Part 25 surveillance
Part 25 certification basis support for surveillance equipment
This support aligns a surveillance installation's evidence with the Part 25 certification basis of the transport-category aircraft it serves. Suppliers and modifiers installing transponders, ADS-B transmitters, and their antennas use it when the box meets its standard but the installed function has to answer a transport-category basis, where diversity antenna behavior, position-source integrity, and interaction with the rest of the aircraft are judged more strictly. The work examines installed transmitter performance, the integrity of the position feeding the broadcast, antenna diversity and placement, and the installation substantiation, then ties each to the basis paragraph it addresses. You receive a surveillance basis map, a list of open items, and an ordered plan to close them.
When this review is needed
- A transponder or ADS-B installation on a transport-category aircraft needs its evidence tied to the Part 25 basis.
- Antenna diversity, common on transport-category airframes, has to be substantiated for pattern coverage and switching behavior.
- The position source feeding the broadcast has to meet the integrity the transport-category surveillance function demands.
- A finding asks how the installed surveillance function meets a named Part 25 paragraph across the airframe.
The problem
Transport-category surveillance installations usually run diversity antennas, top and bottom, and the interaction of the pair, the switching behavior, and the combined pattern is exactly what the box-level test never covered. The position source feeding an ADS-B broadcast has to carry the integrity the transmission advertises, and on a large airframe the antenna and cable geometry are more complex than the box case assumes. These installation-level items are where the Part 25 surveillance basis concentrates, and they sit outside the equipment qualification the supplier arrives with.
What gets reviewed
- The Part 25 basis paragraphs the surveillance function answers at the aircraft's amendment level
- Installed transponder and ADS-B transmitter performance across the diversity configuration
- Antenna diversity, switching behavior, and the combined pattern top and bottom
- Integrity of the position source feeding the broadcast against the advertised quality
- Installation substantiation for interference, power, and interaction with other systems
- The separation between demonstrated installed performance and box-level performance assumed to carry over
What gets validated
- Each surveillance basis paragraph has installed-configuration evidence, including the diversity behavior
- Antenna diversity switching and the combined pattern are substantiated top and bottom
- The position source integrity matches the quality the ADS-B broadcast advertises
- Transmitter performance is confirmed at the installed antennas and cabling, not the box connector
- Interference and electromagnetic compatibility with adjacent systems is addressed for the actual install
Evidence normally required
- The certification basis for the transport-category aircraft and its amendment level
- The transponder and ADS-B equipment performance and qualification reports
- The position-source description and its integrity characteristics
- Antenna placement and diversity design, cable runs, and any pattern analysis or test
- Installation substantiation data for power, bonding, and system interaction
Common discrepancies
- Diversity switching behavior or the combined pattern substantiated for one antenna but not the pair
- A position source whose integrity falls short of the quality the broadcast advertises
- Transmitter performance shown at the box connector rather than at the installed antennas
- Electromagnetic interference with an adjacent system left unaddressed for the large-airframe install
What is at stake
A surveillance package that maps to the box standard but not the transport-category installation basis leaves the diversity behavior and position-source integrity to surface as findings. A diversity pattern that turns out short, or a position source that undershoots the advertised quality, can force antenna rework or a source change on a large airframe, where the rework reaches deep into the aircraft and lands late in the program.
Move from findings to resolution
Identify gaps against the means of compliance.
How the work runs
Set the installed basis
Establish the Part 25 basis paragraphs the surveillance function answers for this aircraft and its amendment level.
Trace the diversity install
Point transmitter, position-source, and both-antenna evidence at the paragraphs and mark where only single-antenna or box data exists.
Substantiate pattern and integrity
Check the combined diversity pattern, switching behavior, and position-source integrity for the installed configuration.
Prioritize the gaps
Order the open items by the airframe rework each would force and deliver a basis map for the matrix.
What the buyer receives
- A basis map linking each relevant Part 25 paragraph to the surveillance installation evidence
- A gap list emphasizing diversity behavior and position-source integrity
- A closure sequence ordering the gaps by the airframe rework each would force
Who uses the output
- Certification leads assembling the surveillance compliance argument for a transport-category modification
- Engineering teams substantiating antenna diversity and the position source
- Compliance managers answering a finding on the installed surveillance function
How the work fits into the transaction or program
The mapping connects the box performance file to the transport-category installation basis, where diversity antenna behavior and position-source integrity decide compliance. Its gap list drives the antenna and source substantiation, and the basis map feeds the compliance matrix the applicant carries into review.
Start with a single asset
Confirm requirements trace through verification.
Aircraft-specific considerations
Transport-category airframes typically require antenna diversity for surveillance, so the compliance question is about the behavior of the antenna pair and the switching between them, not a single location. The position source available also depends on the aircraft's navigation fit. The mapping is scoped to the diversity configuration and source architecture of the specific aircraft.
Jurisdiction-specific considerations
FAA and EASA surveillance mandates and their position-source integrity requirements differ in detail and phase-in, so a surveillance installation compliant for one basis can leave items open for the other. The mapping notes where the integrity requirement and diversity evidence change between authorities.
Regulatory limits
The work maps surveillance evidence to the Part 25 basis and identifies gaps. It makes no compliance finding, grants no surveillance authorization or approval, and does not determine that the installation is airworthy. The authority decides and the applicant owns the claim.
What this review does not cover
- Granting any surveillance authorization, operational approval, or design approval
- Performing the antenna diversity pattern test or the installed EMC campaign itself
- Any airworthiness determination on the installed surveillance equipment
Specific to this review
- Transport-category surveillance usually runs diversity antennas, so the basis question is the behavior of the pair and the switching, which no single-antenna box test answers.
- ADS-B broadcast integrity is capped by the position source, so a source short of the advertised quality is a leading transport-category finding just as it is at normal category, but with more rework to correct.
- On a large airframe the antenna and cable geometry is complex enough that installed transmitter performance can differ materially from the box connector result.
Sources
U.S. Government (eCFR). Type certificates, STCs (Subpart E), TSO authorizations (Subpart O), PMA (Subpart K), and export airworthiness approvals (Subpart L).
Federal Aviation Administration. FAA type certification process, certification basis establishment, and compliance findings.
SAE International. Development assurance process at aircraft and system level, including requirements capture and validation.
Frequently asked questions
Our surveillance box is approved. Why review the diversity installation against Part 25?
Box approval covers the unit. The Part 25 basis asks how the installed surveillance function performs across a diversity antenna configuration on this airframe, where switching behavior and the combined pattern matter. The mapping shows where box approval still leaves installation basis items open.
Relevant glossary terms
Related pages
Where this fits
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