Okay so I am a bit confused about how this will work with two satellites both using FM modulation on the same frequency. FM has the capture ratio effect which basically means that if one signal is about twice as strong as the other that signal will capture the receiver and the other one may make a little background noise but won't be properly demodulates. If both of them are received at the same power level pretty much you just get noise a.k.a. SO-50 on a crowded day. So if these two satellites are transmitting simultaneously whichever one happens to be stronger at the moment will capture the receiver. Regardless of whether one of them has a audio frequency modulated on the carrier at 315 hertz while the other ones at 375 you're most likely going to not here both of them simultaneously Heard or displayed on a PSK waterfall.
This may not be an issue in the future as the satellites drift apart from each other but right now as it seems to me they are pretty darn close so if they were both on they would be in the beam width of any directional antenna.
Aside from the frequency of the PSK carrier itself as modulated on the FM, I would also have to assume that the call sign of the PSK data beacon is hopefully different. The PSK I decoded this morning had a beacon call sign of W3ADO-5. I would have to assume there is a different beacon call sign for both. In my reading I have not seen what the difference between the two would be.
Looking forward to soon trying to test an uplink on 28.120 and see if I can hear myself. This looks like a good challenge.
73. Tom. N5HYP
... how will this work with two FM satellites on the same frequency?
The idea is to serve users no matter which spacecraft is in view. And to not have the users have to always change frequencies and track different objects. Imagine a camper in the field. Just point your beam towards AOS generally and set +5 KHz tuing and when a satellite comes into view, you can use it.
Many things separate them: 1) Distance 2) Time 3) Doppler 4) Users antenna beamwidth
When they are 5 minutes apart, the beam headings will be over 90 degrees apart (10 to 15 dB separation), the Doppler will be typically 10 KHz (10-15 dB separation) for a total of 20-30 dB or more to prevent capture effect.
Since an orbit is 95 minutes long, and the satellites will drift, then 95% of the time, this condition of at least 5 minute separation, will be met.
PSAT PSK (W3ADO-5) was built over 3 years ago and is hardwired to power up by default (maximum chance it will work despite spacecraft packet link or other failures)
BRIC PSK (W3ADO-6) was built just last year and has independent control by Brno designers (can be off while sats are adjacent) It also powers up by default (again, for maximum availability in case of bus failure).
So I think the choice pretty much meets the goals of maximum availability to users and as independent of spacecraft failures as we could make them.
Hope that Helps. Bob, WB4aPR
-----Original Message----- From: AMSAT-BB [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Tom schuessler Sent: Sunday, May 24, 2015 12:00 AM To: amsat-bb@amsat.org Subject: [amsat-bb] PSAT and Bricsat FM PSK on same frequencies
Okay so I am a bit confused about how this will work with two satellites both using FM modulation on the same frequency. FM has the capture ratio effect which basically means that if one signal is about twice as strong as the other that signal will capture the receiver and the other one may make a little background noise but won't be properly demodulates. If both of them are received at the same power level pretty much you just get noise a.k.a. SO-50 on a crowded day. So if these two satellites are transmitting simultaneously whichever one happens to be stronger at the moment will capture the receiver. Regardless of whether one of them has a audio frequency modulated on the carrier at 315 hertz while the other ones at 375 you're most likely going to not here both of them simultaneously Heard or displayed on a PSK waterfall.
This may not be an issue in the future as the satellites drift apart from each other but right now as it seems to me they are pretty darn close so if they were both on they would be in the beam width of any directional antenna.
Aside from the frequency of the PSK carrier itself as modulated on the FM, I would also have to assume that the call sign of the PSK data beacon is hopefully different. The PSK I decoded this morning had a beacon call sign of W3ADO-5. I would have to assume there is a different beacon call sign for both. In my reading I have not seen what the difference between the two would be.
Looking forward to soon trying to test an uplink on 28.120 and see if I can hear myself. This looks like a good challenge.
73. Tom. N5HYP _______________________________________________ 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
participants (2)
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Robert Bruninga -
Tom schuessler