Radar-Altimeter
Radar altimeter or an on-board radar altimeter based radar apparatus is known, which measures the exact altitude of a satellite or an aircraft in accordance with the radar method. It is part of the larger aircraft navigation equipment, measures the altitude above ground by means of short electromagnetic waves and complements the usual barometric altimeter. Radar altimeter work very accurately especially at low altitude and measure contrary to the barometric altimeters not the height above sea level ( QNH), but the real height above ground.
Radar altimeter with FMCW method
Use radar altimeter for low altitudes usually the FMCW principle. Since these devices are usually fixed to the aircraft, would at a sharp bundling of transmission energy in a fixed direction by rolling and pitching of the aircraft, a measurement errors. Therefore antennas are used at a relatively low directivity, the radiation- safe regardless of the attitude toward the ground. In the radar signal processing only the slightest difference measured frequency is used for height calculation. This always points to the reflecting surface with the shortest distance to the aircraft.
To allow a relatively large transmission power, transmitter antenna and receiver antenna are installed as far apart in the wings. The body forms an additional shield between the two antennas and thus prevents direct cross- coupling of the transmitting antenna to the receiving antenna.
When low-level flight can be adjusted continuously in military aircraft on radar altimeter warning level. If it falls below this level, the pilot will (optical or acoustic) warned by a signal. For instrument approaches by military or commercial aircraft, a minimum is also set, in which the radar altimeter gives an alarm. If the pilot the runway is not in sight then, he has to start. The radar altimeter is provision for a CAT II or CAT III approach ( see also: instrument landing system); the grounds for such an approach is for mapped ( Precision Approach Terrain Chart; PATC ) in order to interpret the display correctly.
Radar altimeter in pulsed operation
A radar altimeter of an aircraft used in a large amount, or by a satellite, it is not the FM-CW method is used, but the pulse radar method. Due to the particularity that no single point is illuminated, but a surface, the echo signal is deformed during the reflection. Considering the altitude of the satellite, this deformation may already be so important that a special radar signal processing is required. The illuminated area is increased gradually until in the middle of the illuminated area of the transmit pulse has ended, while at the edge of the illuminated area does not begin until the reflection of the transmitted pulse.
The formerly steep edge of the transmitted pulse is no longer visible in the echo signal. The former short transmitted pulse is lost in the rising edge of the echo signal. The pulse duration of the echo signal may increase to a multiple of the duration of the transmit pulse. As a measuring point for the timing for the delay time measurement of the point is used, on which passes a rising edge of the echo signal in the pulse roof. This is especially in the use of intra-pulse modulation a big problem, since different portions of the modulation superimposed postponed. Phase modulation is radar altimeter therefore not practical since the phase encoding pulse is lost at the latest in the roof of the echo signal by interference.