The instrument of choice
By Jim Sparks
Altitude has always been a No. 1 concern of aviators. Even in the worst case, if an aircraft had enough altitude, a safe landing could be planned and carried out. Pressure-sensitive altimeters have always been the instrument of choice to provide flight crews with their critical data. Barometric altimeters per regulation have to be accurate within a range of 20 to 250 feet depending on surrounding pressure altitude. The fact that this device is sensitive to changes in pressure referenced to a sea level environment make it blind to the rise of mountain ranges or other obstacles to the flight path and with a 20-foot accuracy at sea level, would most likely make a precision landing with sole dependency on this device a bumpy situation. In fact changes in atmospheric pressure from one side of an airport to another can provide a noticeable altitude error.
The radio altimeter sometimes referred to as a radar altimeter is designed to provide an accurate reference, in some cases within 2 feet, of the aircraft above the ground. In most cases in general and commercial aviation this device is operational up to 2,500 feet above ground level (AGL).
History and benefits
Lloyd Espenschied first created the radio altimeter in 1924 but it took 14 years before Bell Labs was able to put the device in a form that was adaptable for aircraft use and utilized an FM frequency.
Although a valuable stand-alone information source, radio altitude can also be supplied to ground proximity warning systems (GPWS) or terrain awareness and warning systems (TAWS). In addition, information is often supplied to flight guidance systems to enhance flight deck visual queues used by the flight crew during takeoff, instrument landings, or go around conditions. In many cases radio altimeter systems will also provide information to alter gain authority in automatic flight controls plus it is a primary sensor in the control of auto landing systems. Radio altitude may even be used by airframe systems to trigger landing gear warnings or even control systems that have an altitude stipulation.
How it operates
A receiver transmitter (RT) working in conjunction with separate transmit and receive antennas makes up the majority of the system hardware. Operation requires the RT unit to send a signal to the transmit antenna which is then directed to the ground. When the signal hits the ground it is reflected back up to the receive antenna. The RT then performs a time calculation to determine the distance, as the altitude of the aircraft is proportional to the time required for the transmitted signal to make the round trip.
The frequency modulated (FM) signal produced by the RT unit is not tunable, at least from the flight deck. In fact the operating frequency range is 4,250 to 4,350 MHz. The basis for the calculation is predicated on the stipulation that a transmitted 4,300 MHz signal will return as a 4,300 MHz signal, but during the time it takes for the signal to travel to the ground and back the transmitter frequency has increased. The difference between the transmit and receive frequencies is directly proportional to the height of the aircraft above the ground at a rate of 40 Hz per foot. A customized preset is fitted to the system to enable the radio altimeter to be installed in a wide range of aircraft where the height above the ground where touchdown occurs can be selected. This point is often calculated as when the main wheels contact the runway. Often there are no controls for the radio altimeter other than a circuit breaker. The system may become operational as soon as aircraft power is supplied. In many cases it is considered a prudent maintenance practice to deactivate this system prior to conducting most airframe maintenance with electrical power. In some aircraft installations a weight off wheels circuit will arm the radio altimeter transmitter once the aircraft is in flight.
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