PilotAware
British Forum => General Discussion => Topic started by: tfede on October 14, 2015, 08:49:01 pm
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Hi,
does the system detect also trasponders in C mode (only ID+Height) ?
Federico
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No, at least not at the moment. It is looking for ADS-B traffic only.
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There are two reasons why it won't work with Mode C
a) Transponders with Mode C only transmit in response to a radar scan. So if out of radar coverage (eg below 3000 feet in much of the UK), they don't emit anything at all.
b) Mode C doesn't include any position information other than altitude, whereas ADS-B includes full GPS position and other data (eg callsign). So in order to know where the signal is coming from, you need some expensive antennas and electronics that can detect the direction and estimate the distance based purely on signal strength. There are some TCAS/TAS systems that can do this, but you'd be looking at £10K+ for anything including useful directional information. The cheaper portable units (eg Zaon) were said to have unpredictable performance. (http://www.flyingmag.com/avionics-gear/portablehandhelds/zaon%E2%80%99s-pcas-xrx-collision-avoidance-system)
So we really need more aircraft to be fitted with any/all of ADS-B, F***M and PilotAware to be able to track other traffic inexpensively.
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b) is the principal issue in my opinion. I routinely fly in airspace where there is no radar service on offer yet it is rare for my transponder to show no interrogations at all, even below 1000ft. Just because there are no service providers doesn't mean that there is no radar coverage.
FWIW I think that ADS-B will become the system of choice in the fullness of time. Position reporting systems such as P3i and Fl*** which use the license-free ISM band will always be limited by legal power constraints and the need for yet another antenna capable of seeing the whole sky. As relatively few GA aircraft have transponders with extended squitter capability, let alone ADS-B in, I suspect that time is still well into the future. In the meantime, PAW looks like the best option.
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Yes - it's a cheap gateway to ADS-B in (and 'out' if you have a mode S transponder) available right now.
The peer to peer transceiver side is a great bonus and should give a nice solution for non mode S aircraft until the ADS-B panacea arrives!
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Wouldn't it be nice if we were allowed to let PAW transmit ADS-B messages!
I suppose I can see why this might be frowned upon - the integrity of the whole ADS-B infrastructure would be at the mercy of bods with dodgy soldering irons and limited understanding - but the counter argument is what is the point of ADS-B if it is too expensive for small aircraft to be able to participate in!
Rant over!
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Wouldn't it be nice if we were allowed to let PAW transmit ADS-B messages!
I suppose I can see why this might be frowned upon - the integrity of the whole ADS-B infrastructure would be at the mercy of bods with dodgy soldering irons and limited understanding - but the counter argument is what is the point of ADS-B if it is too expensive for small aircraft to be able to participate in!
Rant over!
Hi Rob,
anyone else pitch in, but I am pretty sure you will be able to do this, once I get the code resurrected to provide NMEA messages out to the transponder.
I understand the rules are now relaxed so that you can do this SO LONG as you set the integrity level bit accordingly on your transponder.
Thx
Lee
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Just got the reply, Does this help on the rules.
"We will add your name to the list of participants. You will need to follow the LAA process for implementing the minor modification and you must check the SIL and SDA of your transmissions are set to zero. This would normally entail a test flight within the NATS’ ADS-B coverage, which is south and east of lines from Bristol to Stafford then across to Skegness, however, NATS are working with the LAA and the CAA to determine how we can perform a nationwide check."
Now to get the installation signed off and I'm good go for the testing.
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I was thinking more of having a home made ADS-B transmitter without the need for a commercially supplied mode S transponder, but your point is well taken.
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It'll be interesting to see how all this pans out.
Using extended squitter in SSR transponders for ADS-B was a bit of a cludge in the first place. It was just a convenient way of using a frequency that was already allocated to aviation and could relatively easily be adapted to support ES, thus enabling existing aircraft infrastructure to be used. In the US there is already concern that ES will result in too much traffic on 1090MHz and they are proposing a separate, non-SSR frequency of 978MHz for GA aircraft.
If that were to be adopted here then it would open up the idea of an ADS-B only transceiver, which could be produced far more cheaply than a Mode-S + ES transponder. Such a unit would not need any user controls, therefore requiring no panel space and could be put anywhere in the aircraft within reason. It would still need an external antenna and probably have a transmit power in the 10s of watts PEP level, rather than a few hundred mW. It's the sort of thing a company like Trig would jump at.
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The Reply
"This would normally entail a test flight within the NATS’ ADS-B coverage, which is south and east of lines from Bristol to Stafford then across to Skegness,"
Long way for Westmorland Flyer to go for a test and me from Mona. Inverness ???
Is interesting. If FR24 and Pilotaware can do better than this there must be an opening for a feed to NATS. The potential of a large amount of receivers gives potential for error checking on a grand scale.
Lee is it possible to decode enough of the ADS-B for the web interface to identify the target ( reg or hex ) and display the long and lat and altitude that is being sent out ? This would make for a useful piece of test kit.
Doug
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Hi Lee
I am eager to get the PAW NMEA messages output to input to my FUNKE TRT800 Mode S trnspndr so more than willing to be a tester for this for the PAW community.....
Cheers
Bryan
Ps: you expect it to be in the next software release ?
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If that were to be adopted here then it would open up the idea of an ADS-B only transceiver, which could be produced far more cheaply than a Mode-S + ES transponder. Such a unit would not need any user controls, therefore requiring no panel space and could be put anywhere in the aircraft within reason. It would still need an external antenna and probably have a transmit power in the 10s of watts PEP level, rather than a few hundred mW. It's the sort of thing a company like Trig would jump at.
This is what the NATS LPAT device does if I understand it correctly, albeit still on 1090 ES.
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Hi All,
There was a question a while back about handling bearingless traffic.
I am at least partway forward in showing something ...
I guess the question is what should the limits be set to for relative height ?
the rings can be displayed as Red/Amber/Green so three height levels could be used.
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What sort of range can you detect targets out to?
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What sort of range can you detect targets out to?
This is the black art I am dealing with at the moment.
There is no range in the message, simply signal strength and altitude.
The idea is to convert signal strength to range, to be honest I think this is fraught with inaccuracies, but I am willing to investigate.
Thx
Lee
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The OGN people show a method for calibrating dongles which I guess would be needed to do range estimation based on signal strength:
http://wiki.glidernet.org/wiki:raspberry-pi-installation (http://wiki.glidernet.org/wiki:raspberry-pi-installation)
The very first thing you need in order to receive the signals is to be on the correct radio frequency.
Cheap SDR receivers use cheap crystals and their frequency tolerance is about +/-50-100ppm.
At 868MHz 100ppm error makes you 86.8kHz away from the correct frequency, while frequency deviation for FLARM signals is +/-50kHz.
Technically, the software receiver can search a wide range of frequencies for radio packets but this inflicts lot of CPU, thus can only be done for stronger CPU boards, but not for Raspberry PI.
Thus you need to know how much off is the crystal of your DBV-T dongle before you proceed.
You can measure the crystal with the gsm_scan tool, run it like this:
./gsm_scan --ppm 50 --gain 20
it should receive some GSM broadcast channels and measure the frequency correction.
Notice the GSM frequency with a strong broadcast channel, even better: with one or two directly adjacent channels.
Adjust the gain (--gain) and initial crystal correction (--ppm) for best reception of as many as possible channels with consistent correction measurement.
GSM signals are very strong, thus too much gain is not good. You need to find an optimal setting here.
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I've tinkered with this on my spare pi using my 2 dongles. One is 15ppm out, the other 30ppm
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Well if you could detect mode c at say 3, 1 & 0.5nm (of course accepting it's may not be accurate ) that would be incredibly valuable.
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The idea is to convert signal strength to range, to be honest I think this is fraught with inaccuracies, but I am willing to investigate.
Completely agree. It will be nearly impossible to determine range using signal strength with even the slightest nod to accuracy. Amongst the many problems is the fact that the transponder antenna is on the underside of the aircraft - the obvious place, since it is working a ground radar. Signals above or even along the plane of the aircraft will be severely attenuated compared with those below the plane, making range determination on signal strength alone nearly impossible. Without directional antennas it isn't possible to determine the bearing either. I don't think it's worth the effort.
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sorry let me clarify - it is the vertical height difference I am interested in, eg
+/- 250ft RED
+/- 500ft AMBER
+/- 1000ft GREEN
The diameter of the circle is the indication of range (derived from signal strength)
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The idea is to convert signal strength to range, to be honest I think this is fraught with inaccuracies, but I am willing to investigate.
Completely agree. It will be nearly impossible to determine range using signal strength with even the slightest nod to accuracy. Amongst the many problems is the fact that the transponder antenna is on the underside of the aircraft - the obvious place, since it is working a ground radar. Signals above or even along the plane of the aircraft will be severely attenuated compared with those below the plane, making range determination on signal strength alone nearly impossible. Without directional antennas it isn't possible to determine the bearing either. I don't think it's worth the effort.
The trick I am hoping to use is as follows.
Gather and store the ADS-B messages as references,
this will give me an indication of strength for relative distance/height.
this provides me a look up table averaging strength against the distance/height.
on reception of a non ADS-B message, index the table of known (ADS-B) messages for the best estimate.
So in effect the receiver becomes 'self calibrating' using the well defined ADS-B data, to estimate the non ADS-B data
Determining bearing is totally impossible, that is why I said 'bearingless traffic', although I may well be able to use
signal value increasing/decreasing.
I think this is worth a shot, please keep to youselves, I am still waiting for the patent grant on the self calibrating receiver idea :)
Thx
Lee
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Clever Clever Clever :)
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sorry let me clarify - it is the vertical height difference I am interested in, eg
+/- 250ft RED
+/- 500ft AMBER
+/- 1000ft GREEN
The diameter of the circle is the indication of range (derived from signal strength)
Sounds good also. :D
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Yes, that is indeed a cunning plan and it will be very interesting to see how it works out in practice.
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And where is this in the hierarchy of priorities :)
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The trick I am hoping to use is as follows.
Gather and store the ADS-B messages as references,
this will give me an indication of strength for relative distance/height.
this provides me a look up table averaging strength against the distance/height.
on reception of a non ADS-B message, index the table of known (ADS-B) messages for the best estimate.
I don't want to rain on your parade and it is indeed a smart approach, but its success hinges on a chicken-and-egg problem. At this time the ADS-B traffic is mostly from airliners at 30000+ ft. Those use transponders with significantly higher output than most GA planes (they signal level is much higher than our transponders) and fly where we don't. Using those as a reference may give the false impression that a relatively weak Mode-C is still at safe distance.
The number of usable ADS-B references at VFR / GA flight altitudes is almost non-existent. In fact I am getting phone calls asking why my rinky-dink toy plane shows up on FR24 between the big boys..
The lack of ADS-B equipped GA planes is one of the reasons for PAW has its own ARF transceiver in the first place. If there were enough ADS-B equipped GA planes then you wouldn't need to set up ARF transmitters to announce your location.
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Hi Rob
Its a good point, I had not realised there was such a wide variation in the output power on the transponders. So the only thing that can be reported is proximity to vertical separation.
Well if anything at all.
Thx
Lee
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Mode C deconfliction will be important to encourage mass adoption.
My Zaon PCAS does a very accurate job detecting range and vertical separation.
Can you reverse engineer their system Lee?
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It's certainly true that the transponders in CAT are generally more powerful than those fitted to (low end) GA but I doubt the difference is more than a few dB. Any attempt to compute distance from signal strength will anyway come up against the inverse square law, meaning that closer signals will be considerably stronger. This reduces the impact of power differential though it doesn't of course eliminate it.
The point about few ADS-B aircraft in "our" space" is absolutely correct. As I said earlier, I think in the fullness of time this will change but it needs a low cost ADS-B solution. I think if that were to happen then GA would adopt it because, unlike Mode-S, 8.33kHz and so on, it has an obvious and strong benefit to GA. In that scenario PAW is in an excellent position to become the combined ADS-B out/ADS-B in device of choice for GA.
I'm looking with interest at the current ADS-B trials. Not much use to me up here in Cumbria but a step in the right direction for sure.
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Well if anything at all.
One thing you can use to filter is that ADS-B signals reporting > 250kts are probably not usable as a reference?
I'd be interested to know how many of the people on this forum have Mode-S transponders, how many of those are Extended Squitter capable, and how many actually have that enabled. In The Netherlands, Mode-S is mandatory, but even so I still see only few enabling ES.
Would that be something for a poll on the forum?
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On filtering ADS-B: That might be a good option but at the moment it would result in almost all traffic being filtered out.
On Transponders: I fitted a GTX328 Mode-S transponder to my aircraft during build in 2009. Unfortunately I took what was, with the 20:20 vision of hindsight, probably the wrong decision to not go for the more expensive GTX330. At the time I saw no prospect of non certified GPS equipment being permitted to provide ADS-B out data. A decision I am now coming to regret!
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Hi Lee,
The Manual for our Becker Mode S box, states antenna output powers as follows (presumably same spec for all makes);
Class 1 (useable above 15,000'); 125W (21dBw)
Class 2 ( GA useable to 1500'); 70W (18.5 dBw)
You indicate that with mode S targets you can interrogate position and height, and measure signal strength, so far so good. Presumably there will be an output power spec that applies to Mode C transponders, also, plus height info can presumably be decoded off the transmission. So perhaps you will be able to obtain some kind of distance comparison, factoring in inverse square law power reduction?
Mode C was always going to be difficult owing to the lack of position info. I would hope that owners with this type of transponder would carry PAW also, due to its low cost and be conspicuous to other GA via the P3i transmissions. As noted in a previous post, the Zaon units will detect Mode A and C transponders by cleverly timing the transponder transmissions and appling various algorithms to give range and relative height.
To answer Rob's post, I believe my Mode S is ES capable, but it is not yet enabled. I intend applying for the LAA minor mod asap, once Easy VFR is compatible with PAW,(and I am actually able to see traffic) to enable me to use ADSB-Out via the PAW USB/RS232 output. ADS-B IN will be courtesy of PAW!
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Caution Becker.. See my post here:
http://forums.flyer.co.uk/viewtopic.php?f=1&t=93283&p=1341692&hilit=becker#p1341692 (http://forums.flyer.co.uk/viewtopic.php?f=1&t=93283&p=1341692&hilit=becker#p1341692)
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On power, most standard form factor transponders output around 200W peak power (the RMS power is tiny, perhaps 5W max). The Trig TT31 is slightly unusual in that it is 250W peak power. I think the big boys run about 400W peak. Although it sounds like a big difference, it's only about 7dB between the lowest and the highest power and much less than that between commonly used units, which won't make a lot of difference to close-in signal strength.
On Mode-S, no positional information is sent from the aircraft in any of Modes A, C or S. Positional information is only available when Extended Squitter is supported in the transponder and ADSB out data are sent to it.
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The free air path loss increases about 6dB going from 1km to 2km distance. That makes 7dB difference in signal strength quite important for distance estimation. When you look at Mode-C and Mode-S transponders in the field, there are some with low output of <100W, others produce around 250W. Becker has the ATC4401 in low and high power versions.
Another issue for field strength is the antenna installation. The difference between a short run of good quality cable and a decent antenna, or a long run of cheap RG-58 with a poorly matched antenna can easily mean 4 times stronger signals from the same transponder. Certified planes will have more repeatable signal strengths when things are properly installed. With experimentals your mileage may vary (for real this time).
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Maybe a simple answer would be just to set an alarm when a signal strength is received over a trigger threshold level. Although if you suddenly get an alarm and not knowing where to look may cause a distraction?
Steve
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On Transponders: I fitted a GTX328 Mode-S transponder to my aircraft during build in 2009. Unfortunately I took what was, with the 20:20 vision of hindsight, probably the wrong decision to not go for the more expensive GTX330. At the time I saw no prospect of non certified GPS equipment being permitted to provide ADS-B out data. A decision I am now coming to regret!
Problem is, Garmin "sold" us the idea of the 330 at the time as being ES capable. Now they want another huge chunk of money to upgrage them to ES. I think all 330 (non ES) owners in Europe should get together to have a class action against Garmin for mis-selling the transponders. Maybe we could get 330s upgraded to ES for free, how they should have been in the first place.
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I *think* that when I was looking at the GTX330 back in 2009 it did include ES. At least there was no indication anywhere that this was an option that had to be bought separately. Perhaps they changed that somewhere along the line? Or maybe I just didn't investigate sufficiently. Whatever, Garmin seems to have seriously lost the plot on transponders and, I might add, Nav/Com transceivers. If 8.33 ever happens then I'll be replacing my SL30+SL40 combination with something Trig-like I expect.