17 November 2020
ADS-B Notice of Requirement revision 2 and Draft AC91-24 revision 1 released


The NSS team is pleased to announce that the highly anticipated revision to Notice of Requirement NTC91.258 regarding ADS-B has now been released by the CAA.

The revision contains several updates and clarifications. The three main changes are;

  1. Position Source equipment that does not meet the four listed TSO standards is now potentially acceptable as part of an ADS-B system. The equipment still needs to be assessed by the Certification Team and approved by the Director before being able to be used.
  2. Power Requirement clarifications – as per TSO-C112 and ICAO Annex 10 volume IV guidelines, the power requirements for an ADS-B system have now been clearly defined.   
  3. Update of message set elements table – in the previous version there were some elements that did not align which caused some confusion. These have now been corrected.

The revision has been released with a Summary of Submissions from the public consultation period, and explanatory notes are also available. See here for the updated Notice, and here for the Summary of Submissions and Explanatory Notes.

In addition to this, the CAA has also released a draft revision to AC91-24 (ADS-B Systems) for public consultation. See here

This revision includes a proposed method of approving suitable position source equipment that may potentially be acceptable under the new Notice of Requirement, and should benefit many interested participants. There are also new and expanded sections drawn from international sources and in response to common queries to the CAA. Please note that this proposal is not active until the final version has been released following the end of the public consultation period on the 18 December 2020.  

We encourage all participants to review the changes and provide their feedback.

If you have any general ADS-B queries, please email For certification queries, please email