CST GPS & GNSS Surveying Technologies 2 — Questions and Answers
Question 1: Which GPS error source is caused by satellite signals reflecting off buildings or other surfaces before reaching the receiver antenna?
- Ionospheric delay
- Multipath error (Correct answer)
- Tropospheric delay
- Satellite clock error
Correct 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.
Question 2: How does the ionosphere primarily affect GPS signal accuracy?
- It blocks the signal completely under heavy solar activity
- It changes the carrier frequency of the signal
- It slows the signal and introduces ranging errors proportional to electron content (Correct answer)
- It rotates the signal's polarization, causing antenna mismatch
Correct 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.
Question 3: Which receiver capability most effectively eliminates ionospheric delay errors in GPS surveys?
- A large ground plane antenna
- Dual-frequency capability measuring L1 and L2 simultaneously (Correct answer)
- Reducing the total observation session length
- Increasing the elevation mask angle above 20 degrees
Correct 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.
Question 4: Why do GPS surveyors set an elevation mask angle, typically between 10 and 15 degrees?
- To restrict data collection to periods of low sun angle
- To exclude low-angle satellites whose signals traverse more atmosphere and are prone to multipath (Correct answer)
- To limit the vertical range of the antenna tilt sensor
- To define the maximum terrain slope for kinematic surveys
Correct 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.
Question 5: What is ephemeris error in GPS, and what causes it?
- Receiver clock drift caused by temperature fluctuations
- Inaccuracy in the broadcast satellite orbital position and velocity data (Correct answer)
- Antenna phase center offset from the physical antenna mount
- Signal degradation caused by moisture on the antenna radome
Correct 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.
Question 6: What is the purpose of setting a minimum PDOP threshold in GPS data collection?
- To ensure the receiver records a minimum number of epochs per point
- To guarantee that only observations with favorable satellite geometry are accepted (Correct answer)
- To set the antenna height cutoff for leveling tolerance
- To filter out data collected during periods of rain
Correct 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.
Question 7: 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?
- WAAS (Wide Area Augmentation System)
- DGPS marine beacon service
- VRS (Virtual Reference Station) network (Correct answer)
- LORAN-C radionavigation
Correct 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.
CST GPS & GNSS Surveying Technologies 2
Which GPS error source is caused by satellite signals reflecting off buildings or other surfaces before reaching the receiver antenna?