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General Discussion / Re: Transceiver Interference
« on: February 03, 2016, 09:02:52 am »
The first few pages, before it drifts off into marketing spiel, is one of the best SDR receiver primers I've ever seen. Thanks Paul!
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General Discussion / Re: Transceiver Interference
« on: February 02, 2016, 06:32:32 pm »
Virtually all receivers employ what is know as the "superheterodyne" (superhet) design. The incoming signal is mixed with a very low power locally created signal to produce an intermediate frequency where most of the signal amplification takes place. This locally created signal is known as the "local oscillator" and it is, in effect, an extremely low power transmitter.
Dongles like the one we use actually convert down to baseband where digital signal processing (DSP) is used to render the required signals. The superhet concept still applies, so in theory, at least, it is possible for interference from the local oscillator to occur. It is, however, an extremely improbable effect. It would only occur in the event of a fairly significant malfunction in the dongle's electronics, as the local oscillator should not be tuned to anywhere near any of the frequencies that we use. I don't discount it by any means but there are other places I'd be looking first.
Dongles like the one we use actually convert down to baseband where digital signal processing (DSP) is used to render the required signals. The superhet concept still applies, so in theory, at least, it is possible for interference from the local oscillator to occur. It is, however, an extremely improbable effect. It would only occur in the event of a fairly significant malfunction in the dongle's electronics, as the local oscillator should not be tuned to anywhere near any of the frequencies that we use. I don't discount it by any means but there are other places I'd be looking first.
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General Discussion / Re: Mode C/S
« on: December 26, 2015, 07:13:55 pm »
Interesting Alan, thanks for that.
Yes, I agree there should be no great difficulty with mixing ADS-B out squitter from an ADS-B out device with Mode-S radar replies from a separate transponder, with the proviso of receiver front end protection/blocking/recovery times for the transponder. I guess the ADS-B transmitter could also operate at significantly lower peak power than the transponder, as we're not looking for vast range.
Yes, I agree there should be no great difficulty with mixing ADS-B out squitter from an ADS-B out device with Mode-S radar replies from a separate transponder, with the proviso of receiver front end protection/blocking/recovery times for the transponder. I guess the ADS-B transmitter could also operate at significantly lower peak power than the transponder, as we're not looking for vast range.
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General Discussion / Re: Mode C/S
« on: December 26, 2015, 07:01:01 pm »Transponders transmit on 1090Mhz but receive on 1030Mhz so can't see any blow up caused by that.Yes, you're right of course, although 60MHz separation at 1GHz is not a lot for the front end filtering to work on. I have a feeling that if our ADS-B in dongles had their antennas mounted on the underside of the aircraft in the line of fire from the transponder antenna then they would be less than happy!
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General Discussion / Re: Mode C/S
« on: December 25, 2015, 10:23:13 am »I'm afraid dedicated ADS-B out is only any use if you don't have any sort of transponder.That's actually not quite correct. Most CAT and quite a lot of high end GA carries more than one transponder for redundancy, however only one can be in use at a time. One reason for this is that the transmitter of transponder 1 would blow up the receiver of transponder 2 and vice-versa, since they are operating on exactly the same frequency. Another issue is having two transponders both squawking the same aircraft ID, squawk code, etc. from the same position. Jolly confusing to radars and their controllers!
CAA/NATS will not permit more than one 1090Mhz transmission device in an aircraft.
Transponders are really hungry, as well as expensive.Modern Mode S transponders aren't particularly greedy. Typical average power consumption is well under 1A at 12V which is easily within the capabilities of a small battery to provide for a few hours of flight. A non-squawking ADS-B out transmitter would require much the same power budget.
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General Discussion / Re: Transceiver Interference
« on: December 24, 2015, 11:35:00 am »
Questions that do not appear to have been asked: Does the interference only appear on 122.6MHz? Does the nature of the interference change/disappear on adjacent/distant channels? If it is RF-borne interference then a bit of tuning around can reveal much about the interference source and how (indeed if) it is getting into your radio.
Other questions that might help to pin down what is causing the problem include:
* Are you sure that the interference was being picked up as RF by the radio and not by induction into audio lines? That can happen very easily with unshielded audio lines close to noise sources in a small cockpit.
* What was the nature of the interference? Regular pulses (how frequent?) or random crud?
I spend a lot of my time as an air traffic engineer trying to track down EMC issues like these and they can often be quite intractable. The two starter questions are always "what exactly is generating the interference signal" and "where is it getting into my equipment". The answers are seldom simple! Often a new EMC problem turns out to be an issue with the equipment suffering the interference rather than the putative source of the interference.
Other questions that might help to pin down what is causing the problem include:
* Are you sure that the interference was being picked up as RF by the radio and not by induction into audio lines? That can happen very easily with unshielded audio lines close to noise sources in a small cockpit.
* What was the nature of the interference? Regular pulses (how frequent?) or random crud?
I spend a lot of my time as an air traffic engineer trying to track down EMC issues like these and they can often be quite intractable. The two starter questions are always "what exactly is generating the interference signal" and "where is it getting into my equipment". The answers are seldom simple! Often a new EMC problem turns out to be an issue with the equipment suffering the interference rather than the putative source of the interference.
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General Discussion / Engineering?
« on: December 23, 2015, 05:16:17 pm »
What is the significance of "Engineering" displayed below a user's name/avatar? 
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General Discussion / Re: EASA Aircraft
« on: December 04, 2015, 10:11:35 am »
It's understandable that the avionics engineers are jittery. It flies directly in the face of everything they know, namely that aircraft avionics MUST be certified and uncertified kit is the work of the devil, sure to fail and bring the aircraft crashing to the ground. I've met plenty that think like that and it's not surprising because all their training reinforces that view. When I was first looking at moving from owning a C of A aircraft into the Permit/LAA world I was assured by my then engineer that it would all end in tears.
It's not surprising, either, that there were no C of A aircraft in the trial. Even with the minor mod fee waived, it would be an expensive job (simple task, time consuming paperwork), with no certainty that another expensive job would not be needed to undo the mod afterwards. Add that to the engineers' reluctance to recognise the validity of uncertified kit and it just became too hard and difficult.
The situation should be different now, with the trial completed and the LAA allowing all its aircraft to continue using uncertified GPS into ADS-B out. It probably needs some sort of formal statement from the CAA that the same will apply to all C of A aircraft.
It's not surprising, either, that there were no C of A aircraft in the trial. Even with the minor mod fee waived, it would be an expensive job (simple task, time consuming paperwork), with no certainty that another expensive job would not be needed to undo the mod afterwards. Add that to the engineers' reluctance to recognise the validity of uncertified kit and it just became too hard and difficult.
The situation should be different now, with the trial completed and the LAA allowing all its aircraft to continue using uncertified GPS into ADS-B out. It probably needs some sort of formal statement from the CAA that the same will apply to all C of A aircraft.
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General Discussion / Re: Flyer Live 28th-29th November - 1 day to go
« on: November 29, 2015, 12:34:10 pm »
Yes, I was quite impressed with how many hands went up. Certainly over one third of the audience. The other 2/3rds will no doubt be on board before long!
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General Discussion / Re: Flyer Live 28th-29th November - 1 day to go
« on: November 28, 2015, 07:27:19 pm »
Great to meet so many of you at the Flyer Live show today. The PAW stand was very busy with lots of interest and the afternoon presentation to a packed "room" was excellent. Well done Lee and team.
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General Discussion / Re: flarm internal antenna
« on: November 21, 2015, 06:21:32 pm »
Forming the antenna elements into a spiral is a well known way of reducing the overall length of the antenna. There's no such thing as a free lunch, however - the resultant antenna is less efficient than the full sized, straight wired dipole. To be honest, I am a little surprised that Flarm has resorted to this technique as the full-sized antennas at these frequencies are pretty small anyway.
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General Discussion / Re: flarm internal antenna
« on: November 21, 2015, 04:20:46 pm »the FLARM dipole is 145mm, I thought the best length for PAW was 164mm?It depends what velocity factor (VF) you use. VF varies, amongst other things, with the aspect ratio of the antenna - thickness of elements vs. length, the conductor material/plating and the type of insulation, if any.
The free space half wavelength for 868MHz is 173mm. The calculator you're using supposes a VF of ~0.95 (164mm), which I would say is a bit high. The VF could be as low as 0.85 (147mm). I tend to the view that the Flarm antenna is slightly short and that the calculator value is somewhat too long. I find a VF of around 0.9 (155mm) is usually about right when designing VHF/UHF antennas. Won't make a great deal of difference in the grand scheme of things.
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General Discussion / Re: NMEA out to transponder.
« on: November 21, 2015, 01:26:07 pm »
Steve: you have PM.
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General Discussion / Re: NMEA out to transponder.
« on: November 21, 2015, 12:42:47 pm »Any other suggestions on approach gratefully received.It sounds like you're covering all the bases there Steve. I'm the Senior Air Traffic Engineer at Carlisle Airport and as such have various useful contacts in CAA SARG. Generally when I engage with them on GA issues they are very happy to put me in contact with the relevant key personnel. For example I've been doing quite a lot of work on the 8.33kHz saga. If you think this might provide you with another route in to CAA or if I can help with your workload then I am happy to assist.
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General Discussion / Re: NMEA out to transponder.
« on: November 21, 2015, 12:29:32 pm »The NEMA GGA message in particular defines if the position fix is:I did some tests today with the GPS/GLONASS USB Dongle outside with a clear view of the southern sky and found that it does see EGNOS and it sets the DGPS flag accordingly:
0 Fix not available or invalid
1 GPS SPS Mode, fix valid
2 Differential GPS, SPS Mode, fix valid
3 GPS PPS Mode, fix valid
I've analysed the outputs from a Garmin 196 and the GPS/GLONOS UBS device used on the PilotAware.
GPS/GLONASS USB Dongle
$GPGGA,113156.00,5126.31393,N,00103.39777,W,1,08,1.01,59.2,M,46.7,M,,*78
$GPGGA,122133.00,5434.29993,N,00237.87591,W,2,09,1.10,210.7,M,49.1,M,,0000*46
The NMEA sentence above is a copy and paste of PAW's output to the transponder, so PAW is correctly sending the DGPS flag to the transponder.
It must be said though that I had to get the GPS well out into the clear before it picked up the EGNOS satellite. I'm not sure one could rely on it with the GPS/GLONASS USB Dongle on the coaming - unless you only fly ever south that is
