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Old 12-03-2009, 10:59   #1
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Iridium questions

Hello,

I'm involved in a project that uses Iridium for communication, and I have some questions regarding its services:

1# How long does it takes to acquire a stable signal suitable for data transmission?

2# Above the 40 Km of altitude will I be able to receive the signal properly? AFAIK Iridium satellites orbit at 35Km of altitude and use directional antennas pointed to the ground land.

3# Iridium claim its data transmission is 2.4 Kbps, does this really match with the real data transmission rates?

4# Is it worthless to compress the email before sending it? Or its not necessary because Iridium compress data by itself.

5# How long should I take in order to send a 72 KB email?


Thank you in advance!
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Old 12-03-2009, 11:28   #2
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#5
2.4Kbps is 2,400 bits per second - with 10 bits per byte - (8 data bits and a start and a stop bit)
That's 240 bytes per second
72,000 bytes/240 bytes/second = 300 seconds / 60 seconds/minute = 5 minutes.

This is if you get the full throughput. 2.4 Kbps is a very slow speed speed even for dialup.

I'm sure someone will correct me as I may have been retired too long to do the calculations correctly. :>)
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Old 12-03-2009, 12:23   #3
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I use an Iridium for data transmission and the 2400 Baud rate posted is almost illusory; with protocol overhead and dropped packets the effective rate of transmission is closer to 1200 Baud or less. The compression software that Iridium supplies doesn't work with Vista systems, but the internal modem is a Hayes compatible one that supports hardware compression automatically and gets an effective rate of about 50% compression with text e-mails.

I have spent 10 minutes online to send 100Kb of pure text e-mails to my POP3 server.

Above 40KM of altitude? That would be flight level 13122 - what are you doing in space? Iridium functions at airline flight levels, but the software has a tough time compensating for the doppler frequency shifts for a fast-moving object; it has enough work to do computing shifts for stationary phones.
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Old 12-03-2009, 12:24   #4
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Curious, what is this project that has you at 40Km altitude?
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Old 12-03-2009, 13:12   #5
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Iridium Satellites at 780 KM not 35KM

For question 2, the iridium satellites orbit at 485 miles or 780 KM above the earth, and according to this link below they are used in the International Space Station(319 KM to 346 KM), so they should work 40 KM I would think.

http://iridium.mediaroom.com/file.ph...8.pdfFINAL.pdf

edit: Guess if I read the doc I would see the cosmonauts use them AFTER they land. Duh. Sorry.
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Old 12-03-2009, 14:00   #6
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I have read that Iridium have their own server. would it be faster to use their own server? and how long would it take to down load a GRIB files apx 300kb using their server or another server?
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Old 12-03-2009, 16:43   #7
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For email over satphone: check out UUplus.com. The subscription fee will be quickly saved on connection charges.

So yes, it is much better to send compressed data which is exactly what UUplus does for your email. It actually uses the good old Unix UUCP protocols with ZIP compression/decompression.

hanschristian38: absolutely correct, I am impressed with your math ;-) It would be 11 bits per byte if a parity bit is used but I don't think it is.

cheers,
Nick.
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Old 12-03-2009, 19:36   #8
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There's also XGate compression from GMN-COMPRESSED WIRELESS SATELLITE EMAIL, WEATHER, VESSEL TRACKING and at least one other provider compatible with Iridium...
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