--- title: "GPS principles, components, applications" subject: "Surveying II" chapter: 8 key: "semester_4/surveying_ii:gps-principles-components-applications" source: https://www.bctengineer.com/q/semester_4/surveying_ii/gps-principles-components-applications site: BCT Engineering --- # GPS principles, components, applications Surveying II · Chapter 8: Field Astronomy and GPS - Asked in: 2068 Bhadra, 9 marks - Video: [GPS ! Global Positioning System! GPS Surveying introduction! Surveying](https://www.youtube.com/watch?v=rf8XxKXyHw0) — Smart Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/surveying_ii/gps-principles-components-applications - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-8 ## Question What is GPS? Write about the principles of GPS and its components and applications. ## Short answer GPS uses trilateration with signals from at least four satellites to determine position and clock offset. Its segments are space (24+ satellites), control (monitoring stations), and user (receivers). Applications include control surveys, topographic mapping, setting out, deformation monitoring, navigation, GIS data collection, and disaster management. ## 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. #### Applications - Control surveys and geodetic networks; establishing benchmarks. - Topographic and cadastral surveys; setting out of roads, bridges and buildings. - Navigation of vehicles, aircraft and ships; fleet tracking. - Deformation monitoring of dams, bridges and landslides; crustal motion and earthquake studies. - Photogrammetry (camera position and ground control) and GIS data collection. - Disaster management, search and rescue, and precise timing. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which are the three segments of GPS? A. Satellite, ground and orbit segments B. Space, transmission and receiver segments C. Space, control and user segments D. Master, monitor and user segments only Answer: C. GPS consists of the space segment (satellites), the control segment (master and monitor stations) and the user segment (receivers). (Key point: GPS segments) 2. How does a GPS receiver find its position? A. By measuring angles to satellites using a theodolite B. By triangulation from two ground base stations C. By matching photographs with a stored map D. By measuring pseudo-ranges to satellites from signal travel time (trilateration) Answer: D. The range is rho = c(tr - ts); ranges to several satellites fix the position. (Key point: Trilateration principle) 3. 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) 4. Which is an application of GPS in civil engineering? A. Measuring the soil bearing capacity B. Deformation monitoring of dams, bridges and landslides C. Determining the grade of cement D. Testing the strength of concrete cubes Answer: B. GPS is used for control surveys, setting out, navigation and deformation monitoring of structures. (Key point: Applications) ## 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× - [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× - [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× - [GPS field procedure and components](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Agps-field-procedure-components) — Surveying II, chapter 8