Definition
A precision approach and landing guidance system that uses microwave signals to provide pilots with azimuth (lateral) and elevation (vertical) information for the final approach to a runway. It was developed as a transitional system between the older Instrument Landing System (ILS) and the planned full Microwave Landing System (MLS), offering ILS-like guidance using microwave frequencies.
Plain English
A short-lived landing guidance system that used microwave signals to help pilots line up and descend to a runway. It was a stepping-stone meant to bridge the gap between the older ILS and a more advanced microwave system that was being developed.
Context Anchor
Seen in FAA abbreviation lists, instrument approach information, equipment descriptions, and NOTAMs related to landing-system availability.
Derivation
‘Interim’ comes from Latin meaning ‘in the meantime’ — it signals that this system was always intended as a temporary solution. ‘Microwave’ refers to the high-frequency radio waves used (shorter wavelengths than the VHF/UHF used by ILS), which allowed for more flexible and accurate guidance signals.
Why Pilots Care
Most pilots will only meet this term when reading older documentation. Knowing it was a transitional system helps you recognize it as a historical reference rather than something you’ll fly today.
Intuition Check
“Interim” does not mean informal or unreliable here. It means the system follows an approved temporary standard.
Example Sentence 1
The ISMLS was deployed at a limited number of airports as a bridge toward the full microwave landing system that was expected to replace ILS.
Example Sentence 2
Older airports sometimes still reference ISMLS procedures in their approach charts.