Messages

406

General Discussion / Re: ARF alternatives.. XBee 868 ?

« on: November 05, 2015, 06:35:13 pm »

Is it possible to configure the XBEE 868 Pro to serve instead of the ARF board. With the 350mw power that should give suitable range probably at least 20km even with a 1/4 wave vertical air/air.

I think that module has been declared EOL according to the Digi website ...   "EOL in Progress – Product is not for new design. Please refer to XBee RF Modules for our latest products."

Whatever it is it's a critical part for PAW, I only hope it's fully certified module with decent supplier support.  The added benefit of these is the suppliers can very easily tweak them for the different standards & frequencies used around the world - that makes it easy for PAW to be used, legally, virtually anywhere.

There are many alternatives, even fully certified and supported modules are under £30 each from companies that design and make them as their core business.

407

General Discussion / Re: Mode C

« on: November 04, 2015, 04:20:02 pm »

are you able to read the ALT info from the transponder response to the ground stations or are you just trying to work it out?

The information I have available from Mode C/S is signal-strength and pressure-altitude.
For ADS-B I also have the position Lat/Long

The table I created is information gathered from ADS-B data received by PAW, loading up the values of horizontal difference.

The question is whether this can be used as a lookup table for mode C/S where all you have available is signal-strength and pressure-altitude

The FA document for testing ADS-B Out installations has acceptable peak power to be +21dBW to +27dBW, in each quadrant - so it could be different in each quadrant just as long as all four are within the limits above.  That is quite some potential difference to contend with just there, let alone the variance in Mode C transponder output in the GA fleet too.  Throw partial antenna shielding as the relative axis of the aircraft change over time and it all gets very fuzzy very quickly.

Also Mode C returns require triggering via an active radar ping, so depending on altitude there may also be poor coverage to even trigger the transponder in the first place. 

My fear would be too many false positives and it will be start to be ignored, too few and it could be considered useless and also ignored.  But how do you know what are true alerts or not unless you visually confirm each one?  There is a potential here for technology to be a hinderance rather than a help IMHO.

Some of the reviews of the, now defunct, ZAON units highlight the unpredictability of the results.  There must be a reason why they folded?

408

General Discussion / Re: Transmit power and range

« on: October 29, 2015, 10:11:29 am »

...the ISM band the duty cycle can be no more than 10% (per hour) so that shouldn't be an issue.  A device can transmit for a total of no more than 36 seconds in any given hour.

10% would be 360 seconds but perhaps there is an overarching limit of 36s/hour as well, effectively making the long term maximum duty cycle 1%?

The rules give a maximum single length of a transmission to be 36 seconds, with a min 3.6 second gap between.  There are various tables around with all this stuff in.

I was wondering about "interference " from other ISM sources. Am I being overly simplistic in my thoughts that when PAW receives transmissions from non-PAW sources, data will be unrecognisable as not conforming to PAW protocols, therefore nothing is output to display or otherwise?

The ARF will ignore transmissions if they are not on its 'net', in the ARF setup there is a PANID setting - unless it gets the same ID it simply throws the packet away and Lee never even sees it.  I believe they also throw the packet away if it fails a CRC check too, again before Lee even sees it in PAW.  Current this is set to the default of 5AA5 so it's possible if someone else is using an ARF with the default too Lee may see some packets that don't relate to P3i.

409

General Discussion / Re: Transmit power and range

« on: October 29, 2015, 09:44:14 am »

Have people run a scanner where they are testing to check what else is transmitting and at what duty cycle ?

Trouble with 869.4Mhz is 500mw could end up the base level.

example:

http://xlsystems.co.uk/html/869mhz.html

If a few of those were within 10 miles or so....

just a thought.

Yes, and around Cambridge there is something else either on 869.4 or very close by.  From memory it transmitted briefly every couple of minutes.
User the ISM band the duty cycle can be no more than 10% (per hour) so that shouldn't be an issue.  A device can transmit for a total of no more than 36 seconds in any given hour.

410

General Discussion / Re: Transmit power and range

« on: October 29, 2015, 08:39:29 am »

I'd assumed the TX carrier is switched off whilst the ARF is in Rx mode due to Tx/Rx switching in the ARF. Accept phase differences between TX signals potentially a problem.

Yes, it is, the CC1190 on the ARF flips between Tx and Rx based on a signal received from the small SRF board on the ARF module.  When in Rx is operates as a high gain receiver, when in Tx a power amp.  If you add anything external to the ARF to try and boost things etc. it would also need to 'see' these control signals to be able to switch the amp on and off.

(Reading open micros.org description of ARF/SRF including TI admission about unpredictable behaviour, had me slightly concerned and made me wonder if some of the variability the testers are seeing might be caused by such factors as well as RF propagation issues)

I've not seen that admission, but would not be surprised if production variances caused enough of a problem to explain some of the oddities.

411

General Discussion / Re: Transmit power and range

« on: October 28, 2015, 11:10:13 pm »

Yes, the ARF in PAW is using the "high power" (sic) bit of the band. What we need and what we can get are unfortunately likely to be poles apart once metalwork gets in the way. The problem with bolting on amps (transmitter or receiver) is the need for T/R switching so that both can use the single antenna. All is possible, of course but the technology isn't commonplace (i.e. cheap). The ARF already has a PA-cum-RF amplifier that includes the T/R switching fabric. (Edit - I see that Jerry has covered some of these points whilst I was doing my one fingered typist trick.)

I'm working up to an experiment. I shall place PAW #1 in the visual control room at my local airfield, then trundle off with PAW #2. In the aircraft I shall have a) the standard whip antenna lurking on the coaming, b) a dipole taped to the canopy, and c) a quarter wave ground plane antenna protruding through a convenient hole in the underside of my aircraft's all metal fuselage (don't ask...!). I'll then do some tests, flying from and to the airfield with the different antenna configurations. All I need is for it to stop raining... should be an interesting test.

I'm not sure what alternatives to the ARF Lee may be considering. If these do not achieve any improvement in transmit power, I wonder if a narrow band ( or even broad band) power amplifier connected to the ARF TX output, might be the way to go (dependent also on TWF's experiment described above).

My thoughts are;
1) Existing ARF antenna retained and used for ARF receive (and low power TX) as at present, via the end feed SMA connector.

2) ARF pig-tail coax connected to ARF output U/FL connector (effectively "T" ee'd off (1) above), and connecting to power amplifier input, via cable's SMA female end.

3) Amplifier output connected to additional antenna (for high power transmit).

4) Separate (high power i.e. 450 mw) antenna and existing ARF antenna gets around Tx/Rx switching referred to by TWF.

Is this too much of a bodge, could it work? Would the "T"  reduce the ARF receive sensitivity? I consider the two TX antennas could assist "position diversity" as called for, if connected to PAW using coax cables.

Regards,

Chris

Not sure this would work, the additional amplification would probably slightly alter the phase of the signal, so you could make things worse for receivers at shorter range as the two signals contend.  Any additional amp also needs to know when to turn on & off, that signal is available on the ARF board to tell the CC1190 what mode it's in but not easily accessible.  An Amp will just amplify until you tell it otherwise, the carrier will still transmit.

Other modules exist, Lee and his team are looking at options, for me any replacement should be certified for a number of reasons including repeatable quality.  Certified doesn't mean expensive, they can be had for within a few £'s of the ARF.  The big players in this area make them by the truck load for use in all sorts of things, saw a few at an electronics show last week and that was on one stand.  Whats also handy is the offer of assistance with antenna selection and/or design to get a good match where needed.

412

General Discussion / Re: Transmit power and range

« on: October 28, 2015, 08:30:18 pm »

I've not used this setting in PAW, are these RSSI values -ve?  In general RSSI numbers are better the closer to 0.  You would be better to have the units further apart when testing, or in different rooms so there is some attenuation between them, 10 feet isn't that far even at the power of the ARF.

413

General Discussion / Re: Transmit power and range

« on: October 28, 2015, 08:06:38 pm »

I did some highly scientific testing tonight... 

Tests
10ft apart
No aerials on either -                   IS 77          IM 70
1 stubby aerial horizontal –         IS 88          IM 57
2 stubby aerials horizontal –       IS 69          IM 41
2 stubby aerials 1 hoz 1 vert –    IS 65          IM 35
2 stubby aerials vertical –            IS 52          IM 23
1 stubby, 1 long both vertical –  IS 46          IM 23
2 stubby vertical + adptr -          IS 75          IM 47

I couldn't get two long aerials to work... not sure why, but does this mean that 2 right angle adapters (in my last example) would be pretty poor? Seems to be worse that 2 stubby aerials horizontally.

Hi Stephen,

There is clearly a marked difference in the two units when all should be equal, in particular here
2 stubby aerials horizontal –       IS 69          IM 41
2 stubby aerials vertical –            IS 52          IM 23

just out of interest, I know it is hassle, are you able to unplug your ARF and swap, I would expect the two readings above to be the same for master and slave but they are wildly different!.

I have become so dissatified with the ARF :-(

Thx
Lee

As the Master always seems lower than Slave, have you tried to swap Master and Slave settings over and see what the results are?

414

General Discussion / Re: Transmit power and range

« on: October 28, 2015, 07:13:48 pm »

True 5 miles Is possible with around 90mw I think the ARF is capable of. What we need is a Rock Solid signal even when theres line of sight issues cockpit positioning etc.

Theres only a 25khz section of the band that permits 500mw can someone confirm the ARF is transmiting in that section.

Given 400+ mw and a half decent antenna that negates the need for decent ground planes unless you have a metal aircraft and want to start cutting holes in it... (not forgetting Mod Status) I believe 5miles minimum certainty contact is achievable.

Assuming licence restrictions are OK we need a bolt on narrow band amplifier accepting the input of the ARF module or even the XRF which would be cheaper. Shouldnt be too difficult I wouldn't have thought except for cooling issues if running a continuous amp.

Another thought is how good is the ARF receiver would a decent antenna and a preamp work or would we swamp the ARF RX.

Light bulb has just gone off..... must contact a friend who used to work in this field designing kit for the mobile phone network and P2P communication systems....

Yes, the ARF operates within the licence free ISM high power band.

The ARF is two products combined.  An SRF module (the small square board) which is the actual Tx/Rx IC and is a transmitter in it's own right.  This SRF is mounted to another PCB with a combined Tx Booster and Rx high gain amplifier.  This assembly is called ARF by WirelessThings.  Basically it is a TI CC1101 & CC1190 loosely following the TI reference design.  The only thing that can be done with the ARF module is antenna choice.  The TI reference design and the associated documents state the limitations of the design, they are part of the 'low cost' Ti Sub 1GHz chipset range.  Other variants are available that can run at higher powers - these are often deployed within the certified modules available from various (huge) companies.

The last RF stuff I was really involved that I can mention with was the deployment of AirWave for the Police, that is all based on TETRA.  The implementation was "fun", especially the integration into everything else. Ah, the memories....

415

General Discussion / Re: RF Board Progress

« on: October 23, 2015, 08:06:48 pm »

Just reading an article in Aircraft Owner & Pilot.....talking about Project Eva and a collaboration between AOPA, NATS, Funke Avionics and Trig Avionics and there development of a similar unit to the PAW.....Just a little concerned that if this goes ahead and with FLARM there are too many units starting to appear and no uniformity. This will further fragment the cohesive nature of what we are trying to achieve....Different aircraft will be surely transmitting with different protocols and probably frequencies making it all very messy....

There is a common format to broadcast this information, which as ADS-B, that is the worldwide standard.  However it requires some expensive gear to be able to handle it in a compatible way with all the other infrastructure.  Project Eva (Electronic Visibility via ADS-B) is based around ADS-B, and how to make that more accessible with more affordable equipment.

As fas as I know there is only one other 'broadcast' protocol for local traffic awareness, which is FLARM, and that is encrypted.  What PAW via the P3i protocol archives is the ability to have a free and open protocol for local traffic awareness.  The PAW box implements this protocol, others may also decide to support P3i as being an open protocol anyone can join in.  It is this openness that should encourage adoption for the benefit of all, at a more affordable price point than the closed FLARM system.

Anything where a fair chunk of development is out in the open will have hurdles to overcome.  What is really good is that with the testing to date issues are being identified quickly, looked into, and Lee and his team are not afraid to take a step back and re-think such a key element.  Congratulations to them for doing that, all too often people will push on and just make things worse for all in the longer term. 

416

General Discussion / Re: RF Board Progress

« on: October 23, 2015, 06:22:30 pm »

I don't know a lot about the hardware available and what open standards there are in this type of application, but is there a possibility of selecting a generic type that has multiple suppliers, in the same way as the TV dongle where there are multiple suppliers using the same chipset.

Being tied into a proprietary type available from only one supplier looks like it could make the PAW project a bit of a hostage to events.

There is some competition within the packet radio market, and the various chipsets don't play well together.  To get ease of integration with the Pi then a module is the way to go, if you want to roll your own hardware then anything is possible.  There are few generic solutions as underneath they are all based on one chipset or another handling the packet radio element.  The ARF is based on Texas Instrument, F***M uses an NXP set, none of these chipsets were designed to integrate.  Receiving is easy with the likes of the Software Defined Radios (aka the TV dongle) whilst transmittiing is another ballgame and requires some care to get right.

The majority of the really good radio hardware modules would require mounting to be of use to PAW, as the professional modules are designed to be designed into systems.  I suspect that once Lee settles on a radio sub-system he will offer it via the impending webshop on a suitable shield for the Pi.  What is key is what ever the solution is it has to be a high quality unit to get reliable performance, which for me would mean a certified module from one of the big players.  This also has the benefit of production assurance.

I was quoted recently for a fully certified packet radio module in quantities of 10, 100, and 1000 units so availability wouldn't be an issue.  Even the per 10 price was within a few pounds of the ARF so quality modules are available at very competitive price points.  Such suppliers make huge numbers of modules, for worldwide markets, so the risk is low of there being issues for the foreseeable future.

417

General Discussion / Re: PilotAware Unit Trials

« on: October 23, 2015, 11:10:22 am »

bryannortje

Please contact Jeremy Curtis directly.

I remember another forum post where a problem was found with charge 2, I don't know what it was - but I think its to do with start up consumption - whether this will cause the problems you describe I don't know - but you really do need to talk directly with Jeremy.

Regards

dave

The early version had a firmware issue, when the stepped consumption as the likes of PAW booting could be restricted.  This was first seen with an iPad Mini and the "smart" lightning cable doing things passive cables don't, a firmware fixed sorted it out.

The current Charge2 is a more efficient power supply, with even fewer emissions than previously.  It was also put through formal testing by an avionics company to obtain an EASA minor mod approval, with luck the paperwork should be through soon.

I've swapped bryannortje's unit out for a new one (or will when I get it shipped out to him today) rather than just update the firmware.

418

General Discussion / Re: RFSolutions RF Board

« on: October 22, 2015, 09:15:43 pm »

At an electronics design show today I saw several fully certified radio modules that would be easy to interface to the Pi with good specs.  Price range similar to the current ARF with the benefit if full support with the EU.  Depending on supplier some would also help with antenna designs too.

Certified is handy to keep interference down and also to help with immunity from anything lurking around RF wise.  Manufacturing tolerances are also tighter, which is good with lots of radios in a net.

Lots of nice test gear too, but thats another story....

419

General Discussion / Re: Long Marston Test 17 October

« on: October 21, 2015, 11:14:30 am »

Good points Jeremy, thanks. I thought the ARF could run 500mW on 869.4MHz so my apologies for assigning it higher performance that it deserves.

When you say +18dB, I assume you mean +18dBm (since dB is a relative term). 18dBm is 63mW, 20dBm is 100mW. Even 27dBm, aka 500mW - the maximum legal power in the ISM band - is only going to offset 9dB of losses, which in the context of antenna gain/loss and, especially, in terms of shielding losses is not much, sadly. I don't know what the ARF receiver sensitivity is but I'd guess it's within a few dB of -107dBm, so not a lot of room for improvement there either.

I fear that any solution that does not include external antennas is always going to disappoint.

Yes, that should all be dBm.  The TI reference design for the chipset used in the ARF is -107dB, as the booster IC also has an RX amp.  The ARF follows this reference to an extent, but has been altered to fit the form factor.  There is no data sheet for the ARF, so the actual details are unknown.

420

General Discussion / Re: Long Marston Test 17 October

« on: October 21, 2015, 10:35:41 am »

2. is not practical. 30dB is an increase in power of 1000 times. The ARF runs, it is said, 500mW, so we would need 500W to do it this way. Quite apart from licensing issues, that sort of power is a) dangerous (think microwaves again), b) requires a lot of kit to produce it and c) would need significant battery/generator capacity to support it.

All very true, but the ARF runs at circa 90mW, it cannot produce any more, the reference design from TI recommends a maximum of +18dB, while the radio toolset (RFStudio) used to set them up tops out at +20dB as an available setting. It could well be less depending on the power supply, cables, antenna etc.

Numbers still not great, but with closer to +27dB at the source and a receive sensitivity of circa -107dB that some other potential packet radio modules have there is scope for improvement.