UAG Navigation and GPS Systems 2 — Questions and Answers
Question 1: GPS signal multipath error occurs when:
- The satellite signal travels directly from satellite to receiver
- The GPS receiver tracks too many satellites simultaneously
- The GPS signal reflects off surfaces before reaching the receiver (Correct answer)
- The receiver loses contact with WAAS ground stations
Correct answer: The GPS signal reflects off surfaces before reaching the receiver
Multipath error occurs when GPS signals reflect off buildings, terrain, or other surfaces before reaching the receiver, causing inaccurate position calculations.
Question 2: What is Position Dilution of Precision (PDOP) in GPS navigation?
- The reduction in GPS accuracy caused by poor satellite geometry (Correct answer)
- The time delay between GPS signal transmission and reception
- The error introduced by atmospheric interference
- The difference between WAAS and standard GPS accuracy
Correct answer: The reduction in GPS accuracy caused by poor satellite geometry
PDOP quantifies how the geometric arrangement of visible satellites affects position accuracy; a low PDOP (under 4) means favorable satellite geometry and better accuracy.
Question 3: Which type of GPS interference involves intentionally broadcasting false GPS signals to mislead receivers?
- Multipath interference
- Ionospheric scintillation
- GPS spoofing (Correct answer)
- Signal jamming
Correct answer: GPS spoofing
GPS spoofing is the deliberate transmission of counterfeit GPS signals to deceive receivers into calculating false position, velocity, or time information.
Question 4: When converting a true course to a magnetic course, a pilot in an area with 10° East magnetic variation should:
- Add 10° to the true course
- Subtract 10° from the true course (Correct answer)
- Multiply the true course by the variation factor
- No adjustment is needed; GPS uses magnetic north
Correct answer: Subtract 10° from the true course
With East variation, subtract the variation from the true course to obtain the magnetic course (East is least — subtract; West is best — add).
Question 5: What is dead reckoning as applied to UAS navigation?
- Navigating solely by reference to visual landmarks
- Estimating position using last known position, speed, heading, and elapsed time (Correct answer)
- Using only GPS for primary navigation without cross-checking
- A redundant navigation method using two GPS receivers
Correct answer: Estimating position using last known position, speed, heading, and elapsed time
Dead reckoning involves calculating a current position estimate based on a previously known position, applying known speed, heading, and time elapsed since that position.
Question 6: Ionospheric disturbances (such as solar flares) primarily affect GPS signals by:
- Physically damaging satellite hardware
- Causing signal delays that introduce position errors (Correct answer)
- Permanently removing satellites from service
- Reducing the number of GPS satellites in view
Correct answer: Causing signal delays that introduce position errors
Solar activity causes the ionosphere to vary, which changes the speed of GPS radio waves passing through it, introducing timing errors that translate into position inaccuracies.
Question 7: A UAS remote pilot notices the aircraft's GPS-based position display shows a location 50 meters from the actual position. What is the most likely cause?
- The UAS is flying too high for GPS signal acquisition
- Poor satellite geometry (high PDOP) or signal multipath (Correct answer)
- The FAA has deliberately degraded GPS accuracy in the area
- The UAS battery level is affecting GPS receiver performance
Correct answer: Poor satellite geometry (high PDOP) or signal multipath
Large position errors are most commonly caused by poor satellite geometry (high PDOP) or multipath interference from reflective surfaces near the operating area.
UAG Navigation and GPS Systems 2
GPS signal multipath error occurs when: