L10: Global Positioning System Flashcards
What are the different navigational systems from different countries/regions?
NAVSTAR - USA (what GPS refers to)
Galileo - EU
Not full world coverage:
Glonass - Russia
IRNSS - India
Bei-Dou - China
Define GPS
A satellite navigation system used to determine the ground position of an object
Discuss the history of GPS
Developed by the US military in 1960s
Over next few decades, expansion into civilian use
Accuracy greatly improved since initial development
What is the purpose of GPS in surveying?
To use satellites to determine 3D positions of stations for engineering purposes
What are the three segments of GPS (describe each)?
SPACE: constellation of satellites transmitting radio signals to users
CONTROL: ground stations that track the satellites, monitor their transmissions, perform analyses, and send commands; master control in Colorado Springs but also alternate master control station as well as command/control antennas and monitoring sites
USER: all civil and military users of the system; nowadays in many commercial projects
Discuss the satellites needed and their orbits
For all points on Earth, need at least 4 GPS satellites in view at all times
Currently 31 operational at altitude of 20 200 km
Orbit Earth every 12 hours
Different ages of satellites
What is the method of locating a point that GPS uses?
Trilateration
Describe the process of trilateration of GPS
Each satellite sends of signal to receiver on Earth which will get time of the signal being sent and from that calculate distance to satellite which is in a known position
First three satellites with known distances allow for three spheres to be drawn, which give two possible intersection points, but Earth’s surface defines which one is used
Satellites on atomic clocks but not phones so some error exists - constant for all satellites so just one unknown; fourth satellite to calculate the unknown and compensate accordingly
How do the principles of relativity impact the accuracy of GPS?
Special relativity: due to their high speed, the atomic clocks on the satellites will slow down by 7 microseconds per day
General relativity: due to experiencing only 1/4 of Earth’s gravity, the atomic clocks will speed up by 45 microseconds per day
Overall, the clocks will be off by 38 microseconds per day
BUT a computer makes adjustments to the clock so that GPS remains accurate
What are two types of surveying with GPS (briefly describe each)?
Static: set up one station and record data for just that station
Stop and go kinematic: set up central station but also side station that is moved around and measurements taken for 2-3 minutes
