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Author Topic:   Aircraft AIS

jimh posted 01-01-2014 09:45 AM ET (US)
In the current issue of the radio amateur journal QST, there is an interesting article about automatic identification systems for aircraft and how to build a receiver for monitoring them. I mention this for two reasons:
--automatic identification for aircraft is similar to the marine AIS, but actually seems to be lagging behind in deployment; and
--the resources needed to receive the aircraft signals appear to be still in the experimenter's realm, while inexpensive marine AIS receivers are now common.
The aircraft system similar to AIS is called ADS-B. That acronym stands for Automatic Dependent Surveillance-Broadcast. ADS-B is a radio broadcast transmission from an aircraft that contains the present position of the aircraft as determined by its own GNSS receiver. It also transmits its identification and other flight parameters. This is precisely analogous to Marine AIS, in which a vessel broadcasts its position as determined by its GNSS receiver, its MMSI, and data about its course and speed.
Each aircraft transmits its data, called ADS-B OUT. Aircraft also have receivers to listen for other aircraft's transmission that they will receive directly without relay. This is called ADS-B IN.
The aircraft data is transmitted in the 1,090-MHz band; marine AIS is sent on 162-MHz. Because aircraft are typically operating at altitudes of more than 5,000-feet, the range of reception for a ground station can be rather wide. For example, an aircraft at 20,000-feet will have a radio horizon of 200-miles. This suggests that a ground station could be able to directly receive ADS-B transmission from aircraft from 200-miles or more distant.
The QST article shows how to construct a homebrew ADS-B receiver by using a software defined radio technique and a very low-cost digital video broadcast terrain (DVB-T) receiver with USB interface, originally intended as a television receiver for digital television in Europe. These DVB-T USB sticks sell for as little as $15. By using software-defined radio (SDR) techniques, the article shows how the digital TV receiver can be converted into an aircraft ADS-B receiver.
If you are an ARRL member and have registered for digital access to QST magazine, the article can be read on-line from
http://www.nxtbook.com/nxtbooks/arrl/qst_201401/index.php#/40
The whole notion of a software defined radio is a rather amazing concept in modern radio communications. As someone who grew up tuning a general coverage receiver that had a dozen or more vacuum tubes and weighed about 70-lbs, the concept of a radio on a USB stick is still amazing. That you can buy one for $15 and use it for a purpose completely different from its intended use is similarly amazing.
Regarding the implementation of the ADS-B broadcasts from aircraft, it seems like this is a bit slow in coming. The date for mandatory carriage of ADS-B is still in the future, in 2020. Marine AIS has been madatory for about a decade already, and the scope of vessels that may be required to have one is planned to increase.
Of course, you need a bit more than the $15 USB radio stick to get this to work. A modern computer with considerable data processing power is also needed to implement the ADS-B receiver. And a reasonably good antenna for the 1,090-MHz band is also required.
I don't know if there exists a large group of plane watchers that would be analogous to ship watchers or boat nerds, as they are sometimes called. In the Marine AIS realm we already have relatively low-cost, under $200 receivers that are dedicated to receiving Marine AIS transmissions. It will be interesting (for me, at least) to see if low-cost ADS-B receivers are produced for a hobbyist market for aircraft. I think a real ADS-B device, that is, one that could satisfy all the FAA regulations for use on an aircraft, will likely be much more expensive, probably at least one or two orders of magnitude more costly.
jimh posted 01-01-2014 09:57 AM ET (US)
For an example of a relatively inexpensive and portable Marine AIS monitoring system, see my prior article at
http://continuouswave.com/whaler/reference/AIS_ShipSpotter.html
The Marine AIS receiver shown in that article is also a software-defined radio.
dfmcintyre posted 01-01-2014 08:37 PM ET (US)
"or two orders of magnitude more costly...." You're used to the cost of marine electronics.
As someone who does deal with aircraft avionics, you're off. Try more like three to four orders.
Regards - Don
phatwhaler posted 01-02-2014 07:10 PM ET (US)
Perhaps this will end up being a more attainable "TCAS" for smaller planes.
swist posted 01-03-2014 07:00 PM ET (US)
jimh writes "Regarding the implementation of the ADS-B broadcasts from aircraft, it seems like this is a bit slow in coming. The date for mandatory carriage of ADS-B is still in the future, in 2020. Marine AIS has been madatory for about a decade already, and the scope of vessels that may be required to have one is planned to increase."
Technological innovations in aircraft systems are notoriously slow to be implemented. GPS was not commonly used for aircraft navigation (or at least not as a primary navigation system) until well after its use in other transportation segments.
Some of this is legitimate, as safety has to be an overriding concern - and acceptance testing takes time. However, there is also a huge bureaucracy in place that makes these things take a lot longer than they should.
jimh posted 01-06-2014 03:20 PM ET (US)
There does seem to be a tremendous amount of legacy technology in aircraft electronics. I think their radios are still using full-carrier amplitude modulation, or AM. This is also known as AM or Ancient Modulation.
dfmcintyre posted 01-06-2014 05:26 PM ET (US)
Yes, still AM. And I'd estimate that the cost to changeout all aircraft and ground stations throughout the globe would be in the trillions.
And for what gain (nil pun) ?
Regards - Don
jimh posted 01-07-2014 11:06 AM ET (US)
When Curtis Lemay was in charge of the Strategic Air Command branch of the USA Air Force in the late 1950's, I believe they instituted the use of what was then considered a break-through technology in channelized radio voice communication known as single-sideband suppressed carrier or SSSC or SSB. See
http://www.arrl.org/files/file/Technology/pdf/McElroy.pdf
The advantage of SSB over AM was most apparent on difficult radio path links, when fading was a problem. For air-to-ground communication that is typically on a line of sight path, there probably is not a whole lot of difference between AM and SSB, with AM probably being clearer and easier to implement by a wide margin.
A significant problem with SSB is the "Donald Duck" voice quality if the receiver and transmitter are not precisely tuned to the same frequency. AM transmission and reception does not suffer from this. To get the kind of frequency stability and precision tuning need for SSB in a channelized radio service was probably not really possible in the first days of flight and aircraft radio. I can understand how they went to AM--heck FM had not really been invented at the time of manned flight.
Ships tended to rely on radio-telegraphy for communication due to the typical long distance between home base and the ship. I don't know when the VHF Marine Radio service began to use FM. It might be a good topic for a historical perspective.
dfmcintyre posted 01-07-2014 11:47 AM ET (US)
Jim -
I'm going out on the limb, and guess that the switch from AM to FM started mid to late 60's from what I recall from cruising with the folks, and the equipment on board.
One of the items thar always puzzled me was the instance of introducing "channels" vs just using the frequency number in the raw. Avaition has been using the raw frequency for, oh, four or five decades, without much confusion...
Don
Chuck Tribolet posted 01-07-2014 03:17 PM ET (US)
JimH: If you get into researching marine radio history, you should look
into TBS (Talk Between Ships) which was used to communicate
bridge-to-bridge by the US Navy during WW-II. It was 60-80MHz,
and I think FM.
Chuck
djahncke posted 01-07-2014 03:56 PM ET (US)

Jim,
If you want some info on historical use of ships radios you might check out the "Inland Marine Radio History Archive". They have alot of information on ship board radio use from the early CW to VHF.
http://www.imradioha.org/index.htm
swist posted 01-08-2014 09:32 AM ET (US)
ATC cumbersome use of frequencies probably does not bother most commercial traffic, where for the most part they know in advance who they will be talking to and when and what their frequency is.
But you do hear occasional mistakes being made if you listen to ATC chatter.
The nice part of using channel numbers is that they are "virtual" and the underlying transport frequencies could be changed (for example to accommodate new technology) without impacting the channel numbers.
jimh posted 01-08-2014 10:20 AM ET (US)
Chuck's citing of Talk-between-ships or TBS was mentioned in a book I am presently reading, Engineers of Victory, which analyzes certain battles in World War II with regard to how engineering solved problems that lead to victory. The ability to have VHF voice transmission between warships escorting a convoy was cited as an advantage. If high-frequency (HF) transmission were used, it would have been more easily detected.
Earlier marine radio was mostly at medium frequency, 2,000-Kcs or kilocycles, as they used to say, and was amplitude-modulated full-carrier radiotelephony. It was only recently--actually in August 2013--that the USA Coast Guard said they were going to stop keeping a radio watch on 2,182-kHz, the equivalent of our present VHF Channel-16.

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Author	Topic: Aircraft AIS
jimh	posted 01-01-2014 09:45 AM ET (US) In the current issue of the radio amateur journal QST, there is an interesting article about automatic identification systems for aircraft and how to build a receiver for monitoring them. I mention this for two reasons: --automatic identification for aircraft is similar to the marine AIS, but actually seems to be lagging behind in deployment; and --the resources needed to receive the aircraft signals appear to be still in the experimenter's realm, while inexpensive marine AIS receivers are now common. The aircraft system similar to AIS is called ADS-B. That acronym stands for Automatic Dependent Surveillance-Broadcast. ADS-B is a radio broadcast transmission from an aircraft that contains the present position of the aircraft as determined by its own GNSS receiver. It also transmits its identification and other flight parameters. This is precisely analogous to Marine AIS, in which a vessel broadcasts its position as determined by its GNSS receiver, its MMSI, and data about its course and speed. Each aircraft transmits its data, called ADS-B OUT. Aircraft also have receivers to listen for other aircraft's transmission that they will receive directly without relay. This is called ADS-B IN. The aircraft data is transmitted in the 1,090-MHz band; marine AIS is sent on 162-MHz. Because aircraft are typically operating at altitudes of more than 5,000-feet, the range of reception for a ground station can be rather wide. For example, an aircraft at 20,000-feet will have a radio horizon of 200-miles. This suggests that a ground station could be able to directly receive ADS-B transmission from aircraft from 200-miles or more distant. The QST article shows how to construct a homebrew ADS-B receiver by using a software defined radio technique and a very low-cost digital video broadcast terrain (DVB-T) receiver with USB interface, originally intended as a television receiver for digital television in Europe. These DVB-T USB sticks sell for as little as $15. By using software-defined radio (SDR) techniques, the article shows how the digital TV receiver can be converted into an aircraft ADS-B receiver. If you are an ARRL member and have registered for digital access to QST magazine, the article can be read on-line from http://www.nxtbook.com/nxtbooks/arrl/qst_201401/index.php#/40 The whole notion of a software defined radio is a rather amazing concept in modern radio communications. As someone who grew up tuning a general coverage receiver that had a dozen or more vacuum tubes and weighed about 70-lbs, the concept of a radio on a USB stick is still amazing. That you can buy one for $15 and use it for a purpose completely different from its intended use is similarly amazing. Regarding the implementation of the ADS-B broadcasts from aircraft, it seems like this is a bit slow in coming. The date for mandatory carriage of ADS-B is still in the future, in 2020. Marine AIS has been madatory for about a decade already, and the scope of vessels that may be required to have one is planned to increase. Of course, you need a bit more than the $15 USB radio stick to get this to work. A modern computer with considerable data processing power is also needed to implement the ADS-B receiver. And a reasonably good antenna for the 1,090-MHz band is also required. I don't know if there exists a large group of plane watchers that would be analogous to ship watchers or boat nerds, as they are sometimes called. In the Marine AIS realm we already have relatively low-cost, under $200 receivers that are dedicated to receiving Marine AIS transmissions. It will be interesting (for me, at least) to see if low-cost ADS-B receivers are produced for a hobbyist market for aircraft. I think a real ADS-B device, that is, one that could satisfy all the FAA regulations for use on an aircraft, will likely be much more expensive, probably at least one or two orders of magnitude more costly.
jimh	posted 01-01-2014 09:57 AM ET (US) For an example of a relatively inexpensive and portable Marine AIS monitoring system, see my prior article at http://continuouswave.com/whaler/reference/AIS_ShipSpotter.html The Marine AIS receiver shown in that article is also a software-defined radio.
dfmcintyre	posted 01-01-2014 08:37 PM ET (US) "or two orders of magnitude more costly...." You're used to the cost of marine electronics. As someone who does deal with aircraft avionics, you're off. Try more like three to four orders. Regards - Don
phatwhaler	posted 01-02-2014 07:10 PM ET (US) Perhaps this will end up being a more attainable "TCAS" for smaller planes.
swist	posted 01-03-2014 07:00 PM ET (US) jimh writes "Regarding the implementation of the ADS-B broadcasts from aircraft, it seems like this is a bit slow in coming. The date for mandatory carriage of ADS-B is still in the future, in 2020. Marine AIS has been madatory for about a decade already, and the scope of vessels that may be required to have one is planned to increase." Technological innovations in aircraft systems are notoriously slow to be implemented. GPS was not commonly used for aircraft navigation (or at least not as a primary navigation system) until well after its use in other transportation segments. Some of this is legitimate, as safety has to be an overriding concern - and acceptance testing takes time. However, there is also a huge bureaucracy in place that makes these things take a lot longer than they should.
jimh	posted 01-06-2014 03:20 PM ET (US) There does seem to be a tremendous amount of legacy technology in aircraft electronics. I think their radios are still using full-carrier amplitude modulation, or AM. This is also known as AM or Ancient Modulation.
dfmcintyre	posted 01-06-2014 05:26 PM ET (US) Yes, still AM. And I'd estimate that the cost to changeout all aircraft and ground stations throughout the globe would be in the trillions. And for what gain (nil pun) ? Regards - Don
jimh	posted 01-07-2014 11:06 AM ET (US) When Curtis Lemay was in charge of the Strategic Air Command branch of the USA Air Force in the late 1950's, I believe they instituted the use of what was then considered a break-through technology in channelized radio voice communication known as single-sideband suppressed carrier or SSSC or SSB. See http://www.arrl.org/files/file/Technology/pdf/McElroy.pdf The advantage of SSB over AM was most apparent on difficult radio path links, when fading was a problem. For air-to-ground communication that is typically on a line of sight path, there probably is not a whole lot of difference between AM and SSB, with AM probably being clearer and easier to implement by a wide margin. A significant problem with SSB is the "Donald Duck" voice quality if the receiver and transmitter are not precisely tuned to the same frequency. AM transmission and reception does not suffer from this. To get the kind of frequency stability and precision tuning need for SSB in a channelized radio service was probably not really possible in the first days of flight and aircraft radio. I can understand how they went to AM--heck FM had not really been invented at the time of manned flight. Ships tended to rely on radio-telegraphy for communication due to the typical long distance between home base and the ship. I don't know when the VHF Marine Radio service began to use FM. It might be a good topic for a historical perspective.
dfmcintyre	posted 01-07-2014 11:47 AM ET (US) Jim - I'm going out on the limb, and guess that the switch from AM to FM started mid to late 60's from what I recall from cruising with the folks, and the equipment on board. One of the items thar always puzzled me was the instance of introducing "channels" vs just using the frequency number in the raw. Avaition has been using the raw frequency for, oh, four or five decades, without much confusion... Don
Chuck Tribolet	posted 01-07-2014 03:17 PM ET (US) JimH: If you get into researching marine radio history, you should look into TBS (Talk Between Ships) which was used to communicate bridge-to-bridge by the US Navy during WW-II. It was 60-80MHz, and I think FM. Chuck
djahncke	posted 01-07-2014 03:56 PM ET (US) Jim, If you want some info on historical use of ships radios you might check out the "Inland Marine Radio History Archive". They have alot of information on ship board radio use from the early CW to VHF. http://www.imradioha.org/index.htm
swist	posted 01-08-2014 09:32 AM ET (US) ATC cumbersome use of frequencies probably does not bother most commercial traffic, where for the most part they know in advance who they will be talking to and when and what their frequency is. But you do hear occasional mistakes being made if you listen to ATC chatter. The nice part of using channel numbers is that they are "virtual" and the underlying transport frequencies could be changed (for example to accommodate new technology) without impacting the channel numbers.
jimh	posted 01-08-2014 10:20 AM ET (US) Chuck's citing of Talk-between-ships or TBS was mentioned in a book I am presently reading, Engineers of Victory, which analyzes certain battles in World War II with regard to how engineering solved problems that lead to victory. The ability to have VHF voice transmission between warships escorting a convoy was cited as an advantage. If high-frequency (HF) transmission were used, it would have been more easily detected. Earlier marine radio was mostly at medium frequency, 2,000-Kcs or kilocycles, as they used to say, and was amplitude-modulated full-carrier radiotelephony. It was only recently--actually in August 2013--that the USA Coast Guard said they were going to stop keeping a radio watch on 2,182-kHz, the equivalent of our present VHF Channel-16.