Correct Answer : Determining ground speed
Explanation : The primary function of a Doppler radar is to determine the velocity vector of an aircraft with respect to the ground. The velocity component can be integrated into distance travelled and hence Doppler radar can be used as a primary sensor for dead reckoning.
Correct Answer : Doppler shift
Explanation : The velocity is determined by measuring the Doppler shift of microwave transmitted from the aircraft in several narrow beams pointed toward the surface at relatively steep angles, backscattered by the surface and received by the Doppler radar receiver.
Correct Answer : Transmitter power is high and requires ground assistance
Explanation : The Doppler system is self contained, it requires no ground based stations or satellite transmitters. The airborne transmitter power requirements are extremely small, which leads to low weight, size, and cost of equipment.
Explanation : In Doppler radar, there is a change in reflected frequency when there is relative motion between the transmitter and the receiver. This change in frequency is called the Doppler shift and is proportional to the relative speed between the transmitter and receiver.
Correct Answer : Janus
Explanation : Since the three orthogonal components of velocity are of interest, a minimum of three noncoplanar beams are required to measure the three components. Since such a beam configuration has both forward and rearward looking beams it is called Janus configuration, named after the roman god who has the ability to look backward as well as forward.
Correct Answer : 4
Explanation : Although only three beams are required to provide the three components of velocity, most modern Doppler radars employ four beams, because of the planar array antennas generally four such beams. The fourth beam shift can be combined with that of the third to obtain a more accurate component of velocity.
Correct Answer : When landing tail is closer to the ground
Explanation : When an aircraft lands it has to do the flare maneuver where the pilot pitches the aircraft’s nose up to slow down the plane. Radio altimeter measures precise distance from the ground and since the tail is closer to the ground than the nose, to measure the minimum distance it is placed in the tail.
Correct Answer : 5000ft
Explanation : Radio altimeters are generally used for measuring altitude during landing or take off. A radio altimeter gives the distance between the aircraft and the ground and not the pressure altitude. Maximum range of a radio altimeter is 5000ft but is usually switched to at 2500ft.
Correct Answer : Altitude marking radar
Explanation : Altitude marking radar are generally low altitude altimeter designed specifically to provide mark signals at specific altitudes for initiation of an automatic operation such as fuze triggering on submunitions or chute opening on lunar landing systems.
Correct Answer : Provide range to the nearest return
Explanation : To provide a range to the nearest return within the bounds of the antenna beam, many modern radar altimeters incorporate the gate in a pulse modulated radar or a filter in a frequency modulated radar over the leading edge of the return.
Correct Answer : 4.2 to 4.4 GHz
Explanation : The frequency band of 4.2 to 4.4 GHz is assigned to radar altimeters. This frequency band is high enough to result in a reasonable small sized antenna to produce 40° to 50° beam but significantly low so that attenuation by rain is minimum.
Correct Answer : Input power is around 100W
Explanation : The early pulsed radars were evolved to 5W solid state transmitters incorporating receiver pre-amplifiers. They also provided a high degree of reliability, low probability of intercept, small size, and high accuracy.
Correct Answer : Triangular
Explanation : To reduce the errors due to the Doppler shift of the return, a triangular waveform is usually used to modulate the transmitter. Thus a positive Doppler shift will produce a negative frequency error on the rising modulation slope. By averaging the frequency count the error can be minimized.
Correct Answer : Receiver low noise amplifier
Explanation : The transmitter power depends on the sensitivity of the receiver. The receiver low noise amplifier typically has a 2 to 3 dB noise figure, resulting in a sensitivity level that allows relatively low transmitter power.
Correct Answer : Position and altitude can be measured
Explanation : High precision stellar inertial navigation systems have been developed with automatic daylight and night star tracking system. These are highly useful for military aircraft in that they provide accurate position and attitude information, are autonomous, non radiation and are invulnerable to jamming.
Correct Answer : Polaris
Explanation : The north star or Polaris can be used to estimate latitude in the northern hemisphere using the horizon as a local level reference at the observers position. This type of navigation was used by the ancient mariners.
Correct Answer : Celestial fix
Explanation : Navigators on early transoceanic flights used sextants to manually measure star angle with respect to their local vertical. Using the line of sights form two or more stars, along with a star catalog and accurate time reference, the position in Earth latitude and longitude can be deduced. This was known as celestial fix.
Correct Answer : Star catalog
Explanation : The application of Kalman filter begins with the precise mathematical definition of the star observation. The inertial line of sight to any given star is precisely known from the star catalog.
Correct Answer : Kalman filter
Explanation : Navigational systems use Kalman filtering techniques to integrate all the sensed navigational data to obtain the best estimate navigation solution. The use of Kalman filter to integrate stellar measurements requires that star observations and errors be correctly modeled.
Correct Answer : Star tracker
Explanation : The star sensors have a narrow field of view and maintain a centered star image through the gimbal drive commands and are called star trackers. They have day and night tracking capability, given reasonable cloud cover conditions.
Correct Answer : Correction of drift error
Explanation : Inertial navigational suffer from drift after long hours of using. Since the position error growth of these free inertial navigation systems was excessive on long flights, it was natural to periodically update their position with position updates from manual star fixing.
Correct Answer : Catadioptric
Explanation : Telescopes are of three types: refractive, which uses lenses as their primary focusing elements, reflective, which uses a curved mirror for focusing, and catadioptric, which mix mirror and lens systems.
Correct Answer : Diffraction limited image
Explanation : The image of a star at the focal plane of the telescope with perfect, unobstructed optics is a bright spot surrounded by a series of concentric dark and light rings. This image is called a diffraction limited image.
Correct Answer : Optical aperture
Explanation : Each design of a telescope is characterized by an optical aperture, an effective focal length, and a field of view. The optical aperture is the physical diameter of the first element in the telescope.
Correct Answer : Star brightness
Explanation : The threshold level of star detection is a function of star brightness and noise variance and is performed on subsum of pixels of a size to contain star image.
Correct Answer : CCD
Explanation : The use of CCD arrays allows a small instantaneous field of view for sky background, for improved signal to noise ratio, while maintaining a large telescopic field of view.
Correct Answer : Charge coupled devices
Explanation : The photovoltaic or photoconductive detectors are packaged as charge coupled devices. These can produce an array of small cells or pixels. CCD is a closely spaced monolithic array of metal insulator semiconductor capacitors.
Correct Answer : Quantum efficiency
Explanation : Only a certain number of photons arriving at the detector are converted to signal generating electrons. The quantum efficiency of the detector is defined as the ratio of the photons converted to signal generating electrons to the total number of photons arriving.
Correct Answer : Intensity of observing star
Explanation : The power from the sky background at the focal plane of a telescope depends on the background radiance, the effective entrance area, and the field of view of the telescope.
Correct Answer : Size of the star
Explanation : Star signal power is the effective area of the telescope times the irradiance of the star. It depends on the spectral irradiance of the stellar light, the spectral response of the light detector, and the brightness of the star.
Correct Answer : Angular size of image
Correct Answer : f/ number
Explanation : The ratio of focal length to aperture is called the f/ number. For example, an f/10 telescope may have a 30 in. focal length and a 3 in. aperture.
Correct Answer : Optical transmission losses
Explanation : For anti-reflection coated optics, the optical transmission losses may be 1% or 2% at each air to glass or glass to air optical control surface within the telescope, including windows or sensor covers or surfaces. For uncoated optics, the loss per surface is about 4% to 5%.
