--- title: "Short note: GPS principle and components" subject: "Surveying II" chapter: 8 key: "semester_4/surveying_ii:gps-principle-components" source: https://www.bctengineer.com/q/semester_4/surveying_ii/gps-principle-components site: BCT Engineering --- # Short note: GPS principle and components Surveying II · Chapter 8: Field Astronomy and GPS - Asked in: 2079 Jestha, 4 marks; 2078 Poush; 2078 Baisakh; 2077 Chaitra, 4 marks; 2074 Bhadra, 4 marks; 2073 Magh, 4 marks; 2073 Bhadra, 4 marks; 2072 Asoj, 4 marks; 2072 Magh, 4 marks; 2071 Bhadra, 4 marks; 2071 Magh, 4 marks; 2070 Bhadra, 8 marks; 2066 Magh (old course), 8 marks; 2065 Kartik (old course), 8 marks; 2065 Chaitra (old course), 8 marks; 2081 Chaitra, 4 marks; 2080 Chaitra, 4 marks; 2076 Baisakh, 4 marks (asked 18 times) - Video: [What is GPS/GNSS](https://www.youtube.com/watch?v=WjAAvaBwoxQ) — Rami Tamimi | Professional Surveyor - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/surveying_ii/gps-principle-components - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-8 ## Question Write a short note on the working principle and components of GPS. ## Short answer GPS uses trilateration based on signal travel time from at least four satellites to solve for position and receiver clock error. Components: space segment (24+ satellites), control segment (monitoring/upload stations), and user segment (receivers). ## Answer A GPS (Global Positioning System) is a satellite-based radio navigation system, owned by the USA, which gives three-dimensional position (latitude, longitude, height) and time anywhere on or near the Earth in all weather, day and night. #### Components (segments) ```text Space segment (24+ satellites, ~20,200 km) | signals Control segment <--> User segment (master & monitor (receivers) stations, uplink) ``` 1. **Space segment:** at least 24 satellites in six orbital planes inclined at 55°, at an altitude of about 20 200 km, with an orbital period of about 12 h. Each satellite sends signals on carrier frequencies L1 (1575.42 MHz) and L2 (1227.60 MHz), carrying the ranging codes and the navigation message (satellite position and clock). 2. **Control segment:** a master control station, monitor stations and ground antennas; they track the satellites, compute orbits (ephemeris) and clock corrections, and upload them to the satellites. 3. **User segment:** the GPS receivers (handheld, geodetic or surveying types) and their users; the receivers pick up the signals and compute the position. #### Working principle GPS works by **trilateration** using the travel time of the signal. The satellite sends a signal stamped with the time of transmission and its own position. The receiver notes the time of arrival, and the distance (pseudo-range) is $$ \rho = c\,(t_r - t_s) $$ where $c$ is the speed of light, $t_s$ the transmission time and $t_r$ the reception time. Each range puts the receiver on a sphere centred on the satellite. Three satellites would fix the position (three unknowns $X, Y, Z$), but the receiver clock is not as accurate as the atomic clocks in the satellites, so a fourth unknown (the clock error $\delta t$) is added. **At least four satellites** are required to solve for $X, Y, Z$ and $\delta t$: $$ \rho_i = \sqrt{(X_i - X)^2 + (Y_i - Y)^2 + (Z_i - Z)^2} + c\,\delta t,\quad i=1,\dots,4 $$ The accuracy of a point depends on the satellite geometry (DOP), atmospheric delay, multipath and receiver quality. Using two receivers removes most of the common errors: code-based differential GPS (DGPS) gives sub-metre to metre accuracy, and carrier-phase relative positioning (static or RTK) gives centimetre to millimetre accuracy. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why are at least four satellites needed? A. To get time zones B. To solve X, Y, Z and receiver clock error C. Three can't be seen D. To measure height only Answer: B. Four unknowns need four equations. (Key point: Minimum satellites) 2. Pseudo-range is: A. c/(t_r - t_s) B. (t_r + t_s)/c C. c x t_s D. c(t_r - t_s) Answer: D. Speed of light times travel time. (Key point: Pseudo-range) 3. Which set lists the three segments of GPS? A. Space, atmosphere, user B. Space, control, user C. Control, receiver, ground D. Orbit, signal, user Answer: B. GPS has space, control and user segments. (Key point: GPS segments) 4. Orbit altitude of GPS satellites: A. About 500 km B. About 36 000 km C. About 2000 km D. About 20 200 km Answer: D. 24+ satellites in six planes at about 20 200 km, period about 12 h. (Key point: Space segment) ## Close variants set in other exams - [GPS, components and working principle](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Agps-components-working-principle) — Surveying II, chapter 8, asked 19× ## Related in Surveying II - [GPS principle and components, GIS applications](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Agps-principle-gis-applications) — Surveying II, chapter 8, asked 2× - [GPS principles, components, applications](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Agps-principles-components-applications) — Surveying II, chapter 8 - [GPS field procedure and components](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Agps-field-procedure-components) — Surveying II, chapter 8 ## Related in other subjects - [Development stages and PRoR working principle](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Astages-pror-working-arrangement) — Hydropower Engineering, chapter 2 - [Working principle of a digital telephone exchange](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-telephone-exchange-principle) — Telecommunications, chapter 4, asked 4× - [DC motor working principle and torque equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-motor-principle-torque-equation) — Electrical Machines I, chapter 4, asked 4× - [Pelton wheel: components and working](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Apelton-wheel-working-components) — Fluid Machines, chapter 3