Everything connects to everything. In this case, we are talking about the AirSport Altitude Alerter, aircraft transponder and altitude encoder; plus the Air Traffic Control ground equipment and TCAS gear now used in airliners. In order to understand how the AirSport Alerters work, we need to discuss a little about the system overall.

First there was radar. It bounces a radio signal off the aircraft, and measures how long it takes to make the trip. This works, not only on planes, but on trucks and weather and so on. To keep extraneous echoes off the controller's scope, the FAA developed ATCRBS, or Air Traffic Control Radar Beacon System. The "beacon" is what we call a transponder. It isn't radar at all, but it comes up on the same scope and the controller calls it radar. ATCRBS sends out an "interrogation" and the transponder in the aircraft replies. It is this reply that is used by the AirSport Altitude Alerter.

There are two types of interrogations and two replies. If the interrogation is Mode A, the reply is a coded pulse stream consisting of the four digit Squawk code. Mode A means squawk. But if the interrogation is Mode C, the transponder replies with coded pulses containing the altitude.

Mode C interrogations also come from TCAS. This collision avoidance equipment installed in airliners asks other aircraft for their altitude. It is possible to be out of range of ground based radar, but still be responding to an airliner with TCAS.

All transponders use the same data encoding scheme and transmit on the same frequency of 1090 MHz. This commonality is what allows you to use the AirSport in any aircraft. The receiver in the AirSport Alerter is tuned to 1090 MHz and it picks up the data.

There is no difference between Mode A and Mode C data. They are identical. The ground equipment asks for a squawk and they interpret the reply pulses as a squawk. Or, they request altitude and decode the reply as altitude. It's easy for them, since they know what they requested.

Inside the Airsport Alerter, it is a more difficult task. Sorting Mode A from Mode C is an important part of the microcomputer's job. There are 4096 squawk codes, and 1280 of them also decode as altitudes. This gives a range, in 100 ft steps, from -1200 to 126,700 ft.

The Airsport Alerter produces audible warnings via the headset and/or aircraft audio system. It also provides a direct, audible warning from the internal horn located on the back panel. The headset tones use a variety of distinctive sounds, and are adjustable in volume. The HORN switch on the front panel turns all the sounds off and enables the pilot to quickly silence the Airsport unit when needed.

At 900 ft. prior to reaching the selected target, a single tone sounds. When the target is reached, a single tone is again heard. After reaching target altitude, any deviation outside the tolerance zone produces warnings. The tolerance zone is user selectable. If the aircraft climbs above or below the tolerance zone, a musical trill is heard for each one hundred feet of deviation from target. These fly up and fly down tones are repeated every ten seconds until the altitude is within tolerance.

Inside the PRO is the 1090 MHZ receiver, a microcomputer, and the circuitry required to display the data, make the warning tones, and so on. The antenna is contained within the case.

The Airsport Altitude Alerters are very easy to use and will work in any aircraft, with any headset, in nearly any location. There are a variety of other features that you will use every time you fly, such as setting a Descent Altitude and calculating your Rate of Climb.

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