[amsat-bb] P3D Dreaming

[Jeff Davis <jeff@xxxxxxxxxxxxxx>]

Reading the accounts of the Mode-S downlink from UO-36 (during a pass at
the UK Colloquium) has got me dreaming of P3D.

It's been a long, long time since we've got to play with Mode S. I
used to think it was quite a bother when AO-13 switched from Mode B,
which I was equipped for, to Mode S--even though it was only for a small
part of its orbit!

That belief changed with the acquisition of some Mode S equipment! :-)

As far back as 1992, James Miller, G3RUH was saying that Mode S should
be the *preferred* downlink for Phase III satellites. Of course, we
won't know the precise transponder schedules for P3D until it becomes
operational, but I'd venture a guess that the 2.4Ghz downlink will
quickly become popular.

A few of G3RUHs excellent points, made way back when, are worth
repeating (http://www.amsat.org/amsat/articles/g3ruh/115.txt):

1. 145 MHz is noisy.  In some places it's so noisy as to be virtually
unusable.  I don't think there is much disagreement about this.  Noise
arises from almost everything electrical you can think of.  Transport,
appliances, weather, Sun, sky, QRM, splatter, tele-switches, computers;
the list is endless.  And it's getting worse every year.  I haven't even
included noise of the receiver as it's usually swamped by the forgoing. 

2. If you had to choose between your huge 145 MHz downlink antenna, and a 
tiny 60cm lightweight, compact, suitcase sized, no-tune, elegant dish that 
you could genuinely hitch to a balcony; which would you have?

JM went on to say:

: So, where does that get us?  We have established that 145 MHz noise =
: 1200K,  and on 2.4 GHz  it's 120K.   That's a factor of 10x quieter at
: least. 
:
: It means that for a given spacecraft e.i.r.p., your ground station
: antenna on S-band can be 10 times smaller than for the 2 metre band. 
:
: Why?  Because less noise means less signal needed. Received signal
: strength is directly proportional to the capture area of the antenna.
: Capture area is proportional to physical size.
:
: That "10 times smaller" means less mechanical engineering, less windage,
: less cost,  less maintenance, less environmental impact, greater
: portability and so on and on.

Which is precisely why Chris Jackson, G7UPN said of the UO-36 demo:

> The demo was perfect. At AOS the satellite was behind the lecture
> theatre but pointing to the building opposite the lecture entrance the signal
> was very strong and fully quieting on reflection with a BW of 35 kHz
> receiver. 
> It was easy to find the signal as it was so strong. 
> Later the signal was end stopping the s-meter when overhead and very
> steady. 
> So it looks like the power output of the satellite is much too strong
> for a normal reception on a normal antenna. 
> A 10 dB reduction would give the same results. 
> The audience was of course very impressed with the strong signal. 

And Peter Guelzow, DB2OS added:

> Freddy's S-Band Downlink Demo was a real "eye-opener" for all. 
> It gave a pretty good idea what we can expect to see on P3-D 
> and Freddy said that the dish antenna he demonstrated was already 
> "overkill".. Many people understood why P3-D will be the "Easy-Sat".. 

And that, mind you, was with a whopping 1 Watt down from UO-36 with the
antenna off-pointing by several degrees!

When I think back to the glory days of AO-13 I recall hour-long QSOs with 
stations on other continents, as the bird neared apogee the antennas barely
had to move to track the satellite. Doppler was nil for seemingly long
periods of time. Weekly Nets with worldwide check-ins...

Man, I just keep dreaming about P3D! :-)

73,
-- 
jeff davis, n9avg
http://n9avg.org/

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