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Posted: 10/29/2004 11:53:00 PM EDT
[#1]
I would think it has something to do with reliability issues, and maybe even a time delay type of thing.
When i fly and I am under a IFR flight plan they could or need to issue an ammeded clearance or traffic advisory in a matter of seconds and sometimes a sec could me a tragidy or a reagular day. (sorry if that stuff didnt make sense causne its been a while since i have had something to drink and i had a bunch tonight |
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Posted: 10/30/2004 12:10:26 AM EDT
[#2]
Possibly because it would be waaay to hard to get the rest of the world to change over all its equipment?
Just my opinion.... |
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Posted: 10/30/2004 12:23:29 AM EDT
[#3]
Is satellite communication always slow? Seems like the amateur radio voice satellite were always pretty fast back when I was into the hobby. Not to mention, they're over the ocean, and I never hear urgent messages regarding traffic, just weather and that isn't urgent IMO, since they have weather radar and other equipment telling them what's ahead. I don't know that's why I ask. On HF, I hear the controllers having to ask more than once for what the pilot may have said. HF is very noisy and propagates differently depending on the frequency and time of day, ect...
A few sats with several freqs would be easy for everyone to talk on. Not much of an equipment change unless the frequencies are out of the air band. Just seems like todays technology would have changed how oceanic traffic was handled. |
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Posted: 10/30/2004 9:19:20 AM EDT
[#4]
plain and simple money. Bandwidth cost more and the retro fit would be very expensive.
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Posted: 10/30/2004 10:56:31 AM EDT
[#5]
I do believe transoceanic and transpolar flights use HF radio communication. Intracontinental flights use VHF.
WRT to satellites, I would imagine the Doppler shift problem with satellites would only be exacerbated with aircraft traveling 500+ mph. |
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Posted: 10/30/2004 10:58:14 AM EDT
[#6]
We have a winner!! |
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Posted: 10/30/2004 12:41:21 PM EDT
[#7]
Wooohooo! I win! I'll take the redhead.. the one with the big boobies and legs up to her neck. Yep, that one. |
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Posted: 10/30/2004 12:43:11 PM EDT
[#8]
YOU CAN'T HAVE THE RED HEAD!!!!  Of course, to be honest, she isn't MINE... I just like her lots!!  |
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Posted: 10/30/2004 1:12:10 PM EDT
[#9]
I guess GPS works by magic then? |
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Posted: 10/30/2004 1:20:04 PM EDT
[#10]
No, GPS was developed specifically for military uses, including fast moving aircraft and even faster GPS-guided missiles. Ordinary satcom gear wasn't designed to handle that.
CJ |
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Posted: 10/30/2004 1:20:48 PM EDT
[#11]
I guess Verizon Airfone works by magic over the ocean too... |
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Posted: 10/30/2004 1:21:17 PM EDT
[#12]
Well, technically, YES. There are what, 24 or 26 GPS birds operating, and to get a position, you need 2 signals, minimum. You are also fighting distance. There is nothing blocking the line of sight for the GPS signals that are above the a/c horizon so signal reception is good. Ground based signals are subject to that evil nasty propogation stuff. Generally speaking, the higher the frequency, the shorter the range. ETA: If you've ever used a GPS and looked at the signal screen, you note that the signal strength varies, even when stationary. And also remember, GPS is a one way system. Not two way comms. Also, bandwidth and signal integrity factors in. What is your GPS sample rate? Continous, once ever 2 seconds? It all plays a factor, adds weight, and costs money |
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Posted: 10/30/2004 1:22:01 PM EDT
[#13]
What does this have to do with doppler shift? |
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Posted: 10/30/2004 1:24:41 PM EDT
[#14]
The doppler shift is less when the signal source is directly above you, or high on the horizon than when it is closer to your elevation, on a 3-D plane. ETA: I take it none of you ever studied Astronomy at the collegiate level?? |
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Posted: 10/30/2004 1:32:52 PM EDT
[#15]
Actually you need only one satellite to get timing, you need 3 satellite signals to get a 2-dimentional positional fix and 4 to get a 3-dimentional location. |
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Posted: 10/30/2004 1:36:44 PM EDT
[#16]
Out of a bank of four radios and a handheld. I'd like one to work right over time. Heck, the most stable radio receiver I've got was the ADF.
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Posted: 10/30/2004 1:38:50 PM EDT
[#17]
That's cuz yer ADF is on the low end of the HF band. AND it's handy to listen to Rush when you are on a mid-day flight! 
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Posted: 10/30/2004 1:42:11 PM EDT
[#18]
You got it...heh... heh. My radios spent more time on the bench then in the air. They had so many yellow tags that the radio weighed an extra pound. 
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Posted: 10/30/2004 1:44:01 PM EDT
[#19]
Thanks for the replies.
I know that higher frequencies are less reliable (GHz range), would it not work better to use lower range frequencies like VHF or lower? Why do satellites (their engineers) always insist on using such high freqs? |
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Posted: 10/30/2004 1:44:30 PM EDT
[#20]
Gee...let me think... AIRfone...the very NAME of it suggests something, don't you think? Like for use in air travel? Could be!   |
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Posted: 10/30/2004 1:47:52 PM EDT
[#21]
Cuz higher freq's are not as subject to atmospheric interfereance caused by sun. Also, higher freq in a line of sight application requires less power and is easily directable with a small, tiny beam antenna, whereas HF antenna's are huge. What, the VLF station in Northern WI that just shut down, it's radiating element was 7 MILES LONG!!!!! |
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Posted: 10/30/2004 1:54:43 PM EDT
[#22]
Ah, good question. Besides having to stay within FCC frequency bands designated for satellite communication use, the signals of higher frequency propagate more easily (using less power) than lower frequencies carrying the same information. Also remember that frequency and wavelength are inversely proportional so your GPS, if using a lower frequency, would require a significantly longer antenna. In conclusion, 1. Because the FCC says so 2. Ease of radiation (GPS signals have to travel over 9,000 miles) and 3. smaller antennas make handheld devices more convenient Edited to say: Damn you nationwide and your cat-like reflexes -Wingnut Satellite, Wideband, Telemetry and Space Systems Technician USAF 2E131 "Geek it 'till it MHz" |
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Posted: 10/30/2004 2:03:04 PM EDT
[#23]
Sheesh, many Part 121 aircraft STILL have nothing but VOR/ADF navigation. No GPS. You think the airlines will spring for SatCom?
Then there is the STC/337 issue with retrofitting all those aircraft with new equipment. It's not like installing a new car stereo - there is lots of FAA red tape involved. The overriding factor though is $$$$. HF has worked fine all these years, why spend the money to upgrade? |
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Posted: 10/30/2004 2:09:41 PM EDT
[#24]
Yeah, folks around here think "Class 3" is expensive... try "FAA APROVED" 
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Posted: 10/30/2004 2:44:54 PM EDT
[#25]
Yeah, being in ham radio , I understand certain things about radio. You mentioned how HF antenna have to be large in most cases to work well and have gain, however, what are these aircraft using for antennas? The don't deploy some large dipole when using HF, LOL. I could just see a long wire hanging off their tail, hehe. Since the lower the frequency the larger the antenna, how do they manage to get any signal in say the 75 meter area?
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Posted: 10/30/2004 2:46:30 PM EDT
[#26]
Wow we have a lot of geniuses on here today. The question was, why don't airlines use satellite for coms. The supposed answer was "Doppler shift!" My comment was "Airfone works". Your comment "DUh! That was designed for airlines!" Wow, give yourself a big smack on your giant forehead. |
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Posted: 10/30/2004 2:51:45 PM EDT
[#27]
Plenty of aircraft DO do that... I don't know about airliners, but Navy and Air Force aircraft have done that for years. The Navy TACAMOs have to to communicate with the submarines at VLF and ELF. |
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Posted: 10/30/2004 3:05:26 PM EDT
[#28]
true, true |
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Posted: 10/30/2004 3:24:56 PM EDT
[#29]
Yep. If you are in a small plane crossing the atlantic, you will usually have a wire antenna on a reel that you lower through the floor or out the window. |
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Posted: 10/30/2004 4:38:26 PM EDT
[#30]
Sorry, maybe I had to lay it out in slightly more obvious terms. Airfone works because its system was specifically designed to compensate for the doppler shifts that can be expected with satellite equipment (any radio equipment, actually) on a modern airliner. I don't know exactly how Airfone does it. If they're operating on a lower frequency range, doppler compensation is simpler, and I have no idea what frequencies they run on. The higher the frequency, the greater the effects of any given speed-induced doppler shift. CJ |
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Posted: 10/30/2004 5:30:12 PM EDT
[#31]
No, No and NO.
1) HF comms on aircraft is simply because it's cheap and "relatively" reliable. There is no need to string an antenna out the back for normal HF. The skin of the aircraft acts as the antenna. I've talked with ground stations in Nevada while over the Med. 2) There is absolutely ZERO problem with satcom on an aircraft. Doppler has no bearing on it whatsoever. The shift on an aircraft going 500 kts is negligible. Think about it: before the advent of doppler radar, radars had no problems tracking targets, even the signal was still doppler shifted... We use satcom all the time, and they operate using a UHF radio. In the past it was only for text/data, but now it's voice. And the time lag is imperceptable. ETA: I just noticed this was post # .308 |
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Posted: 10/30/2004 5:36:42 PM EDT
[#32]
The costs of new tech are prohibitive due to the fucking FAA
Fucked Again and Again |
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Posted: 10/30/2004 5:43:35 PM EDT
[#33]
I know a retired Air Forc e radio tech. He told me of them using an antenna 10K feet in length they would let out the back of a C-130. They were experimenting in the 20 to 50 Hz range. Talk about a long wave ...
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