We seek to understand and document all radio transmissions, legal and otherwise, as part of the radio listening hobby. We do not encourage any radio operations contrary to regulations. Always consult with the appropriate authorities if you have questions concerning what is permissable in your locale.

Author Topic: OTHR 11244  (Read 2657 times)

07code04stalker1776

  • Guest
OTHR 11244
« on: March 29, 2013, 2017 UTC »
Getting OTH Radar on 11244. Sounds a bit off freq. How could I determine the actual frequency of origin?

Offline Token

  • Global Moderator
  • DX Legend
  • *****
  • Posts: 1791
    • View Profile
Re: OTHR 11244
« Reply #1 on: March 30, 2013, 0129 UTC »
How to tell the frequency of OTHRs will depend on what hardware you are using.  What kind of receiver do you have?

Also, what kind of OTHR is it?

T!
T!
Mojave Desert, California USA

07code04stalker1776

  • Guest
Re: OTHR 11244
« Reply #2 on: March 30, 2013, 0241 UTC »
Yeasu FT-817 and a mobile antenna.

here is an example similar in nature.

https://soundcloud.com/search?q=cyprus%20othr%20-%2050%20sweeps%20sec

Offline Token

  • Global Moderator
  • DX Legend
  • *****
  • Posts: 1791
    • View Profile
Re: OTHR 11244
« Reply #3 on: March 30, 2013, 0255 UTC »
That sound sample does not seem to work here.  But the description matches the PLUTO radar on Cyprus Island.

Here is one of my videos of PLUTO  http://www.youtube.com/watch?v=tU6UTZUCvsc

The above video is the most common mode for PLUTO, 50 Hz rep rate and 20 kHz wide, but it does have other modes.  Note that PLUTO almost always ends up on a center frequency that is some multiple of 5 kHz, so the the freq ends in a 5 or 0.  Also, when PLUTO shifts freq and comes up on a new freq there is a dead carrier for the first several seconds before the FMCW sweeps start.

T!
T!
Mojave Desert, California USA

Offline skeezix

  • Global Moderator
  • DX Legend
  • *****
  • Posts: 3587
  • Minneapolis, MN
  • What does 'RNO stand for?
    • View Profile
Re: OTHR 11244
« Reply #4 on: March 30, 2013, 0301 UTC »
The above video is the most common mode for PLUTO

So THAT's what that thing is. Been wondering about that.

Minneapolis, MN

Offline Token

  • Global Moderator
  • DX Legend
  • *****
  • Posts: 1791
    • View Profile
Re: OTHR 11244
« Reply #5 on: March 30, 2013, 1424 UTC »
Getting OTH Radar on 11244. Sounds a bit off freq. How could I determine the actual frequency of origin?

Determining the center frequency of a radar signal with a  traditional superhet like the FT-817 will depend a little bit on what kind of radar it is.

In the case of the radar that it probably was in this case, the PLUTO radar off of Cyrpus, it is pretty easy.  PLUTO is an FMCW radar.  This means Frequency Modulated Continuous Wave.  Continuous wave, in this case, is the true meaning of the word.  In radio we get used to calling Morse Code “CW” and that is wrong, typical Morse (no modulation, carrier turned off/on to indicate the dits and dahs, also called A1A) is more correctly called “ICW”, for Interrupted Continuous Wave.

PLUTO uses a specific type of FMCW called LFMCW, Linear FMCW.  This is actually the most commonly used HF radar / sounder waveform.  This simply means that the modulation frequency that generates the FMCW is linear, or if you were to plot it on a graph it would be a straight line with a specific slope to it.  The slope is the “chirp rate” of the signal.  The chirp rate of an FMCW radar can tell you the velocity range of targets the radar is designed to detect.  You can tell a lot of basic information, with any radar, based on the transmitted signal, maximum possible range, probable target velocity range, probable minimum target size, minimum detection range, potential to track nth time targets, and range resolution, these are some of the basics that can be guestimated at with a received radar signal, some of them can be determined quite accurately.

A picture will help here, this is the PLUTO common waveform, 50 Hz rep rate, 20 kHz width, 1 MHz chirp rate, as seen when you break it down close enough to see each sweep.  This is the same waveform as was used in the video that I posted from my YouTube channel:



Using the vertical cursors and looking at the “dF” (delta frequency) value in the upper right corner we can see it is 20 kHz wide.  Yes, the dF value says 20058, not 20000, however that is caused by the sample rate used to write this display, it would not allow the cursors to land exactly on 20000 Hz.

Using the horizontal cursors and sampling across 10 cycles we can see that the “dT” (delta frequency) value in the upper right corner says 00:00:200.165.  Again, the written rate of this window would not allow the cursors to land on exactly 200, but that is what it should have been.  In this case the 00:00:200.165 means 200.165 milliseconds, or 200 milliseconds for 10 cycles, so 20 msec per cycle.

The important thing to take away here is that the transmitter stays on continuously, it sweeps from the lower freq to the upper and immediately goes back to the lower and again sweeps up, in an ever repeating sawtooth pattern.  Because the transmitter never pauses between pulses this is CW.  The fact that the signal sweeps in frequency during each sawtooth makes it FM, or frequency modulated.  The fact that the sweep is a straight line means the FM rate is linear.

So, what does all of this have to do with determining the frequency using a traditional superhet receiver like the FT-817?  Knowing the shape helps understand why you must do what you must do to find center frequency.  The following applies to FMCW radars like PLUTO.

If you tune anyplace within the FMCW signal you will hear what sound like chirped pulses.  If your IF bandwidth is 2.8 kHz wide you will hear a little more than 2.8 kHz of the 20 kHs wide signal.  Everyplace within the signal will sound about the same, with no detectable “peak” in the signal.  If you are in USB as you tune up in frequency the signal should end fairly abruptly as you find the edge of the signal.  As you tune down the signal should appear to taper off more as you find the edge.  You can use this to your advantage.

Starting from someplace within the signal select USB.  Tune upwards in frequency until the signal falls off and note the frequency.  Now select LSB and tune down in frequency until the signal falls off about the same amount, again note the frequency.  This is mostly done by ear, but the S meter can help also.  Subtract the lower frequency from the upper frequency, this should give you a value pretty close to the width of the signal.  Then divide the width by 2 and either subtract it from the upper frequency or add it to the lower frequency, this will result in the center frequency.

This sounds like a pain in the butt, but really it is quick and easy after you have done it once or twice.  And you now have two parameters to help ID what the radar is, its bandwidth and its center frequency.  By using a simple audio program you can determine the sweep rep rate.  With rep rate and bandwidth you can mathematically determine chirp rate.

Frequency, bandwidth, and rep rate are all ways to help ID which radar you are hearing.  Some radars have limited frequency ranges, some radars always use the same rep rate or bandwidth.  And no, there are no good online references to which radar has which rates/BWs/etc.  You just have to gather that information as you can.

Hope that helps,
T!
T!
Mojave Desert, California USA

07code04stalker1776

  • Guest
Re: OTHR 11244
« Reply #6 on: March 30, 2013, 2144 UTC »
That is awesome information. I wish I could learn more. What is the best way to accomplish this Token? Is there a class or reference material some where?

Offline Token

  • Global Moderator
  • DX Legend
  • *****
  • Posts: 1791
    • View Profile
Re: OTHR 11244
« Reply #7 on: March 31, 2013, 0308 UTC »
That is awesome information. I wish I could learn more. What is the best way to accomplish this Token? Is there a class or reference material some where?

Well, you could join the military and become something like an EWO or a CTT, those classes cover the subject pretty well ;).  If you look online and find the Ohio State “Introduction to Radar Systems, 2011” that is a pretty good starting point.  Merrill Skolniks “Radar Handbook” is also a good place to start, and he also has an “Introduction to radar systems” (different book from “Radar Handbook”) that is pretty good.

Most of the above focuses on microwave radar, not HF.  However pretty much any technique that can be used at microwave can and is used in HF also.  When looking at text’s on the subject just keep in mind the limitations of HF and bandwidth.  For example you are not going to see a 100 nanosec pulse used in HF, probably.  No matter that you might desire the range resolution of such a pulse, the fact that it would have a 3 dB bandwidth on the order of 10 MHz means that it would be impractical, but not impossible, in the HF range.

T!
T!
Mojave Desert, California USA