The system is used at a large number of airports all over the world, and allows planes to land safely even when visibility is zero because of snow or fog.
When the plane approaches the airport, the pilot aligns it with the runway by following the direction beam. Then, as the plane passes each of the marker beacons, the pilot verifies that the plane is at the correct altitude at the current distance from the runway.
The hardware used in this system consists of various high-quality items, and each major component is included twice in the system. This hardware redundancy makes it possible for the system to work even when a component fails. Of course, a component failing is not the normal situation, and if this happens, the system has to be serviced. The point here is that if a component does fail, the system will still function until it's serviced. The component failure is immediately reported to the air traffic controllers at the airport, who will take the appropriate action.
The event logging subsystem keeps track of everything that happens within the context of the marker beacon. Normally this is simply the result of a series of tests that is continuously executed. As long as everything is functioning as intended the system keeps track of test results for the last 24 hours or so. If something is not functioning as intended, i.e. an alarm condition occurs, the system keeps track of this as well.
The RF transmitter monitoring subsystem continuously compares the radio waves output from the redundant transmitters. As long as the two signals output are within a predefined range of acceptable deviation, this part of the system is considered to function. If the range is exceeded, an alarm condition is initiated, and the necessary steps are taken.
I also made a Windows application for simulating normal operation, as well as hardware failures and other alarm conditions. Using this application, we were able to do thorough tests of most of the software components that went into the final system. Of course, other tests were done as well.