[amsat-bb] S-band on Eagle
wb4gcs at amsat.org
Thu Sep 7 18:40:37 PDT 2006
Ladies and gentlemen:
I have been reading the comments regarding the Eagle S-band downlink
decision. I herewith respond to some.
All along, the Eagle project has put a premium on hard science, rigorous
peer review, and dispassionate analysis.
All along we knew there might or would be some changes to what we've
With several other issues on the move or decided, we took a few days in
late June to hammer out, based on science, the transponder situation.
We did multiple, independent link analyses, and all came to the same
conclusion. We validated interference predictions with measurements
that have been taken. The conclusion is that S-band is marginally
useful as a downlink today (unusable in many locations), and by the time
Eagle is launched, and through it's lifetime, will be unusable.
Personally, I do not like this decision. I have S-band equipment that I
was looking forward to using.
Personally, I can live with this decision, and defend it to myself, let
alone anyone else, because I am convinced that it is technically sound.
As one of you commented this evening, AO-40 wasn't exactly a rock
crusher on S-band. It worked, and adequately, at least on CW, but was
not great. As I reflect on my personal AO-40 experiences, I now
understand that there are 2 reasons for my disappointment. Local noise
was one, and transponder distortion was another -- the latter will be
corrected by SDX. S-band on AO-40, by necessity not choice, got many of
us out of our comfort zones and convinced us that microwaves really
aren't that hard. Let's enter the realm of S up and C down with that
REMEMBER THAT AMSAT IS DEVELOPING THE GROUND STATION ALONG WITH THE
You will not be left on your own to develop ground equipment. You will
not be asked to mortgage your house; affordability by the masses is a
key component of the design for the entire system.
Comparison with AO-51 is an "apples and oranges" thing. The path losses
are much different. FM is very unlike CW/SSB or wideband digital
modes. Remember that a goal of the Eagle system is that everything is
on all the time. No more matrix switches, no more schedules. We need
up and downlinks to be useful throughout the entire orbit.
Why did we "regress" to U/V? For one reason, many of you asked for it,
and some of us worked hard to somehow incorporate that capability in
response to member's desires. The foot in the door was the utility of a
V-band beacon in case everything else goes sour. The forcing function
was as follows. A goal of Eagle has always been some kind of hand-held
or jump bag portable ground station capability for entry into the
emergency area while the tsunami waters are receeding and the hurricane
winds are down to gale force. Many of us thought we could take
advantage of the gain in small antennas to do this on the microwaves.
Rigorous link analysis led us to the conclusion that the best place to
do this is U/V -- both from a RF/DC power perspective, and from a link
standpoint. This leads to the digital and "traditiona"l transponder
package for U/V, implemented in SDX.
Why not also include a (switchable) S-band downlink in parallel with the
V-band? Remember, no switches. Also, we run into an antenna space
problem. We've already increased the size of the spacecraft from where
we started, and for many reasons, do NOT want to build something as
large as AO-40.
A couple of you have commented this evening that perusing EaglePedia has
not revealed the details of the decisions. That is because, while Bob
McGwier (N4HY) and I have been working on the report of the meeting, it
is not public. We've been working hard on it since June, in and around
other AMSAT, ham radio, and life events. Shortly after I push send on
this message, I'll begin my final look at that document prior to public
Before I go spend time with the spousal unit TONIGHT, I will finish it
and publish it. I will send an announcement on the amsat-bb that it is
public. To read the report, go to the EaglePedia main page, and select
project index. From there, select Team and Meetings, and there will be
several options, not all public. The San Diego meeting minutes will be
obvious and public.
Several of you have commented, "If S-band is unusable, why is it flying
on P3E?" Fair question, but I can't answer for AMSAT-DL. I DO know
that the P3E system design is considerably older and farther along than
Eagle. Eagle design decisions were made based on the best information
in June of 2006.
Aspects of this discussion have been worthy of /dev/null. More aspects
of this discussion have raised valid points and reasonable questions,
deserving of response, which is why I've been composing this note for
the last four hours or so.
A few have raised the old allegation of the builders doing what they
want, without regard to users needs. This is not true. While the
desire to advance the state of the art (part of amateur radio's
justification for existence and allocation of valuable spectrum) leads
to enthusiasm and study of potentially useful new techniques, this
desire had no impact on our analysis-based decisions. We started with
requirements and services, and explored what would deliver.
I thank all of you for your interest and support. Your Eagle team is
working hard to develop a system that will serve our needs, as well as
advance the state of the art. As promised, we are making decisions
based on sound, peer-reviewed science, not anybody's opinions or desires.
Please read the report, study the spreadsheets, download the excel file
and play with it. If you can, make your own measurements and put those
numbers into the spreadsheet. I think you'll understand how these
decisions were reached.
After contemplating the report, send me your questions. I'll either
respond or forward to he who can best respond. Another fundamental
tenet of the Eagle project is openness, so you have a right to a
response to a rational question. Please recognize that we're volunteers
too, and subject to the realities of day jobs, families, and a real
life. Response may not be instantaneous.
Thanks again and very 73,
James A. Sanford, PE
Eagle Project Manager
wb4gcs at amsat.org
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