Satellite Systems and Frequencies Revision Notes | A Level AQA Physics

1. Why Satellite Communication Matters

Picture a ship in the middle of the Atlantic Ocean, hundreds of kilometres from the nearest land. No copper cable can reach it, no fibre-optic link runs beneath it — yet it can stream live weather data, send distress signals, and browse the internet. That is the power of satellite communication.

Satellite systems are a core part of modern data communication infrastructure, sitting alongside wired and wireless terrestrial links. At A Level, you need to understand how these systems are structured, which frequencies they use, and how they compare with other transmission media on the grounds of data rate, cost, and security.

2. Satellite System Types and Typical Frequencies

Not all satellites orbit at the same altitude, and that choice of orbit has direct consequences for the frequencies used, the signal delay experienced, and the coverage area achieved. There are three main orbit categories you need to recognise.

GEO satellites are the most familiar: a single satellite covers roughly one-third of the Earth's surface and appears stationary from the ground, making dish alignment straightforward. The trade-off is a one-way propagation delay of around 120 ms (approximately 240 ms round-trip) — noticeable in voice calls and problematic for real-time applications.

LEO constellations (such as those used for modern satellite broadband services) use many satellites to provide continuous coverage, with much lower latency — typically 20–40 ms — because the signal travels a far shorter distance. The higher frequency Ka-band allocations used by some LEO systems support greater bandwidth, though rain fade becomes a more significant issue at these frequencies.

3. Up-Link and Down-Link Frequencies — Why They Must Differ

Here is where the physics gets genuinely interesting. A satellite must receive signals from the ground (the up-link) and simultaneously transmit signals back to the ground (the down-link). If both used the same frequency, the satellite's own powerful transmitter would swamp its own sensitive receiver — a problem called de-sensitisation.

Think of it like trying to hear a whisper while shouting at the top of your voice. The transmitted signal is orders of magnitude stronger than the incoming signal the receiver is trying to detect. Even with shielding, using identical frequencies makes it practically impossible to separate the two.

By using different frequencies for up-link and down-link, the satellite's on-board filters can reject the transmitted frequency and pass only the received frequency to the receiver circuitry. This frequency separation is built into every satellite communication standard.

In practice, the up-link frequency is typically higher than the down-link frequency. For example, in the Ku-band, the up-link sits around 14 GHz while the down-link sits around 11–12 GHz. The up-link uses a higher frequency to ensure separation from the down-link, preventing de-sensitisation; the additional path loss this incurs is compensated by high-power ground station amplifiers, which satellites cannot match due to their power constraints from solar panels and battery capacity.

The frequency separation also allows the use of a transponder on the satellite: a device that receives the up-link signal, shifts it to the down-link frequency, amplifies it, and retransmits it. This frequency conversion is the core function of every communications satellite.

4. Comparing Transmission Media

Satellite links are one option among several. Real communication systems choose between copper wire, optical fibre, terrestrial wireless (microwave or radio), and satellite links depending on the application. The three axes of comparison the specification requires are: data transmission rate, cost, and security.

Optical fibre stands out for raw data rate and security. Because light is confined within the fibre core, it does not radiate outward, making passive interception extremely difficult. Any physical attempt to tap the fibre causes measurable signal loss, which can be detected. This makes fibre the preferred medium for high-security, high-bandwidth backbone links.

Copper wire remains cost-effective for short-distance connections — within a building or across a local area network — but its bandwidth degrades sharply with distance, and it is vulnerable to electromagnetic induction-based eavesdropping. Shielded cables reduce but do not eliminate this risk.

Satellite links excel where no terrestrial infrastructure exists: remote regions, maritime routes, and disaster-relief scenarios. Their main disadvantages are the high cost of the space segment, the latency introduced by GEO orbits, and the fact that the signal is broadcast over a wide footprint — anyone within that footprint with suitable equipment can attempt to receive it. Encryption is therefore not optional in satellite communications; it is a fundamental requirement.

5. Evaluating Media for Specific Scenarios

Exam questions often ask you to evaluate or suggest a suitable medium for a described scenario. The key is to weigh the three criteria — data rate, cost, and security — against the specific constraints of the situation. There is rarely one universally correct answer; the skill is in the justification.

Consider a bank transferring financial transaction data between two city-centre offices 500 metres apart. Data rate requirements are high, security is critical, and cost is secondary to reliability. Optical fibre is the clear choice: it offers the highest bandwidth, is extremely difficult to intercept, and over 500 metres the installation cost is manageable.

Now consider a research station in Antarctica needing to send daily weather data to a European meteorological centre. No fibre cable runs to Antarctica, terrestrial microwave links are impractical over thousands of kilometres, and copper wire is out of the question. A satellite link — despite its cost and latency — is the only viable option. The relatively low data volume (daily weather reports) means the lower data rate of a satellite link is acceptable.

6. Worked Evaluation: Choosing a Transmission Medium

Evaluation questions at A Level require you to argue both sides and reach a supported conclusion. The following worked example models the structure examiners reward.

Worked example: Evaluating Transmission Media for a Remote Island Broadband Link

A small island 200 km from the mainland has no existing cable infrastructure. A telecoms company must provide broadband internet to 500 households. Evaluate whether a satellite link or a submarine optical fibre cable is more suitable, considering data rate, cost, and security. [6 marks]

1. Distance: 200 km over sea. Users: 500 households. No existing infrastructure. Need: broadband internet. — Before comparing media, pin down what the scenario actually demands. This prevents generic answers that ignore the context — a common reason for losing marks.
2. Submarine fibre: capable of Tbps aggregate capacity, easily shared across 500 users at high individual speeds. Satellite (GEO): shared bandwidth of tens to hundreds of Mbps across all users; LEO constellations offer higher rates but still shared. Fibre wins on data rate. — [B1] for correctly stating fibre offers higher data rate. Note that 'shared bandwidth' is the key concept for satellite — as more users connect, each gets less.
3. Submarine fibre: very high capital cost for 200 km of undersea cable, specialised installation vessels, landing stations. Satellite: high ongoing bandwidth cost but no cable installation; terminal dishes needed per household or at a central hub. For 500 households, satellite may have lower upfront cost but higher long-term running cost. — [B1] for distinguishing capital cost (fibre higher) from ongoing cost (satellite higher per Mbps). Examiners reward this distinction.
4. Submarine fibre: signal confined within cable; physical tapping requires cutting the cable (detectable); very secure. Satellite: signal broadcast over wide footprint; any receiver within coverage area can attempt interception; encryption is essential but adds complexity. — [B1] for correctly identifying fibre as more inherently secure and explaining why satellite requires encryption as a compensating measure.
5. Fibre offers superior data rate and security but at very high capital cost. Satellite offers lower upfront cost and faster deployment but lower data rate per user and inherently lower security. For 500 households requiring reliable broadband, fibre is preferable long-term if funding allows; satellite is preferable if rapid deployment or limited capital is the priority. — [B1] for a supported conclusion that acknowledges the trade-off rather than declaring one option universally superior. Avoid absolute statements — examiners penalise them.

Answer: Optical fibre provides higher data rates and greater inherent security, making it preferable for a permanent high-demand installation. However, the very high capital cost of a 200 km submarine cable means satellite is a viable alternative where budget or deployment speed is the primary constraint, provided encryption is implemented to mitigate the security risk.

7. Exam Technique and Common Mistakes

The three specification requirements in this topic — satellite system types and frequencies, up/down-link frequency separation, and media comparison — each have characteristic exam traps. Addressing them directly is the difference between a mid-grade and a top-grade answer.

Across this lesson, every specification requirement connects to a single underlying theme: communication system design always involves trade-offs. Satellite systems use separated up- and down-link frequencies to solve the de-sensitisation problem; the choice between satellite, fibre, copper, and wireless links involves trading data rate, cost, and security against the constraints of the application. Recognising and articulating those trade-offs is precisely what A Level exam questions in this topic are designed to assess.