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Re: to many downlinks on 145.825Mhz



On Sun, 30 Dec 2001, ROBERT TURLINGTON wrote:

> the choice of 145.825Mhz as a downlink for new satellites is now
> unacceptable ,for most of today and parts of yesterday the orbits of
> UOSAT11,PCSAT&STARSHINE3 have overlapped ,even with a beam it has been
> extremely difficult to sort out one lot of data from another.
> If the new sats had been offset by say+-5khz from 145.825Mhz or so these
> clashes would not have happened I think! So could all satellite constructors
> take note and try and coordinate via AMSAT for example.
> regards Robert G8ATE

To discriminate multiple FM downlinks you need at least 20 KHz separation.
Since there are only 10 such channels in the entire 145.8 to 145.999
satellite subband it is necessary for satellites to share frequencies.
Although you will occassionally see two such satellites in view at the
same time, the probabilities of collision are extremely small:

PCsat to UO-11               0.6%
PCsat to Starshine           0.6%
Starshine to UO-11           0.06%
Starshine to UO-11           0.06%
UO-11 to PCsat or Starshine  3%

And if you include the use of a beam and the FM capture effect, the
potential for interference is even less.  Yes, you may hear
interference on a few orbits every few months, but out of the 1350 orbits
since launch, its a very small problem compared to the advantages of
saving spectrum...

WB4APR@amsat.org, Bob

PCsat WEB  page     http://www.ew.usna.edu/~bruninga/pcsat.html
ISS-APRS FAQ:       http://www.ew.usna.edu/~bruninga/iss-faq.html
CUBESAT Designs     http://www.ew.usna.edu/~bruninga/cubesat.html
APRS SATELLITES     http://www.ew.usna.edu/~bruninga/astars.html


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