ADS-B, ADS-C from the ATC point of view
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ADS-B, ADS-C from the ATC point of view
Hi all,
As an airline pilot, I only know things by reading the FCOM and picture it with my own imagination when it is not related to the aircraft.
Sometimes it is like the blind drawing an elephant by touching a part of its huge figure, then the picture comes out blurry.
It is about ADS-B and ADS-C.
The things that I 'think' I know
1. ADS-B uses XPNDR while C uses ATSU ( i.e. FMS via HF, VHF, or SATCOM)
2. ADS-B needs a special receiver to see the aircraft while C needs link to ATS service provider who provides data received from thier own antennas(HF, VHF or SAT)
The things that I don't know
1. How it is displayed on their screens. Are they use different screens for B and C?
2. For ADS-B, it sends aircraft information like selected ALT, HDG, SPD according to FCOM. Are they really able to see those to monitor how pilots are complying their instruction? If that is true, how it is displayed the MANAGED Value (i.e. dashed value on Airbus FCU)
3. Does a waypoint name matter on the FMS? ADS-C sends data from the FMS with other data. Let's take a random route with half degree coordinate for example. The name of the waypoints on the FMS are normally 5 digit and there's universal agreement how to code the LAT/LONG coordinates into 5 digit. However when the half degree is on the Latitude, there's no such an agreement as far as I know. In this case, can I simply name it any 5 digit or just leave it to FMS default value which is 7 digit without a hint of half degree coordinate (i.e. when insert N0500.0/W13330.0, FMS names it 05N133W).
I hope someone could kindly explain. Thank you in advance.
Cheers
As an airline pilot, I only know things by reading the FCOM and picture it with my own imagination when it is not related to the aircraft.
Sometimes it is like the blind drawing an elephant by touching a part of its huge figure, then the picture comes out blurry.
It is about ADS-B and ADS-C.
The things that I 'think' I know
1. ADS-B uses XPNDR while C uses ATSU ( i.e. FMS via HF, VHF, or SATCOM)
2. ADS-B needs a special receiver to see the aircraft while C needs link to ATS service provider who provides data received from thier own antennas(HF, VHF or SAT)
The things that I don't know
1. How it is displayed on their screens. Are they use different screens for B and C?
2. For ADS-B, it sends aircraft information like selected ALT, HDG, SPD according to FCOM. Are they really able to see those to monitor how pilots are complying their instruction? If that is true, how it is displayed the MANAGED Value (i.e. dashed value on Airbus FCU)
3. Does a waypoint name matter on the FMS? ADS-C sends data from the FMS with other data. Let's take a random route with half degree coordinate for example. The name of the waypoints on the FMS are normally 5 digit and there's universal agreement how to code the LAT/LONG coordinates into 5 digit. However when the half degree is on the Latitude, there's no such an agreement as far as I know. In this case, can I simply name it any 5 digit or just leave it to FMS default value which is 7 digit without a hint of half degree coordinate (i.e. when insert N0500.0/W13330.0, FMS names it 05N133W).
I hope someone could kindly explain. Thank you in advance.
Cheers
Last edited by ABUSMAN; 6th Mar 2023 at 08:57.
Air Traffic Controllers are invariably at the bottom of the food chain when it comes to the latest equipment; just because you might have it in your aircraft doesn't mean to say all controllers can receive and display the required information.
Not many even have Mode S displays.
Not many even have Mode S displays.
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Complex questions and I’m sure lots of folk on here will know much more technical detail, but here goes ….
1. Ads-B is ‘ATS Surveillance’ and each ATM surveillance system has ways to display aircraft ‘targets’ on the situation display. Frequently what’s displayed may comprise aggregated surveillance data from a number of sources and types (e.g. primary, SSR, Mode-S, ADS-B, and from many discreet sources) with the display symbols indicating quickly to the controller what types of data this being displayed to them. It’s often the case with surveillance ‘tracked’ systems that the actual source of the data is not immediately clear to the controller (though this can be interrogated by them within a few clicks), but it depends on complexity and type of operation being provided exactly what the controller is looking at (eg approach, Enroute etc.).
2. Downlinked Aircraft Parameters (DAPs) may be available for display by controllers but may not always be displayed (because of the rule set or controller preference). They are often used by the wider ATM system in the background, alerting controllers only when pre-programmed business rules are triggered (e.g. when your selected flight level differs from the cleared flight level within the ATM system). All of the ads-b message set is clearly defined within eurocae documentation and it also sets the values and tolerances that can be accepted for each parameter. The ATM safety case sets out what steps are taken to assure that only data with an acceptable providence is used, and also what the system does when this isn’t achieved. In my experience some very clever engineers rigorously analyse this data throughout the data lifecycle (ie before it’s commissioned and routinely whilst in operation) to assure the data is fit for purpose. This is part of the overall safety case and approvals approach. I don’t know what data is used or not, as my focus is often ‘what’s available, how can I use to improve safety, reduce controller workload, and improve the service quality provided to airlines’ - you’d need an engineer for that question.
3. Yes, these matter - a lot ! In some operational areas, the ads-c contract exchanges messages at log-on and also during routine reports to download the route as held in the FMS. It compares this route with what’s held in the ATM system and alerts the controller to a risk your aircraft may go ‘the wrong way’ and that is essential as the ATM system may have predicted possible conflicts based on the planned trajectory it’s holding. It can also cause controllers to question pilots about their routing and could cause some confusion in the air (eg ‘why am I being asked about my routing, surely they have my flight plan’ etc.). On coordinate inputs, ARINC424 sets out the short code use of coordinates and the standards that should be used. In some airspaces, often oceanic and continental offshore where intervention capability isn’t as timely as vhf, separation standards are predicated on flying to the right location, and contingency procedures and things like SLOP are predicated on a consistent use of ARINC424. With such standards being as small as 15nm, failure to correctly code your waypoint could put you many miles away and potentially head-on with someone else with a very large closing speed - that’s not a good look (!). Basically, this is airmanship (for woman and men - I can’t recall the right none-gendered term) 1-0-1, if you’ve standards to comply with, understand them, be competent to apply them, cross-check your work, and remember that others are relying on your doing that (and equally, you are relying on them too).
Like I say, others will know more than me, but that’s my 2c worth, offered with best intentions, and with the hope others can improve it with their knowledge and experience. RT
1. Ads-B is ‘ATS Surveillance’ and each ATM surveillance system has ways to display aircraft ‘targets’ on the situation display. Frequently what’s displayed may comprise aggregated surveillance data from a number of sources and types (e.g. primary, SSR, Mode-S, ADS-B, and from many discreet sources) with the display symbols indicating quickly to the controller what types of data this being displayed to them. It’s often the case with surveillance ‘tracked’ systems that the actual source of the data is not immediately clear to the controller (though this can be interrogated by them within a few clicks), but it depends on complexity and type of operation being provided exactly what the controller is looking at (eg approach, Enroute etc.).
2. Downlinked Aircraft Parameters (DAPs) may be available for display by controllers but may not always be displayed (because of the rule set or controller preference). They are often used by the wider ATM system in the background, alerting controllers only when pre-programmed business rules are triggered (e.g. when your selected flight level differs from the cleared flight level within the ATM system). All of the ads-b message set is clearly defined within eurocae documentation and it also sets the values and tolerances that can be accepted for each parameter. The ATM safety case sets out what steps are taken to assure that only data with an acceptable providence is used, and also what the system does when this isn’t achieved. In my experience some very clever engineers rigorously analyse this data throughout the data lifecycle (ie before it’s commissioned and routinely whilst in operation) to assure the data is fit for purpose. This is part of the overall safety case and approvals approach. I don’t know what data is used or not, as my focus is often ‘what’s available, how can I use to improve safety, reduce controller workload, and improve the service quality provided to airlines’ - you’d need an engineer for that question.
3. Yes, these matter - a lot ! In some operational areas, the ads-c contract exchanges messages at log-on and also during routine reports to download the route as held in the FMS. It compares this route with what’s held in the ATM system and alerts the controller to a risk your aircraft may go ‘the wrong way’ and that is essential as the ATM system may have predicted possible conflicts based on the planned trajectory it’s holding. It can also cause controllers to question pilots about their routing and could cause some confusion in the air (eg ‘why am I being asked about my routing, surely they have my flight plan’ etc.). On coordinate inputs, ARINC424 sets out the short code use of coordinates and the standards that should be used. In some airspaces, often oceanic and continental offshore where intervention capability isn’t as timely as vhf, separation standards are predicated on flying to the right location, and contingency procedures and things like SLOP are predicated on a consistent use of ARINC424. With such standards being as small as 15nm, failure to correctly code your waypoint could put you many miles away and potentially head-on with someone else with a very large closing speed - that’s not a good look (!). Basically, this is airmanship (for woman and men - I can’t recall the right none-gendered term) 1-0-1, if you’ve standards to comply with, understand them, be competent to apply them, cross-check your work, and remember that others are relying on your doing that (and equally, you are relying on them too).
Like I say, others will know more than me, but that’s my 2c worth, offered with best intentions, and with the hope others can improve it with their knowledge and experience. RT
Thanks, that's a useful exposition.
One point of potential confusion:
Downlinked Aircraft Parameters (DAPs) aren't actually part of ADS-B. Both are sent via the ATC transponder, but DAPs only in response to interrogation from the ground, whereas ADS-B is broadcast ("squittered") continuously.
One point of potential confusion:
Downlinked Aircraft Parameters (DAPs) aren't actually part of ADS-B. Both are sent via the ATC transponder, but DAPs only in response to interrogation from the ground, whereas ADS-B is broadcast ("squittered") continuously.
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Thanks, that's a useful exposition.
One point of potential confusion:
Downlinked Aircraft Parameters (DAPs) aren't actually part of ADS-B. Both are sent via the ATC transponder, but DAPs only in response to interrogation from the ground, whereas ADS-B is broadcast ("squittered") continuously.
One point of potential confusion:
Downlinked Aircraft Parameters (DAPs) aren't actually part of ADS-B. Both are sent via the ATC transponder, but DAPs only in response to interrogation from the ground, whereas ADS-B is broadcast ("squittered") continuously.
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Thank you very much "Roger That" for your reply. It really helped deepening my understanding. Let me ask a bit more to clear still hanging fog in my brain.
1. To display ADS-B data of the target on the ATM screen, does it need special atenna or is it just a software upgrade of existing SSR?
2. Regarding the name of the waypoint, now I get why it does matter. ADS-C downloads what's in the FMS and compare the route with the filed route in the ATM. Fair enough. However it still doesn't answer the issue that I raised.For the NAT HLA, all coordinates that commercial aircrafts use, there're univeral 5 digit names of those using agreed coding rules as described in ARINC424. So it is not a problem as long as this is used both in the FMS and ATM system.
Problem comes when using the random route which include coordinates like N0500.0 E13530.0 (Half degree on the Longitude). It is neither a whole number nor a half degree LAT coordinate that NAT HLA uses. From a pilot point of view, we file a flight plan using a coordinate and insert this coordinate into the FMS. When inserting this coordinate, FMS fix name is created either 7 digit (05N135E) by default which doesn't show half degree or any name defined by a pilot. As a result it can't be the same between ATM(N0500.0 E13530.0) and FMS(Either 05N135E or ABCDE).
I hope my point is well delivered and any input would be highly appreciated.
1. To display ADS-B data of the target on the ATM screen, does it need special atenna or is it just a software upgrade of existing SSR?
2. Regarding the name of the waypoint, now I get why it does matter. ADS-C downloads what's in the FMS and compare the route with the filed route in the ATM. Fair enough. However it still doesn't answer the issue that I raised.For the NAT HLA, all coordinates that commercial aircrafts use, there're univeral 5 digit names of those using agreed coding rules as described in ARINC424. So it is not a problem as long as this is used both in the FMS and ATM system.
Problem comes when using the random route which include coordinates like N0500.0 E13530.0 (Half degree on the Longitude). It is neither a whole number nor a half degree LAT coordinate that NAT HLA uses. From a pilot point of view, we file a flight plan using a coordinate and insert this coordinate into the FMS. When inserting this coordinate, FMS fix name is created either 7 digit (05N135E) by default which doesn't show half degree or any name defined by a pilot. As a result it can't be the same between ATM(N0500.0 E13530.0) and FMS(Either 05N135E or ABCDE).
I hope my point is well delivered and any input would be highly appreciated.
Thank you very much "Roger That" for your reply. It really helped deepening my understanding. Let me ask a bit more to clear still hanging fog in my brain.
1. To display ADS-B data of the target on the ATM screen, does it need special atenna or is it just a software upgrade of existing SSR?
2. Regarding the name of the waypoint, now I get why it does matter. ADS-C downloads what's in the FMS and compare the route with the filed route in the ATM. Fair enough. However it still doesn't answer the issue that I raised.For the NAT HLA, all coordinates that commercial aircrafts use, there're univeral 5 digit names of those using agreed coding rules as described in ARINC424. So it is not a problem as long as this is used both in the FMS and ATM system.
Problem comes when using the random route which include coordinates like N0500.0 E13530.0 (Half degree on the Longitude). It is neither a whole number nor a half degree LAT coordinate that NAT HLA uses. From a pilot point of view, we file a flight plan using a coordinate and insert this coordinate into the FMS. When inserting this coordinate, FMS fix name is created either 7 digit (05N135E) by default which doesn't show half degree or any name defined by a pilot. As a result it can't be the same between ATM(N0500.0 E13530.0) and FMS(Either 05N135E or ABCDE).
I hope my point is well delivered and any input would be highly appreciated.
1. To display ADS-B data of the target on the ATM screen, does it need special atenna or is it just a software upgrade of existing SSR?
2. Regarding the name of the waypoint, now I get why it does matter. ADS-C downloads what's in the FMS and compare the route with the filed route in the ATM. Fair enough. However it still doesn't answer the issue that I raised.For the NAT HLA, all coordinates that commercial aircrafts use, there're univeral 5 digit names of those using agreed coding rules as described in ARINC424. So it is not a problem as long as this is used both in the FMS and ATM system.
Problem comes when using the random route which include coordinates like N0500.0 E13530.0 (Half degree on the Longitude). It is neither a whole number nor a half degree LAT coordinate that NAT HLA uses. From a pilot point of view, we file a flight plan using a coordinate and insert this coordinate into the FMS. When inserting this coordinate, FMS fix name is created either 7 digit (05N135E) by default which doesn't show half degree or any name defined by a pilot. As a result it can't be the same between ATM(N0500.0 E13530.0) and FMS(Either 05N135E or ABCDE).
I hope my point is well delivered and any input would be highly appreciated.
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Thank you very much "Roger That" for your reply. It really helped deepening my understanding. Let me ask a bit more to clear still hanging fog in my brain.
1. To display ADS-B data of the target on the ATM screen, does it need special atenna or is it just a software upgrade of existing SSR?
2. Regarding the name of the waypoint, now I get why it does matter. ADS-C downloads what's in the FMS and compare the route with the filed route in the ATM. Fair enough. However it still doesn't answer the issue that I raised.For the NAT HLA, all coordinates that commercial aircrafts use, there're univeral 5 digit names of those using agreed coding rules as described in ARINC424. So it is not a problem as long as this is used both in the FMS and ATM system.
Problem comes when using the random route which include coordinates like N0500.0 E13530.0 (Half degree on the Longitude). It is neither a whole number nor a half degree LAT coordinate that NAT HLA uses. From a pilot point of view, we file a flight plan using a coordinate and insert this coordinate into the FMS. When inserting this coordinate, FMS fix name is created either 7 digit (05N135E) by default which doesn't show half degree or any name defined by a pilot. As a result it can't be the same between ATM(N0500.0 E13530.0) and FMS(Either 05N135E or ABCDE).
I hope my point is well delivered and any input would be highly appreciated.
1. To display ADS-B data of the target on the ATM screen, does it need special atenna or is it just a software upgrade of existing SSR?
2. Regarding the name of the waypoint, now I get why it does matter. ADS-C downloads what's in the FMS and compare the route with the filed route in the ATM. Fair enough. However it still doesn't answer the issue that I raised.For the NAT HLA, all coordinates that commercial aircrafts use, there're univeral 5 digit names of those using agreed coding rules as described in ARINC424. So it is not a problem as long as this is used both in the FMS and ATM system.
Problem comes when using the random route which include coordinates like N0500.0 E13530.0 (Half degree on the Longitude). It is neither a whole number nor a half degree LAT coordinate that NAT HLA uses. From a pilot point of view, we file a flight plan using a coordinate and insert this coordinate into the FMS. When inserting this coordinate, FMS fix name is created either 7 digit (05N135E) by default which doesn't show half degree or any name defined by a pilot. As a result it can't be the same between ATM(N0500.0 E13530.0) and FMS(Either 05N135E or ABCDE).
I hope my point is well delivered and any input would be highly appreciated.
2. This is where I start to run out of steam a little as I work with some technically outstanding colleagues who just know the detail of this stuff (so I don’t need to !). Let’s do this iteratively though … have a look in https://www.icao.int/EURNAT/Pages/EU...F142A1E5BC9%7D . Specifically have a read through 2018_003(Rev01) as it contains descriptions of the interactivity using ARINC424. Bear in mind that ATC use what is flight planned and generally don’t use ARINC424 to display anything (remember ARINC424 like many things was developed as shorthand because of mobile database sizes. ATM systems don’t suffer from the same challenges as they generally have bigger memory capacities and they only need to cover their geographical areas, not over the entire route of every flight etc.).
it’s my understanding that ARINC424 reflects what is displayed on the FMS, not the actual coordinate as that can be interrogated - but that’s a pilot thing not a ground thing (which is more my thing). Your dispatch should provide a copy of the ATC flight plan so you know what we’ve got. The missing clarity is what the airborne systems do with the short code because I don’t believe when we receive data from ads-b and ads-c it’s in ARINC424 standard notation. . Again using the previous link, have a read at bulletin 2017-002_Revision 051 that contains the following:
“NOTE: In NAT airspace where unnamed, half-degree of latitude waypoints are used, waypoint display labels can be misleading (minutes can be truncated or rounded) and/or the FMC can create a generic label). It is imperative that crews check the expanded coordinates of all oceanic waypoints. Custom waypoints in aircraft navigation databases for half-degree of latitude points should use “Hxxyy”as introduced in Supplement 23 to ARINC 424 paragraph 7.2.5. See NAT OPS Bulletin 2018-003, “Waypoint Insertion/Verification Special Emphasis Items.”
Once you have a read, please ask another question and I’ll see if I can answer further. If you wish to take it offline, feel free to DM and we can find a way to have your questions answered. Meantime, hope this helps.
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Wow~I'm speechless to see your tremendous knowledge. Many thanks.
Too many information to take in, so firstly let me summarize it.
1. ADS-B data can be either picked up by ATC on the ground or by Satellite which are fed to a contracted subscribers(ATC, or Flight Radar24 etc.). The first case requires antennas and processors etc. to display targets while the second case only requires processors to assemble data obtained from data provider such as Aireon.
2. ATC only care about what's written on the filed flight plan. If it is a LAT/LONG coordinate, it is LAT/LONG not ARINC424. However ATC do receive ARINC424 standard notation in reality which they are well aware of.
Okay!
To clarify my previous question, let's take an example in the second last page of NAT OPS Bulletin2018_003(Rev01).
Filed route contains a waypoint 5230N02000W.
FMS can only display this waypoint as N52W020 due to its capability(Max 7 digit)
1. In this case, when ATC picks up FMS route, Half degree is missing. So my understanding is that to avoid this confusion it's better to use a special format that ARINC424 use which includes letter 'H' to represent a half coordinate. I stand here to be corrected.
2. Ultimate question (Reason why I started this thread)
If a filed route contains a waypoint 5200N02030W, FMS can only display this waypoint as N52W020. Again, this is not representing a real coordinate. Unfortunately there's no special format to represent Longitude half degree coordinat in the ARINC424 standard. Pilot always crosscheck the waypoint coordinate to see if the filed coordinate is correctly inserted into this waypoint. However there's no way that ATC can check whether this waypoint is correctly representing a filed coordinate by only looking at the name of the waypoint.
I hope my question is better described this time. Again, any comments are highly welcomed.
Too many information to take in, so firstly let me summarize it.
1. ADS-B data can be either picked up by ATC on the ground or by Satellite which are fed to a contracted subscribers(ATC, or Flight Radar24 etc.). The first case requires antennas and processors etc. to display targets while the second case only requires processors to assemble data obtained from data provider such as Aireon.
2. ATC only care about what's written on the filed flight plan. If it is a LAT/LONG coordinate, it is LAT/LONG not ARINC424. However ATC do receive ARINC424 standard notation in reality which they are well aware of.
Okay!
To clarify my previous question, let's take an example in the second last page of NAT OPS Bulletin2018_003(Rev01).
Filed route contains a waypoint 5230N02000W.
FMS can only display this waypoint as N52W020 due to its capability(Max 7 digit)
1. In this case, when ATC picks up FMS route, Half degree is missing. So my understanding is that to avoid this confusion it's better to use a special format that ARINC424 use which includes letter 'H' to represent a half coordinate. I stand here to be corrected.
2. Ultimate question (Reason why I started this thread)
If a filed route contains a waypoint 5200N02030W, FMS can only display this waypoint as N52W020. Again, this is not representing a real coordinate. Unfortunately there's no special format to represent Longitude half degree coordinat in the ARINC424 standard. Pilot always crosscheck the waypoint coordinate to see if the filed coordinate is correctly inserted into this waypoint. However there's no way that ATC can check whether this waypoint is correctly representing a filed coordinate by only looking at the name of the waypoint.
I hope my question is better described this time. Again, any comments are highly welcomed.
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Wow~I'm speechless to see your tremendous knowledge. Many thanks.
Too many information to take in, so firstly let me summarize it.
1. ADS-B data can be either picked up by ATC on the ground or by Satellite which are fed to a contracted subscribers(ATC, or Flight Radar24 etc.). The first case requires antennas and processors etc. to display targets while the second case only requires processors to assemble data obtained from data provider such as Aireon.
2. ATC only care about what's written on the filed flight plan. If it is a LAT/LONG coordinate, it is LAT/LONG not ARINC424. However ATC do receive ARINC424 standard notation in reality which they are well aware of.
Okay!
To clarify my previous question, let's take an example in the second last page of NAT OPS Bulletin2018_003(Rev01).
Filed route contains a waypoint 5230N02000W.
FMS can only display this waypoint as N52W020 due to its capability(Max 7 digit)
1. In this case, when ATC picks up FMS route, Half degree is missing. So my understanding is that to avoid this confusion it's better to use a special format that ARINC424 use which includes letter 'H' to represent a half coordinate. I stand here to be corrected.
2. Ultimate question (Reason why I started this thread)
If a filed route contains a waypoint 5200N02030W, FMS can only display this waypoint as N52W020. Again, this is not representing a real coordinate. Unfortunately there's no special format to represent Longitude half degree coordinat in the ARINC424 standard. Pilot always crosscheck the waypoint coordinate to see if the filed coordinate is correctly inserted into this waypoint. However there's no way that ATC can check whether this waypoint is correctly representing a filed coordinate by only looking at the name of the waypoint.
I hope my question is better described this time. Again, any comments are highly welcomed.
Too many information to take in, so firstly let me summarize it.
1. ADS-B data can be either picked up by ATC on the ground or by Satellite which are fed to a contracted subscribers(ATC, or Flight Radar24 etc.). The first case requires antennas and processors etc. to display targets while the second case only requires processors to assemble data obtained from data provider such as Aireon.
2. ATC only care about what's written on the filed flight plan. If it is a LAT/LONG coordinate, it is LAT/LONG not ARINC424. However ATC do receive ARINC424 standard notation in reality which they are well aware of.
Okay!
To clarify my previous question, let's take an example in the second last page of NAT OPS Bulletin2018_003(Rev01).
Filed route contains a waypoint 5230N02000W.
FMS can only display this waypoint as N52W020 due to its capability(Max 7 digit)
1. In this case, when ATC picks up FMS route, Half degree is missing. So my understanding is that to avoid this confusion it's better to use a special format that ARINC424 use which includes letter 'H' to represent a half coordinate. I stand here to be corrected.
2. Ultimate question (Reason why I started this thread)
If a filed route contains a waypoint 5200N02030W, FMS can only display this waypoint as N52W020. Again, this is not representing a real coordinate. Unfortunately there's no special format to represent Longitude half degree coordinat in the ARINC424 standard. Pilot always crosscheck the waypoint coordinate to see if the filed coordinate is correctly inserted into this waypoint. However there's no way that ATC can check whether this waypoint is correctly representing a filed coordinate by only looking at the name of the waypoint.
I hope my question is better described this time. Again, any comments are highly welcomed.
the standards and guidance are described within bulletins for each respective airspace. Whilst that’s confusing and a lot of work, the general guidance can be found within ICAO Doc10037 (Global Operational Data Link (GOLD) Manual). An OUT OF DATE copy can be found at https://www.skybrary.aero/sites/defa...34.pdf#page114. Chapter 4 deals with flight crew procedures and the relevant paragraph may be 4.4.6 which deals with CPDLC reporting procedures.
Within NAT HLA the big units automate the use of UM137 (Confirm Assigned Route) messages and expect to receive your DM40 by way of response. The ground atm systems compare what’s received with the safe trajectory stored within the ground system and alert the controller if they don’t match. Controllers use these alerts to prevent Gross Navigation Errors (and intentions), in effect preventing safety risk events.
The reference here I think answers your question ( I had to do a bit of digging to find the reference) which essentially means that 424 is used on board to accommodate database and display screen size constraints, but it permits full access to coordinates. When dealing with ATC, they want to know these coordinates and aren’t interested or can’t accept ‘424 - flight deck shorthand’ in communication.
HTH
