Note ref. doppler w/ UHF iGate listener for ISS Packet
Hello everyone.
For anyone who has an interest in using the digipeater on the ISS now that it has switched to 70cm, I wanted to pass along some non-scientific findings.
Of course it's great to see your packets digipeated from the ISS but part of the fun also is showing up on the ariss.net website which lists digipeated packets that were received by a ground station setup as an "iGate" to forward that traffic over the internet (resulting in a listing on the ariss.net website).
It is my understanding that a couple of factors are contributing to a reduction in the number of digipeated packets successfully making it to the ariss.net site.
For one, there are less iGates able to receive on 70cm. That issue can be resolved over time as people naturally add the ability to listen on 70cm with an iGate app.
But another issue that several folks have pointed out is that with the more substantial doppler shift at 70cm, a receiver set to a single frequency will miss much of the traffic sent back down from the ISS.
I have never run an iGate and wasn't completely sure what was involved, but with a number of SDR-equipped Raspberry Pi's around here I wanted to put one more set of ears out there for UHF packet.
I'm sure there are any number of ways to setup a Pi as an iGate; I used the instructions at: https://github.com/wb2osz/direwolf/blob/master/doc/Raspberry-Pi-SDR-IGate.pd...
But how to setup a Pi is not why I wanted to post this. Of course I initially setup my Pi-iGate to listen on the published frequency of 437.550. However, I received very few packets from the ISS.
It crossed my mind that, at the point of closest approach, the doppler frequency is changing quite rapidly. So, I thought that I might improve my count of iGated packets if I listened on a frequency that would be viable for a longer period of time. Using (WD9EWK) Patrick's doppler adjustment guide as a reference, I chose to change my listener to 437.555.
Again, this is not scientific, but I am now receiving more packets on the Pi-iGate than I was before. Only time will tell if this continues. Of course the better solution would be to program the Pi to adjust for doppler throughout the entire pass and there IS code online to provide that compensation. However, I have not been able to get that working.
If anyone is interested, here is the script that I run to activate the iGate listener on my Pi... it runs in the background so that I can log off from the Pi and just leave it running stand-alone.
----------------------------------------------------- #!/bin/bash nohup rtl_fm -f 437.555M - | direwolf -l /home/pi/iss-uhf-log/ -c /home/pi/sdr.conf -r 24000 -D 1 - > /dev/null 2>&1 & # -----------------------------------------------------
Hope someone finds this informative and at least one of you might be prompted to add a 70cm iGate to your shack!
73,
-Scott, K4KDR Montpelier, VA USA
If your receiver is equipped with AFC, like the Icom IC-910, it will correct for doppler without the need to do it in software...
George, KA3HSW
----- Original Message ----- From: "Scott" scott23192@gmail.com To: amsat-bb@amsat.org Sent: Sunday, November 13, 2016 1:48 PM Subject: [amsat-bb] Note ref. doppler w/ UHF iGate listener for ISS Packet
Hello everyone.
For anyone who has an interest in using the digipeater on the ISS now that it has switched to 70cm, I wanted to pass along some non-scientific findings.
Of course it's great to see your packets digipeated from the ISS but part of the fun also is showing up on the ariss.net website which lists digipeated packets that were received by a ground station setup as an "iGate" to forward that traffic over the internet (resulting in a listing on the ariss.net website).
It is my understanding that a couple of factors are contributing to a reduction in the number of digipeated packets successfully making it to the ariss.net site.
For one, there are less iGates able to receive on 70cm. That issue can be resolved over time as people naturally add the ability to listen on 70cm with an iGate app.
But another issue that several folks have pointed out is that with the more substantial doppler shift at 70cm, a receiver set to a single frequency will miss much of the traffic sent back down from the ISS.
I have never run an iGate and wasn't completely sure what was involved, but with a number of SDR-equipped Raspberry Pi's around here I wanted to put one more set of ears out there for UHF packet.
I'm sure there are any number of ways to setup a Pi as an iGate; I used the instructions at: https://github.com/wb2osz/direwolf/blob/master/doc/Raspberry-Pi-SDR-IGate.pd...
But how to setup a Pi is not why I wanted to post this. Of course I initially setup my Pi-iGate to listen on the published frequency of 437.550. However, I received very few packets from the ISS.
It crossed my mind that, at the point of closest approach, the doppler frequency is changing quite rapidly. So, I thought that I might improve my count of iGated packets if I listened on a frequency that would be viable for a longer period of time. Using (WD9EWK) Patrick's doppler adjustment guide as a reference, I chose to change my listener to 437.555.
Again, this is not scientific, but I am now receiving more packets on the Pi-iGate than I was before. Only time will tell if this continues. Of course the better solution would be to program the Pi to adjust for doppler throughout the entire pass and there IS code online to provide that compensation. However, I have not been able to get that working.
If anyone is interested, here is the script that I run to activate the iGate listener on my Pi... it runs in the background so that I can log off from the Pi and just leave it running stand-alone.
#!/bin/bash nohup rtl_fm -f 437.555M - | direwolf -l /home/pi/iss-uhf-log/ -c /home/pi/sdr.conf -r 24000 -D 1 - > /dev/null 2>&1 &
#
Hope someone finds this informative and at least one of you might be prompted to add a 70cm iGate to your shack!
73,
-Scott, K4KDR Montpelier, VA USA _______________________________________________ Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
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Good idea. Another thing you can do is anticipate where all the other igates are and choose a fixed doppler for the region you can hear better than other IGates. Though without knowing what the other gates are doing, it is a multivariable problem... Ill leave that to the tedious...bob
-----Original Message----- From: AMSAT-BB [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Scott Sent: Sunday, November 13, 2016 2:48 PM To: amsat-bb@amsat.org Subject: [amsat-bb] Note ref. doppler w/ UHF iGate listener for ISS Packet
Hello everyone.
For anyone who has an interest in using the digipeater on the ISS now that it has switched to 70cm, I wanted to pass along some non-scientific findings.
Of course it's great to see your packets digipeated from the ISS but part of the fun also is showing up on the ariss.net website which lists digipeated packets that were received by a ground station setup as an "iGate" to forward that traffic over the internet (resulting in a listing on the ariss.net website).
It is my understanding that a couple of factors are contributing to a reduction in the number of digipeated packets successfully making it to the ariss.net site.
For one, there are less iGates able to receive on 70cm. That issue can be resolved over time as people naturally add the ability to listen on 70cm with an iGate app.
But another issue that several folks have pointed out is that with the more substantial doppler shift at 70cm, a receiver set to a single frequency will miss much of the traffic sent back down from the ISS.
I have never run an iGate and wasn't completely sure what was involved, but with a number of SDR-equipped Raspberry Pi's around here I wanted to put one more set of ears out there for UHF packet.
I'm sure there are any number of ways to setup a Pi as an iGate; I used the instructions at: https://github.com/wb2osz/direwolf/blob/master/doc/Raspberry-Pi-SDR-IGate. pdf
But how to setup a Pi is not why I wanted to post this. Of course I initially setup my Pi-iGate to listen on the published frequency of 437.550. However, I received very few packets from the ISS.
It crossed my mind that, at the point of closest approach, the doppler frequency is changing quite rapidly. So, I thought that I might improve my count of iGated packets if I listened on a frequency that would be viable for a longer period of time. Using (WD9EWK) Patrick's doppler adjustment guide as a reference, I chose to change my listener to 437.555.
Again, this is not scientific, but I am now receiving more packets on the Pi-iGate than I was before. Only time will tell if this continues. Of course the better solution would be to program the Pi to adjust for doppler throughout the entire pass and there IS code online to provide that compensation. However, I have not been able to get that working.
If anyone is interested, here is the script that I run to activate the iGate listener on my Pi... it runs in the background so that I can log off from the Pi and just leave it running stand-alone.
----------------------------------------------------- #!/bin/bash nohup rtl_fm -f 437.555M - | direwolf -l /home/pi/iss-uhf-log/ -c /home/pi/sdr.conf -r 24000 -D 1 - > /dev/null 2>&1 & # -----------------------------------------------------
Hope someone finds this informative and at least one of you might be prompted to add a 70cm iGate to your shack!
73,
-Scott, K4KDR Montpelier, VA USA _______________________________________________ Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
El 13/11/16 a las 20:48, Scott escribió:
But another issue that several folks have pointed out is that with the more substantial doppler shift at 70cm, a receiver set to a single frequency will miss much of the traffic sent back down from the ISS.
[...]
It crossed my mind that, at the point of closest approach, the doppler frequency is changing quite rapidly. So, I thought that I might improve my count of iGated packets if I listened on a frequency that would be viable for a longer period of time. Using (WD9EWK) Patrick's doppler adjustment guide as a reference, I chose to change my listener to 437.555.
Hi Scott and all,
Another thing that you can do is to correct for Doppler in software (obvious).
If you're using an FM radio, this may me more complicated because you need to set up CAT, but if you're using an SDR, it's just a matter of using the appropriate software.
Since you're asking this specifically for iGates, we can assume that you have an internet connection on your receiver to fetch TLEs automatically.
Then you can use whatever Doppler correction tool you like to be in tune during all the ISS pass. For instance, this command line tool:
https://github.com/cubehub/doppler
should be useful, especially for embedded systems such as a Raspberry Pi.
The disadvantage of doing Doppler correction is that you can only do it for one satellite. In 145.825 there is the problem that there are several satellites you want to receive, several of them may be in view simultaneously, and they will have different Doppler offsets. Still, you could setup the system so that if there's only one satellite in view you correct for that satellite and if there are several you either prioritize and correct for one of them, take an average of the corrections, or just do no correction. This is more complicated, of course.
However, on 437.550 you only have the ISS, so it makes much sense to track Doppler correction only for the ISS, and this is much easier than handling several satellites.
So, nothing new here. What I mean is that automatical Doppler correction for the ISS using an RasberryPi and RTL-SDR is not that hard (and much better solution than listening on a fixed frequency). Come on guys, we can do it!
73,
Dani EA4GPZ.
Great tips, Dani - thanks for the input!
I tried the code from https://github.com/cubehub/doppler but ran into difficulty with the dependencies installing correctly. But I really should revisit that since it is the best solution.
I had NOT, however, considered the possibility of multiple APRS-capable satellites on UHF being visible at the same time. That is ugly, but as you mentioned we only have the ISS to accommodate for now on 70cm.
Good info!
-Scott, K4KDR
=================================================================
-----Original Message----- From: Dani EA4GPZ Sent: Monday, November 14, 2016 10:26 AM To: amsat-bb@amsat.org Subject: Re: [amsat-bb] Note ref. doppler w/ UHF iGate listener for ISSPacket
El 13/11/16 a las 20:48, Scott escribió:
But another issue that several folks have pointed out is that with the more substantial doppler shift at 70cm, a receiver set to a single frequency will miss much of the traffic sent back down from the ISS.
[...]
It crossed my mind that, at the point of closest approach, the doppler frequency is changing quite rapidly. So, I thought that I might improve my count of iGated packets if I listened on a frequency that would be viable for a longer period of time. Using (WD9EWK) Patrick's doppler adjustment guide as a reference, I chose to change my listener to 437.555.
Hi Scott and all,
Another thing that you can do is to correct for Doppler in software (obvious).
If you're using an FM radio, this may me more complicated because you need to set up CAT, but if you're using an SDR, it's just a matter of using the appropriate software.
Since you're asking this specifically for iGates, we can assume that you have an internet connection on your receiver to fetch TLEs automatically.
Then you can use whatever Doppler correction tool you like to be in tune during all the ISS pass. For instance, this command line tool:
https://github.com/cubehub/doppler
should be useful, especially for embedded systems such as a Raspberry Pi.
The disadvantage of doing Doppler correction is that you can only do it for one satellite. In 145.825 there is the problem that there are several satellites you want to receive, several of them may be in view simultaneously, and they will have different Doppler offsets. Still, you could setup the system so that if there's only one satellite in view you correct for that satellite and if there are several you either prioritize and correct for one of them, take an average of the corrections, or just do no correction. This is more complicated, of course.
However, on 437.550 you only have the ISS, so it makes much sense to track Doppler correction only for the ISS, and this is much easier than handling several satellites.
So, nothing new here. What I mean is that automatical Doppler correction for the ISS using an RasberryPi and RTL-SDR is not that hard (and much better solution than listening on a fixed frequency). Come on guys, we can do it!
73,
Dani EA4GPZ.
participants (4)
-
Dani EA4GPZ -
George Henry -
Robert Bruninga -
Scott