Correct Answer : Maximum effective aperture
Explanation : Maximum effective aperture is defined as the ratio of terminating impedance to the power density of the incident wave. It is also called the effective area of an antenna and is given by Dλ/4π.
Correct Answer : Noise figure
Explanation : The factor by which a receiver fails to reach the theoretical internal noise limit is often expressed as a ratio, in decibels, and is known as the noise figure. It can also be described as the ratio between the noise power output of a practical receiver and the noise power output of an ideal receiver.
Correct Answer : 3MHz
Explanation : Up to 3MHz, an appreciable amount of energy follows the curvature of the earth and are called as the ground wave. Above 3MHz, the radio waves do not follow the curvature of the earth and are used for a line of sight communication.
Correct Answer : Vertical polarization
Explanation : At lower frequencies, along the surface of the earth, vertical polarization is universally used. This way minimum signal is radiated into the ground.
Correct Answer : Unpredictable path
Explanation : The sky waves makes some types of long range communication possible, but is of less value to navigation system because it’s transmission path is unpredictable. A rough location where the sky waves might get reflected can be determined by Snell’s law.
Correct Answer : Day/night changes
Explanation : Propagation of ground waves depends on several additional factors. Some of which are conductivity and dielectric constant of earth. Weather changes like thunderstorms and lightning strikes also create extra atmospheric noise and degrade signal.
Correct Answer : Pseudoranging
Explanation : In this method, the user’s receiver and the reference station are assumed not to be synchronized in time. By measuring several such pseudoranges, the user’s three dimensional position and its time offset can be determined.
Correct Answer : Absolute error
Explanation : The absolute or predictable error is the error in determining position relative to an earth referenced coordinate system. The earth frame is one such coordinate system that has its origin at the Earth’s center of mass and its axes fixed on Earth.
Correct Answer : Multipath effects
Explanation : Serious distortion of signal modulation may occur due to different paths simultaneously travelled by the signal between transmitter and receiver. These are called multipath effects.
Correct Answer : Ground based
Explanation : Ground based direction finders take bearings on airborne transmitters and advice the aircraft of its bearing from the ground station. Such stations can afford the necessary complex equipment, but the operation is cumbersome and time consuming, and requires airborne transmitter and communication link.
Correct Answer : Very low error
Explanation : Airborne direction finder or homing adapters take bearing from ground transmitters. These direction finders typically can afford only the simplest of systems and must, therefore, tolerate large errors.
Correct Answer : Rotating loop
Explanation : The rotating loop is driven by a motor and forming part of a servo system that rotates the loop until a null is found and then stops. They are sometimes referred to as the radio compass and are housed inside plastic structures that do not reflect radio waves.
Correct Answer : Horizontal polarization is assumed
Explanation : The loop antenna is a rectangular loop of wire whose inductance is resonated by a variable capacitor. The signal is assumed to be vertically polarized and the horizontal antenna pattern is a figure of eight. When the loop is 90° with respect to the transmitted signal the currents induced are of same amplitude and phase.
Correct Answer : 63Mhz above or below the received frequency
Explanation : The Ground beacon in the DME receives the airborne pulses, and after a 50μs delay, retransmits them back to the aircraft on a frequency 63Mhz above or below the airborne transmitting frequency. Here the frequency received by the DME is the Airborne transmitting frequency.
Correct Answer : Reduce interference
Explanation : Pulse systems can easily interfere with each other even if they are of different frequencies. Paired pulses are used in DME to reduce interference with other pulsed systems.
Correct Answer : 12?sec
Explanation : The interrogation pulses are in pairs. Each pulse is placed 12μsec apart with each pulse lasting 3.5μsec. The pulse pair repetition rate ranges between 5 pulse pairs per sec to a maximum of 150 pulse pairs per sec.
Correct Answer : Alford loop
Explanation : Each radiator in the ground station transmitter is an Alford loop. The Alford loop generates a horizontal polarized signal having the same field pattern as a vertical dipole.
Correct Answer : Sense antenna
Explanation : The horizontal pattern of the loop antenna is a figure of eight which has two null positions 180° apart. This ambiguity will cause the system to give the same indication whether it is pointing toward a station or away from it. A sense antenna can be added when the signal ambiguity must be resolved.
Correct Answer : 30rps
Explanation : The VOR sends out two signals. One is a highly directional signal that rotates at a frequency of 30 rps and an omnidirectional signal. The omnidirectional signal is pulsed every time the directional signal coincides with north.
Correct Answer : Counterpoise
Explanation : The marker beacons use an array of half wave radiators that are aligned in line with the airway. A wire mesh counterpoise below this array can be used to reinforce the upward beam by increasing the signal strength in a particular direction.
Correct Answer : Goniometer
Explanation : The goniometer has two sets of fixed windings at right angles to each other, each set connected to one loop. It combines the output from the loop antenna into miniature a magnetic field in which a rotor can operate.
Correct Answer : Cone of silence
Explanation : The NDB works on an omnidirectional antenna which has the radio pattern of a donut. The zone on the middle which is directly above the antenna is called as the cone of silence. In this region, no signal is transmitted.
Correct Answer : No mountain and night effect
Explanation : NDB uses ground waves which follow the curvature of earth. They are inexpensive and independent of accuracy issues but suffer from night effect and mountain effect. Mountain effect is the distortion of ground wave in terrain of nonuniform character.
Correct Answer : 75MHz
Explanation : Marker beacons were developed for better fixes along the airways in addition to directional information. They operate at 75MHz and radiate a narrow pattern upward from the ground.
Correct Answer : VOR
Explanation : Loran-C, Omega, Decca, and Chayka are the hyperbolic navigational systems whereas the VOR, DME falls under the point source navigational systems.
Correct Answer : Both measure the phase difference
Explanation : Omega and Decca measure the phase differences between the signals transmitted from pairs of stations while the Loran-C and Chayka measure the time difference between the signal from two or more transmitting stations.
Correct Answer : Subjected to site errors
Explanation : The Loran stands for Long range navigation and uses ground waves with pulsed technique to avoid sky wave contamination. Being a hyperbolic system, it is not subjected to the site errors of point source systems.
Correct Answer : 3
Explanation : Loran consists of transmitter stations in groups forming chains. At least three transmitter stations make up a chain. One station is designated as the master while the other is called secondaries.
Correct Answer : Earth-Ionosphere wave guide
Explanation : In the wave guide model of VLF wave propagation the region in which the Omega signals are confined is known as the Earth-Ionosphere wave guide.
Correct Answer : Constant length transmission periods
Explanation : The Omega system has variable length transmission periods. This makes it possible for users to synchronize an Omega receiver to the signal format with no additional extra information.
Correct Answer : Acts as relays
Explanation : Each station has a signal monitoring facility that is situated 20 to 50km away from the antenna. These stations perform functions such as monitor signal performance, provide the required data to phase synchronize the stations, and detecting solar terrestrial events that cause shifts in the propagated signal phase.
Correct Answer : PGTR identification
Explanation : PGTR or Pulse group time reference identification is the process of ensuring that the receiver is operating on the ground wave of the signal. The basic principle behind it is that ground waves travel faster than sky ways.
Correct Answer : Blanking
Explanation : The Loran signal format can be modified to accommodate a single transmitter station in two chains. This is accomplished by permitting transmission for one of the chains to take precedence over the other when the signal format calls for simultaneous transmission in both chains. This function is called blanking.
Correct Answer : 100kHz
Explanation : Each station transmits signals that have standard pulse leading edge characteristics. Each pulse consists of a 100kHz that rapidly increases in amplitude in a prescribed manner and then decays at a rate that depends on the particular transmitter.
Correct Answer : Diurnal
Explanation : The diurnal variations are short term propagation effects caused primarily by local weather changes and day/night transitions along the signal path. Variations in a refractive index of the atmosphere versus height from the ground contribute to the short term propagation errors.
Correct Answer : Blink
Explanation : The secondary stations blink to notify the user that a master secondary pair is unusable. Blink is repetitive on/off pattern of the first two pulses of the secondary signal.
Correct Answer : Rho-Rho-Rho
Explanation : There are some Loran-C users who do not employ Loran-C in a hyperbolic mode but rather in the direct range rho-rho-rho mode. The rho-rho-rho process involves a minimum of three transmitter stations and the use of an iterative computation to obtain fix.
Correct Answer : Cost efficient
Explanation : Direct ranging Rho-Rho mode requires a minimum of two stations, a highly stable user frequency standard and precise knowledge of the time of transmission of the signal. The use of this mode is limited by the high cost of stable frequency standard.
Correct Answer : Millington’s method
Explanation : In Millington’s method, the signal between the transmitter and the receiver is broken down into finite segments of different conductivity levels, based on conductivity maps. The incremental phase delay is then computed as a function of range and conductivity for each path segment summed and averaged to provide an estimate of ASF.
Correct Answer : No seasonal errors
Explanation : The corrections using differential Loran-C are generally valid for the co-relation distance of approximately 100 miles from the reference station. Real time co-relations remove both seasonal and diurnal errors can be broadcast.
Correct Answer : PATCO
Explanation : Dual redundant pulse amplitude and timing controllers or PATCO accept timing signals from the timer and derive from this all the signals needed by the transmitter. Signals generated by the PATCO include start triggers, charging triggers, digital amplitude reference signals, amplitude compensation signals, and megatron reference trigger.
Correct Answer : Reduce interference from other frequencies
Explanation : Loran-C signal reception can be impaired by interference from other signal broadcast on slightly different frequencies. To avoid degradation of S/N associated with these interfering sources, Loran -C sets are equipped with notch filters.
Correct Answer : 1000mi
Explanation : Chayka is a pulse phase radio navigation system similar to the Loran-C system. By using ground waves at low frequencies, the operating range is 1000mi and by using pulse techniques, sky wave contamination can be avoided.
Correct Answer : Crystal controlled
Explanation : A typical Decca chain consists of a master station and three slave stations. A station has a 2kW crystal controlled transmitter feeding a 300ft antenna.
Correct Answer : Continuous wave
Explanation : At each Omega station, continuous wave signals on four common frequencies and one station unique frequency. The signal frequencies are time shared among the stations so that a given frequency is transmitted by only one station at any given time.
Correct Answer : Helix
Explanation : The RF signal that is to be transmitted is transferred to the ‘Helix,’ a large helical coil that acts as a coarse tuning device for the antenna. The helix is equipped with separate taps for each signal frequency transmitted.
Correct Answer : Hanging tower
Explanation : Since the Omega stations transmit in very low frequencies, the antennas are the largest physical structures in the stations. Three types of antennas are employed in the Omega system: grounded tower, insulated tower, and the valley span.
