# Orbital Mechanics Questions and Answers – Ground Tracks

This set of Orbital Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Ground Tracks”.

1. What is the projection of satellite’s orbit on earth’s surface called?
b) Ground tracks
c) Course
d) Satellite revisit period

Explanation: The projection of a satellite’s orbit onto the earth’s surface is called its ground track. At a given instant, one can imagine a radial line drawn outward from the center of the earth to the satellite. The point at which this line pierces the earth’s surface is called ground track.

2. Which of the curve does the ground track of a low earth satellite resemble?
a) Sine curve
b) Cosine curve
c) Tangential curve
d) Secant curve

Explanation:During each satellite’s orbit, it reaches the maximum and minimum latitude while passing the equator twice. Thus, the ground track of the satellite in low earth orbit resembles a sine curve.

3. In which frame of reference is the ground track measured?
a) Frame of reference of ground observer
b) Frame of reference of the sun
c) ECI frame of reference
d) Heliocentric frame of reference

Explanation: The satellite’s imaginary path as traced on the earth’s surface is called ground track. This is measured in reference to the ground’s observer. The observer will see the point at which satellite passes directly overhead (zenith).

4. In which of these orbits does the local time share half of its time in ascending and descending halves?
a) Molniya orbit
b) Polar orbit
c) Sun- Synchronous orbit
d) Tundra orbit

Explanation: The unique property of the low altitude circular sun-synchronous orbits is that the satellite’s ground track has one local time on its ascending half and another local time (12 h away) on its descending half. The two local times remain the same for the entire mission.

5. What is the ground track shape for a satellite in a geosynchronous orbit?
a) Spiral
b) Zig-zag
c) Circular
d) Figure eight

Explanation: In a geosynchronous orbit, the orbit period of the satellite is equal to the Earth’s rotation time period. This leads to the ground track having a figure eight shape over the fixed location of the earth which crosses the equator twice.

6. What is the ground track shape for a satellite in a geostationary orbit?
a) Single point
b) Circular
c) Figure eight
d) Circular

Explanation: Geostationary orbit is a type of geosynchronous orbit which has the orbital period same at the time period of the earth’s rotation. This means that the satellite always appears to be at the same point overhead the earth at all times. Thus, its ground track is a single stationary point.

7. If the perigee and apogee lie in the same equatorial plane, then which symmetry is exhibited by the ground tracks?
a) 90 deg vertical symmetry
b) 180 deg rotational symmetry
c) Axis symmetry
d) Translation symmetry

Explanation: When the perigee and apogee lie in the same equatorial plane, the argument of perigee is zero. In that case, the ground track of the satellite in the orbit appears to have a 180-degree rotational symmetry about the orbital nodes i.e. it appears both same above and below the equator of the Earth.

8. The Molniya orbit exhibits a ground track having 180 deg rotational symmetry about the equator.
a) True
b) False

Explanation: Molniya orbits are highly eccentric orbit which does not have apogee and perigee in the same equatorial plane. The Molniya orbits have a argument of perigee of approximately -90 deg and apogee at 63 deg. Due to this, the satellite is in the northern hemisphere for a longer time compared to the southern hemisphere. Thus, there isn’t any rotational symmetry of the ground tracks.

9. Which angle is represented in the figure given below for the satellite orbiting the earth?

a) Central angle
b) Elevation angle
d) Horizon angle

Explanation: The angle ε is the elevation angle which is the angle of the satellite above the horizon as seen by the observer. For the communications satellite, this value ranges from 2-10 degrees meanwhile the reconnaissance satellite’s elevation varies from 30-45 degrees.

10. If the Earth’s central angle is 58.3 deg and the angle of elevation for a satellite is 8.9 deg, then what is the value of the nadir angle?
a) 31.7 deg
b) 40.6 deg
c) 139.4 deg
d) 22.8 deg

Explanation: Given, λ = 58.3 deg, ε = 8.9 deg
The nadir angle is given from the relation: λ = 90 – η – ε as shown in the figure. Nadir angle is the angle of the subsatellite point which is to the edge of the field of view.
58.3 = 90 – η -8.9
η = 22.8 deg

Sanfoundry Global Education & Learning Series – Orbital Mechanics.

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