up for the losses? Well, if you
double the power, you only gain 3 dB improvement (decibels are in logarithm scale).
You need about 100,000 times more power for the 200 KHz wave to have the same energy
at the end of that 2000 feet round-trip as 50 KHz! Now we know why our structure
and side scanners have much more limited range than our lower frequency sounders.
BTW, one way to cheat is to have a narrower beam
as frequencies go up. This focuses the energy more (higher “directivity”) and like
focusing a light beam, gives us more equivalent power. Side-scan and structure scan
transducers do this to an extreme with very narrow angle in one direction and standard
transducers do the same by narrowing the high frequency beam. The latter comes at
the cost of reduced coverage of area that the beam sees.
While we are computing losses, we should also compute
the main factor which is distance related. As the sound waves move away from the
transducer, they literally balloon in size. They keep expanding and that expansion
means we lose energy per unit of surface area. The loss is 6 decibels per doubling
of distance. Computing the same loss for 1000 feet deep target, we get a loss of
37 dB. This loss is frequency independent and would need to be added to the numbers
above to get our combined loss. For the 50 KHz signal depth dominates compared to
frequency related loss (37 dB vs. 5 dB). When we jump to 200 KHz, the frequency
dependent losses become very significant at 49 dB for a total of 37+49 = 86 dB compared
to just 43 dB for 50 KHz.
A quick summary then is that
your sounder has two metrics of resolution: one that is based on the path between
the transducer and the target. And the other, is how large of a target you can resolve.
You have had some control of the latter by choosing your sounder frequency but not
the former. Note also that these are absolute limits of performance. What you actually
get is degraded from these due to equipment limitations.
Now that I have gotten you sufficiently depressed
over how sounders work
,
let me give you some relief with respect to this new CHIRP technology.
CHIRP Technology
As you may have heard, CHIRP technology was originally
developed for radars. The term stands for
” Compressed High Impact Radar Pulse.”
Its development dates back to classified work and
use during WWII era. The technique became declassified around 1960 so its use became
widespread including application in sounders. The needs of the two domains are very
similar as radars and fish finders both care about discriminating between targets
and having good distance performance.
So what is the idea here? Recall that we detected
the range/distance to our target in traditional pulse sounders using time. We measured
how long it took for the pulse to return to us and that told us the distance since
we know the propagation speed of sound through water. CHRIP replaces time with frequency.
We still have a pulse. But within that pulse, we don’t use one frequency but rather,
a range of frequencies. Instead of 50 KHz tone, we start at 40 KHz and keep incrementing
to 60 KHz before the pulse ends. To measure “distance” now, we look at which frequencies
the target excites. In the example I just gave, we have 20,000 different frequencies.
That gives us a lot of steps to use to resolve the distance from our target.
A remarkable thing happens when we use a frequency
sweep this way. Our range resolution
becomes completely independent of pulse length!
If I have 20,000 frequency
steps, you can make the pulse 10 times bigger or smaller and I would still have
that many steps. With standard pulse sounder which in this context is called a “narrow
band” sounder (since it uses a single tone vs many in CHIRP), our resolution is
= speed of sound in water * Pulse length / 2. With CHIRP, our resolution = speed
of sound in water / (2 * Bandwidth). Bandwidth is the range of frequencies. In the
above example of 40 to 60 KHz, we have 20,000 Hz (20 KHz) of bandwidth. Assuming
a pulse length of 0.5 millisecond, our standard sounder has a range resolution of
1.28 feet or 15 inches. A CHIRP sounder using the same pulse but with that 20 KHz
of bandwidth will shrink that number to just 1.5 inches!!! This is why you see statements
like this in the Simrad BSM-2 brochure:
”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.”
Indeed we have taken our range resolution from unit
of feet to unit of inches. The several feet part comes if you use longer pulse than
my example above which reduces the range resolution of pulse sounders but not CHIRP.
CHIRP Gain
In the above example, if we divide 15 inches by 1.5,
we get a figure of merit of 10. That is the gain in range resolution achieved by
using CHIRP. Our pulse got “compressed” by that factor. I put compressed in quotes
because our pulse has not really shrunk. The sounder is still sending the same pulse
length as it would have if CHIRP was not used. But from practical point of view
the system is acting as if its pulse had shrunk proportional to the bandwidth of
our CHIRP frequencies.
Mathematically we can compute the gain by multiplying
our pulse width by the bandwidth. In our example, 0.5 milliseconds is 0.0005 which
times 20,000 gives us the same gain of 10.
This analysis gives us
one of the key metrics in CHIRP performance: system bandwidth.
We saw how the CHIRP
sounder resolution is computed by taking speed of sound in water and dividing it
by two times bandwidth. Life gets better and better as our bandwidth increases.
Let’s take the “high CHIRP” range of frequencies available to us in new CHIRP sounders
at 130 KHz to 210 KHz. The bandwidth is the difference between them or 80 KHz. Performing
the math results in amazing range resolution of just 0.4 inches! Our CHIRP gain
using the same 0.5 msec example as before has jumped from 10 (for the 40 to 60 KHz
range) to 40 (for 130 to 210 KHz).
Note that
improved target resolution
works at all depths/distances.
In the above example, you would have a target resolution of 0.4 inches whether
you are in 10 feet of water or 10,000. A traditional sounder may increase its pulse
length to penetrate deeper water, which puts it at even worse disadvantage compared
to CHIRP. For this reason, often people think of CHIRP as a deep water technology
but that is not correct. CHIRP will outperform a pulse sounder at all depths.
CHIRP Power
Expanding on the last topic, synonymous with CHIRP
is this notion of increased power to penetrate deeper water. Yet the theory of CHIRP
is all about range resolution. So how do we get more power to penetrate deeper water?
The answer is simple: since CHIRP’s range
resolution is not dependent on pulse length anymore, we are free to boost that as
much as we like. As I noted earlier
the power in the water is = output power * pulse width. If we have a 1,000 watt
sounder and we pulse it at 1 msec, then the effective amount of power into water
is 1000 * .001 = 1 watt. To get more power in the power we could increase the pulse
length but then that reduces our resolution. We could increase the power sent to
the transducer but that has a cost in power consumption and beefier transducer to
handle the additional peak power. These problems go away with CHIRP. Pulse length
can be increased with no ill effects on range resolution. So that becomes the ticket
to get much better depth performance.
The Simrad BSM-2 for example has a max pulse length
of 70 milliseconds. If this were a conventional pulse sounder, the range resolution
would be an ugly 179 feet! Anything within that column of water could not be separated
than any other. Turn on CHIRP at high range with 80 KHz of bandwidth and our vertical
resolution reduces to a remarkable 0.4 as if the pulse was very narrow. Taking the
ratio of these two resolutions we see that our CHIRP gain is an incredible 1.5 million!!!
In other words, if you wanted to build a traditional sounder that had the same 0.4
inch in resolution, its power would have had to be 1.4 million times higher to achieve
the same performance as CHIRP.
A traditional sounder may have a “long” pulse length
of just 5 msec. Compared to that, our 70 msec pulse in the Simrad BSM-2 is 14 times
longer. If the 5 msec sounder had 1,000 watts of power, we could outperform it with
just 100 because that is equivalent to 100x14 = 1,400 watts. The 5 msec sounder
will have a range resolution of 7.7 feet vs. our CHIRP sounder of 0.4 inches. All
around greatness! The BSM-2 is rated at 250 watts by the way so using the same multiplier
it would have a performance equiv. to 3,500 watts.
As another example, the Raymarine CP450C has a pulse
length of 80 msec which gives it a similar performance to Simrad BSM-2 with respect
to resolution. They spec a “nominal power” rate of 1,000 watts. I have no idea what
“nominal” means. If that is RMS power then it has four times the power of BSM-2.
Alas, I could not find any specs for the maximum
pulse length of Garmin GSD-26. Nor did I find such data for the Furuno DFF1-UHD.
What is stated is that the Furuno has a high range is 175 KHz to 225 KHz which gives
us a bandwidth of 50 KHz rather than 80 KHz in Simrad/Raymarine/Garmin. This degrades
the target resolution from 0.4 inches to 0.6 inches. Not much to lose sleep over
although it is interesting that they have gone for a shorter range here. Better
be careful to pick a transducer also that is optimized for that range (i.e. maximum
of 225 Khz vs 210).
On the low range, Furuno specs +- 20 KHz which gives
us double the bandwidth at 40 KHz (Simrad and others are 20 KHz). This shrinks the
low CHIRP resolution form 1.5 inches to 0.8 inches. So it seems that is where Furuno
is trying to differentiate by providing better target resolution at the low frequency
range, in essence optimizing for deeper water performance.
Performance Comparison
For all the advantages of CHIRP, it is remarkable
that there is no side-by-side comparison with older technology. Researching beyond
recreational sounders I did find some good comparisons. Here is one from the paper,
“FM slide (chirp) signals: a technique for significantly improving the signal-to-noise
performance in hydroacosutic assessment systems” by Ehrenberg and Torkelson. Two
systems are compared. One is a pulse based system with a pulse length of 0.18 millisecond
giving it a range resolution of 5.5 inches (top row). The other is CHIRP with 10
KHz of bandwidth giving it a range resolution of 3.1 inches. So roughly equal in
range resolution. The difference then is how much better the signal to noise ratio
is due to longer 5 millisecond pulse length in CHIRP (bottom row):
The top row is traditional pulse sounder. The lower
row is CHIRP. Notice the much longer pulse length in CHIRP which gives its much
increased power in this comparison. There is only one target and is being shown
in the form of that one spike in each graph (below the pulse line). Our goal is
to get the cleanest pulse above the noise floor as to make it easy and clear for
us to find our targets of interest (fish, structure, etc). The ratio of that peak
of that spike and the variations around it is our "signal to noise ratio" or S/N
for short. The higher the S/N, the better in electronic systems. The term "noise
floor" is used to refer to average level of noise.
The left pair is in the conditions of little noise
(e.g. clear water and shallow depth). There is little to separate the performance
of CHIRP vs pulse although the CHIRP is ever so slightly cleaner with respect to
noise floor. The pair on the right represents addition of 10 dB worth of noise (e.g.
due to deeper targets or higher frequency in use). The pulse sounder on top right
shows immediate pain in the form of increased noise whereas the CHIRP in the bottom
right is barely impacted with very clean return of that one target.
Let’s increase the noise even more:
The increase of 10 more dB of noise creates havoc
for the pulse system on top left. No longer can one tell where the target is as
it is lost in sea of noise pulses. When noise is this high, turning up the volume,
i.e. sounder gain, will do you no good since it will increase the level of noise
and our target return at the same time so you will get a lot of clutter with your
target lost in them.
In sharp contrast to above, the CHIRP response on
bottom left is going about its business, barely registering the noise. Our sharp
target response is still there, well above the noise floor.
Raising the noise another 10 dB finally pushes the
CHIRP system to its limit although one can still make up the main target pulse as
it has higher amplitude than noise (bottom right). The pulse system lost the battle
in the last round and shows totally random response. We clearly see the benefits
here: CHIRP while giving us slightly higher resolution (3.1 inches vs. 5.5), managed
to provide 30+ dB of advantage in signal return over the pulse system. Such increased
power can be put to use for example in the form of using the higher frequencies
in our sounder at much lower depths. Or maintain bottom while at speed and deeper
sea floor.
Here is another example that I found online (click
here for the larger image
http://www.omg.unb.ca/Ksidescan/images/K320_chirp.gif): :
These are side-scan images using 200 KHz. The left
is a traditional pulse system which as indicated, uses a 0.1 millisecond pulse.
The middle image increases the pulse length to 3.0 milliseconds which smears the
detail as expected. The right image is CHIRP. It keeps the same timing (slightly
boosted to 3.2 milliseconds) but uses CHIRP with 8 KHz bandwidth. Even with that
modest bandwidth there is sharply improved contrast which is the result of much
better signal to noise ratio. We get superb resolution and signal strength.
This is another good example of what a chain looks
like under water:
Without CHIRP:
With CHIRP:
I think case is closed as far as superiority of CHIRP
.
Transducer
The business end of our sounder is the transducer.
We can’t just change things up stream and expect it to follow. Using the speaker
analogy, you can’t send high frequencies to your woofer and expect it to play them
at the same amplitude as low frequencies.
Traditional transducers have been designed around
pulse sounders so naturally they have narrow bandwidth. If you all you are going
to transmit is 200 KHz, there is no sense in having a transducer which goes down
to 150 KHz. Same for frequencies above 200 KHz. For this reason the response of
standard transducers is very “peaky.” They have their highest output at the target
frequency and output starts to drop fairly quickly as you move to either side of
that frequency.
The standard in the industry is to measure the bandwidth
at the point where the output drops by a factor of two. In decibel units, this is
a “-3 dB” point. If we divide the target frequency by this bandwidth number we get
the system’s “Q.” Here is an example picture showing it in Wiki:
Working through an example, let’s say we have a 200
KHz transducer which has its response dropping by a factor of two at 198 KHz and
202 KHz. The difference between these is 4 KHz. If we divide 200/4 we get a Q of
50.
In my previous math with high CHIRP I assumed a range
of 130 to 210 KHz for a bandwidth of 80 KHz. The center frequency for this unit
is (130+210)/2 which is equal to 170 KHz. Our Q therefore is 170/80 = 2.1. Clearly
this calls for a very different transducer than the above example. We are in need
of a “broadband” transducer. Airmar has a range of transducers with such wide responses/low
Q to pair up with our sounders. They say their minimum spec is for Q of 3 which
is slightly worse standard than what we need here.
Side Lobes
There is a side effect in CHRIP technology in that
the recovered pulse has shadows around it (“side lobes”). In technical terms the
response generates a sinc() function which has a sharp pulse in the middle representing
our target but has spurious smaller pulses going forever on the left and right.
If not dealt with, we could get ghost images around the target edges or have it
contribute incorrectly to the amplitude of the return from close by targets. This
is easy to deal with by various techniques with the most common being lowering the
pulse amplitude at the beginning and end of our frequency sweep (so called Window
function). This will nicely reduce the side lobes but has a negative effect in that
it lengthens the effective CHIRP pulse width. There are other techniques with different
tradeoffs with respect to reduction of the side lobes and increase in pulse width.
Which one your sounder has? Nobody knows since it is not stated. So computing the
effect of this modification is unknown. If I were to throw a number out there it
would be to de-rate our CHIRP gain by 1.5. So our 0.4 inch resolution may in reality
be 0.6 inches.
Doppler Effect
When used in Radars, CHIRP resolution can be degraded
by the Doppler effect. Doppler as you may know is a phenomenon where frequencies
shift up or down based on velocity of the target. It is used in such things as radar
speed guns. Since CHIRP measures range using a set of frequencies, anything that
modifies them would create an error. Computing this error for a sounder is difficult
since we have to make assumptions regarding the angle of our target relative to
our sounder transducer. I have a model for it but do not yet have confidence in
the results to share it. For now, I don’t think it is a serious issue. Just in case,
if you want the most accurate range determination, do so at slow speed. Worst case
scenario is target moving up and down relative to the boat.
Display Resolution
We have talked about the wonderful resolution that
CHIRP provides down to half an inch or so. As noted, this is the floor of our system
resolution and not what you may actually achieve. For example, if you are running
in 480 foot of water and your display has 480 pixels vertically, each pixel represents
data from 1 foot. So don’t expect to see half inch resolution. Fortunately we can
zoom in and when we do, we get truly more detail since our underlying sounder has
finer resolution than our display. Raymarine makes hay out of this by claiming its ”TruZoom™ mode” to extract more features as you zoom in.
In reality every CHIRP system has that.
Broadband Sounders
Many pages later we get to the motivation that led
me to do this research: exactly how much better a CHIRP sounder is against the last
generation “broadband” units? In literature and industry outside of recreational
marine the term broadband is synonymous with CHIRP. The reason is that in CHIRP
we fill our pulse with many frequencies as opposed to a single frequency in traditional
sounder. Therefore by definition we have broadened the range of frequencies that
exists and hence the name. The recreational marine industry however has tried to
bifurcate the market into CHIRP and non-CHIRP while seemingly using the same technology
name for both. What is the reality here? I am not 100% sure but will show you my
thinking. I invite your feedback.
To get answers here is maddeningly difficult. We
have been given little beyond generic terms to characterize the performance of broadband
sounders. There are some clues however. For example in the marketing material for
both Simrad for BSM-1 and same technology built into Lowrance/Simrad devices they
tout two things: long pulses and low output power (“whisper into water”). By now
you should be familiar with these concepts and how long pulses do indeed increase
effective power. You should also know the problem: long pulses create poor range
resolution yet these companies rave about the increased resolution. The math simply
does not allow one to have the claimed improved resolution and long pulses to coexist
unless one thing is used: CHIRP! Yes, I said it. These broadband units are indeed
using CHIRP as their technology name broadband indicates. Garmin for example in
its GSD 24 material says they gained 6X better resolution than other technologies.
That kind of gain cannot be attained without very short pulses which would reduce
depth capability.
The story is more complicated though. Recall that
our newest devices with CHIRP in their name support wide bandwidth as high as 80
KHz and require similar bandwidth transducers to pair with them. The latter is not
part of the requirements of broadband sounders. They appear to work with older “narrow
band” sounder transducers just the same. So did I just invalidate my theory of broadband
sounders being CHIRP? If they are using “single frequency” transducers, then surely
can’t be using them with frequency sweeps that CHIRP requires. Turns out they can!
Let me explain.
Traditional transducers do indeed have high Q meaning
that they have narrow bandwidth. But narrow bandwidth does not mean a single frequency.
Take the common Airmar P66 transducer. At 50 KHz, it has a Q of 24 and at 200 KHz,
its Q is 30. Dividing 50 KHz by 24 results in 2 KHz worth of bandwidth. At 200,
the bandwidth climbs yet again to 6.7 KHz. These are heck of a lot larger than a
single frequency!
What this means is that we can indeed use a CHIRP
frequency sweep to drive traditional transducers. All we have to do is limit our
bandwidth so that we don’t go past their usable (-3 dB point) response. Since CHIRP
resolution is proportional to bandwidth, we lose some of our performance advantage
but not all. As a comparison, our BSM-2 in its high range has 80 KHz of bandwidth
which results in range resolution of 0.4 inches. The “broadband” unit using the
P66 transducer would have 6.7 KHz bandwidth resulting range resolution of 5.1 inches.
That is still quite respectable as without using CHIRP, the same sounder will have
15 to 60 inches of range resolution depending on pulse length used, resulting range
resolution improvement of 3 to 12X. Go back to the sidescan example I provided earlier
that that used 10 KHz of bandwidth at 200 KHz and the noticeable improvement it
provided in signal return. CHIRP at any bandwidth decouples us from pulse length
allowing us to increase that at will to get better power into water. This may be
the reason why companies like Garmin claim “6X” better target resolution for its
GSD 24.
So it appears that we are being sold the same story
twice under different names. If so, we have had this so called “baby CHIRP” all
along in the form of broadband sounders at pretty reasonable costs. The units now
designated as CHIRP when coupled with high performance broadband transducers, simply
up the game and give us as much gain yet again. How much better is a ratio of the
bandwidth of each system. We know the bandwidth of new devices as they are specified
in their range of frequencies supported. For the old, we can guess per above. Using
this type of analysis for example, the Simrad BSM-2 is 80/6.7 or 12X finer range
resolution than Simrad BSM-1/sounder built into Lowrance/Lowrance multifunction
devices.
Now that the cache of CHIRP has set it, we are seeing
the term used instead of broadband. A good example is the Raymarine Dragonfly. With
the entire unit+transducer costing less than a broadband transducer alone, it is
for certain that its CHIRP bandwidth is low and the unit likely is no different
than traditional broadband sounders despite the “CHIRP” designation.
While not related to increasing range or depth capabilities,
high-end CHIRP units have doubled their data paths/processing. They can drive low
and high frequencies sweeps at the same time. The advantage is that display rate
should improve since we are not sending one set of tones versus others as traditional
sounders do. And the data is better time synced. Down side is increased power usage
and cost.
Finally I recall seeing a thread here where some
people thought CHIRP is no different than tuned sounders. Tuned sounders allow the
user to dial in the frequency as opposed to it being 50 or 200 for example. By now
it should be clearly why this is not CHIRP. CHIRP gets is power by using a sweep
of frequencies. Having a single frequency, no matter what it is set to, will not
create the same advantage. Likely the tuning feature of that class of sounder is
used to dial in the right frequency as to get to the desired target depth/type better.
Summary
CHIRP technology allows
a sounder to sharply reduce its effective pulse length and with it, gain much higher
resolution with respect to determining the position of a target.
That gives it resolution
down to sub inches vs. feet for traditional sounders.
Importantly, the range resolution does not depend on pulse length anymore allowing
the designer to increase that parameter in order to get more power into the water.
More power means deeper penetration and better quality target returns. The improvements
here are very significant.
For good or bad, we have had the first installment
of this technology in our broadband sounders at much lower cost. Latest generation
CHIRP systems then double down and improve the performance further.
With wide spread use of CHIRP it is important to
seek out its key performance metric which is bandwidth and pulse length. These parameters
differentiate one CHIRP system from another.
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