
How Radar Works - lowdanie
https://www.daniellowengrub.com/blog/2019/10/26/radar
======
jcims
The HB100 [0] and CDM324 [1] are very inexpensive little CW doppler radar
modules at 10GHz and 24GHz respectively. Feed them 5v and send the mixer
output to a small microphone amp/input jack and you can record the frequency
beat of most objects in motion.

[0]
[https://www.amazon.com/dp/B00FFW4AZ4](https://www.amazon.com/dp/B00FFW4AZ4)

[1]
[https://www.amazon.com/dp/B07WH67J9W](https://www.amazon.com/dp/B07WH67J9W)

This is what it sounds like when you toss a quarter in front of an HB100.
[https://www.youtube.com/watch?v=8riretP8ylE](https://www.youtube.com/watch?v=8riretP8ylE)

The bowtie looking part is the actual flight, the other part right after that
is it bouncing off the counter.

Spinning same quarter -
[https://www.youtube.com/watch?v=5lnYvJoxRak](https://www.youtube.com/watch?v=5lnYvJoxRak)

Shining it at parts of a ceiling fan -
[https://www.youtube.com/watch?v=tIiFvByf1CQ](https://www.youtube.com/watch?v=tIiFvByf1CQ)

~~~
GhettoMaestro
It amazes me how cheap these application-specific chips have become - it opens
up so many new potentials. I can only guess that something like this would
cost $1000+ roughly 10 years ago?

~~~
blattimwind
No, not really, doppler radar based proximity sensors (used in e.g. baths)
cost about 40 € more than 10 years ago. It's not complicated technology.

~~~
scoot
What's an e.g. bath?

~~~
gregoryl
e.g. is
[https://en.wiktionary.org/wiki/exempli_gratia](https://en.wiktionary.org/wiki/exempli_gratia)

which you can pretty much read as "for example, [example(s)]"

~~~
scoot
Sorry, I should have guessed my comment would be lost on those who don't know
how to use "e.g." correctly.

~~~
gregoryl
Just assuming you were being earnest (perhaps learning english!) as there
wasn't much to gain from interpreting it another way.

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benburleson
This brings back a lot of memories. One of my proudest technical achievements
was as an intern: an assignment to figure out how many unique emitters were in
a field of data picked up by some antenna. The data was provided on a CD with
not a lot of other info.

I ended up finding a program some college researcher had published for
matching DNA patterns, and between a little data massaging and software
hacking, I was amazed when results actually made sense.

As an intern, I never knew exactly who provided that CD or where the program
went from there, but it was a fun problem to work on for a while.

~~~
jacquesm
These cross-domain hacks are very satisfying.

~~~
sbmthakur
Is there a list somewhere about such hacks?

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rrss
Thanks OP, this is a nice resource.

One note, though. The presentation of IQ demodulation seems to mix up the real
and imaginary components of the analytic signal with the electric and magnetic
fields. Even if you skip the analytic signal and just generate a straight up
real signal (with zero imaginary component), there will still be a magnetic
field.

This seems important because the current presentation suggests the complex
signal representation is somehow specific to electromagnetic signals, while it
is really just a mathematical convenience applicable to any signal (EM, sound,
wave on jump rope, etc).

~~~
lowdanie
Thanks a lot for the comment - you are absolutely right.

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m0zg
Early nukes used an interesting radar design to trigger at a preset distance
above ground: there was a long coil of cable inside the nuke, and the fuze
would emit random electromagnetic noise towards the ground. The reflected
noise would then be received by an antenna, and the original emitted signal
would be delayed by the coil by a certain predetermined time delta. This
allowed the nuke's fuze to compute autocorrelation (and therefore detect
distance) using entirely analog methods, and in a way that's impossible to
jam, because autocorrelation on random signal is pretty darn robust (it's a
delta function for ideal white noise).

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dijit
There was a good talk about how radar (and lasers) are weaponised by police to
detect the speed of vehicles (and countermeasures to that) at defcon this year
too.

[https://youtu.be/vQtLms02PFM](https://youtu.be/vQtLms02PFM)

Might be interesting too.

~~~
blattimwind
> how radar (and lasers) are weaponised by police to detect the speed of
> vehicles

How exactly is this process "weaponisation"?

~~~
whatshisface
If your radar gun is strong enough, it will push the car forwards. Once the
plasma and gaseous metals have dissipated you can then ticket the driver for
speeding even if they weren't speeding before you measured their speed.

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pps43
It's a good start, but stops short of the most interesting parts. The signal
to noise section should at least mention the root of forth power in the radar
equation, as this is one of the key limitations. Beam forming with mechanical
scanning or phase arrays is also important from the practical standpoint, as
so is the relationship between wavelength, antenna size and angular
resolution. Finally, at least the concept of how the autocorelation function
of a good pulse should look like is worth mentioning, with examples of
complementary or pseudorandom sequences .

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drno123
There is fantastic webinar (consisting of 5 videos) from TI which covers CW
and FMCW radar techniques: [https://training.ti.com/intro-mmwave-sensing-fmcw-
radars-mod...](https://training.ti.com/intro-mmwave-sensing-fmcw-radars-
module-1-range-estimation?context=1128486-1139153-1128542)

~~~
watersb
Wow, thanks!

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jorblumesea
Math went over my head, does anyone have a primer or intro for some of the
math involved here?

~~~
dboreham
Kreyszig: [https://www.oreilly.com/library/view/advanced-engineering-
ma...](https://www.oreilly.com/library/view/advanced-engineering-
mathematics/9780470458365/)

~~~
mcguire
Well, that's the deep end...

I'm having to translate it from TeX, so I don't know, but you might want The
Scientist and Engineer's Guide to Digital Signal Processing By Steven W.
Smith.

[http://www.dspguide.com/](http://www.dspguide.com/)

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tzs
For the constant velocity case, the graphic shows the transmit pulse as having
two cycles, and the received pulse as having four cycles of higher frequency,
with the received pulse being the same length as the transmit pulse.

Shouldn't the received pulse have the same number of cycles as the transmit
pules, and the received pulse be shorter than the transmit pulse (or longer in
the case of the target moving away instead of toward the source)?

Or does something non-intuitive happen (probably because of special
relativity) resulting in the received pulse gaining energy from the
reflection, which is reflected (pun intended) as the pulse being longer than
you'd expect classically?

~~~
lowdanie
You are right that the graphic is a highly simplified cartoon of what the
actual received wave looks like - I should have stated that in the post.

Regarding the question, it is true that the received pulse will be shorter or
longer than the transmitted one (depending on whether the object is moving
towards or away from the sensor) but since we are assuming that the speed of
the object is significantly less than the speed of light, we're making the
simplifying assumption that this difference is negligible. People sometimes
refer to this as the "stop and hop" assumption since in practice we are
assuming that the object "stops" at point x when the radio wave is emitted and
then "hops" to x + dt*v once the wave has been received dt seconds later.

Regarding the number of cycles, you are again correct that the image
represents a gross oversimplification. One reason for this is that since the
carrier frequency is so much higher than the doppler frequency, the true graph
of the received wave would not be noticeably different than the transmitted
one. It is easier to plot a realistic graph of the demodulated wave which you
can see in the plot titles "samples of the demodulated pulse".

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soufron
To me, the maths of the radar are the easy part.

But the physics of it? That's something that seems entirely more complex to
me.

Is it really? Could any of you explain how you emit the radio wave, and how
you detect it back? In terms of hardware?

~~~
packet_nerd
It starts with an oscillator circuit producing an alternating current sine
wave at the desired frequency. The oscillator circuit often involves a crystal
in order to generate an accurate and stable frequency.

The alternating current is then feed down a transmission line, which in the
most basic case is just 2 wires side by side. Being side-by-side, the current
going "up" in one wire is balanced by the current coming "down" in the other
so that at a distance from the transmission line, the radiation from the two
wires (mostly) cancel each other out. Often instead of side-by-side, one wire
is often wrapped around the other with an insulator separating them (coax),
but the same principal applies.

The transmission line then feeds into the antenna. A basic antenna is a piece
of wire exactly 1/2 wavelength long. Electric pulses "bounce" off the wire's
ends, sort of like how if you tether one end of a rope and then shake the
other, the "waves" going down the rope will "bounce". The length of the wire
being 1/2 wavelength is key because it takes a wave exactly one cycle to start
from one end, bounce off the other, and return to the start. Just as the first
wave is bouncing off the start end, it gets a new "bump" from the transmission
line. This allows a lot of current to build up in the antenna, sort of like
how a kid on a swing can use several small well timed kicks to swing himself
much higher than just a single big one could. Of course, antennas can be much
more complicated than just a dipole, for example, you could place several
dipoles in a row and feed them in phase so that their transmitted signals add
together at points perpendicular to the row, and cancel each other at points
in line with the row. Or, you could use a parabolic dish to concentrate the
signals in one direction.

In radar, the radio waves travel out, bounce off objects in the their path,
and a small fraction of the original signal comes back to the antenna.
Depending on the design the radar might have a separate receiving antenna, or
it might receive on the same antenna it used for transmitting. From a radio
perspective, a receiving antenna is exactly the same as a transmitting
antenna, only, it's the radio waves coming in through space that cause it to
resonate instead of the other way around in a transmitting antenna.

Finally, the signal comes down the transmission line and into a receiver
circuit which analyzes and extracts whatever data (in the radar case, it
deduces distance and speed by looking at the elapsed time and frequency
shifts).

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zxcvbn4038
I’ve always wanted a way to trigger the radar detractors that people use in
their cars. I’ve noticed for a while that when a bunch of people suddenly move
to the right lane there is usually a cop with a speed gun around, so sending a
fake pulse periodically seems a sneaky way to clear the traffic ahead (but
would only work when there was something solid to reflect the pulse back,
sorry Kansas). I’ve not found anything on Google or Ali Express that looks
like it might help me pull this off - nobody wants to build a Rasphberry Pi
speed gun it seems.

~~~
acomjean
I don't think your allowed in the US to send fake pulses. Something FCC...

Otherwise I'd think you'd see a lot of radar dector jammers on the market.

~~~
GhettoMaestro
Laser and radar jammers do exist and is usually targeted at high end car
folks. It’s not legal obviously.

~~~
denysvitali
Radars jammers aren't legal (because radio waves are regulated by the FCC),
but laser jammers are actually legal in most of the US (because light is
regulated by the FDA):

[https://youtu.be/vQtLms02PFM](https://youtu.be/vQtLms02PFM) : popped out
today on my YouTube feed, pretty relevant to this whole discussion (and also
really worth talk)

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et2o
Of course I knew that radar measured time of flight of radio frequencies, but
it never occurred to me to actually look into the math. This was presented
incredibly well.

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sandpaper26
While this is a good intro, a lot of radar work now relies not on square
pulses but linear frequency modulated (LFM) waveforms. This greatly expands
the number of image formation algorithms you can use, depending on system
requirements.

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unnouinceput
Side note: Same technique can be used to track a cell phone, except GSM
frequencies are higher so the tracking distance is considerably shorter then a
normal radar. But still good enough to do a city wide track of a person of
interest.

~~~
blattimwind
GSM frequencies (800, 1800 MHz) are about an order of magnitude lower than
typical radar frequencies (~5-20 GHz).

~~~
touisteur
Mode S is 1030/1090 MHz, and L-band/S-band Primary Surveillance Radars ('real'
radars) are still quite in use, especially in ground-based surveillance or Air
Traffic Management. In fact 4G and wimax were threatening civilian radar
bands.

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pron
Next level: [https://en.wikipedia.org/wiki/Synthetic-
aperture_radar](https://en.wikipedia.org/wiki/Synthetic-aperture_radar)

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ak217
Thanks, this is a nice introduction.

Two typos in this sentence: _One way to work around this problem is to use a
non uniform spaceing of the pulses which is none as staggering._ (spacing,
known)

~~~
yesenadam
Even better:

One way to work around this problem is to use a non-uniform spacing of the
pulses, which is called _staggering_.

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itcrowd
Lovely introduction to pulsed radar systems. Would love a similar piece on
continuous wave systems (fmcw, for example). Thanks for this!

~~~
GhettoMaestro
Is CW used for radar applications? (I am only familiar with pulsed radar
sets).

~~~
blattimwind
AFAIK not much in practice because it can only detect speed and has no real
way of detecting distance. Most radar applications want to detect distance,
usually enhanced to position, and speed as well. You can't really do that with
CW.

~~~
rrss
FMCW (mentioned by 'itcrowd) can determine distance.

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choxi
I've been watching a lot of those USS Nimitz UFO videos lately, so this is
very timely

