AQA A-Level Physics Paper 2, June 2023: Question 13

Properties of Geostationary Satellites

📌 What this question tests

This multiple-choice question assesses your fundamental understanding of gravitational fields and orbital mechanics, specifically the defining physical characteristics and operational limitations of a geostationary orbit.

Orbital plane: Must be directly above the Earth's equator.
Orbital period and angular speed: Must match Earth's rotational period (24 hours / 86 400 s) and rotate in the same direction (west to east).
Coverage footprint: Line-of-sight constraints meaning a single satellite cannot view or transmit to polar regions or the opposite hemisphere.

Question 13 • Multiple Choice [1 Mark]

Detailed Option Analysis

Which statement is not true for a satellite in a geostationary orbit?

✅ Correct Answer: Option D

"Signals from the satellite can be sent to any point on the Earth's surface during one orbit."

This statement is false (making it the correct selection):

Because the satellite is fixed relative to a single point over the equator, it only ever has a direct line of sight to roughly 40% of the Earth's surface.
It can never transmit signals to the opposite side of the globe or effectively reach high-latitude polar regions.

💡 Why Options A, B, and C are True

A is true: The center of any orbit must coincide with Earth's center of mass. To remain above the exact same point on the surface at all times, the orbit must lie in the equatorial plane.
B is true: Angular speed is given by ω = 2π / T. Since both Earth and satellite complete one full revolution in the same time period T, ω_satellite = ω_Earth.
C is true: By definition, the orbital period T is equal to the Earth's sidereal rotational period (~24 hours).

Deep-Dive & Exam Strategy

📐 Key Physics Relationships

For any circular orbit around mass M:

Centripetal = Gravitational Force:
m ω² r = G M m / r²
Orbital Radius:
r³ = G M / ω² = G M T² / (4π²)
Fixed Parameters:
Because T = 24 h is fixed, a geostationary orbit can only exist at one unique radius: approximately 4.23 × 10⁷ m from the centre of the Earth (altitude ~ 3.6 × 10⁷ m ).

🧠 Exam Technique: Negative Phrasing

Spot the negative trigger: The word not is bold in the question. Read each option as a True/False statement first:
• A → True
• B → True
• C → True
• D → False → Select D!
Underline "not" on the physical paper to avoid instinctively picking the first true fact you read.

❌ Common Errors & Misconceptions

Linear vs Angular Speed: Confusing angular velocity (ω) with linear orbital speed (v). While ω is identical to a point on the equator, its orbital speed v is much higher because v = ωr and its radius is much larger.
Polar Coverage: Assuming that because the satellite is high up, it can "see everywhere". A minimum of 3 equidistant geostationary satellites are required to provide near-global coverage, and even then, polar coverage is poor.

Mark Scheme Breakdown: Selecting D awards [1 mark]. Any other choice scores 0.