SDRSharp - compensating for satellite doppler

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Friseal
Posts: 3
Joined: Fri Jul 07, 2017 11:58 pm

SDRSharp - compensating for satellite doppler

Post by Friseal » Sat Jul 08, 2017 12:10 am

I'm an amateur radio operator trying to use SDRSharp for receiving LEO satellites transmitting SSB. Is there a simple way to configure SDRSharp to handle more precise tuning to compensate (either manually or automatically) for doppler shift?

When trying to tune currently, the sideband width prevents me from gradually shifting the center frequency to compensate for doppler shift. As the signal drifts, I can't keep clicking it to follow without having to click past the bandwidth sliders and thereby losing the signal completely. Is there a way to overcome this?

Sorry if this is a repeat post. I searched but didn't find anything specific that related. Thanks for any suggestions/assistance.

BackHive
Posts: 17
Joined: Wed Sep 02, 2015 10:48 pm

Re: SDRSharp - compensating for satellite doppler

Post by BackHive » Sat Jul 08, 2017 9:57 pm

Friseal,
For LEO satellites the received frequency needs to be constantly corrected due to doppler shift. It's completely impractical to do this by hand.. which is what you've been doing. The task is normally handled by a satellite tracking program, with up to date elements (https://www.space-track.org, http://celestrak.com/NORAD/elements/ etc.) for the satellite(s) you are interested in and has the ability to connect to your SDR software .
I use the excellent WxTrack by David Taylor.... (http://www.satsignal.eu/software/wxtrack.htm). For frequency correction, this program works in conjunction with a plugin for SDRSharp (http://www.satsignal.eu/software/DDETracker.html) that allows it (and other similar sat tracking programs like Orbitron etc) to connect to and constantly correct the receive frequency as the satellite passes overhead. However, as great as WxTrack is, it can only correct the frequency every 2 seconds maximum. It was designed originally to also connect to an antenna rotor, which it does perfectly.
Some time ago I contacted David and he was kind enough to develop a small program called FastWxTrack... (http://www.satsignal.eu/software/beta.htm)
This program uses the element sets and frequency info txt files that WxTrack requires/uses (updateable by the user) to make frequency corrections to SDRSharp 20 times a second !! I use it very regularly and find it indispensable. When you select the satellite you want to track, it gets the rx frequency and mode, makes the correction and sends it to SDRSharp. Or, after selecting the satellite, you can input the frequency and mode yourself and make up or down corrections on the fly. It's a brilliant little program that does the job perfectly.
I hope this helps.
B/H

rtlsdrblog
Site Admin
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Joined: Mon Nov 19, 2012 11:54 pm

Re: SDRSharp - compensating for satellite doppler

Post by rtlsdrblog » Sun Jul 09, 2017 2:18 am

You could also just simply use the WFM mode, and make the IF bandwidth wide enough to cover the entire doppler shift.

Also, as an alternative to BackHives method you can also try Orbitron with SDR# http://www.rtl-sdr.com/rtl-sdr-tutorial ... te-images/

Friseal
Posts: 3
Joined: Fri Jul 07, 2017 11:58 pm

Re: SDRSharp - compensating for satellite doppler

Post by Friseal » Tue Jul 11, 2017 4:55 am

Thanks, guys. I think I have a solution to my problem, but I appreciate your responses. The FM Wide isn't going to work with linear satellites transmitting SSB, and Orbitron will only track one frequency at a time. You can't change frequency in SDR# and have the doppler shift calculations continue. With the linear transponders, multiple signals can be sent/received across the birds, so the frequencies aren't necessarily fixed for each contact. I think I can manually adjust the tuning for each contact by changing the shift to 1 Hz instead of the default 10 Hz. I will try that and see how it goes.

rtlsdrblog
Site Admin
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Joined: Mon Nov 19, 2012 11:54 pm

Re: SDRSharp - compensating for satellite doppler

Post by rtlsdrblog » Tue Jul 11, 2017 11:39 am

Ah I thought it was NOAA, not SSB, my bad I misread.

But aren't the auto doppler correcting plugins that we mentioned working for you though? If you get your PC timing accurate with NTP, and use up to date TLEs they should give you accurate frequencies.

Friseal
Posts: 3
Joined: Fri Jul 07, 2017 11:58 pm

Re: SDRSharp - compensating for satellite doppler

Post by Friseal » Wed Jul 12, 2017 11:02 pm

Oh yes. The doppler shift works fine if I don't try to adjust the frequency manually. But that's just it. The linear transponder birds use a range of frequencies to receive and re-transmit instead of a fixed frequency.

For example, FO-29's uplink is 145.900 - 146.000 Mhz, and the downlink is 435.800 - 435.900 Mhz. The midpoints are 145.950 and 435.850, so if the bird hears something on 145.950 up, then it will re-transmit that on 435.850. But things change slightly if you move up or down on the uplink. At 145.940, FO-29 re-transmits on 435.860 -- and vice-versa for 145.960 up and 435.840 down. Etc.

So if I set Orbitron at the middle frequency, and then find that I need to change the downlink to 435.840, I can't do that on the fly and still have the doppler shift calculations handle the changes as needed. I have to go back to Orbitron and re-enter the new frequency everytime, and that doesn't make operation very easy when trying to work a contact.

If I'm mistaken, please let me know as I would certainly like to get this working smoothly. It is well worth the trouble to not have to buy another radio transceiver just for full-duplex operation. :D

Username
Posts: 402
Joined: Sun Oct 09, 2016 7:27 am

Re: SDRSharp - compensating for satellite doppler

Post by Username » Thu Jul 13, 2017 9:16 am

SDR for reception and a Radio for Transmit? :lol:

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