
Homemade synthetic aperture radar - lispython
http://hforsten.com/homemade-synthetic-aperture-radar.html
======
shas3
This is incredible stuff! One of the less-hyped slow revolution is what's
happening in non-wearable sensing. Some of it is driven by the consumer-
ization of advanced radar systems, a technology that was the domain of the
military and space/satellite comm.

Some companions to this work:

1\. Wifi sensing from academia, particularly in the ubiquitous computing
community- Shwetak Patel's group in Washington [1], Dina Katabi's group in MIT
[2], Patwari's group in Utah [3], etc.

2\. MIT OCW on how to build a radar system with advanced [4] capabilities
[http://ocw.mit.edu/resources/res-ll-003-build-a-small-
radar-...](http://ocw.mit.edu/resources/res-ll-003-build-a-small-radar-system-
capable-of-sensing-range-doppler-and-synthetic-aperture-radar-imaging-january-
iap-2011/)

[1]
[https://ubicomplab.cs.washington.edu/](https://ubicomplab.cs.washington.edu/)

[2]
[http://groups.csail.mit.edu/netmit/wordpress/projects/projec...](http://groups.csail.mit.edu/netmit/wordpress/projects/projects/)

[3] [http://span.ece.utah.edu/neal-patwari](http://span.ece.utah.edu/neal-
patwari)

[4] I use 'advanced' in the sense that it is way, way, more sophisticated in
its use of math and signal processing/machine learning/linear algebra
algorithms than your standard automotive speed monitoring radar.

------
pdabbadabba
Previous discussion of a similar project, by [evidently] the same person:
[http://hforsten.com/6-ghz-frequency-modulated-
radar.html](http://hforsten.com/6-ghz-frequency-modulated-radar.html)

As before, this is totally awesome---until the FCC (or other national spectrum
regulator) shows up at your door...

Here is an assortment of things that the author might be interfering with in
the neighborhood of his 5.5 GHz carrier frequency:
[https://wirednot.wordpress.com/2014/01/07/what-else-is-in-
th...](https://wirednot.wordpress.com/2014/01/07/what-else-is-in-the-5-ghz-
spectrum-hint-its-not-just-weather-radar/)

Some of this is the sort of stuff you really don't want to interfere with--
both for safety reasons, and because of the likelihood of enforcement.

Next time I suggest that the author look into whether his jurisdiction will
grant him an experimental license so he can do this legally. In many
jurisdictions that is cheap and easy (unless, of course, you want to operate
in a band where the interference risk is just too high).

Edit: I see the author is in Finland. Unfortunately for the author, I'm
guessing this changes nothing since these frequencies are regulated both
nationally and internationally. But it does mean that there is probably some
local variation in the exact services that might be subject to interference.
And who knows: maybe in Finland the 5.5 GHz band is a "do whatever you want at
any power level you want" band. But I doubt it...

~~~
falcolas
Or he has his amateur radio license and is licensed to operate at higher power
levels.

Even unlicensed, he has up to 25mw of power and 8.5db antennas (which with
25mw or less can give you upwards of a mile of clean signal), which may be
sufficient. Hard to say.

~~~
pdabbadabba
Does an amateur license allow you to operate a RADAR system? I doubt it. I had
thought that amateur operators were somewhat limited in the sorts of hardware
and operations they are permitted. (E.g., type acceptance of equipment when
operating outside designated amateur bands.)

And in the U.S. at least the bands around 5.5 GHz (the so-called U-NII-2 and
U-NII-2 extended bands) are subject to lots of interesting rules for
unlicensed operation, including the ability to detect and avoid incumbent
RADAR signals.
[https://transition.fcc.gov/bureaus/oet/ea/presentations/file...](https://transition.fcc.gov/bureaus/oet/ea/presentations/files/oct14/51-New-
Rules-for-UNII-Bands,-Oct-2014-TN.pdf)

Of course, I know literally nothing about the Finnish rules that actually
apply here...except that, thanks to the ITU, they probably bear some
resemblance to the U.S. rules.

~~~
semi-extrinsic
Radar systems are fairly common on boats larger than ~30 ft, and you can
operate those without any license at all. The latest generation devices are
all broadband, working on the principle that if you spread your broadcast over
a wide enough range, the power you're broadcasting on any given band is very
low, i.e. legal and not causing interference for others. Maybe this guy is
working off the same principle?

~~~
pdabbadabba
You don't always need an operator license but, when you don't, you typically
do need properly certified equipment. (This is also why you don't need a
license to use Wi-Fi.) The rules vary by service and by frequency band.

I'm no expert on boat RADARs, but I'd bet that, 1) they operate in bands
allocated for that sort of use and 2) that they have been either type accepted
or certified by the FCC to comply with whatever rules govern their band of
operation.

------
sidfarkus
This is pretty awesome. I used to write processing software for airborne SAR
and often wondered what it would take to just strap some stuff to my car and
do it. Unfortunately blasting EM radiation in almost any useful frequency is a
pretty big no-no without going out to the middle of nowhere.

------
annacollins
I’m not understanding how moving along a single axis can generate a 2D image.
Is there a second axis that moves at a right angle to the flight path (or
leadscrew)? I must be missing something..

~~~
vonmoltke
A radar returns a vector of measurements along the line of sight of the beam.
These are in-range measurements from a particular position and time. SAR
stitches together a series of these in-range measurements taken at different
positions and times to create a single 2D image of the static elements of a
scene.

~~~
annacollins
That means the second axis is generated by measuring the distance (slant
range) from the antenna to each point. right ?

~~~
function_seven
Right. If the radar is held stationary, it can produce a one-dimensional
"image" of what is in its line-of-sight. Move it sideways and you generate the
2nd axis.

It's a little more complex than that, because the field of view is not a
perfectly narrow beam, but same idea.

------
mjb
Nice. The down-range focus on the fenceposts is very good. Cross-range focus
isn't as good, still impressive for a such a low-cost system. It'd be very
interesting to see some images of a more target-rich environment.

Radar, SAR, and especially passive multistatic SAR are technologies that I'm
sure we're going to see more of in future. Passive SAR (or transmitter-of-
opportunity SAR) can be computationally expensive, but pieces seem to be there
to build something that works really well.

------
ju-st
I'm still hoping that someone talented does a similar project with the HB100
radar sensor (costs $5) :)

~~~
vonmoltke
I haven't used it, but from reading the datasheet it appears to be nothing but
an RF motion sensor. The output is simply a voltage level reflecting the
magnitude of the Dopper shift the sensor measured. There is no ability to
perform range measurement or range gating, so all it will tell you is if
something is moving within its (very wide) field of view.

------
vt240
Very cool, I built a variant of the MIT kit a few years ago. It was a lot fun.

------
mcnamaratw
That's awesome!

------
spitfire
I'd hire this guy. This, the homemade GPS guy, and the homebuilt apollo
computer guy are all the sorts of people you want on your team to teach the
young bucks a thing or five.

So I'd hire him.

~~~
egeozcan
I don't get the Young Turks[1] reference. What do you mean?

[1]:
[https://en.wikipedia.org/wiki/Young_Turks](https://en.wikipedia.org/wiki/Young_Turks)

edit: I assume that was a typo as it was edited to "young bucks" later.

~~~
DanBC
Young turks is idiom to refer to usually young people who rebel against
authority. I'm not sure it fits here.

Whippersnappers is perhaps a better fit.

~~~
egeozcan
Oh, thanks for the clarification. I'm Turkish and didn't know of this usage.

~~~
droithomme
The news talk show, founded by a Turkish-American, that you are familiar with
and link to is actually meant to be a humorous reference to the idiomatic
usage. The idiomatic usage used to be well known, but these days people are
more familiar with the news show and the idiomatic usage is somewhat archaic
now. It's not a negative reference, more a respectful acknowledgement of
toughness and persistence - one doesn't want to mess with young turkish
warriors unless they are ready to face a competent adversary.

~~~
makeset
It's actually a reference to the early-20th-century Ottoman political reform
movement.

[https://en.wikipedia.org/wiki/Young_Turks](https://en.wikipedia.org/wiki/Young_Turks)

