[amsat-bb] Re: P3E transponder and launch

Tony Langdon vk3jed at gmail.com
Fri Mar 23 13:31:49 PST 2007

At 11:28 PM 3/23/2007, Patrick McGrane wrote:
>Greetings from pat n2oeq
>I have been looking forward to the launch of P3E which I thought was 
>going to be this year.
>I am concerned about the germans use of the SDR technology over 
>william Liejanier's transponder design.
>I am of the belief that signal processing presents a loss in the 
>path, please correct me if I'm wrong.

Why would it be a loss?  SDR represents an opportunity to improve the 
passband characteristics, and increase the dynamic range of the 
transponder, as well as make other tweaks.  Depending on the design, 
there is also the potential to upload new passband characteristics 
and tweak the transponder while it's in space (sort of the equivalent 
to hitching a ride on a rocket and making a few tweaks :) ).

>Williams Hamsat transponder is excellant and made me very interested in P3E.
>Besides making the voice unnatural, are other modes allowed through 
>the SDR transponder?

Why would a SDR make voice unnatural?  All the SDR is doing is 
replacing the traditional analog filters with digital ones, which 
gives much steeper skirts.  The DSP also offers the opportunity to 
deal with alligators and maybe even dynamically share the transponder 
power equally between all stations.

>Does the Helaps system compensate for the loss?

HELAPS is a high efficiency linear PA.  Needed, regardless of whether 
the transponder is analog or digital.

>I'm of the Keep it simple discipline as once its up there nothing 
>can be done to fix it.

On the other hand, what the system offers is the potential to 
dynamically configure the passband.  Got non ham QRM sucking power 
from part of the passband?  Just notch it out, for example.

While it is good to have old style analog transponders available as a 
standby, we do need to look to the future and embrace new 
technology.  What is the point of spending millions and occupying our 
bands to put up another satellite with 1970's RF technology, when 
there are experiments we can fly using the latest technology?  I have 
nothing against a tried and true analog transponder as a backup (if 
space and weight constraints permit, that's prudent), but I do like 
to see new technology used as well.

73 de VK3JED

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