Avionics Mcqs - Set 17

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.

ANSWER: True

Explanation:
Hyperbolic navigational systems are called so because of the hyperbolic lines of the position they produce rather than the circles and radial lines associated with the system that measures distance and bearing.

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.

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.

ANSWER: False

Explanation:
Loran is a hyperbolic navigational system that came into usage right before the outbreak of world war II. The Loran-C uses ground waves at low frequencies, thereby securing an operating range of over 1000miles, independent of line of sight.

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.

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.

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.

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.

ANSWER: Pulse trailing edge

Explanation:
The pulse trailing edge is defined as that portion of the Loran-C pulse following the peak of the pulse or 65μsec after the pulse is initiated. The pulse trailing edge is controlled in order to maintain spectrum requirements.

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.

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.

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.

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.

ANSWER: Composite Omega

Explanation:
The composite Omega technique reduces the diurnal variation but does little to reduce the wide variation in phase behaviour exhibited by paths of equal length over substantially electromagnetic/geophysical environments.

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.

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.

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.

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.

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.

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.

ANSWER: 2

Explanation:
Each transmitter station is physically divided into two groups of units to provide system redundancy. At the appropriate interfaces switching units are provided between them.

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.

ANSWER: True

Explanation:
In the Omega system, each station transmits 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.

ANSWER: True

Explanation:
Each HCG or half-cycle generators contributes to the power contained in the Loran-C pulse. 32 HCGs comprises the standard set. The basic set can be expanded in multiples of eight HCGs.

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.

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.

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.

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.

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.

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.

ANSWER: Position and acceleration of satellite

Explanation:
All of the satellite navigation equations have either satellite position or velocity as a variable. The linear independence of the equations, which dictate the observability of the navigation solution, is a function of the relative position of the satellites in orbit. Thus the position and the velocity of satellites is of primary importance.

ANSWER: Line of nodes

Explanation:
The line of nodes is the intersection of the orbital plane with the Earth’s equatorial plane. The ascending node and the descending node are the points where the satellite crosses the equatorial plane.

ANSWER: False

Explanation:
The ascending node is the point where the satellite crosses the Earth’s equatorial plane from the southerly latitude to the northerly.

ANSWER: 6

Explanation:
Six independent constants are needed to specify the nominal orbit. These can be the three components of position and velocity at any instant of time and are used in the orbital mechanics equations.

ANSWER: Line of apsides

Explanation:
The points where the satellite is closest and farthest from the Earth is called the perigee and apogee respectively. The line connecting the apogee and the perigee is called the line of apsides.

ANSWER: Motion of a passing comet

Explanation:
Perturbations cause the orbital plane to oscillate and rotate and vary the satellite from its elliptical orbital path. These forces include spherical asymmetrical components of Earth’s gravitational field, Lunisolar perturbations, air drag, magnetic and static electric forces.

ANSWER: True

Explanation:
The navigational messages from a satellite include parameters describing the satellite’s clock offset and drift, which are predicted by the control segment. Any instability in the satellite’s clock causes this prediction to be in error, thus resulting in range and range rate errors in the user’s navigation solution.

ANSWER: True

Explanation:
Satellite clock errors, including those caused by SA dithering, are completely eliminated by DGPS, except for the SA dithering effects due to delays in estimating, broadcasting, and making the DGPS corrections.

ANSWER: Reduces velocity

Explanation:
The major sources of errors are when the signal pass through the atmosphere. The troposphere causes the propagation velocity of the signal to be slowed, compressing the signal wavelength.