Doppler DF

[BigMojo]

Does anyone use one for UDF or Mobile DF'ing? Just been doing some reading on the PicoDopp, interesting theory and wondering how well it would work in our application.

[floridacyclist]

Are you on the Picodopp Yahoo group? http://groups.yahoo.com/group/picodopp

Just tell me who you are (you know, the secret decoder ring and all LOL) and I'll let you in.

Seriously, we just make potential members tell us something so we know they're not a SPAMbot

[BigMojo]

I'm not...can't get on it at work either... our Firewall admin thought it would be a good idea to not let anyone use anything with "yahoo" in it... 

[ES forever!]

Well I have several of the Doppler systems dopplers, it can interface to the computer  and displays the heading and it actually plots a solution as you gain more angles that are shown on the map. the draw back is that the software is not a moving map system.

I looked at th pico the deviation on the heading was too great for me and there is no interface to any software.

[_]

Well I have several of the Doppler systems dopplers, it can interface to the computer  and displays the heading and it actually plots a solution as you gain more angles that are shown on the map. the draw back is that the software is not a moving map system.

I looked at th pico the deviation on the heading was too great for me and there is no interface to any software.

How much do those systems cost?

[♠SARKID♠]

Bump

I've spent much of the day looking at and researching Doppler and came across this PicoDopp system.  I pose the same question as was at the point of thread start.  How well would this work for us?  Would an AM 121.5 signal even be traceable through the FM system?  With the new 406 beacons, would this even be worthwhile considering 121.5 is now such a low power?

[SarDragon]

There's a Yahoo! Group for the unit. You can ask them all the Qs you want.

http://groups.yahoo.com/group/PicoDopp/

[♠SARKID♠]

Okay, lets just say the concept of Doppler in general then.  I know we have our own radio experts here that can shed some light.

[SarDragon]

OK, an FM receiver will receive an AM signal, but there is a significant loss of sensitivity.

As for 406 vice 121.5, the latter will still be easier to DF because of the constant signal. The 406 data burst is very difficult and time consuming to DF.

[Gunner C]

OK, an FM receiver will receive an AM signal, but there is a significant loss of sensitivity.

As for 406 vice 121.5, the latter will still be easier to DF because of the constant signal. The 406 data burst is very difficult and time consuming to DF.

Why is that?  Does it burst intermittently?  How often is the burst?  I found that 243.0 was easier and more accurate than 121.5.  Linear logic would tell me that 406 would be even better.  Obviously not!

[♠SARKID♠]

The data burst is only a small fraction of a second long.  Ive been chatting with the picodopp creator and he mentioned that the system requires a quarter to a half of a second to pick up on a carrier.  With the length of the data burst, the doppler wouldnt have time to engage, let alone have time to accumulate any useful data.

[Gunner C]

That makes sense!  Isn't there a weak 121.5 signal sent along with this?

[♠SARKID♠]

Yes there is.  Its not as strong as the old beacons, but its there for the close-in work.

[desertengineer1]

I think there's a bit of a misnomer in the name.  The design doesn't use doppler.  It measures signal strength of individual beams in an electronically rotated phased array.  More antennas = more accuracy, but requires additional switching and measurement capability.

[Major Lord]

I think there's a bit of a misnomer in the name.  The design doesn't use doppler.  It measures signal strength of individual beams in an electronically rotated phased array.  More antennas = more accuracy, but requires additional switching and measurement capability.

Respectfully, I disagree. The FM receivers used in Doppler systems don't care about signal strength above the threshold for FM capture. The electronically rotated antenna array measures the phase changes introduced as a function of the rotational "speed"  ( an audio frequency ) as each antenna is synthetically spun in relation to the signal origination point. This is a different process that the TDOA (time difference of arrival) or  amplitude divergence, and is truly using the Doppler effect. To use an analogy, it would be as if we drove a very quiet car in a large circle around a car track, and listened to a single noise source in the grandstand: the apparent frequency of the noise source would  change as we approached and again as we passed. This is the same phenomenon exhibited by a passing train's horn changing in apparent frequency as it passes a fixed point. Now I will grant that the amplitude of the trains' horn also changes in amplitude as it reaches and passes a fixed point ( the inverse/Sq law of energy is still the law, even under the current administration) but the amplitude differences in a Doppler array are for the most part ignored, since with a distant signal and closely spaced antenna, the percentage of change is not within the practical real-time measurement capabilities. The phase change is readily apparent, Ergo, Doppler vs. Signal strength.

The primary problems with Doppler systems for weak signal finding is twofold; One , any given antenna in the array is switched on for a very short time, largely because PIN switch diodes are used to turn each antenna on in turn, creating the synthetic rotation effect. Two, an FM receiver must be used, since the phase modulation used by Dopplers is not supported by AM receivers. This means that you need to have a signal strength sufficient to achieve FM capture. This is pretty hard to do with the anemic 121.5 transmitters on board these next generation ELT's. A number of users have reported Dopplers as very useful in flatlands for low power signal direction finding, so there may be some hope.

Major Lord

[SarMaster]

We have several units in FLWG.  They work on 406, 121.5 and just about everything else.  Right now we have 3 of them in fixed locations linked to the internet.  So i can sit at home and watch my computerscreen with google and see all the triangulation points on the map.....

USCG uses a similar system but with a 27M pricetag!

[♠SARKID♠]

Wow, high speed!  How do you link the program onto the internet, and how do you set it up for what I would assume is google maps?  Frankly the GPS software that is reccommended for the system seems a bit sketchy to me.

[♠SARKID♠]

A bit of an update, I've purchased one of the units and am about halfway through the construction.  I've finished the antenna platform and now only need to put the boards in enclosures and wire everything up.

[a2capt]

(Wasn't this thread just randomly locked with no explanation sometime back?)

I have a Roanoke Doppler and the antenna array for that thing seems to be the major weak link of the system. Even with it built with capacitors so that distance does not matter, it's still a finicky device. But just setting up the antennas proves to be a pain.

As for the battery, I do have to say .. I used a "dead" Mac Powerbook G4 battery that I charged with a universal charger and that thing still works after 4 years.

I'll be interested in what comes of this as I'd consider ditching that thing for something else. The other I looked at was the Ramsey kit.

[♠SARKID♠]

Okay, so it took a lot longer than expected, but I have my PicoDopp up and running.  Its all set for a field test (which won't be easy given the foot of snow we just got).

[AirAux]

Let us know which unit you got and where and how much and how it works.  I have thought this was the way to go for a while, but didn't see much interest in it.  Thanks.

[Major Lord]

My distributor sells this one as a kit for a good low cost Doppler kit.

http://byonics.com/dsp-rdf/

Major Lord

[321EOD]

Does anyone have any feedback on using the RAMSEY DDF unit?

Has anyone successfully replaced the really poor antenna array/magnets that come with the kit?

Photos would be appreciated (especially any vehicle installs)

Thanks

[a2capt]

I got a Roanoke Doppler and thought about working the antenna array I made for it, into the Ramsey one, as I decided that the Ramsey stuff would not last more than one use. If the antennas didn't depart.

[Major Lord]

I tried the Ramsey kit, and found it to be less than ideal. The antenna array as provided is just plain awful, but, there is no reason you could not improve on their array with a more robust build.

Major Lord

[♠SARKID♠]

Well I finally took my PicoDopp out for its first field test today.  I have to say that the performance is less than stellar.  It is very inaccurate on 121.5 and gives such a wide range of bearings that at a distance of even a few hundred yards its useless.  I'm going to have to tinker, talk with the creator, and figure out if its a problem with my setup or something.  I've been working on this far too long to let it go down in flames.

[Major Lord]

What are you using as a receiver?

Major Lord

[♠SARKID♠]

What are you using as a receiver?

Major Lord

A RadioShack Pro-95 scanner.

[Major Lord]

Confirming that you were able to switch the aircraft band range into the FM receive mode? If so, what kind of detection range were you getting with just the scanner alone? ( no Picodopp antenna array?)

Major Lord

[♠SARKID♠]

Oh dear lord I'm an idiot.  I forgot to switch it into FM.  *Bangs head against desk*

[Major Lord]

Naaah, happens all the time with people using Dopplers on Aircraft bands. I did not have great luck with Doppler on 121.XXX, but maybe the Picodopp will work better then the commercial Marine unit, and the Ramsey, I tried. The Picodopp looks cool!

Major Lord

[wingnut55]

And you are using FM for what? Aircraft com and ELTs are AM,  121.775 is our practice beacon.

[♠SARKID♠]

And you are using FM for what? Aircraft com and ELTs are AM,  121.775 is our practice beacon.

The doppler requires the receiver to be in FM to create the necessary modulation and tone.  You lose all the audio, but a carrier is a carrier and it can still be tracked in FM.

[Major Lord]

You should still be able to hear the characteristic sweep tone even using FM demodulation, since there is a phase-shift modulation element to the signal as well. No signal is really AM or FM in a perfect sense, they contain aspects of multiple forms of modulation, depending on your mathematical model for viewing them. As a practical matter, you will find that you can talk to an aircraft radio with an FM ham radio operating out of band, although it won't sound good! (Note: This is against the rules-only use for emergencies!)

One of the limiting factors of a Doppler for small signal direction finding is the requirement for an FM receiver. You have to have enough signal level for FM capture, generally much more than you need for AM demodulation. The antenna array in a switched antenna system (like the Doppler) is also inherently lossy.

What are you using for a Beacon?

Major Lord

[♠SARKID♠]

I'm just using the standard pointer cadet beacon.

[a2capt]

It was clippy, but when VX-150's were still usable within CAP, and when you do the wide open modification instead of just the MARS mod, you can enter anything from 100 to 200 mhz. Of course, the radio didn't tune that whole spectrum but some got fairly down into the aviation band.  So much so that they were very usable in close proximity to talk to overhead aircrews who had no FM. I had a chance to cherry pick radios after I modified a whole bunch and have a couple that work down as far as 121.395 which also makes them useful for DF'ing up close, when you need an additional piece of equipment to do hangar elimination.

At one place I used to work, they were always picking up SoCal Approach on a particular radio one part of the building. (A radio tuned to an album rock FM station for music in the tech shop), sometimes the chatter got quite  busy and someone would call me to ask what it meant. The station in question was 101.5, IMHO. Quite "far" from the nearest voice frequency of 118.000 and higher. Even the VOR's begin at 108.000, so there must have been some harmonic reasoning having to do with the exact positioning of the radio, it's antenna, the surrounding terrain and a remote station for the approach sector.

I've got a Roanoke Doppler that I've used with the output from the VX-150 as well, and had decent results with it. The hassle with that thing is the antenna array that I over-built.