[NSP-GWG] VDB Considerations (for the ground too) and the RTCA Avionics MOPS FRAC

[barbara.clark at faa.gov]

Colleagues -

As you know, RTCA is in their Final Review and Comment (FRAC) period for the VHF compatibility updates to DO-253D that have been the subject of coordination between ICAO NSP and RTCA SC-159 for some time. There is a note for Annex 10 that corresponds to a note in the MOPS that I'd like to bring to your attention. Below are text extracts from both documents.

Proposed Change for Annex 10 (See Note 1 below) - taken from the NSP/5 Meeting Report:

[cid:image001.jpg at 01D4C50F.A5AC97C0]

Here is the corresponding note in the avionics MOPS FRAC version:

[image.png]

You will notice that there is an attempt in the MOPS language to be more specific regarding the exposure of the receiver to excess undesired power. I expect further clarification of this note to occur as a part of the RTCA FRAC process.

For example, Joel Wichgers intends to submit a comment on this FRAC MOPS note to address three items that he feels needs further clarification in the note:
1) "3 opportunities" should be "at least 3 opportunities",
2) need to exclude more than just the period of excess undesired power (i.e., also need to exclude receiver recovery time and the asynchronous nature of the excess undesired power to the VDB time slot structure) , and
3) the 3 opportunities applies for the MFR for Type 1 of 0.001 (i.e.., 1 in 1000 full length messages), and does not apply to 3 opportunities when the MFR is 60%.
There are similar issues with the SARPs note.

Joel Wichgers and Daniel Domey discussed these concerns, and I understand they may propose to clarify the MOPS note during the FRAC comments resolution as follows:

Change from: "e.g., the opportunity to receive three Type 1 messages in every 3.5 second window (Section 2.3.11.5.2.1.1) without excess power for GAST C."

Change to: ", e.g., the opportunity to receive at least three Type 1 messages in every 3.5 second window (Section 2.3.11.5.2.1.1) for GAST C operations.  The opportunity to receive messages excludes periods where the MFR requirement of 1 failed message per 1000 full length messages (Section 2.2.5) does not apply (e.g., during periods of excess undesired power plus any associated receiver recovery time and time period for the asynchronous nature of the excess undesired power to the VDB time slot structure)."

Thus, the entire MOPS note would read as follows:

Note:  This requirement supports brief excessive power received during ILS localizer and VOR overflight.  The duration of the excess power is limited by the continuity of the operation (refer to Appendix K.8), e.g., the opportunity to receive at least three Type 1 messages in every 3.5 second window (Section 2.3.11.5.2.1.1) for GAST C operations.  The opportunity to receive messages excludes periods where the MFR requirement of 1 failed message per 1000 full length messages (Section 2.2.5) does not apply (e.g., during periods of excess undesired power plus any associated receiver recovery time and time period for the asynchronous nature of the excess undesired power to the VDB time slot structure).  For GAST D operations where the Type 1 and Type 11 messages are transmitted at their minimum broadcast rate (reference Section 2.4.1 in RTCA/DO-246()), no excess power is allowed during the portion of the operation when the timeout for these messages is 1.5 seconds (Section 2.3.11.5.2.1.2).  A VDB undesired signal never exceeds the maximum allowed power of the desired VDB signal within the service volume.

There is an additional consideration for the original SARPs note regarding its consistency with the continuity analysis (aligned with the paper presented during the November NSP meeting and included (with modifications) into Appendix K of the MOPS.  It seems more restrictive on the ground station than it needs to be (perhaps especially for systems using antenna diversity). The SARPs note indicates that brief outages are not allowed for GAST D operations when the timeout is 1.5 seconds.  That statement is not supported by the continuity analysis.  It is however true to state that brief outages are not supported when the Type 1 and Type 11 messages are transmitted at their minimum broadcast rate.  The continuity analysis has shown that as long as there are 3 message reception opportunities that satisfy a MFR of 1 in 1000 full length messages in each and every timeout period (sliding window) for the Type 1 and Type 11 messages, then continuity can be satisfied.  Message Type 2 is slightly different, and the degraded MFR can be used to support meeting the continuity.  Joel Wichgers has a FRAC comment on the MOPS that will propose an addition to the end of Section Appendix K.8 that will describe how the 60% degraded MFR can be used to support meeting the continuity associated with Type 2 messages that have a 60 second timeout.

I'd like to encourage interested people to follow this process, whether you comment or not. Comments are being accepted through 25 February and, as stated above, at least one comment is expected on this note.  It will be discussed during the WG-4 comments resolution meeting during the first week of March.

- Barbara

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://host412.hostmonster.com/pipermail/nspcsg_aeronav.net/attachments/20190215/ac85121b/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image001.jpg
Type: image/jpeg
Size: 35661 bytes
Desc: image001.jpg
URL: <http://host412.hostmonster.com/pipermail/nspcsg_aeronav.net/attachments/20190215/ac85121b/attachment.jpg>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image002.png
Type: image/png
Size: 182108 bytes
Desc: image002.png
URL: <http://host412.hostmonster.com/pipermail/nspcsg_aeronav.net/attachments/20190215/ac85121b/attachment.png>