[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Differences between ISS and normal packet.



BIG DIFFERENCES BETWEEN ISS and TERRESTRIAL PACKET              de WB4APR

Regarding performance of your ISS packet station on an OMNI antenna, there
are many factors that make this totally different from Terrestrial packet
and why many plug-n-play packet set ups will not work well with ISS:

RANGE:  ISS is from 400 to 2000km away at ALL times.
1) Everyone, everywhere gets the same "best" signal (within 10 dB)
2) There will be fades much greater, but no one does any better
3) Conversly, each station on your local BBS, gets orders of magnitude
different signals (30 to 60dB, thats 1000 to 1,000,000 times stronger)
depending on where they live...  Ranges are 1 to 100 miles...

LESSON:  Saying "it works" on a local BBS is a meaningless measure.

QUALITY:  90% of packet signals on the air are poor to marginal.
1) Most packet users just plug-n-play to speaker and Mic
2) If it works to the closest BBS, to them, "it works".
3) Most did not adjust XMIT Audio to be below compression in the MIC
circuits, nor check or adjust deviation, nor did they adjust the
equalization in the TNC.
4) Most have not checked their rigs for frequency accuracy in years.

LESSON:  A setup to where "it works", might still have performance that
is 20dB or more worse than it COULD BE.

DOPPLER:  The ISS has over 6 KHz of Doppler during the pass.
1) On the ground, the best deviation for the best performance will
   just fill the passband with signal
2) This same signal cannot tolerate any frequency offsets or the
   signal distorts as it goes out of the passband.
3) From ISS, the signal is out of the center 2 Khz of your
   pass band 90% of the TIME!
4) Modern rigs have very tight filters.  Good for ground, Bad for Space.

LESSON:  1200 baud SPACE links on 2 meters must use reduced deviation
levels so that even with 3 KHz of frequency offset, ALL of the modulation
engergy is still within the passband.... OR Tune your rig in 1 Khz steps.
Unfortunately, it sounds to me like the ISS downlink has a pretty "normal"
deviation meaning you may have to tune up or down 1 or 2 KHz to get a good
packet in your modern tight filter radio...  Most FM rigs wont tune such
small steps.

EQUALIZATION:    The age old controversy of FM pre/de-emphasis.
1) TNC's connected to the Speaker/Mic produce a packet signal that is
pre-emphasized and similarly demphasized on receipt. 
2) TNC's connected to the Discriminator and Modulator produce equal
level tones and the tones are independent of the speech processing
circuits.  

LESSON:  Attempts to communicate between systems set up by #1 or #2 will
always suffer a 6 dB skew in their tones (and performance with noise 
will be much worse.  I have not yet had a chance to look at my
discriminator output on a scope when ISS was in view, but I suspect it is
optimized for plug-n-play (spkr/mic) operation.

CONGESTION:  Too many people
1) The number of users you hear direct might be say 1 (within 20 miles)
2) Raise the ISS to 400km high and the same density of users equates
   to 10,000 other folks transmitting at the same time to ISS.
3) The ISS is half duplex during a 10 min pass so the ALOHA channel
   capacity can optimally support less than 100 successful packets per
   pass.

LESSON:  Your objective for success should be to get one packet through.
Later we can improve the throughput once we get "everyone" organized...

WHAT CAN YOU DO:
1)  Stop transmitting when you have one success.  Let someone else try
2)  Reduce your attempts to randomly once evey 2 minutes or so.
3)  Reduce your TNC audio level below speech circuit clipping
4)  Reduce your deviation noticibly below terrestrial packet norms
5)  Be sure your rig is accurate within 1 KHz of the exact freq
6)  TUne +/- 3 KHz at start and end of pass
7)  Minimize COAX loss.

ETC.  THink of this as the good opportunity to set up your packet station
properly.  Everyone gets the same signal on average.  If you aren't seeing
what others are seeing, fix it...  If ISS isnt seeing you, ANY of the
above could be wrong...

de WB4APR, Bob
  

----
Via the sarex mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe sarex" to Majordomo@amsat.org



AMSAT Top AMSAT Home