Sonar

Sonar or sound navigation and ranging is based on using sound to detect and locate objects.

In passive sonar we listen to the sound of the environment and detect and possibly locate objects from this.

In active sonar we transmit a sound pulse of some kind and use the echo to detect and locate objects. In principle this is a simple process where we emit sound and measure the amount of time it takes for the sound to travel to the target, reflect off the target and travel back to the receiver. This time-delay tells us how far away the target is. The strength of the received echo indicates how strong the reflection of the target is. By plotting this information, an image of the seafloor can be created.

If you want to learn more about sonar systems I recommend the book Sonar Systems, which is distributed under a creative commons licence and is free to download and use. If you reside in Oslo, there are a few courses at the University of Oslo that are relevant; Namely Imaging, Digital signal processing, Digital signal processing II and Signal processing in space and time.

Multipath

Sound does not just reflect back to our receiver, but scatters in all directions. Sometimes this scattering hits the bottom and surface a number of times, and then makes it back to the receiver. This is illustrated in the figure below. These signals that have taken other paths than the direct path is added as noise to the received signal. These multiple paths vary in strength depending on a number of factors, and are referred to as multipath. 

The notation for multipath used here is “b” for bottom and “s” for surface. Capital letters indicate a single non-specular reflection, while a small letter indicates a specular (mirror-like) reflection. 

Several methods exist for reducing the effect of multipath and I have grouped them into two groups by where in the process they work and mentioned the most important ones. See table 1.1 for an overview.

Data collection

Using multiple sensors for data collection has been shown to improve the range significantly. (article) This is, however, at the expense of resolution (as the bandwidth is limited) and the system complexity. It is not an adaptive method, and thus it might not perform well in very rough and varying terrain.

With an interferometric sonar system it is possible to use the spatial coherence as a measure of data quality, and thus get a measure of obtained imaging range. This coherence is mainly a measure of multipath interference, and by modeling the ocean environment it is possible to simulate what could have been done better. For instance we could have sent some of the energy in a different direction, or listened in a different direction on reception. If the system has an adjustable transmit beam in the vertical direction, e.g. by electronic steering of the beam, it would be possible to adapt the transmit beam while in the water in such a way that the strongest multipath interference is reduced. This is the main subject of my master thesis work.

Data processing

Traditional delay-and-sum (DAS) beamforming, super-resolution techniques that improve DAS and image enhancement are all methods that are used to improve the available data. Thus they may be easier to implement, but can not improve imaging beyond the actual maximum data available in the signal. The data collection methods can, since they are either collecting more signals or adapting before collecting the signal to improve the amount of data available in the signal.

Future work

It is important to improve data during collection rather than trying to improve flawed data. Data processing methods can even be applied as a second layer of improvement after improving the data collection.

Among the data collection methods, the multiple sensor setup has been shown to work, but it imposes some limitations on the sonar system design. I will look into adjusting the transmit beam and other sonar parameters to reduce multipath in my thesis entitled Environmentally Adaptive Sonar on an Autonomous Underwater Vehicle.

Please contact me or leave a comment if you are interested in discussing this subject and reading the complete essay, in which I look into the different methods mentioned in table 1.1.

]]> http://oycob.com/wp/2012/03/08/the-multipath-problem-in-sonar-imaging-of-the-seafloor/feed/ 0 http://oycob.com/wp/2012/03/07/the-power-of-introverts-stop-the-madness-for-constant-group-work/ http://oycob.com/wp/2012/03/07/the-power-of-introverts-stop-the-madness-for-constant-group-work/#comments Wed, 07 Mar 2012 18:43:39 +0000 Ole Jacob Lorentzen http://oycob.com/wp/?p=123 Very good TED-talk by Susan Cain, who makes a case for the introvert in us.

What’s so bad about it? It is a bill that is only on the agenda because of lobbying from the industry. Most supporters of the bill are themselves supported by the very same industry. (claims taken from the video below) Corruption? No, American politics – it’s unfortunately how the system works. And they can tolerate a certain part of that as long as the bills are sensible. This bill however is simply put a bill that allows the American industry to censor the Internet without really involving the legal system. Censorship does not belong in the free world, nor does the principle guilty until proven innocent. But that is what a desperate billion dollar industry is trying to employ.

If you want to learn more about this check out Wikipedia’s FAQ and do a google search. Unless you’re an american citizen there is not much active action you can take other than staying informed. This TED-video featuring Clay Shirky posted today better explains why we should all care, not just americans. He also points out how this is not a single incident, but rather just another step in a process that has been going on for quite some time. And there is no reason to believe that they will give up any time soon.

I tried to dig up some recent articles on the matter actually discussing the technology involved, but didn’t find too much at first glance. Will look more and update if I find something interesting. I found two books that seemed relevant at first, but then fell through: MRI-guided Focused Ultrasound Surgery (seems to be for practitioners and not about the actual technology involved) and a compendium which seems to be just an aggregation of Wikipedia articles.

Anyway, check out the video – Yaov Medan presents some amazing successful cases.

I’ve always been fond of PCs using Linux for server applications and Windows for desktop. With Windows 7 I felt like Microsoft finally got it completely right, and I still do.

Still, I have always believed that Apple produces the very best hardware. Even though I’ve never been fond of OSX, it does provide the full package without compatibility issues and driver hassles from the windows and (especially) linux worlds. So I have decided to give Lion a try.

First impression is wow, great gestures using the amazing touchpad. The full-screen apps utilizing separate desktops is a dream, and I’m glad I waited with the switch until this was available – I don’t think I would’ve liked it much without it. Why can’t I have this on my iPad?

Hardware-wise it is just what I expected. Neat, sleek and polished. And quick. I experience no speed difference between my old (better) i5 2.5GHz with intel SSD and the macbook air. I haven’t really put it to the test yet, but then again I don’t usually do…

One thing I do miss is the quick navigation everywhere with the keyboard. On Windows 7 I barely use the touchpad because navigation and use is so quick without it. This might just be an issue of adapting though, and I just discovered the neat cmd+space=spotlight shortcut, that helps a lot.

Bottom line; very nice piece of hardware and software. Still not as quick in OSX as I were in Windows, but adapting. Will give it some more time before resorting to bootcamp if necessary ;D Just kidding. Or am i?

• not numbering the frontpage
• numbering the table of contents (TOC) with roman numerals
• numbering the remaining pages with arabic numerals

That evening I got an e-mail from one of them (Jørgen) who has now done some research. His master thesis template does this “automatically”. It simply defines where the \frontmatter{} begins and where the \mainmatter{} begins.

Trying this in my own article LaTeX threw errors at me. It didn’t know these commands. Some googling made me realise that these are only available in book-like document types, ergo not in the article type. So if you’re using a type that supports this, well then you can stop reading right about now!

If not, then you might be interested in what I found out on google when investigating this further. I’m simply going to put this out there in code which should be pretty self-explanatory:

% frontpage:
\thispagestyle{empty} % to avoid numbering the current page
% note that the page is still counted, so you need to reset the counter properly if that is not intended. If you use \pagestyle, the style will be set for the entire document. Then you will need to set it back with something like \pagestyle{plain} (or something more exotic)

% TOC:
\pagenumbering{roman} % roman numerals
\setcounter{page}{1}  % reset counter

% Regular content:
\pagenumbering{arabic} % arabic numerals}
\setcounter{page}{1} % reset counter

And this works! You can do the same with sections too, but it requires a different approach. Setting the counter value is the same, just replace page by section/subsection/subsubsection. To define the numerals to be used with sections you need to do this redefinition:

\renewcommand \thesection{\roman{section}}
\renewcommand \thesubsection{\roman{section}.\roman{subsection}}
\renewcommand \thesubsubsection{\roman{section}.\roman{subsection}.\roman{subsubsection}}

Et voilà!

Note that if you want big roman numerals (e.g. VI instead of vi) you can use Roman instead of roman.

Tonight Team Telefonica arrived first in Cape Town with 40 knots of gusting winds pushing their Volvo Open 70 boat into port. What an amazing race!

I find sailing to be a big source of inspiration and am thinking about picking it up myself as a hobby, though with nothing near a Volvo Open 70!

I have been following the Volvo Ocean Race the last month and what a competition. Amazing teams go up against natures wildness in an effort to go around the earth as fast as possible. Three boats have either had their masts smashed to pieces or suffered huge hull damage, which is the case of Team Sanya. This is a though race. Over the years five people have lost their lives during racing. Ocean racing is a dangerous sport.

After reading in the Norwegian sailing magazine Seilas (no. 9, 2011) that norwegian navigator Aksel Magdahl is participating with Team Sanya I guess nationalism gives me a team to support. I just hope their hull will be good as new and ready for the next in-port race in Cape Town and the second leg to Abu Dhabi, which will be more of a sprint into Abu Dhabi due to the pirate threat off the African coast. Looking forward to the start on december 11th!

In Seilas they also stated that Stavanger is interested in applying to be a Volvo Ocean Race port in 2018. To succeed they would probably also have to put together a participating team since cities that participate with a boat is much more likely to be chosen. Now THAT would be exciting… Check out the article which contains more info.

You can follow the race at the official website and even create your own boat and join the fun in the Volvo Ocean Race Game!

On a side note; I would love to learn more about the actual boat technology. Wonder where information about this can be found, if it is not guarded as trade secrets.