# Avionics Questions and Answers – Orbital Mechanics and Clock Characteristics

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This set of Avionics Multiple Choice Questions & Answers (MCQs) focuses on “Orbital Mechanics and Clock Characteristics”.

1. Which of the following is the most fundamental for satellite navigation?
a) Power source in satellite
b) Position and acceleration of satellite
c) Distance from the Earth
d) Total number of satellites in the sky

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.

2. What is the line connecting the intersection of the orbital plane with the Earth’s equatorial plane called?
a) Critical point
b) Intersection plane
c) Line of nodes
d) Differential line

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.

3. When the satellite crosses the equatorial plane from the southerly latitude to the northerly, the point is called descending node.
a) True
b) False

Explanation: The ascending node is the point where the satellite crosses the Earth’s equatorial plane from the southerly latitude to the northerly.
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4. What is the number of independent constants required to specify the nominal orbit?
a) 6
b) 5
c) 2
d) 7

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.

5. The line connecting the perigee and apogee is called as?
a) Differential line
b) Line of apsides
c) Line of maximum reception

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.
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6. Which of the following is not one of the perturbations that make the satellite to vary from its elliptical path?
a) Solar activity
b) Asymmetrical components of Earth’s gravitational field
c) Air drag
d) 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.

7. The clocks are important in satellite navigation as they are potential sources of error.
a) True
b) False

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.

8. Differential GPS corrects for satellite errors.
a) True
b) False

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.

Sanfoundry Global Education & Learning Series – Avionics. 