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Re: lower frequency downlinks



Pat:
Thank you for your note.  As the Eagle Project Manager, I will attempt 
to respond from the perspective of Eagle; I do NOT speak for the AMSAT 
Board of Directors.  Some thoughts:

1.  The Eagle design does not currently include an HF transmitter.  The 
functional requirements to which Eagle is being designed are available 
for your perusal on the AMSAT web site.

2.  The Eagle design is not fixed in stone, and is in fact evolving.  
The firmest aspect is the Functional Requirements Document.  Even that 
is subject to change as continued science and analysis tells us what is 
feasible and what is not.  The intra-spacecraft communications system is 
fairly firm, because it's been well vetted and tested and will fly on 
P3E.  Similarly, the IHU, the brains of the satellite, is well along for 
the same reasons.

3.  As you postulate, and as others wiser than me have explained, an HF 
transmitter is not practical for an HEO satellite.  Your successful HF 
downlink experiences were on LEO satellites, some with large power 
budgets.  These are very different than Eagle, a high orbiter with a 
limited power budget.  Unlike the LEOs you've worked on HF, Eagle will 
provide DX ragchew capability.

4.  You asked for the subject to be bantered about; I believe that you 
have accomplished your objective.  Good for you!

5.  The incremental difference in cost between an HF downlink and 
VHF/UHF downlink is tiny compared to the total costs of construction and 
launch, so this is not a factor in what we build. 

6.  As I indicated above, the Eagle design is not in concrete, but it 
currently does not include an HF transmitter.  As rigorous science leads 
us to a design which will succeed on orbit, I will keep your request for 
an HF transmitter in mind.  If an HF opportunity presents itself, we'll 
consider it.  Candidly, for the reasons above and articulated on the 
amsat-bb by others, I don't think it will prove practical.  
Nevertheless, YOU HAVE BEEN HEARD.

7.  As to your difficulties copying UHF downlinks with a crossed-yagi:  
This sounds like my experience during the majority of the last Field 
Day!  Despite a top-quality antenna, low loss coax, and a 
state-of-the-art preamp at the antenna, I didn't hear the birds very 
well at all.  I finally figured out that my az/el system was 20 degrees 
out of calibration in azimuth -- twice the tolerable error based on the 
gain and beamwidth of my antenna.  When I manually corrected for that, 
all was well.  I suggest you check it all -- coax loss, preamp 
performance, azimuth calibration, elevation calibration, tracking 
accuracy, currency of your keplerian data, and accuracy of the clock in 
your tracking computer/system.  Believe me, "been there, done that!"  In 
my time on satellites, I've gooned each of those factors at least once.  
Fundamentally, the system you describe should hear the UHF LEO downlinks 
LOUD.  Keep plugging away at it.

8.  In one of your notes, you refer to a "universally usable 
satellite."  I'm not sure what that means.  I will tell you that the 
FUNCTIONS to be implemented by Eagle take into account the desires 
expressed in a recent survey of AMSAT members.  As someone on the -bb 
has indicated, the desires of the builders, who seek new endeavors and 
challenges, are also taken into account.  I personally believe that the 
current functional requirements for Eagle strike a good balance.  If 
anything, the exprsessed desires of the membership have been given a 
higher priority than anything else except hard science, whose laws are 
not subject to repeal or negotiation. 

Pat, I hope that, along with others, I've answered your questions and 
concerns.  If not, please tell me what I can clarify.

I thank you for your interest in and support of Eagle.

Very 73,
Jim
wb4gcs@amsat.org

James A. Sanford, PE
Eagle Project Manager





n2oeq@netzero.net wrote:

>greetings from patrick n2oeq 32323
>
>I'm in favor of low frequency downlinks for satellites because of the lower signal or path loss, much less doppler, reception with standard or readily available equipment, and possible use by many more individuals than the super high frequencies.
>
>I enjoyed making contacts on rs-12/13, rs-15, and ao-7.
>I found HF downlinks to be much easier to use whereas I could concentrate on the actual contact.
>
>Is it possible to incorporate an HF transponder into Eagle or is the design etched in stone? Maybe the orbit is too high for such a downlink. Then perhaps, could Amsat banter about the idea of a new satellite with this in mind? I imagine the costs associated with a lower frequency satellite would be less because of the components and designs which are already proven.
>
>Anyway, this is a bulletin board so I thought I would raise the idea.
>
>Thanks and 73, pat
>
>
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