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).