N
N_Jay
Guest
trainman111 said:
Last I looked into it, I thought it was officially on hold.
I could be wrong.
EDIT:
Looks like 2015 now.
http://atwonline.com/magazine/article.html?articleID=1183
Why AM?
- Thread starter w4rez
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Last I looked into it, I thought it was officially on hold.
I could be wrong.
EDIT:
Looks like 2015 now.
http://atwonline.com/magazine/article.html?articleID=1183
I'm in NY oramge county, but kind of far away from stewart. Hoping to catch something coming or going from there. I saw on a milair site to use 50k steps, I will change them to 25 now. I use the air service search and get lots of hits, but far enough away to catch only half the conversation. Using the 785 from my vehicle, so antenna isn't the best either. Gonna post soon on antenna forum, got old RS sat antenna on roof. It's going soon but will use the huge pipe/post there to place the proper base antennas. I'll take photos of whats there and ask how to go about it the best way. Scan conventional, 800 EDACs (NYCOMCO), want good air and milair as well. Think I'll be able to put up a couple of antennas with whats there(3 or 4" pipe abot 15 feet over roof top) now. Thanks for the correction and great info so far.
Bob
Bob
Several points on this subject but this post primarily focuses on performance and compares AM to narrowband FM.
There are multiple reason why AM remains in use for air traffic control; historic, bandwidth, performance etc. Everyone says that FM has a higher signal to noise ratio than AM.
That statement is a half truth, an urban legend that needs to be qualified. There is a vast difference in performance of each depending upon the received signal strength and FM is not always better.
The improvement in S/N ratio for FM in dB over a 100% modulated AM signal can be calculated using the formula dB = 10*Log 3*m^2, where m is the modulation index of the FM signal. The modulation index in FM is calculated by this formula m = deviation / highest mod frequency.
In a typical NFM system like a two-way radio, the deviation is 5 KHz deviation and the highest modulating frequency is 3 KHz so m = 1.667. If you plug 1.667 into the first formula the S/N ratio gain for FM over a 100% modulated AM signal is about 9.2 dB. In an FM broadcast system where the modulation index is 5, the gain is 18.75 dB!.
But there is one caveat, in order to achieve this gain in S/N ratio: all of the significant FM sidebands need to exceed a minimum signal level. (significant sideband is any whose amplitude is 1% or more of the unmodulated carrier. The amplitudes of the significant FM sidebands are not equal and some are very small. ) This is sometimes called the FM improvement threshold but more commonly called the quieting point. As the FM signal level sinks below this threshold, the S/N advantage quickly disappears because the smaller amplitude sidebands drop down below the receiver detection level. The bottom line is that when signals are strong FM will outperform AM by about 9 dB but when signals are weak there is a limit point at which AM then has a higher S/N ratio than FM.
This is because of two factors: 1. FM has multiple sidebands and all of them need to be detectable in order to have the gain in S/N ratio. 2. An FM receiver bandwidth is typically in the 15 KHz range where as an equivalent AM receiver bandwidth can be as narrow as 6 KHz. That fact by itself gives AM an approximate 4.0 dB advantage in noise reduction. This is one of the reasons why single sideband, a form of AM, works best for weak signal Dxing.
To summarize: If the FM signal exceeds the quieting point then it will typically have a higher signal to noise ratio than an equal level AM signal. As the FM signal level drops below this point, the AM signal will eventually have a higher signal to noise ratio.
Also to clarify the noise immunity of AM vs FM, there are theoretically similar limitations between the two systems. There are modulation index conditions in which AM outperforms FM in noise rejection. However in practical systems this isn't the case. Also it is generally considered that atmospheric noise is not a significant factor at VHF frequencies and above.
As far a changing to digital comm as others have mentioned. The Nexcom program is replacing FAA VHF radio equipment nationwide. These are dual-mode equipment, capable of both conventional AM and digital modulation (D8PSK). The digital mode will support both digital voice and data services. From what I have gathered, the digital modes will be first introduced in the super-high sectors because the aircraft that fly in those sectors have more modern systems are therefore more likely to be able to use the digital modes. The MDR (multimode digital radio) equipment is capable of 25 KHz and 8.33 KHz channel spacing
There is a simultaneous but unrelated project underway to replace the UHF comm radio equipment. These use conventional AM and are not digital capable and they are only capable of 25 KHz channel spacing.
Attached are links to data sheets on each.
VHF radio
http://www.acd.itt.com/pdf/CAVU_2100.pdf
UHF radio
http://www.gdc4s.com/documents/D-CM300-10-0706.pdf
D8PSK
http://spacecom.grc.nasa.gov/icnsconf/docs/2002/04/Session_B1-1_Hung.pdf
Dan
wa8vzq@yahoo.com
There are multiple reason why AM remains in use for air traffic control; historic, bandwidth, performance etc. Everyone says that FM has a higher signal to noise ratio than AM.
That statement is a half truth, an urban legend that needs to be qualified. There is a vast difference in performance of each depending upon the received signal strength and FM is not always better.
The improvement in S/N ratio for FM in dB over a 100% modulated AM signal can be calculated using the formula dB = 10*Log 3*m^2, where m is the modulation index of the FM signal. The modulation index in FM is calculated by this formula m = deviation / highest mod frequency.
In a typical NFM system like a two-way radio, the deviation is 5 KHz deviation and the highest modulating frequency is 3 KHz so m = 1.667. If you plug 1.667 into the first formula the S/N ratio gain for FM over a 100% modulated AM signal is about 9.2 dB. In an FM broadcast system where the modulation index is 5, the gain is 18.75 dB!.
But there is one caveat, in order to achieve this gain in S/N ratio: all of the significant FM sidebands need to exceed a minimum signal level. (significant sideband is any whose amplitude is 1% or more of the unmodulated carrier. The amplitudes of the significant FM sidebands are not equal and some are very small. ) This is sometimes called the FM improvement threshold but more commonly called the quieting point. As the FM signal level sinks below this threshold, the S/N advantage quickly disappears because the smaller amplitude sidebands drop down below the receiver detection level. The bottom line is that when signals are strong FM will outperform AM by about 9 dB but when signals are weak there is a limit point at which AM then has a higher S/N ratio than FM.
This is because of two factors: 1. FM has multiple sidebands and all of them need to be detectable in order to have the gain in S/N ratio. 2. An FM receiver bandwidth is typically in the 15 KHz range where as an equivalent AM receiver bandwidth can be as narrow as 6 KHz. That fact by itself gives AM an approximate 4.0 dB advantage in noise reduction. This is one of the reasons why single sideband, a form of AM, works best for weak signal Dxing.
To summarize: If the FM signal exceeds the quieting point then it will typically have a higher signal to noise ratio than an equal level AM signal. As the FM signal level drops below this point, the AM signal will eventually have a higher signal to noise ratio.
Also to clarify the noise immunity of AM vs FM, there are theoretically similar limitations between the two systems. There are modulation index conditions in which AM outperforms FM in noise rejection. However in practical systems this isn't the case. Also it is generally considered that atmospheric noise is not a significant factor at VHF frequencies and above.
As far a changing to digital comm as others have mentioned. The Nexcom program is replacing FAA VHF radio equipment nationwide. These are dual-mode equipment, capable of both conventional AM and digital modulation (D8PSK). The digital mode will support both digital voice and data services. From what I have gathered, the digital modes will be first introduced in the super-high sectors because the aircraft that fly in those sectors have more modern systems are therefore more likely to be able to use the digital modes. The MDR (multimode digital radio) equipment is capable of 25 KHz and 8.33 KHz channel spacing
There is a simultaneous but unrelated project underway to replace the UHF comm radio equipment. These use conventional AM and are not digital capable and they are only capable of 25 KHz channel spacing.
Attached are links to data sheets on each.
VHF radio
http://www.acd.itt.com/pdf/CAVU_2100.pdf
UHF radio
http://www.gdc4s.com/documents/D-CM300-10-0706.pdf
D8PSK
http://spacecom.grc.nasa.gov/icnsconf/docs/2002/04/Session_B1-1_Hung.pdf
Dan
wa8vzq@yahoo.com
w4rez said:
Jeff, you're correct. Localizers, Glide Slopes and VORs are AM modulated and they can't be converted FM because the principles of operation only work with AM. They all use a concept called space modulation which is a form of AM in which the modulation index is directly related to the position of the aircraft relative to the antenna. To give a brief example: A localizer is equally modulated with 20% 90 Hz and 20% 150 Hz signals. When the aircraft is exactly lined up on centerline it receives exactly that 20% 90 Hz & 20% 150 Hz. If it is to the right side of center line the %mod of the 150 Hz increases and that of the 90 Hz decreases. If on the other side of centerline the opposite condition occurs, 90 Hz has the larger modulation index. The indicator in the aircraft is displays the difference in modulation index between the two signals.
Just to throw out one more curve ball to everyone - there is also a form of capture effect that works with AM. It's just not talked about in general communications circles but capture effect is not limited to FM. If it didn't exist, there'd be a whole class of glide slopes that wouldn't work.
Dan
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N_Jay
Guest
wa8vzq said:
And back to my point.
Is there a reliable reference that says (or implies) that aviation radio was intentionally left on AM for other then the difficulty in conversion?
I can't think of any sources that speak directly about not replacing AM but then again, I'm not sure that there was ever any large scale study on the subject until recently. Since the majority of other aviation systems are AM, I doubt that anyone ever took changing from AM to FM as a serious proposal. Heck on some of the HF channels I still hear an occassional aircraft in the south american/latin american countries using full carrier AM for voice comms and here in the US I still hear some pilots complaining about having to buy 720 channel radio gear.
Certainly from a historic perspective, FM wasn't very practical until the late 40's, early 50's. Although there some PD's using FM in the 40's, many were still using the old 16XX KHz dispatch well into the 50's. Then there were all of the lawsuits going on bewteen Armstrong and the various mfg companies that stole his ideas. That probably didn't help a lot. By that time AM was very well entrenched in the aviation industry.
The original FM method as designed by Armstrong was very complex. His system actually generated phase modulation that was converted to FM by audio shaping. Receivers were much more complex as well; in FM even the detector needed to be aligned. You also needed more complex test equipment than with AM. So that all stacked against FM
regardless of advertised advantages.
Below are a few docs from ICAO that address current thinking. Bottom line on these is that spectrum use is one of the key limiting factors for FM. Everyone knew that from the 30's that FM needs much wider bandwidths to make significant advantage of the s/n ratio gains. In that regard as I mentioned in an earlier post, when the signals are weaker FM performs worse than AM. It's the weak signal performance that is more critical.
http://www.icao.int/icao/en/anb/meetings/anconf11/documentation/ANConf11_ip001_en.pdf
http://www.icao.int/anb/panels/acp/meetings/amcp3/item-5f.pdf
Certainly from a historic perspective, FM wasn't very practical until the late 40's, early 50's. Although there some PD's using FM in the 40's, many were still using the old 16XX KHz dispatch well into the 50's. Then there were all of the lawsuits going on bewteen Armstrong and the various mfg companies that stole his ideas. That probably didn't help a lot. By that time AM was very well entrenched in the aviation industry.
The original FM method as designed by Armstrong was very complex. His system actually generated phase modulation that was converted to FM by audio shaping. Receivers were much more complex as well; in FM even the detector needed to be aligned. You also needed more complex test equipment than with AM. So that all stacked against FM
regardless of advertised advantages.
Below are a few docs from ICAO that address current thinking. Bottom line on these is that spectrum use is one of the key limiting factors for FM. Everyone knew that from the 30's that FM needs much wider bandwidths to make significant advantage of the s/n ratio gains. In that regard as I mentioned in an earlier post, when the signals are weaker FM performs worse than AM. It's the weak signal performance that is more critical.
http://www.icao.int/icao/en/anb/meetings/anconf11/documentation/ANConf11_ip001_en.pdf
http://www.icao.int/anb/panels/acp/meetings/amcp3/item-5f.pdf
gcgrotz
Member
I would have nightmares about P25 at 525mph with multipath and phase jitter. 5.4us per mile times 9 miles per second divided by symbol rate....
Just think globally about using AM and converting some bush pilot in Africa or a South pacific island hopper. The new radio would probably cost more than his plane is worth.
Just think globally about using AM and converting some bush pilot in Africa or a South pacific island hopper. The new radio would probably cost more than his plane is worth.
N
N_Jay
Guest
gcgrotz said:
Who said anything about P25?
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