Satellite Study Guide

📖 Core Concepts Artificial satellite – a human‑made object placed in orbit around a celestial body, usually Earth. Satellite bus – the standardized structural platform (e.g., CubeSat bus) that carries payloads and subsystems, lowering cost and design time. Constellation – a group of satellites that work together to provide coverage or redundancy (e.g., Starlink). Orbit types Low Earth Orbit (LEO) – a few hundred km above the surface; ideal for high‑resolution imaging and rapid revisit. Geostationary Orbit (GEO) – circular, 35 785 km above the equator; appears fixed to a ground point, perfect for continuous communications. Sun‑synchronous orbit – combines altitude & inclination so the satellite passes any location at the same local solar time, giving consistent lighting for remote sensing. Launch & propulsion – satellites reach orbital velocity via a launch system; later they use chemical thrusters (hypergolic fuels) or ion thrusters (Hall‑effect, xenon) for orbit insertion, station‑keeping, or deorbiting. Attitude control – thrusters + reaction wheels orient the spacecraft on three axes. Power – solar panels generate electricity; batteries (usually Li‑ion) store it for eclipses; RTGs (radioisotope thermoelectric generators) are reserved for deep‑space missions. Communication transponder – receives a ground‑station signal, amplifies it, and re‑transmits back to Earth. End‑of‑life (EoL) deorbiting – planned removal (often via controlled burn) to limit space‑debris creation. Kessler syndrome – a cascade of collisions in a debris‑dense orbit that can render that orbital region unusable. --- 📌 Must Remember First satellite: Sputnik 1, 4 Oct 1957 (Soviet Union). GEO altitude: 35 785 km (stationary relative to Earth). LEO altitude: 200–2 000 km. Sun‑synchronous orbit ⇒ same local solar time on each pass. Chemical thruster propellants: hydrazine (monopropellant) or MMH/N₂O₄ (bipropellant). Ion thruster propellant: xenon; provides high specific impulse, low thrust. CubeSat size: standardized 10 cm × 10 cm × 10 cm units (1U). Deorbit requirement – mandated in launch licences after the 2010s. Kessler syndrome – cascade risk especially in densely populated LEO shells. Frequency allocation – international bodies assign radio‑frequency bands to prevent interference. --- 🔄 Key Processes Launch → Orbital Velocity Rocket accelerates payload to 7.8 km s⁻¹ (LEO) or 3.1 km s⁻¹ (GEO). Orbit Insertion Chemical thruster burn places satellite into target orbit; fine‑tuned with small burns. Station‑Keeping Periodic thruster firings (chemical or ion) correct drift; reaction wheels maintain attitude. Payload Operations Solar panels generate power → batteries store → transponders handle communications → sensors collect data. End‑of‑Life Deorbit Controlled burn (chemical/ion) lowers perigee into atmosphere, or moves to a graveyard orbit (GEO). --- 🔍 Key Comparisons LEO vs. GEO vs. Sun‑synchronous Altitude: LEO ≈ few × 10² km ► GEO ≈ 3.6 × 10⁴ km ► Sun‑sync ≈ 600–800 km (inclination tuned). Coverage: LEO – fast revisit, narrow swath; GEO – constant over one region; Sun‑sync – consistent illumination. Chemical thrusters vs. Ion thrusters Thrust: high (N) vs. low (mN). Specific impulse (Isp): 300 s vs. > 2 000 s. Use: major Δv (orbit insertion) vs. long‑duration station‑keeping. Solar panels vs. RTGs Energy source: sunlight vs. radioactive decay. Typical use: near‑Earth missions vs. deep‑space, low‑light missions. CubeSat vs. Microsat Standardization: CubeSat follows 1U/3U/6U modules; microsats are larger (10–100 kg) but not necessarily standardized. Geostationary comm. vs. LEO constellations Ground antenna: fixed dish (GEO) vs. tracking antennas (LEO). Latency: 250 ms (GEO) vs. < 30 ms (LEO). Anti‑satellite kinetic vs. directed‑energy vs. RF jamming Mechanism: impact/fragmentation vs. laser/maser beam vs. radio‑frequency interference. --- ⚠️ Common Misunderstandings “All satellites have RTGs.” → Only a few deep‑space probes use RTGs; most rely on solar panels. “Ion thrusters can launch a satellite.” → Ion thrusters provide efficient, low thrust; launch is done by rockets with chemical propulsion. “Deorbiting always means burning up in the atmosphere.” – In GEO, satellites are moved to a graveyard orbit instead of atmospheric re‑entry. “LEO automatically gives high‑resolution images.” – Resolution depends on sensor optics and altitude; too low can cause atmospheric drag. “Kessler syndrome is a single collision event.” → It is a cascade of many collisions over time. --- 🧠 Mental Models / Intuition Orbit speed vs. altitude – Faster in low orbits, slower higher up (think of a race car on a steep hill vs. a flat road). Satellite bus = car chassis – The bus holds everything together; payload is the “passenger”. Constellation = fleet of taxis – Multiple “cars” (satellites) provide coverage everywhere, handing off passengers (data). Kessler syndrome = falling dominoes – One collision creates debris that triggers more collisions. --- 🚩 Exceptions & Edge Cases Sun‑synchronous – Requires precise inclination (≈ 98°) and altitude; not all LEO satellites are sun‑sync. GEO with inclined orbit – Operators may let inclination grow to save fuel, using ground antennas that track the figure‑8 motion. RTGs limited to deep‑space – Not used for Earth‑orbiting satellites due to safety and cost. Planned deorbit vs. graveyard – GEO satellites are placed in a higher “graveyard” orbit; LEO satellites are usually deorbited into the atmosphere. Frequency allocation exceptions – Some scientific missions receive protected bands outside the commercial allocations. --- 📍 When to Use Which Choose orbit High‑resolution imaging, rapid revisit → LEO. Continuous coverage of a region → GEO. Consistent lighting for remote sensing → Sun‑synchronous. Select propulsion Large Δv (orbit insertion, rapid maneuver) → Chemical thrusters. Efficient, long‑duration station‑keeping → Ion thrusters. Pick power source Near‑Earth, abundant sunlight → Solar panels + batteries. Beyond Mars or eclipse‑prone → RTG. Bus type Low‑cost, rapid development → CubeSat bus. Heavier payloads, longer missions → Microsat or custom bus. Communication architecture Fixed ground terminals → GEO transponders. Mobile users, low latency → LEO constellation transponders. --- 👀 Patterns to Recognize “Same local solar time” → Sun‑synchronous orbit. “Appears stationary in the sky” → GEO. “High thrust, short burn” → chemical propulsion. “Low thrust, long duration” → ion propulsion. “Regulatory requirement for deorbit” → post‑2010 launch licences. “Radio‑frequency band allocated by ITU” → frequency‑allocation regulation question. --- 🗂️ Exam Traps Altitude confusion – Some questions may list 35 785 km but label it as “high LEO”; remember that is GEO. Power source mis‑match – A question about a deep‑space probe with solar panels is a trap; RTGs are the correct answer. Propulsion purpose – Selecting ion thrusters for “launch” is wrong; they are for in‑orbit maneuvers. Deorbit vs. graveyard – “End‑of‑life disposal” for GEO satellites is a graveyard orbit, not atmospheric re‑entry. CubeSat size – Not all “small satellites” are CubeSats; a 50 kg microsat is not a CubeSat. Kessler syndrome wording – If a statement says “a single collision will clear a debris‑dense orbit,” it’s false; the syndrome is a cascade, not a clearing. ---