GPS & GNSS Surveying Technologies Flashcards
7 cards from real CST practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 GPS & GNSS Surveying Technologies flashcards as text
Which GPS error source is caused by satellite signals reflecting off buildings or other surfaces before reaching the receiver antenna?
Answer: Multipath error
Multipath error occurs when GPS signals travel indirect paths by bouncing off nearby surfaces before arriving at the antenna, causing the receiver to compute an artificially long signal travel distance.
How does the ionosphere primarily affect GPS signal accuracy?
Answer: It slows the signal and introduces ranging errors proportional to electron content
The ionosphere, a layer of charged particles in the upper atmosphere, slows GPS signals as they pass through it, introducing a range error that varies with solar activity and the amount of ionized particles along the signal path.
Which receiver capability most effectively eliminates ionospheric delay errors in GPS surveys?
Answer: Dual-frequency capability measuring L1 and L2 simultaneously
Because ionospheric delay is frequency-dependent, dual-frequency receivers measuring on both L1 (1575.42 MHz) and L2 (1227.60 MHz) can compute and remove the ionospheric delay from the position solution.
Why do GPS surveyors set an elevation mask angle, typically between 10 and 15 degrees?
Answer: To exclude low-angle satellites whose signals traverse more atmosphere and are prone to multipath
Satellites near the horizon have signals that pass through a much greater thickness of atmosphere and are more susceptible to multipath from the ground; excluding them with an elevation mask angle improves solution quality.
What is ephemeris error in GPS, and what causes it?
Answer: Inaccuracy in the broadcast satellite orbital position and velocity data
Ephemeris error arises when the orbital parameters (predicted position and velocity) broadcast by a GPS satellite in its navigation message differ from the satellite's true orbital position, causing small but systematic ranging errors.
What is the purpose of setting a minimum PDOP threshold in GPS data collection?
Answer: To guarantee that only observations with favorable satellite geometry are accepted
Setting a maximum acceptable PDOP (e.g., reject when PDOP > 4) ensures that position computations only occur when satellite geometry is favorable enough to produce reliable, accurate results.
Which network-based service creates a virtual GPS base station near the rover using data from multiple permanent reference stations, delivering RTK corrections via cellular network?
Answer: VRS (Virtual Reference Station) network
A VRS (Virtual Reference Station) network processes data from multiple CORS stations to synthesize a virtual base station close to the rover's location, providing RTK-quality centimeter-level corrections without a physical base station.