I've been working on the Pirate Radio Annual; it's going to be a long haul & I'm nowhere close to finished with it. I've just been starting to think about articles, etc. for it. One that I was thinking about was TDoA and if any measures can be taken to skew the results.
Are you specifically talking about impacting the ability of hobbyist or hobby networks like the Kiwi TDOA plug in? Or are you worried about professionals?
At the hobby level you may be able to reduce the accuracy, but probably not at the professional level.
* use two transmitters on exactly the same frequency from somewhat different locations that are fed with an Internet audio feed to keep the audio in sync
* use an AM transmitter in one location to lay down a carrier on a frequency and an SSB transmitter in a different location to modulate the carrier
The multi transmitter solution has the most probability of success, as long as they are all the same mode. What is that going to do to the audio though? Propagation delays will produce echoes, even if the audio starts out in sync.
Use of an AM transmitter combined with an SSB signal will not prevent or degrade TDOA. For the purposes of TDOA you can ignore (in fact, most hobby based applications probably do this) the carrier as it has no easily / grossly identifiable features. Instead you look only at the modulation and you correlate events in the modulation between the samples taken.
Other less active possibilities seem to be:
* use of frequencies with a smaller ground wave
* use of antennas with low-angle radiation
* use of directional antennas
Short of reducing the area over which a transmission is heard none of these will be useful. TDOA works regardless of the propagation technique, direct path, ground wave, sky wave, short of backscatter there is little that can be done there. Direct path may be the most accurate, but good results can be achieved with any kind of propagation that does not involve angular multipath or backscatter.
So I also wonder about the general accuracy of the TDoA results while just simply broadcasting on 43m with dipole or a nondirectional antenna. Has anyone noticed that the results are more or less accurate on different frequencies (e.g., 4 MHz vs. 6.9 MHz, etc.)? Any other variables?
The accuracy of Kiwi type TDOA applications is highly variable. The accuracy of professional systems much less so.
Professional sensors can use TDOA or combinations of TDOA and other techniques to plot the source of a transmission at significant distances to very small areas. I have used TDOA and other passive systems capable of plotting the position of a target at 10's of km to within a few meters, at hundreds of km to within a large city lot, and at thousands of km to within a few miles.
Long range TDOA, thousands of km, gets you close, short range TDOA or other techniques get you exact.
TDoA's accuracy is IMHO highly overrated. I am not sure why it would be a concern to pirate operators, the FCC has significantly better technology (as has for a long time).
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As I like to joke "Propagation gives your location away".
I 100% agree when you are talking about hobby TDOA like the Kiwi system.
And yes, it has always been possible to get a good idea where an operator was, at least to the general region. With that information, and if you really wanted to invest the time and expense, you could use other techniques to find the exact location.
Since hobby TDOA became widely available I have plotted, for my own information, the majority of pirates I have heard on HF. I have not shared those results with anyone, and see no reason to, but in general there have been very few surprises, most of the ops appear to be transmitting from about the locations I previously thought. The availability of TDOA generally has not revealed ops locations to me that I did not already suspect.
There's one idea I can think of that might work - use very low power. The weaker you can make your signal, the larger the TDoA error will be. Probably not an ideal solution, however.
Distributed low power transmitters, a network of multiple transmitters working at low enough power to not deliver usable energy to more than a couple of TDOA nodes, would work, but is not practical in this application.
If no more than 2 TDOA nodes can receive a given signal you cannot resolve its location. If the third node is receiving the, seemingly, same signal (or two versions of the same signal, one weak and one significantly stronger) from a different transmitter location then the plotted results will be garbage.
I understand the concern operators have... but think TDoA may be overrated. It's not helping the FCC. I guess if an operator is really concerned about listeners knowing roughly where they are located (as in, in one of these states here, or maybe one of those states over there...) it may be an issue.
This. Professional facilities already have (and have had for some time) the ability to plot with far greater accuracy than the hobby TDOA systems like Kiwi can deliver. The hobby TDOA solutions just bring a "lite" version of that technology to the common man.
If anyone with nation-state pockets is looking at you they will find you, the question is do they have the time, manpower, or desire to look for you?
The other factor I didn’t see mentioned is where some receivers are getting single hop reception and others are ground wave or multiple hops. That will impact the position estimate unless the TDOA algorithm corrects for that.
The varying height of the reflecting ionosphere (which can be different for each receiver site) impacts the accuracy. Picking receiver sites on either side of the day/night terminator could have an impact on accuracy. If you could get receiver stations that all heard the xmtr via ground wave then the position estimate could be very good...but you already have a good idea of where they are then don’t you?
Only if you are trying to get accuracy measured in a few km at extended distances.
And that is a key. No TDOA system, pro or otherwise, in one plot yields a street address at thousands of km distances. But it can get you to within direct path or ground wave receive distances from that far. Then stage 2 is getting to that approximate location and using local techniques (can be TDOA also, or maybe just AOA). Of course, if you start in the general area and know you are there (say by propagation characteristics) then local techniques can be used initially.
T!
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