HM Unit Page



HMU — Ground Based Height Monitoring Units



           A HMU (Height Monitoring Unit) is a ground based system consisting of two main components - HME (Height Monitoring Element) and TMU (Total vertical error Monitoring Unit).

           The HME captures SSR transponder signals from aircraft replying to interrogations from radar stations. The signals contain the information from modes S, C and A transmissions. The HME determines the geometric height and position of an aircraft by comparing the time of reception of the SSR transponder signals at different receiver locations. This information is transmitted to the TMU as one plot per second. The data is collated by the TMU to create a track history of the aircraft passing through the area of coverage. The track information is then combined with meteorological data to evaluate the overall value for Total Vertical Error (TVE).

           When completed, the monitoring process produces TVE (Total Vertical Error), AAD (Assigned Altitude Deviation) and ASE (Altimeter System Error) readings for each aircraft measured. In the European context, Height Monitoring results are automatically transmitted to the EUR RMA at EUROCONTROL Headquarters, Brussels where they are verified and credited to approved aircraft and operators.



EUR Region HMU Locations


           Linz in Austria - 48°12’08”N....014°17’35”E (near LNZ VOR)
           Nattenheim in Germany - 49°56’45”N….006°33’25”E (near NTM VOR)
           Geneva in Switzerland - 46°21’49”N….005°55’34”E (near GVA VOR)


You can check HMUs status here


NATCMA Region HMU Location


Strumble in Scotland – is located near Strumble (STU) VOR below the centerline of UL9 (formerly UG1).  The horizontal coverage area for the HMU is a 13.8 nm radius circle centered on position N51 56 00’ W004 40’ 00


To check the HMU status contact UK NATS at


JASMA Region HMU Location


Setouchi in Japan – 343044N 1341627E. Setouchi HMU coverage is 40 nm.


You can check HMUs status here


NAARMO Region AGHME (Aircraft Geometric Height Measurement Element) Locations


Atlantic City, Wichita, Cleveland, and Phoenix (United States);

Ottawa and Lethbridge (Canada).


You can check AGHMEs status here


AAMA Region ADS-B (Automatic Dependent Surveillance Broadcast) Location




HMU radius of operation are usually 45 nm.