[amsat-bb] Re: Why do the amsats get more and more complex?

[John B. Stephensen]

The purpose of the SDX is to make the analog transponder better. In the 
past, the loudest signal controlled the downlink level and could hog the 
transponder. SDX allows each uplink signal to be isolated and levels 
adjusted to more equally share the downlink. The cost of the satellite is 
proportional to the power consumed by the transmitters so the less efficient 
the transmitter, the weaker the downlink. SDX also allows more efficent 
implementation of a downlink transmitter.

One reason for implementing new bands and new modes is to reduce antenna 
size and allow more people to use the satellite. People in CC&R-restricted 
properties have to negotiate with the homeowner's association to allow 
antennas and the smaller the antenna, the better. For example, in LA in the 
1990s you had a choice of a condominium with restrictions forbidding any 
external antennas and any transmitting antennas (and lots of other stuff) 
for $200,000-300,000 or a house with no restrictions for $400,000-800,000.

When P3A was designed in the late 1970's, U/V was the only practical mode. 
L/S was implemented later to reduce antenna size as ambient noise levels are 
lower in the microwave bands. It does this, but WiFi was invented afterwards 
and eliminates the advantage in many locations. The purpose of digital voice 
is to reduce the required received signal level. When combined with a 
downlink in a WiFi-less microwave band, antenna size could be reduced to the 
minimum.

AO-40 was an anomoly due to the fact that AMSAT-DL was offered a large space 
on a rocket for little money. They rushed to fill the space. P3A, AO-10, 
AO-13, P3E and Eagle are all smaller and about the same size.

73,

John
KD6OZH

----- Original Message ----- 
From: "Steve Meuse" <smeuse at mara.org>
To: "Jason White" <jason at jason.white.name>
Cc: "Amsat BB" <amsat-bb at amsat.org>
Sent: Tuesday, September 19, 2006 17:38 UTC
Subject: [amsat-bb] Re: Why do the amsats get more and more complex?


Jason White expunged (jason at jason.white.name):

> Anyway, I'm just curious why it seems that every new satellite project
> proposed seems to be bigger and more complex than the last? I keep
> hearing about exotic modes and uplink/downlink bands for P3E.. software
> defined transceivers, etc. etc. and what it looks like to me are more
> and more failure points. I understand the need to push the limits of
> technology as a justification for our very existence, but personally I
> feel like the designs are overly complicated and highly priced. I'm not
> ready to switch my earth station to SDRs, for instance.. I'm dubious
> about putting one into orbit.. then again, I'm not skilled enough to
> make those sorts of decisions.


A minor note of clarification, the Software Defined Transponder (SDX) does 
not require groundstations to run SDRs. It's a new method to implement the 
traditional linear transponder design in software.

Now, as for the bigger/better issue, I'll make some observations:

- P3E is purpose built as a technology testing platform for a very specific 
Mars mission, I don't see it as all bells and whistles. (bigger better for 
the sake of bigger better)

- The Eagle design, as it's turning out, *seems* to be much less complicated 
than a AO-40 or P3E, at least in the number of bands and functions.


Then there are the basics, like FCC Part 97 rules:

Specifically...

§97.1 Basis and purpose.

(b) Continuation and extension of the amateur's proven ability to contribute 
to the advancement of the radio art.

(c) Encouragement and improvement of the amateur service through rules which 
provide for advancing skills in both the communications and technical phases 
of the art.


> What I'm getting at is that Oscar 7 proved how reliable older technology
> can be..  For the price of one of the  phase 3 birds  it seems like
> several Mode B linear transponder sats could be put up, or a few more FM
> sats. I personally would much rather see a modest mode B sat in AO-40s
> intended orbital pattern than to try to wrangle parts for microwave.

I've talked about this before, but it's important to remember that AMSAT is 
not a serivce provider. We don't exist to provide a continual supply of mode 
X transponders in Y orbit. We're here to advance the state of the art, IMHO.

(of course, I still hold the right to throw a temper tantrum over mode S 
downlinks)

> Wouldn't it be better to separate out some of the more experimental
> stuff from the old standbys? That way a failure of one whole sat would
> still leave something usable for the same money spent.  My vote would be
> to piggyback a completely independent analog satellite onto P3E "just in
> case".

If you want to build an "old standby" sat, go for it!

The people who are actively building sats aren't interested in that. I don't 
blame them either. But remember, AMSAT isn't stopping ANYONE from putting a 
team together to build a new sat.

> If someone could help me understand why the direction is the way it is
> maybe I could get excited about the bigger sats, but I think you get
> more "bang for the buck" with the smaller less complicated birds. My
> favorite so far is PCSat I. Mostly off the shelf hardware and I had a
> very easy time digipeating APRS through it. One of those in an
> elliptical orbit would be a hoot!


Of course, you would likely need a more robust equipment set to work HEO :) 
It's a give and take, there isn't a perfect solution.


-Steve
N1JFU

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