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# THoughts on minimum ERP...

• Subject: [amsat-bb] THoughts on minimum ERP...
• From: Bob Bruninga <bruninga@xxxxxxxxxxxxx>
• Date: Wed, 28 Jul 1999 18:10:33 -0400 (EDT)

> The Minimum Power required to work the Mir 2-meter PMS is a  very
> elusive calculation....  the most important variable is, How many people
> are calling at the same time.  If the 2-meter channel is perfectly
> clear, and there is NO other traffic on the Channel... ... Then you can
> probably come up with an ERP rating of much less than one (1)  watt.

Yes, it calculates to just about 1 watt horizon-to-horizon.  But that does
tell the user that *in the absence of QRM*, 1 watt is the Link budget.

> Since we are dealing with an FM signal, the signal you need to
> transmit needs to be approximately 10 times stronger than the average
> noise level being  heard at the FM satellite

[which in most cases is the sum of all the other simultaneous users]

> I prefer to base my calculation on experience and the term (Reliable
> Communication).  A one watt HT can ping Mir on a Good day.  However this
> is mostly luck and should be considered Un-reliable communications.
> A 45 watt ERP station will have Reliable Communications with the Mir
> FM system more than 50% of the time.

You have made the point well, that in order for one station (the single
user logged onto the MIR PMS) must be 10 db over everyone else no matter
what that power level is.  So if 45 watts is the general recommendation,
then the user that really needs access should probably run 50W into a 10
dB beam to have 500W ERP.  I think MIR regulars typically run over 100W
into 13 dB antennas or over 2,000 Watts to achieve good reliability.  I
agree that *this is required* for reliable comunications to the single
user that must get in.  But my only point is that everything over, say the
5W level is only needed to overcome to QRM (other users).

I am not arguing with the need for these numbers.  THis is what is
required.  My request for "minimum ERP requirements" is as a benchmark for
knowing the link budget baseline based on the satellites receiver and
antenna gain in the absence of QRM.

We used this fact to our advantage in the June MIR-HT tests.  Since
everyone was running only 5W to a rubber duck, we were all equal players
and we only got digipeated when we did not collide with anyone else. (and
we were successful).  If there was no collision, then our 5W was
just as good as 50 or 500 or 2000.  BUT THE GOOD NEWS IS, that we also did
not interfere with the PMS user that was logged on and using 50 Watts.
His packets (at 50w or more) would capture the channel and his packets
were good over us 5w users.  (Of course, he was still competing with all
the other 50 watt stations).

THus, in effect, we 5w HT users were using *spare channel capacity* that
was not being used at that instant.   At the same time we were not
interfering with any normal activity in progress.  THis in effect is the
kind of "routine-use" I am suggesting as a possible solution for the
single user transponders.

Let everyone compete equally at the minimum (QRM-FREE) power level and
chase grids, ragchew, or just anything they want to do.  They will then be
the "routine" users.  THen if anyone needs to make an emergency call or to
give a priority demo, then all they have to do is go to 50 Watts.  The FM
capture effect is a perfect priority threshold.  If somehow we inculcated
into the satellite operators psyche that only the minimum published ERP
was the norm and that everyone was supposed to compete at that level,
then this would allow those with urgent, or higher priority needs to
override at any time.

I think HAMS would do this if it became standard operating practice and
everyone understood it.  A priority user would just use the word
"prioirty" so that eveyone would understand that he was using hi power.

By knowing that 1) you could do anything you wanted without criticism if
you did it at the minimum recommended ERP and 2) that you personnally,
could get in at ANY time you had to by simply going to 50W, then everyone
is a winnner.

Will it work?  Possibly...  But only if we work hard to get it accepted
as the modus operandi.  Conversly, continuing to promote FM satellites
with no rules, no minimum ERP standards and no operating plan will not.
It only leads to escallation to 2,000 watts as the only reliable uplink...

P.S.  THis dual-use worked well on packet, where the objective of the 5w
stations was only to inject their position (grid) and status while the
main PMS user did his thing.  Being digital, software on the ground
accumulated everything into good orderly fashion.

On a voice bird, it would be quite disconcerting to have multiple pieces
of converstaions, grids, and racgchews on such a hit/miss intersperesed
with "priority" users doing their thing...  But it could work... if
we changed our expectations.   I donno... just a thought..

bob, WB4APR

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