Category : Science and Education
Archive   : SATGEN-0.ZIP
Filename : SATGEN99.PRN

 
Output of file : SATGEN99.PRN contained in archive : SATGEN-0.ZIP
Satgen99 The Elliptical Orbit. A brief personal review by GM4IHJ 18Feb91

Low Earth orbits (LEO) whether circular or elliptical have limited
communications range (4000 to 6000 kms) and limited communications time
(2 to 3 mins max G to W). Mode A links 2m up 10m down are badly upset by
the daytime ionosphere and in UK that means 24 hrs/day in summer. Mode J
2m up 70cm down links are subject to IARU bandplan problems. Microwave
links Mode L 23cm up, 70cm down and Mode S 70cm up 12cm down suffer from
fierce doppler. So only mode B 70cm up 2m down is really useful for LEO.
Albeit with less than optimun DX capability and duration .Geostationary
orbiters may one day give us good DX communications but until we find a
way to pay for these very expensive systems , or get a free piggy back
ride to geostationary orbit, all debate is pointless.

This presently leaves us with the high altitude elliptical orbiter . Both
Oscar 10 and Oscar 13 were built to fly the high altitude Molniya type
orbit which hovers 40,000kms above the Arctic for two roughly 6 hour
periods each day serving most of the Northern hemisphere.The practical
results from Oscar 10/13 leave a lot to be desired. Amsat NA DL
compromised the true molniya in order to provide coverage in the Southern
hemisphere, and this and a series of accidents and failures led to
performance on the principal B mode which - as one Amsat NA BOD
member put it, "is never better than marginal", and he might have added "
depends critically on the size of your wallet". Mode J was beset by
bandplan problems though it worked well, and mode L oscar 10 had a serious
failure, while mode L oscar 13 is working as designed but requires rather
a lot of expensive gear to produce the mode L uplink. By contrast Mode S
70cm up 12 cms down has worked superbly albeit with a very restricted
operating schedule.

At present Amsat DL is organising the building of Oscar 13's successor
Phase III D, for a launch about 3 or 4 years hence. The modes of operation
from Phase III D are still under discussion, clouded in my view by an
enormous amount of Amsat member opinion that wants yet another marginal
mode B outfit. Have they really thought about the alternatives ?

For this writer a decision to go once again for the discredited mode B
would be crazy. No high altitude mode B has worked well and no designer
can suggest any way to change this situation ,because , mode B antennas
have almost no gain on the size of sat we can afford to launch. Yes mode B
is super as with Oscar 7 at low altitude, but not in a Molniya orbit.

Over this winter, this writer has been getting excellent results with a
simple UEK-13 P3C S Band convertor 2400/2402MHz to 144/146MHz made by SSB
Electronics available from Piper Comms 4 Severn Rd Chilton-Didcot OX11 0PW
connected to a home brew helix. Please take a listen to mode S before you
next ask for mode B on Phase III D. This coming summer I will be trying it
with a helical feed to both my 90cm and my 2m dishes . Will report how it
goes later.
PS. Yes mode B is still the way for LEO,but for PhaseIII its mode S and we
should try 2 or 3 day orbit patterns, instead of the compromised wandering
molniyas ( more on that later ) . Brickbats and hate mail to GM4IHJ@GB7SAN
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  3 Responses to “Category : Science and Education
Archive   : SATGEN-0.ZIP
Filename : SATGEN99.PRN

  1. Very nice! Thank you for this wonderful archive. I wonder why I found it only now. Long live the BBS file archives!

  2. This is so awesome! 😀 I’d be cool if you could download an entire archive of this at once, though.

  3. But one thing that puzzles me is the “mtswslnkmcjklsdlsbdmMICROSOFT” string. There is an article about it here. It is definitely worth a read: http://www.os2museum.com/wp/mtswslnk/