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08-20-2015, 04:40 PM
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Join Date: Jan 2005
Location: Twin Cities, MN
Posts: 1,565
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Quote:
Originally Posted by humptybump
Hi Ian,
I don't think any of us are [sufficiently] delusional to believe the SDR+RPi is "the same thing" as a Stratus or GDL39 (as least I'm not  )
At the same time, this project fits nicely with the experimental nature of our operational domain.
Quick update: I recompiled the necessary modules to get the RT5370 based wifi dongles to work. My ground tests successfully connect with ForeFlight (but obviously I am not receiving any weather data).
The local offer still stands for anyone who wishes to test flit this weekend.

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What are you using for an antenna? A properly tuned antenna, oriented vertically, will provide the best reception. Then again, reception on the ground *at all* will be difficult in many places due to LoS restrictions.
__________________
Brad Benson, Maplewood MN.
RV-6A N164BL, Flying since Nov 2012!
If you're not making mistakes, you're probably not making anything
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08-20-2015, 05:16 PM
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Join Date: Feb 2011
Location: USA
Posts: 3,179
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Hi Brad. I just swapped my "quick test" antenna (in the picture) for an antenna tuned for 978Mhz. But I'm sure the majority of the deficiency is that I'm still located on the ground 
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08-21-2015, 09:22 AM
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Join Date: Feb 2011
Location: USA
Posts: 3,179
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DIY Stratux PIREP
I shirked my duties this morning and flew with the Stratux up on my glare shield. As a point of comparison, I have a GDL39 with a dedicated antenna on the bottom of my aircraft back under the RV-8 rear baggage area.
I had no problem getting my iPhone 6P to connect to the Stratux wifi. The first problem came when I tried to get ForeFlight to recognize the Stratux (FreeFlight) device. It took 20 minute of random ideas and then it connected. None of this surprised me since I had the same problem yesterday in my bench. Subsequent boot-ups of the Stratux had no problem with connecting. I'll classify it as one of those random things.
During ground ops and in flight I noticed the ForeFlight "connection" to FreeFlight would drop. This again was random. When I looked closely at the videos linked to the reddit posts I notice it happened to his system as well.
I picked up the first tower at 650' AGL. It was unreliable. I would lose it frequently. The direction and orientation of the aircraft would contribute to the loss. At 2500' AGL I had two towers but that would fluctuate often back to one and zero. By comparison, the GDL39 had five towers. At 3500' AGL I had three towers with the Stratux ranging from 30-50nm while the GDL39 had seven towers from 30-90nm.
I did receive a 4 local radar frames and 1 national radar frame with the Stratux during the test flight. I also received 280 text reports. I checked three local airports and had METAR and/or TAF for each. I don't know the data for the GDL39 but I had radar animations and weather reports for the four airports I checked.
I tested with both a 1/4 antenna tuned to 978 mhz and the little SDR supplied antenna. It's a guess but I think the tuned antenna was better. Both antennas were affected by the orientation of the airplane. I observed a tower 30 miles away while flying toward it and lost that tower when flying away from it.
My initial conclusions are: - antenna location is important - a tuned belly antenna will likely perform better than my tests
- connectivity is variable - I can't compare with a real Stratus unit and my GDL39 is hard wired to my panel and uses Bluetooth to ForeFlight and Garmin Pilot on the iPhone. (for those who were wondering, "yes", ForeFlight on the iPhone immediately recognized my G3X Touch Connext)
- Reception quality is "OK" - I knew going into the tests that the SDR would not be as discriminating and sensitive as the GDL39. As my altitude increased, the Stratux data improved but the GDL39 was working well at 1000' AGL and when the Stratux was at its best, the GDL39 was still 2x better on towers.
Remember, the Stratux is closest to a first generation Stratus 1.
It was a fun experiment and from that measure, it was a success. However, having flown with a first generation GDL39 for three years now, it's not reasonable to consider them in the same category. If you've got the parts readily available or you enjoy Raspberry Pi projects, it's a fun little project.
I captured lots of notes and screen grabs but since I consider this an educational experience, I'll leave those as "an exercise for the student".
Last edited by humptybump : 08-21-2015 at 11:28 AM.
Reason: Typos
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08-21-2015, 10:11 AM
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Join Date: Aug 2011
Location: Austin TX
Posts: 347
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would this antenna work with the SDR dongle?
http://www.deltapopaviation.com/UAT_Antenna.html
Also, would installation on top of the fuse (behind the my sliding canopy when open) be the best place? I have comm antennas under the fuse (below the seats).
Quote:
Originally Posted by humptybump
I shirked my duties this morning and flew with the Stratux up on my glare shield. As a point of comparison, I have a GDL39 with a dedicated antenna on the bottom of my aircraft back under the RV-8 rear baggage area.
I had no problem getting my iPhone 6P to connect to the Stratux wifi. The first problem came when I tried to get ForeFlight to recognize the Stratux (FreeFlight) device. It took 20 minute of random ideas and then it connected. None of this surprised me since I had the same problem yesterday in my bench. Subsequent boot-ups of the Stratux had no problem with connecting. I'll classify it as one of those random things.
During ground ops and in flight I noticed the ForeFlight "connection" to FreeFlight would drop. This again was random. When I looked closely at the videos linked to the reddit posts I notice it happened to his system as well.
I picked up the first tower at 650' AGL. It was unreliable. I would lose it frequently. The direction and orientation of the aircraft would contribute to the loss. At 2500' AGL I had two towers but that would fluctuate often back to one and zero. By comparison, the GDL39 had five towers. At 3500' AGL I had three towers with the Stratux ranging from 30-50nm while the GDL39 had seven towers from 30-90nm.
I did receive a 4 local radar frames and 1 national radar frame with the Stratux during the test flight. I also received 280 text reports. I checked three local airports and had METAR and/or TAF for each. I don't know the data for the GDL39 but I had radar animations and weather reports for the four airports I checked.
I tested with both a 1/4 antenna tuned to 978 mhz and the little SDR supplied antenna. It's a guess but I think the tuned antenna was better. Both antennas were affected by the orientation of the airplane. I observed a tower 30 miles away while flying toward it and lost that tower when flying away from it.
My initial conclusions are: - antenna location is important - a tuned belly antenna will likely perform better than my tests
- connectivity is variable - I can't compare with a real Stratus unit and my GDL39 is hard wired to my panel and uses Bluetooth to ForeFlight and Garmin Pilot on the iPhone. (for those who were wondering, "yes", ForeFlight on the iPhone immediately recognized my G3X Touch Connext)
- Reception quality is "OK" - I knew going into the tests that the SDR would not be as discriminating and sensitive as the GDL39. As my altitude increased, the Stratux data improved but the GDL39 was working well at 1000' AGL and when the Stratux was at its best, the GDL39 was still 2x better on towers.
Remember, the Stratux is closest to a first generation Stratus 1.
It was a fun experiment and from that measure, it was a success. However, having flown with a first generation GDL39 for three years now, it's not reasonable to consider them in the same category. If you've got the parts readily available or you enjoy Raspberry Pi projects, it's a fun little project.
I captured lots of notes and screen grabs but since I consider this an educational experience, I'll leave those as "an excessive for the student".
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__________________
***********************************
--2008 RV-9A
Austin TX
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08-21-2015, 08:09 PM
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Join Date: Jul 2015
Location: -
Posts: 12
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Quote:
Originally Posted by humptybump
I shirked my duties this morning and flew with the Stratux up on my glare shield. As a point of comparison, I have a GDL39 with a dedicated antenna on the bottom of my aircraft back under the RV-8 rear baggage area.
I had no problem getting my iPhone 6P to connect to the Stratux wifi. The first problem came when I tried to get ForeFlight to recognize the Stratux (FreeFlight) device. It took 20 minute of random ideas and then it connected. None of this surprised me since I had the same problem yesterday in my bench. Subsequent boot-ups of the Stratux had no problem with connecting. I'll classify it as one of those random things.
During ground ops and in flight I noticed the ForeFlight "connection" to FreeFlight would drop. This again was random. When I looked closely at the videos linked to the reddit posts I notice it happened to his system as well.
I picked up the first tower at 650' AGL. It was unreliable. I would lose it frequently. The direction and orientation of the aircraft would contribute to the loss. At 2500' AGL I had two towers but that would fluctuate often back to one and zero. By comparison, the GDL39 had five towers. At 3500' AGL I had three towers with the Stratux ranging from 30-50nm while the GDL39 had seven towers from 30-90nm.
I did receive a 4 local radar frames and 1 national radar frame with the Stratux during the test flight. I also received 280 text reports. I checked three local airports and had METAR and/or TAF for each. I don't know the data for the GDL39 but I had radar animations and weather reports for the four airports I checked.
I tested with both a 1/4 antenna tuned to 978 mhz and the little SDR supplied antenna. It's a guess but I think the tuned antenna was better. Both antennas were affected by the orientation of the airplane. I observed a tower 30 miles away while flying toward it and lost that tower when flying away from it.
My initial conclusions are: - antenna location is important - a tuned belly antenna will likely perform better than my tests
- connectivity is variable - I can't compare with a real Stratus unit and my GDL39 is hard wired to my panel and uses Bluetooth to ForeFlight and Garmin Pilot on the iPhone. (for those who were wondering, "yes", ForeFlight on the iPhone immediately recognized my G3X Touch Connext)
- Reception quality is "OK" - I knew going into the tests that the SDR would not be as discriminating and sensitive as the GDL39. As my altitude increased, the Stratux data improved but the GDL39 was working well at 1000' AGL and when the Stratux was at its best, the GDL39 was still 2x better on towers.
Remember, the Stratux is closest to a first generation Stratus 1.
It was a fun experiment and from that measure, it was a success. However, having flown with a first generation GDL39 for three years now, it's not reasonable to consider them in the same category. If you've got the parts readily available or you enjoy Raspberry Pi projects, it's a fun little project.
I captured lots of notes and screen grabs but since I consider this an educational experience, I'll leave those as "an exercise for the student".
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humptybump, you're not using the recommended hardware, correct? The WiFi dongle is $10 and worth it to not deal with issues, they're all hit or miss and the one in the original hardware setup doesn't seem to be causing issues for others.
In terms of reception, try using the antenna that comes with it in its retracted position. Most people report good reception with this. Some guys on beechtalk actually reported better reception than Stratus 2. At 1500' AGL I get a steady 2-3 towers.
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08-21-2015, 09:23 PM
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Join Date: Jul 2015
Location: -
Posts: 12
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Quote:
Originally Posted by dynonsupport
One thing to be aware of with the digitizer dongles is that the sensitivity of these devices is much lower than for dedicated designs. These USB sticks are meant to pick up TV stations that output 1,000,000 watts from 30 miles away, while an ADS-B ground station is 250 watts and can be 100+ miles away.
The dedicated designs like a Stratus, the Garmin GDL-39, or the Dynon ADS-B box have incredible sensitivity because they are specifically tuned to receive only the frequency they are designed for. SDR boxes like the USB TV sticks inherently need to have huge front end bandwidth, which hurts selectivity and sensitivity.
You may easily find that the SDR ADS-B works fine 30 miles away from the ADS-B ground station, but not the 200 miles that a specialized receiver can do.
Now, clearly I'm biased because I work at Dynon and we like our ADS-B box a lot. This sounds like a fun project. Just don't judge the coverage and performance of the ADS-B system based on one of these receivers.
--Ian Jordan
Dynon Avionics
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TV stations in the US transmit at a maximum of 50,000W. I don't know about ADS-B ground stations but don't think your number for that is accurate either.
Sensitivity isn't a great differentiator in this case, it's not like a comm radio where you need to catch a single message. Having recorded some UAT messages with an RTL-SDR dongle, in the span of 30 minutes I got spammed with all local airports' METARs an average of 5 times over. Maybe if it were more sensitive I'd get the exact same METAR message another 5-10 times...
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08-21-2015, 10:33 PM
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Join Date: Jun 2005
Location: Woodinville, WA
Posts: 1,499
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Okei,
UHF TV stations in the USA are allowed up to 5 million watts if analog and 1 million if digital (which they are all now). That's 470 to 890 MHz, and of course we're talking 978 MHz or 1090 MHz, above the TV frequencies.
It's only VHF channels 2-6 that are limited to 45 KW digital and channels 7-13 are 160 KW.
My number for ADS-B ground stations is inaccurate, you are right. Many GBT's are only 25 watts. But the FAA is allowed to tune them up to +58 dBm (630W) to get the appropriate coverage.
The issue with sensitivity is actually quite important if you are flying cross country, where you may actually care what the weather is doing in front of you while relying on the FAA's ground station spacing based on a UAT receiver meeting the TSO sensitivity. Totally agree that if you are only flying locally in a dense area it matters a lot less.
--Ian
Last edited by dynonsupport : 08-21-2015 at 10:44 PM.
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08-22-2015, 01:26 AM
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Join Date: Jul 2015
Location: -
Posts: 12
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Quote:
Originally Posted by dynonsupport
Okei,
UHF TV stations in the USA are allowed up to 5 million watts if analog and 1 million if digital (which they are all now). That's 470 to 890 MHz, and of course we're talking 978 MHz or 1090 MHz, above the TV frequencies.
It's only VHF channels 2-6 that are limited to 45 KW digital and channels 7-13 are 160 KW.
My number for ADS-B ground stations is inaccurate, you are right. Many GBT's are only 25 watts. But the FAA is allowed to tune them up to +58 dBm (630W) to get the appropriate coverage.
The issue with sensitivity is actually quite important if you are flying cross country, where you may actually care what the weather is doing in front of you while relying on the FAA's ground station spacing based on a UAT receiver meeting the TSO sensitivity. Totally agree that if you are only flying locally in a dense area it matters a lot less.
--Ian
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Do you have, by chance, the TSO sensitivity specs for UAT receivers? I'm curious to quantify the difference between the published specs on an RTL-SDR dongle vs that.
EDIT:
Ian, you probably know a lot more about this than I do. ICAO "Doc 9861 AN/460" says:
"All UAT aircraft receivers have the same receiver sensitivity requirements. The receiver sensitivity at the PMP, for 90 per cent message success rate in the absence of interference, is –93 dBm for long UAT ADS-B messages, –94 dBm for basic UAT ADS-B messages and –91 dBm for ground uplink (ground-to-air) messages."
http://superkuh.com/gnuradio/R820T_d...0_unlocked.pdf is the documentation I could find. It shows:
FFT:8k,QPSK,CR:1/2 Sensitivity -97.5dBm
Lower at higher data rates.
QPSK,CR:1/2 I see is 0.62 bits/sec per Hz
The same ICAO doc says the data rate is 1041667 bit/sec for UAT. So that's less than 0.01 bits/sec per Hz.
Am I looking at this the right way? From this it seems like it actually exceeds spec for sensitivity.
1.041667 Mbps.
Last edited by okei : 08-22-2015 at 02:00 AM.
Reason: answering my own question
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08-22-2015, 06:30 AM
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Join Date: Feb 2011
Location: USA
Posts: 3,179
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Quote:
Originally Posted by okei
humptybump, you're not using the recommended hardware, correct?
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Correct. I experimented with what I had on hand.
Quote:
Originally Posted by okei
In terms of reception, try using the antenna that comes with it
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I tested with a tuned antenna and with the SDR's original antenna. There was not a big difference.
The towers received will depend on where they are and what interference is in the area. Comparing different geographic areas will have many variables.
BTW: when I reviewed the reddit author's videos, I noticed he was in the Detroit area and did not see towers until he was around 3000' AGL and a close look shows he edited out more than an hour of video. During a period of maneuvering at just over 300' AGL, he see 7 then 4 then 2 then 3 towers. There is also a brief moment when he too had the "not connected" status. I had the same experiences.
This is a great experimental project and encourange anyone who likes tinkering with Raspberry Pi projects to give it a try. I had fun and learned a few things along the way.
Last edited by humptybump : 08-22-2015 at 08:18 AM.
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08-22-2015, 08:26 AM
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Join Date: May 2005
Location: Croissy-Beaubourg, France
Posts: 224
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Other project
This is another project along the same lines.
www.pilotaware.com
It has a transmitter which might be illegal to use in some countries.
Some good informations on the project's forum.
I don't know if it will work without the transmitter installed, I will ask the author as soon as I get access to the forum.
Depending on which parts I receive first I'll try this one or the stratux.
__________________
Pascal
RV-7A #72588 - F-PSGA
VAF #484 - Donated 2020
Flying
www.notreavion.net
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