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.
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)
Linz – 45 nm
Nattenheim – 45 nm
Geneva – 45 nm
You can find the operational status of the HMU's and more information like the HMU's position and the requirements for a successful monitoring by the HMU's on web page www.eurocontrol.int/eur-rma.