--- title: "Communication via satellite as unguided scheme" subject: "Instrumentation II" chapter: 5 key: "semester_5/instrumentation_ii:satellite-unguided" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/satellite-unguided site: BCT Engineering --- # Communication via satellite as unguided scheme Instrumentation II · Chapter 5: Data Acquisition And Transmission - Asked in: 2076 Chaitra, 4 marks - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/satellite-unguided - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-5 ## Question How can you design the communication system with satellite as an unguided transmission scheme? ## Short answer Use a GEO satellite at C/Ku/Ka band; earth station modulates (e.g., QPSK), up-converts, and transmits via high-gain dish. Satellite transponder shifts frequency and amplifies. Downlink is received with dish, LNB, down-converter, and demodulator. Link budget includes free-space path loss (~196 dB at 4 GHz for GEO). ## Answer Satellite communication is an **unguided (wireless) transmission scheme**: microwave signals are sent through free space from an earth station to a satellite, which relays them to another earth station. #### Design ```text Source -> Modulator -> Up-converter -> HPA -> Uplink dish ~~~ 6 GHz uplink ~~~> Satellite transponder: LNA -> Frequency translator -> TWT/power amp ~~~ 4 GHz downlink ~~~> Downlink dish -> LNB -> Down-converter -> Demodulator -> Destination ``` 1. **Orbit:** geostationary (GEO, 35,786 km) for fixed coverage, LEO/MEO for low delay. 2. **Frequency bands:** C band (6 GHz up / 4 GHz down), Ku band (14/12 GHz), Ka band (30/20 GHz). The uplink uses a higher frequency than the downlink to reduce satellite power. 3. **Earth station:** modulator (QPSK/PSK), up-converter, high power amplifier and parabolic dish antenna; receive side with LNA/LNB, down-converter and demodulator. 4. **Satellite transponder:** receives, amplifies, shifts the frequency (to avoid interference) and retransmits. 5. **Link budget:** free-space path loss FSPL = 20 log d + 20 log f + 92.45 dB (d in km, f in GHz). For GEO at 4 GHz: 91.1 + 12.0 + 92.45 = **195.6 dB**, so high-gain antennas and a low-noise receiver are needed. 6. **Multiple access:** FDMA, TDMA or CDMA to share the transponder. 7. **Delay:** one hop (up and down) is 2 x 119 ms = about 238 ms (round trip about 0.5 s), which must be considered for protocols. 8. Rain fading margins, error control coding and power supply (solar panels) complete the design. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is satellite communication called unguided? A. It uses optical fibre B. It uses coaxial cable C. Microwave signals travel through free space, not a cable D. It needs no antennas Answer: C. The earth station sends microwaves to the satellite, which relays them. (Key point: Unguided scheme) 2. What does the satellite transponder do? A. Only reflects the signal unchanged B. Demodulates and stores the data C. Generates the message D. Receives, amplifies, shifts frequency and retransmits Answer: D. Frequency shifting avoids interference between uplink and downlink. (Key point: Transponder) 3. In the C band, which frequencies are used? A. 6 GHz uplink and 4 GHz downlink B. 4 GHz uplink and 6 GHz downlink C. 14 GHz both D. 6 GHz both Answer: A. Uplink is higher so the satellite needs less power. (Key point: Frequency) 4. What is the free-space path loss for GEO at 4 GHz? A. About 95.6 dB B. About 195.6 dB C. About 20 dB D. About 395 dB Answer: B. FSPL = 91.1 + 12.0 + 92.45 = 195.6 dB. (Key point: Path loss) ## Related in Instrumentation II - [Data logging system for optical, satellite, Bluetooth](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adata-logging-three-schemes) — Instrumentation II, chapter 5 - [Master-slave 8085 bidirectional communication via 8255](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Abidirectional-8085-master-slave) — Instrumentation II, chapter 2 - [Digital transmission system; analog vs digital](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aanalog-digital-transmission-compare) — Instrumentation II, chapter 5 - [Transmission line issues in high speed design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Atransmission-line-issues) — Instrumentation II, chapter 7 - [Case study: proposed system design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acs-proposed-system-design) — Instrumentation II, chapter 10 - [Reducing failure probability in milk scheme](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Amilk-scheme-failure) — Instrumentation II, chapter 7 ## Related in other subjects - [Short note: Satellite communication system](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-satellite-communication) — Communication Systems, chapter 5, asked 2× - [Guided vs unguided transmission media](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aguided-vs-unguided-media) — Data Communication, chapter 4 - [Optical fiber vs satellite transmission](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aoptical-fiber-vs-satellite) — Data Communication, chapter 4 - [Guided and unguided transmission media](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atransmission-media-explain) — Telecommunications, chapter 2, asked 2×