The Mystery of Negative Values in Radar Altimeters When on the Ground
A radar altimeter measures the distance between an aircraft and the ground by sending out radar signals and measuring the time it takes for the signals to return after bouncing off the surface. The altimeter typically displays the height above the ground level.
Understanding the Phenomenon
When a radar altimeter shows negative values on the ground, it can be attributed to several underlying factors. These include:
Reference Level
The radar altimeter may be referencing a specific altitude level, such as mean sea level (MSL) or a barometric altitude. If the aircraft is below this reference level, the altimeter will show a negative value.
Calibration Issues and Errors in Settings
Proper calibration is crucial for accurate measurements. If the altimeter is not calibrated correctly or if there is an error in its settings, it can incorrectly calculate the distance to the ground, leading to negative readings.
Interference or Multipath Effects
Radar signals can bounce off nearby objects or surfaces before returning to the altimeter, which can cause erroneous readings. This can lead the altimeter to misinterpret the distance, potentially displaying negative values.
Software or System Malfunction
A fault in the altimeter's software or hardware can result in incorrect readings, including negative values. This can be a result of various issues such as corrupted data, firmware errors, or hardware failures.
Normal Operation and Nose Angle
During normal operation, a properly functioning radar altimeter should not show negative values when the aircraft is on the ground. However, when the aircraft touches down, the nose is in the air, making the attitude zero. As the nose comes down, the antenna is closer to the ground, hence the negative value readings.
Radio Altimeter Principles of Operation
The Radio Altimeter, or Radar Altimeter, measures the height of the aircraft by sending a radio signal to the ground and measuring the time it takes for the signal to travel to the ground and back. Unlike the pressure altimeter, which relies on air pressure and changes with weather, the Radio Altimeter provides accurate height above the ground.
Signal Range and Limitations
Radio Altimeters have a limited range of operation, with cockpit displays typically showing a minimum of 0ft and a maximum of 2500ft. The actual signal from the sensor can cover a greater range, including small negative values and up to around 5000ft. Beyond this range, the signal becomes too weak and unreliable for use.
Multiple Sensors and System Reliability
Airplanes often have multiple sensors for providing independent sources of information. For example, to ensure the reliability of critical systems, the aircraft may need two systems for the captain and first officer, and three separate sensors for a full autoland system. Each radio altimeter has two separate antennas mounted on the belly of the aircraft: one for transmitting the radio signal and one for receiving.
Visual Explanation
Photographs of these antennas can illustrate their placement. You can typically see four small square antennae in a square pattern on the belly of the aircraft, midway between the wing fairing and the nosewheel bay doors. Closer inspection reveals another pair of antennae mounted flush to the aircraft skin on the centerline.
These signals are used by many systems on the aircraft, such as Terrain Awareness Warning Systems (TAWS) and autoland, which are vital for ensuring the safety of the flight. Understanding the principles and limitations of radar altimeters is crucial for pilots and maintenance personnel to troubleshoot and ensure proper operation.