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@mara.org To: "Jason White" jason@jason.white.name Cc: "Amsat BB" amsat-bb@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@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...
ยง97.1 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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