--- title: "GPS, components and working principle" subject: "Surveying II" chapter: 8 key: "semester_4/surveying_ii:gps-components-working-principle" source: https://www.bctengineer.com/q/semester_4/surveying_ii/gps-components-working-principle site: BCT Engineering --- # GPS, components and working principle Surveying II · Chapter 8: Field Astronomy and GPS - Asked in: 2069 Bhadra, 8 marks - 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-components-working-principle - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-8 ## Question What is GPS? Write the components and working principle of GPS. ## Short answer GPS is a satellite-based navigation system providing 3D positions using trilateration. The space segment comprises 24+ satellites in six planes transmitting L1/L2 signals; the control segment tracks and updates orbits; the user segment consists of receivers. The receiver computes pseudo-range from signal travel time, solving for X, Y, Z and clock error using at least four satellites. ## 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. What does GPS provide? A. Only horizontal position in clear weather B. 3-D position and time anywhere on or near the Earth in all weather, day and night C. Only height above sea level during daytime D. Position only where mobile networks exist Answer: B. GPS is a satellite navigation system giving latitude, longitude, height and time under all weather conditions. (Key point: Definition of GPS) 2. What describes the GPS space segment? A. 12 geostationary satellites at about 36,000 km B. 24 satellites in a single equatorial plane at 2,000 km C. At least 24 satellites in six orbital planes inclined at 55°, at about 20,200 km, sending L1 and L2 D. 6 satellites in polar orbit at 20,200 km with one frequency Answer: C. The constellation has 24 or more satellites in six planes at about 20,200 km with a period of about 12 h, transmitting L1 and L2. (Key point: Space segment) 3. What does the control segment do? A. Receives signals and computes the user's position B. Carries the L1 and L2 carrier signals C. Measures the antenna height at the station D. Tracks satellites, computes orbit and clock corrections and uploads them to the satellites Answer: D. Master and monitor stations track the satellites, compute ephemeris and clock errors and upload them. (Key point: Control segment role) 4. Why are at least four satellites needed? A. Four unknowns exist: X, Y, Z and the receiver clock error B. Each satellite gives only one of the coordinates X, Y, Z, height C. Four satellites are needed to cancel multipath D. Three satellites cannot send the navigation message Answer: A. Three unknowns X, Y, Z plus the receiver clock error need four range equations. (Key point: Four satellites needed) ## Close variants set in other exams - [Short note: GPS principle and components](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Agps-principle-components) — 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 - [Use and working principle of subtense bar](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Asubtense-bar-use-working-principle) — Surveying II, chapter 2 ## 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