PilotAware
British Forum => General Discussion => Topic started by: thearb on October 17, 2015, 05:29:09 pm
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Very successful test today with 5 pilotaware equipped aircraft. None of the power dropouts whatsoever that affected the June tests. Screen shots will be going up on this thread later this evening, many thanks to Ian Fallon, Bryan, Alan Walker, Lee Moore and Keith Vinning.
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Alan Walker departing, rest lining up:
https://www.dropbox.com/s/sr369oplgxqzr9n/2015-10-17%2013.17.18.png?dl=0
2 mins later, note 400FEE now switches to show as G-SARV, note the easyjet in bottom left in third link, proving ads-b feed also operative.
https://www.dropbox.com/s/zpxbigcv2aooo9s/2015-10-17%2013.19.13.png?dl=0
https://www.dropbox.com/s/o2kl09vcat83j6q/2015-10-17%2013.19.47.png?dl=0
https://www.dropbox.com/s/mx1eo4ovhytnxb8/2015-10-17%2013.20.09.png?dl=0
2 mins later:
https://www.dropbox.com/s/i1slodk6xq8ch66/2015-10-17%2013.21.03.png?dl=0
3 still in vicinity 5 mins later:
https://www.dropbox.com/s/trt9t0ria1e0d5o/2015-10-17%2013.31.34.png?dl=0
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I have to say that this enthuses me greatly.
I did tests yesterday but with only one aircraft and results were not so convincing.
Weather looks good mid afternoon tomorrow (Sunday 18th) so I think I'll try and get airborne and persuade someone to fly with my spare Unit too.
Regards and Tanks to all concerned today.
Gerry
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This looks good. ..where were the units located, power supplies used, etc etc ???
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Don't know what others thinks but I reckon using tail number rather than hex code makes a lot more sense for us. SSR is never going to see us anyway..
It also allows us to differentiate PAW from ADS-B
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Yes, all reported altitudes look correct against the ground station (me in green), so that bug seems fixed.
Many of the pilots today commented on the fact that they often couldn't see the aircraft that were showing on the screen, even when they knew where to look outside the cockpit. Explains why the skies seem busier from the ground than when in the air, often we are just oblivious to close aircraft.
Standard PAW unit powered off aircraft AUX USB outlet, sited on coaming.
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Don't know what others thinks but I reckon using tail number rather than hex code makes a lot more sense for us. SSR is never going to see us anyway..
It's an interesting question. I can see why we need to have the ICAO code programmed into PAW so we don't see our own transponder but there's no particular reason for the P3i side to use it. The only problem is that the aircraft registration can be up to (I think) 6 ASCII characters, which is six bytes vs. the three that the ICAO code requires. There isn't room in the 24 byte P3i packet as it stands.
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In the p3i packet the icao code is used as a unique identifier.
We need this in order to discriminate between multiple received packets
Thx
Lee
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So, this being a bit late at night, can someone summarise this as to how far away the P3i data could be seen from the ground and from other aircraft? :)
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I think we collectively agreed that it was in the order of 5 miles air to air.
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An Avidyne TAS600 sees 7 miles so 5 not bad for £100 :)
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In the p3i packet the icao code is used as a unique identifier.
We need this in order to discriminate between multiple received packets
Understand the need for a unique identifier but the aircraft call sign is also unique so I wondered if, field size apart, it could perform the same task.
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If we DO want longer range (not convinced) I guess we could fit a ground plane?
Taken from the DIY OGN tracker build page. They are working 868.3Mhz
If using a plastic box then add 2 or 3 strips of Aluminium or Copper tape each approximately 85mm long from the
SMA connector to act as ¼-wave ground planes
Copper tape on Ebay (http://www.ebay.co.uk/itm/50mmx1m-Copper-Foil-Tape-Pedal-Single-Sided-Adhesive-EMI-Shielding-MFR-/261362221692?pt=LH_DefaultDomain_3&hash=item3cda67027c)
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Chaps.
How do you change from Hex code to Reg (like GSARV Has). I have tried in the configuration page but the minute I press save it auto generates a hex code again?
Any clues Please
Steve
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Chaps.
How do you change from Hex code to Reg (like GSARV Has). I have tried in the configuration page but the minute I press save it auto generates a hex code again?
Any clues Please
Steve
Hi Steve,
This is nothing to do with PilotAware, we suspect SkyDemon is performing a lookup of the HEX code to REG, I need to check with Tim Dawson. OR, Ian is really pumping out ADS-B without his knowledge !
Thx
Lee
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We also undertook some testing yesterday based near March.
Our results were max 3 nm in air. Our ground monitoring station managed only 1000m range! Our best airborne aerial was the whip aerial mounted externally on a Streak Shadow. We think even this was being shadowed by the airframe, since the signal would go and then come back for no reason. We are using 2.5amp mains power supply and juicybits cable for the ground monitor, and Charge 2 with juicybits power cable in the air stations. We don't believe we have power issues. Our range could be similar or marginally worse than you demonstrated. There appear to be many variables when undertaking live testing. We keep getting differing results for no apparent reason! Oh and our ARF's have onboard SMA connector. Although to be fair we can detect no obvious difference in range whether with pigtails or direct SMA connection. Again because of the spread of results.
The range we are demonstrating with the ARF as a P3i transceiver is not currently enough for us, we are all GA types with a collision approach speed of between 200 and 300 kts. We are hopeful that an alternative and higher power transceiver will become available to all, and incorporated in the standard build. We have measured a few of our ARF transmitting outputs and they all measure 89-90 mW and nowhere near the 500mW maximum allowed for under the terms of the licence free rules. We think from Lee's state of the nation address he may be considering the apparent range problem, currently experienced with the ARF, and hope he may look at an alternative with a higher output. However, Our experience with ADSB is very good.
Regards
dave
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I think we were getting a bit more during our air to air tests at Long Marston. Subject to me solving another curiosity, perhaps a lot more. I wouldn't write the ARF off just yet.
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CaptChaos and I also went up to play yesterday.
He had two units, I had one. I could see his one unit from 5 miles or more away, definitely beyond visual range! He swapped units and his other unit wasn't quite so good, I had to get closer to see him. However, he couldn't see me at all.
We landed and had a walk around. He could get the signal from my unit when close up, but lost it when 30-40 yards away. From his weaker unit, it continued to be picked up half way across the airfield, maybe 300 yards away. Still not that far.
So something's going wrong. I measured the voltage on mine which didn't seem to transmit much at all (but I assume the aerial is fine as it was with that unit I was picking up at 5 miles) and it was 4.9V. I think my ARF transmit may be duff. Don't know if JCurtis has any sort of scheme to send these things back?
We then went flying with me using his other unit, the one which had been received on the ground from 300 yards away. Anyway, similar thing, I could see him from 3 miles away or more, but he could only see me when we got within reasonably close visual range.
Anyway, if these things are only outputting 80-90mW even when they're working as suggested in another post, we might be able to do better.
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Fully agree, the ARF is still a good option. 5 miles is obtainable using proper antenna setup. I would think that using a metal housing for the RPi and the 868MHz antenna centered on top might already give enough ground plane to be effective.
Air-to-ground is cute, but for safety it is more important to have good coverage in the horizontal direction you're flying in, plus or minus 1000 ft or so. Transmissions to the ground is power wasted...
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Don't know if JCurtis has any sort of scheme to send these things back?
I have contacted those who have already placed orders for ARFs to be absolutely sure they want to continue with their order. I'm not taking any more orders for the ARF modules.
If the ARF heartbeat is flashing then the module is up and active, bar plugging one into an RF Power meter there is little you can do to test them. The PAW box doesn't start to transmit on the ARF until it gets a GPS fix, so when testing after a power up always ensure you either have a GPS unit attached or CollisionAware active.
The shield is still available, as it gives the Barometric sensor for those who want an easy option to fit one of those.
Anyway, if these things are only outputting 80-90mW even when they're working as suggested in another post, we might be able to do better.
Doing better is an option as 500mW rated, fully certified, packet radio modules are available for a very similar price point and physical size, downside is they won't talk to the existing ARFs.
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I'm giving a PAW UNit to some Radio Ham boys who are very knowledgeable on getting the best out of low power, higher frequency communications.
They will measure output with various aerials I have tried and then start tuning through aerial design and ground plane combinations knowing that gain shouldn't exceed 500mW.
It's worth a try to utilise the ARF route which looks good except for range.
G
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If the ARF heartbeat is flashing then the module is up and active, bar plugging one into an RF Power meter there is little you can do to test them. The PAW box doesn't start to transmit on the ARF until it gets a GPS fix, so when testing after a power up always ensure you either have a GPS unit attached or CollisionAware active.
After poor RF performance, we sited two Awares near each other and started walking. As mentioned, the signal was lost for mine after 30 or 40 yards or so. Can we send the things back to the manufacturer for replacement? Out of the three we had, lt looked like one transmitted 30 or 40 yards, another 300 yards, and the third about 5 miles!
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If the ARF heartbeat is flashing then the module is up and active, bar plugging one into an RF Power meter there is little you can do to test them. The PAW box doesn't start to transmit on the ARF until it gets a GPS fix, so when testing after a power up always ensure you either have a GPS unit attached or CollisionAware active.
After poor RF performance, we sited two Awares near each other and started walking. As mentioned, the signal was lost for mine after 30 or 40 yards or so. Can we send the things back to the manufacturer for replacement? Out of the three we had, lt looked like one transmitted 30 or 40 yards, another 300 yards, and the third about 5 miles!
A few questions:
- Do they all have the same hardware construction technique ?
- Do they all have the same antennas / antenna connectors (SMA soldered vs pigtail etc)
- Are they all soldered as well as each other ;D
AFAIK ground - ground even in clear air will not be as good as air - air (in clear air).
btw do you know about the master / slave test mode ? You can put one in each mode via the Configuration web interface and then the logging will show power levels. Would be interested to see the values you get.
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A few questions:
- Do they all have the same hardware construction technique ?
- Do they all have the same antennas / antenna connectors (SMA soldered vs pigtail etc)
- Are they all soldered as well as each other ;D
Two of them used the JCurtis module, including mine which didn't appear to transmit much at all. I can't comment on the quality of the soldering! :D But my unit which wasn't transmitting for toffee could still receive from 5 miles away, so I don't think it's the antenna or connection. Two of the units have the large antenna, the third has a GSM type antenna, but for the final test we swapped antennae and went flying again but it made little difference.
Air to air range was similar to ground range in that the 300 yard one was picked up from about 300 yards away in the air, and my one was only just picked up when in close formation.
btw do you know about the master / slave test mode ? You can put one in each mode via the Configuration web interface and then the logging will show power levels. Would be interested to see the values you get.
Forgot about that. I'll maybe give that a go next time.
Ah, more info in this thread:
http://forum.pilotaware.com/index.php/topic,160.msg2400.html (http://forum.pilotaware.com/index.php/topic,160.msg2400.html)
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Two of them used the JCurtis module, including mine which didn't appear to transmit much at all. I can't comment on the quality of the soldering! :D
Well, it was soldered with additional flux to ensure good wetting on the joints. The Rx would't have got through if it was too bad, the signal would have been very low at 5nm and a bad joint would have made that much worse. I'll pop it onto a power meter if you would like, but I suspect it will be the same ~90mW as the rest.
Edit: Correct units from dB to mW...
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Sorry - please don't interpret that post as being rude about your soldering am sure it's better than mine by far!
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Sorry - please don't interpret that post as being rude about your soldering am sure it's better than mine by far!
No problem there :) , a connector issue won't resolve the problems with the ARF.
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I'll pop it onto a power meter if you would like, but I suspect it will be the same ~90dBm as the rest.
1 megawatt seems a bit excessive :) 90 mW?
Yep, should be ~90mW, spent to much time flipping between units today. I'll nip back and edit that!
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Hi.
Just want to reiterate a few things.
I had a measurement done on the PA Box -89mW was recorded. I have tested on the ground and in the air all 3 types of build.
Build 1 - The original self build with Digole Regulator board and home brewed wiring using 40 pin IDC block..
Build 2- Jeremy's shield board with Baro and myself soldering another ARF unit to it.
Build 3- Jeremy's shield board with Baro and ARF as the plug and play no soldering option using pigtail and direct SMA for comparison. No difference observed.
The most range I ever had from these units was 4.7nm line of sight to a unit on the ground from 1000'. I have not done an air to air test and flying at 150kts wouldn't want to get anywhere near a potential target either. One aircraft used during testing, had the Aerial mounted externally for testing with a 70Kts airspeed.
More tests are planned at lower airspeeds. Same observations as most of this topic thread indicates-, nothing has been consistent.
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A few questions:
- Do they all have the same hardware construction technique ?
- Do they all have the same antennas / antenna connectors (SMA soldered vs pigtail etc)
- Are they all soldered as well as each other ;D
AFAIK ground - ground even in clear air will not be as good as air - air (in clear air).
Ian the construction techniques are detailed in my air testing post. However, briefly one of mine is soldered by me using Digole reg and the other is the full Jeremy shield presoldered option with on board SMA. The soldered one uses two pig tails, one for the ARF and one for the DVB dongle so I can mount on the case for spiral GSM type aerial. The other one has standard long aerial on the SMA and standard DVB aerial.
The one I soldered (but not for that reason) is detectable at the greatest range. But I could only detect Paul's unit and one of mine when Paul took it in his aircraft when very close. Therefore I wondered if power source was a factor, or blanking or may be both my units don't receive well. It would be good to work through each potential issue to reach a conclusion.
Hope that helps answer your question.
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We have had similar experiences with performance off the ARF with P3i. For example
Flight test 1
Lancair and Falco - never even got off the ground because units wouldn't see each other on the ground let alone in the air. Both are 'composite' aircraft. We did however take the unit out of the Lancair, and then managed a successful ground test of 100m or so. Thought great and reinstalled unit in plane. Immediately no receive for either unit. Unit in Lancair on windshield. Later tested the units on ground against another ground based unit and they all worked. We got around 2 km range upstairs window to upstairs window. So No air to air that day! Decided we need simpler aircraft.
Flight test 2
Eurostar to ground station
1+km range. Lost in circuit at Fenland when aircraft turned away from unit. Not able to establish an air to air test. Unit on rear parcel shelf (in open view).
Flight test 3
Able to establish an air test. Results similar or marginally worse to long marston test.
We have established using the master and slave results that in an ideal environment - 6m range within a kitchen line of sight we can get -20/21 dBm, with all units on a good day. However we have also seen -30dBm in exactly the same locations on another day. Power supply variations appear to make a difference. We also established that in our case replacing the pigtail with the onboard SMA connector (might have) improved reception or transmission and improved our figures by 1 or 2 dBm. But its difficult to say because of the variation in figures received from the testing regime. We also tried a simple dipole aerial at 2 legs of 81mm (seemed to give best result). This also might have improved the figures by maybe 1- 2 dBm. We have also tried a monopole aerial again 81mm long with 165mm ground plane - similar.
The bottom line is that from our tests the aerial and sma connectors have made only very small gains.
We still don't know why we couldn't get the PAW to work on P3i in our complex aeroplanes we are now concentrating on simple planes for further testing! However we have also established that a body (probably alive or dead) in the cockpit will mask the signal received (or transmitted) in our case this was a TOTAL wipe out. Tests again in the kitchen indicate that a hand in front of the aerial will reduce sensitivity by 10 or 15dBm. Standing in front of the aerial has a similar effect. This probably illustrates the point. Lee amongst others has already stated this.
So aerial placement will be vital as well as the largest transmitting power possible on P3i. Lets be honest we have measured 90 mW from our ARF - which is less than 20dBm. If a hand near the aerial can reduce the received signal by 10 or 15 dBm then we are very sensitive to any variation or masking of the signal transmitted or received by anything, and everything. This includes, metal instrument panel, engine, metal fuel tanks, persons in cockpit etc.
The ADSB works brilliantly - because the transmit power of ADSB is far greater. We have been able to watch an aircraft on finals into Fenland from a ground level PAW around 15nm away! This again proves the point - to make the system as good, and as reliable as possible we need:
the largest transmit power available to us -
Probably external aerials properly designed, in order to reduce masking as much as possible. (Powerflarm appears to use two monopole aerials mounted on the aircraft underside)
I hope my comments are considered helpful, we are hoping to do more air to air testing next weekend with the P3i and ARF.
Regards
Dave
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Very interesting observations Dave regarding absorption or fades caused by human body. Could explain why the results appear to depend on direction flown etc. It sounds as though to achieve more power a simple power amplifier might be needed on the P3i ARF transmit side to more closely achieve the 500mw into the antenna. I don't know if anything is likely to be available off the shelf or whether the ARF can be modified easily to connect the output through such an amp set-up. I assume with greater power output from the PAW, there shouldn't be any need to modify the ARF Rx side.
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Very interesting observations Dave regarding absorption or fades caused by human body. Could explain why the results appear to depend on direction flown etc. It sounds as though to achieve more power a simple power amplifier might be needed on the P3i ARF transmit side to more closely achieve the 500mw into the antenna. I don't know if anything is likely to be available off the shelf or whether the ARF can be modified easily to connect the output through such an amp set-up. I assume with greater power output from the PAW, there shouldn't be any need to modify the ARF Rx side.
The ARF is a packet radio IC and a signal booster as a package, no margin there to boost it again without a redesign and a move to a different IC. Alternatives modules exist, with 500mW power and fully certified from other suppliers. Downside is they won't talk to the existing ARFs.
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Hi Jeremy,
Just quickly looked at ARF web page. Apparently it can be used with the XRF (as a preamp?). It states that using XRF at each end a range of 10s of km is possible. The bonus is that the units talk to each other.
Regards,
Chris
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Hi Jeremy,
Just quickly looked at ARF web page. Apparently it can be used with the XRF (as a preamp?). It states that using XRF at each end a range of 10s of km is possible. The bonus is that the units talk to each other.
Regards,
Chris
There are two modules from WirelessThings, the XRF and ARF. If you look at the XRF it has a small daughter board on it - that they call the SRF module.
The SRF module is a Ti packet radio chip with an embedded micro controller, that WirelessThings have written software for to give it an easy to use AT command based serial interface.
The XRF is this SRF module mounted onto carrier to give it the XBee form factor you see.
The ARF is the same SRF radio module combined with a Ti PowerAmp to boost the radio signal (and also increase the sensitivity for reception) again presented in the XBee form factor. The ARF is the highest power radio WirelessThings currently make.
You might be able to get 10k+ with directional high gain antenna on a point to point link, with all the power going to where you need it. You can go a long way with a directional antenna with very little power, I've seen Bluetooth run nearly a mile using, in effect, a glorified Pringles tube.
What we need good omnidirectional broadcast transmission.
So, yes, the ARF could loosely be described as an XRF with an amplifier on it.
Edit: Reading the description on the website, "When used with an XRF the gain overall is around 10db" I think this means the ARF has a signal strength of +10dB over that of the ARF, as it continues to say "using an ARF at both ends will give the greatest range (expected to be 10's Km's)". That 10's of KM's I would expect to see with directional antenna.
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Well, if my reading Sky Demon is correct, I was picking up CaptChaos from about 9nm away when both aircraft were heading directly towards each other. Unfortunately he couldn't see me until we were very close indeed!
So there's some sort of variation here.
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Hi Jeremy,
Thanks for the explanation. I took the web-site description to indicate an XRF at both ends of the ARF! I now realise they meant an ARF at both ends of the radio link.
Whilst I was a Satcomms engineer in years gone by, I'm now extremely rusty..
Nowhere in WT web-description does it seem to quote an output power for the ARF. Is there any mileage in my suggestion that the transmit and receive side could be separated out and an external RF power amp be used to boost the eirp?
Regards,
Chris
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Hi Jeremy,
Thanks for the explanation. I took the web-site description to indicate an XRF at both ends of the ARF! I now realise they meant an ARF at both ends of the radio link.
Whilst I was a Satcomms engineer in years gone by, I'm now extremely rusty..
Nowhere in WT web-description does it seem to quote an output power for the ARF. Is there any mileage in my suggestion that the transmit and receive side could be separated out and an external RF power amp be used to boost the eirp?
Regards,
Chris
I remember calculating geostationary satellite wobble for a tracking project, a good few years ago now, to always keep a dish pointed correctly. That was fun...
The recommended power from an ARF is +18dB according to TI who make the chipset, it's pushed to +20dB which is the maximum recommended limit for the chipset. The CC1190 is a combined TX and RX booster, so it also lifts the RX sensitivity too - it's controlled via a signal line from the CC1110 to flip states.
There is no way to split them out, the ARF is an integrated package based on the TI reference design.
You could get WT to design a new ARF, based on a similar chipset that has a potential higher power. TI state it would need shielding to prevent harmonics leaking out everywhere and would need some work to get right and stay within the legal limits.
For more power it's either a WT redesign, with some assurances over the limits and getting things measured and profiled (i.e. legal in terms of transmission, harmonics etc.), that has the benefit of being backwards compatible.
The other option is use a replacement, fully certified, module from one of the big players, downside doesn't talk to the ARF. Price wise there would probably be about a £10 to £15 hike in the price of the packet radio element of PAW to get to 500mW with support and backup from the manufacturer.
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Well, if my reading Sky Demon is correct, I was picking up CaptChaos from about 9nm away when both aircraft were heading directly towards each other. Unfortunately he couldn't see me until we were very close indeed!
So there's some sort of variation here.
Paul, depends on SD settings - the trend line shows either time or distance - 2,5,10 for each so if you were on distance and I think you said second mark, that equates to 5 NM.
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I won't declare who I picked up until he agrees I disclose his identity. This afternoon from inside an aircraft on the ground at Sywell EGBK I picked up a PAW with solid reception with him around 8nm. My kit was inside an Aluminium Aircraft with the aerials on a table near the window.
Steve
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And I now declare my identity :) By arrangement, I was hunting for a ground station to the north of Wellingborough. I could only detect it when directly overhead at 2000', the range being no more than a few hundred yards. He received only 6 ARF messages from me, and I received only 12 from him, during maybe 10 minutes. However, at the same time, also by arrangement, the_top_pilot was receiving me solidly from Sywell, about 4 nm away! My unit was on the coaming of a Skyranger, with a good view of the horizon. As the_top_pilot said, he went on the track the rest of my flight, at up to 8nm away.
There do appear to be serious problems with the ARF, especially RX sensitivity and repeatability. I suspect that we will have to change to a better specified and supported unit before reliability can be achieved. I note the incompatibility issues, but I am prepared to shell out the equivalent of an hour's flying for a replacement, if that is what is needed. The worry is that there are so many units out there already, and people queuing up to join in. Sorry to be so negative, but I feel I have to voice my concerns.
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In theory, everyone who has built one so far is an experimentalist...so swapping RF modules at this stage may be par for the course.
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OK I've been doing some research following our experiments
In for a penny in for a pound!
Because the system is similar to flarm, and works on the similar frequency I thought I'd look up flarm and powerflarm in google. I will try to keep my findings short, however:
Powerflarm has more power output than flarm to increase range.
Powerflarm personal results seem to indicate 2 - 8km range. Powerflarm claims the order of 5-8miles range - and flarm claim this is enough for collision warning.
Powerflarm has two aerial connections possible - to avoid blanking spots in the aeroplane. The second aerial requires a second licence fee!
At least in the states (different frequency) range was very poor until a band filter was added on the board. Don't know about Europe.
There is a specialist dipole aerial for flarm (PA1) its expensive - £44 claimed to increase the range, especially if both parties use it.
There is published info how to test your (aerial) installation using another flarm and to draw up a 360 degree range or black spot graph for the plane. Doing this and moving the aerial a small amount can often increase range dramatically, even with a small change. We could undertake this ground installation testing too.
Powerflarm appears to be tending to fixed aerial(s) - and there is EASA paperwork to do this on most GA aircraft.
There are suggestions on the net that the glareshield of a GA aircraft is the worst place to locate the aerial! because the signal can be blanked by instrument panel, engine, and occupant(s) of the cabin. There is some suggestion that two aerials are good - say one on the fin and one on the underside?
The bottom line seems to be - get the most power, provide bandwidth filtering, allow for two aerials to give the best chance to avoid blanking, plan your aerial locations carefully and test for 360 degree transmission to get the best result possible - of course Flarm is a commercial product.
Finally; hope that your conflicting traffic has done the same to enable the best range AND DONT FORGET TO LOOK OUT - DONT RELY ON THE ANTi COLLISION SOFTWARE.
Looks like as a community we have a lot of work to do to raise the bar (as they say)
I think a lot of problems with seen and being seen are down to aerial blanking - obviously more power will equal a bit more range.
Dave
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was up for about 3 hours yesterday in the Midlands, routing down from Husbands Bosworth to Shotteswell I picked up a converging contact in my blind spot, from my right at about 45 degrees angle about 10 miles out, twin seneca OXF14, 200 ft below. Eventually he crossed in front of me about a mile ahead, similar height just as I was approaching Shotteswell. Got on the ground and asked my arranged meet if he saw the twin, 'oh yeah, crossed right in front of you'. At no point did I ever catch sight of him.
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My apologies for this lengthy post but I think it is worth trying to explain why we are seeing such wildly different results and then discuss the options that might be available. This is a complex subject, not well understood outside the realms of RF engineers and radio amateurs. There is, however, perfectly good science behind it and I am sorry to say that I am completely unsurprised by the results that we are now experiencing. OK... here goes... deep breath!
Falcoguy's observations are spot on and many of the points have been raised in this forum before. Antenna system gain (loss) and position/shielding are the fundamental issue here and that is exactly the reason why Flarm has had to go to two antennas, preferably externally mounted.
The sums go something like this:
For every doubling of distance the free-space signal loss is about 6dB (i.e. the power drops by a factor of four. This is the inverse square law in action. So, if we start with one unit of energy at 1km, it is down to 0.25 units at 2km, 0.06 units at 4km, 0.015 units at 8km and so on. That's the reduction in signal strength due to distance. It is immutable. We can do nothing about it.
Now lets look at antennas:
The basic 1/4 (quarter)wave antenna is a useful starting point. They are commonplace - our aircraft's VHF antennas are one example. They work well because they are operating against an "infinite" ground plane - the aircraft's fuselage. Those with composite aircraft will have metal plates or aluminium foil strips to achieve a similar effect. The whip antenna that comes with the ARF looks to me like a 3/4-wave device, end fed. This theoretically gives around 3db gain over a 1/4 wave antenna but it has no effective ground plane to work against, so it is probably losing all that apparent gain, perhaps even more. Simply by fixing the ground plane problem we would get at least 3dB antenna gain - the equivalent of about a 50% increase in range. Another way of doing this is to make a dipole antenna. A dipole will always significantly outperform an end fed antenna with a poor ground plane. There are other options too. The antenna can make quite a difference to range.
But the elephant in the room is shielding:
Particularly in a metal aircraft, but composites aren't completely immune, radio waves are severely attenuated by reflective materials such as aluminium. It is very easy to achieve 20dB of attenuation with rather little metalwork, especially at 800MHz. Just look how little metalwork there is between you and the business end of your microwave. Your food gets cooked but you do not! 20dB is the equivalent to a drop in range from, say, 10km to about 1km. If significant metalwork is between your antenna and the other guy's antenna then the losses can easily amount to 40dB or more - the difference between a range of 100km down to just 1km. These are overwhelming numbers! Let's be generous and allow for just 30dB attenuation...
We can recover that 30dB in three ways:
1. Move the antenna(s) to a position that is not shielded;
2. Increase transmit power by 30dB;
3. Increase receive sensitivity by 30dB.
1. is the only practical solution. It is the reason why all installed antennas on aircraft are external.
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.
3. is not practical. The receiver is already very sensitive, close to the limits of technology, so it is difficult to make significant gains here. Notably issues with noise, both internally generated inside the receiver and external, such as solar and ignition noise, make this impractical.
Yes, a combination might be used but by far the biggest gain - and therefore where our attention should be focussed - is in the antenna system and, especially, its position, clear of shielding.
Realistically an external antenna system is the only solution and, since we need to see all around us we most likely need more than one antenna for optimal results, exactly as the folks at Flarm have discovered. This isn't PAW's fault, nor is it a failing of the ARF. It is simply the laws of physics in action.
Sorry for the long missive!
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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.
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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.
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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.
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All the points about antenna location are well made. However, I think that something more fundamental is going wrong here. Recalling my experience yesterday, I had good LOS through the flimsy windshield of a Skyranger to a ground station, from a height of about 2000'. I was in the region for about 10 minutes, and would therefore have transmitted about 300 packets. The ground station received only 6, and I received only 12 from him Simultaneously a ground station 4 miles away could receive me without difficulty (but I never received anything from him). According to my understanding, the inverse square law says that the signal level at the local ground station should have been about 24 db greater than the one 4 miles away. It seems to me that there is a problem with the receivers, or maybe frequency drift?
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I had good LOS through the flimsy windshield of a Skyranger to a ground station, from a height of about 2000'. I was in the region for about 10 minutes, and would therefore have transmitted about 300 packets. The ground station received only 6, and I received only 12 from him Simultaneously a ground station 4 miles away could receive me without difficulty (but I never received anything from him).
If I understand this correctly you had two ground stations, one 2000ft directly below you that struggled and the other four miles away that worked better. If that is correct then it brings into play another aspect of simple whip antennas, which is that they do not work equally well in all directions. Notably, for this case, the antenna will not radiate in its own plane, i.e. along the line of the wire. The radiation pattern of a vertical antenna is rather like a ring doughnut plonked flat around the antenna - lots of signal horizontally through 360 degrees but nothing above or below. Effectively radiation is perpendicular to the antenna wire. The nulls off the ends are often very sharp indeed - a characteristic that is used amongst other things for direction finding.
It is harder to explain why the station four miles away could see you but you could not see it. Don't have an opinion on that at the moment but I think it highly unlikely that frequency drift is an issue - these units are quite broad band after all.
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Yes, definitely something more going on than RF line of sight/obstructions/antenna placement. In the tests CC and I did, one unit could see another unit at 5 miles. The reciprocal unit couldn't see the first one until 40 yards away.
On the ground, we verified this with two units sited together...the one unit dropped out at 40 yards, another at 300 yards.
If we were actually getting 68mW/100mW all the time and everything worked as it should, we'd at least get a mile's notice, and up to 5 miles in good line of sight. I guess we'd get even more with fully working units at 500mW.
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Paul,
There must be some difference in the "build" of the two units that could be investigated. Have you tried Ian Fallons suggestion earlier in this thread of measuring power by way of Master /Slave working?
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Not yet. On the cards!
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I think we were getting a bit more during our air to air tests at Long Marston. Subject to me solving another curiosity, perhaps a lot more. I wouldn't write the ARF off just yet.
Hi Ian,
I'm intrigued, as to this curiosity? Any conclusions as yet?
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Reluctant to commit to this but it does look like my PAW was being picked up overhead Shotteswell by Bryan's near overhead Long Marston over ARF which by my reckoning is about 14nm / 25km
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If that is correct then it brings into play another aspect of simple whip antennas, which is that they do not work equally well in all directions
I approached station A (the near one) from two miles away, from the direction in which it had the best view of the sky. So the slant angle would be not much different than that to station B. I got no response until I was nearly overhead, so I think that rules out the radiation pattern.
A thought occurs to me, do you think that the receivers are being swamped by noise radiated by the Pi, especially in Slice of POD examples, like mine. The ARF is closer to the noisy stuff than the earlier glue pen models.
I am going to do some master/slave tests when I get my second ARF, which I persuaded J Curtis to sell me :)
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btw do you know about the master / slave test mode ? You can put one in each mode via the Configuration web interface and then the logging will show power levels. Would be interested to see the values you get.
Not sure how to do that. I tried and got ARF logging messages on the master like this but have no idea what it means!
aSSRSSIS-58 aMMRSSIM-54
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Model aircraft use 100mW on the 2.4Ghz band and have known control issues if a person get between the pilot and model, and that is at less than a kilometre range. Not surprised that internal and shielded antenna reception is poor between PAW units, not to mention poor antenna setups.
We seem to have two threads running on this issue?
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2.4GHz is absorbed by water-based things such as foliage and people significantly more than is 800MHz, so I am not particularly surprised. It's the reason that 2.4GHz is used for microwaves. 800MHz is actually a rather useful frequency because it is relatively unaffected by water but even 800MHz won't go through metal unfortunately!
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Just as a complete amateur, this may be a stupid question, but would the tv tuner interfere with the ARF at all?
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I can't think of any way in which a TV tuner (PC based? Stand-alone TV?) or indeed any other receive-only kit could interfere with the ARF's operation.
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I mean the one used for picking up ads-b in the PAW system. Sorry, that wasn't too clear.
If I knew what this meant: aSSRSSIS-58 aMMRSSIM-54 it would help, I seem to get higher values without the dvb-t plugged in that when it is in...
I'll do further testing tomorrow with my two units in slave/master config to compare the outputs with and without the dvb-t tuner plugged in.
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2.4GHz is absorbed by water-based things such as foliage and people significantly more than is 800MHz, so I am not particularly surprised...
Thank you TWF, I was forgetting that they are quite different frequencies.
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I seem to get higher values without the dvb-t plugged in that when it is in...
That sounds like it might be a power supply problem. The DVB dongle is quite a greedy beast, which is why it gets so hot.
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Just as a complete amateur, this may be a stupid question, but would the tv tuner interfere with the ARF at all?
It's been requested for the next set of tests that the DVB be removed for air-air testing to see if it makes a difference.
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If I knew what this meant: aSSRSSIS-58 aMMRSSIM-54 it would help
The -58 and -54 are the received signal strength at the slave and master respectively (I think that's the right way round). I believe that the figures should be in the low -twenties, for stations 30m apart, but I could be mistaken. Presumably your stations were separated by building structure at -58 db(i?) ?
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I seem to get higher values without the dvb-t plugged in that when it is in...
That sounds like it might be a power supply problem. The DVB dongle is quite a greedy beast, which is why it gets so hot.
The ARF is powered by the 3.3v regulator, which has a fair bit of headroom, so supply problems shouldn't be an issue. I have never seen the 3.3v supply at anything less than 3.25
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If I knew what this meant: aSSRSSIS-58 aMMRSSIM-54 it would help
The -58 and -54 are the received signal strength at the slave and master respectively (I think that's the right way round). I believe that the figures should be in the low -twenties, for stations 30m apart, but I could be mistaken. Presumably your stations were separated by building structure at -58 db(i?) ?
So lower is better? They're separated by 1 floor in my house. I'll do some more testing then...
power supply isn't an issue, I'm using more than the recommended kit. 1x 4.2amp wall power supply with .5m juicybitz cable, the other is on a 2.4amp battery pack with the same cable.
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To avoid misunderstanding, more positive numbers are better. So -20 is better than -58. Lee, please could we have some advice as to which way round the master/slave figures are, and what to expect at a given distance, say 30 foot?
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To avoid misunderstanding, more positive numbers are better. So -20 is better than -58. Lee, please could we have some advice as to which way round the master/slave figures are, and what to expect at a given distance, say 30 foot?
There is a description here ...
http://openmicros.org/index.php/articles/84-xrf-basics/146-rssimode
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That is very helpful, thanks. Do the max range figures of 88 to 92 apply to the ARF? ISTR seeing that the ARF has a more sensitive RX than the XRF
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my understanding is that once the figures get down to -90, the link becomes unreliable
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I seem to get higher values without the dvb-t plugged in that when it is in...
That sounds like it might be a power supply problem. The DVB dongle is quite a greedy beast, which is why it gets so hot.
The ARF is powered by the 3.3v regulator, which has a fair bit of headroom, so supply problems shouldn't be an issue. I have never seen the 3.3v supply at anything less than 3.25
It won't be a question of voltage headroom but the ability to supply all the current your entire set-up requires.
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It won't be a question of voltage headroom but the ability to supply all the current your entire set-up requires.
If the current supply is insufficient the supply voltage will necessarily drop, or crowbar off entirely. A 3.3v regulator will typically function until the supply voltage has dropped to 4.3v, but the Pi can be expected to fail before that, when the supply reaches the specified minimum of 4.75v.