GPS/GNSS Advanced Concepts
PublicDeep dive into GPS theory, error sources, and survey methods with authoritative citations
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Cards (8)
What is the GPS pseudorange equation?
ρ = R + c(dt - dT) + d_iono + d_tropo + ε Where: ρ = measured pseudorange R = geometric range dt = satellite clock error dT = receiver clock error d_iono = ionospheric delay d_tropo = tropospheric delay ε = noise and multipath — GEOG 862
What are the three GPS segments?
1. Space Segment: 31+ satellites in 6 orbital planes at 20,200 km 2. Control Segment: Master control station, monitor stations, ground antennas 3. User Segment: All GPS receivers (military and civilian) — Van Sickle, Ch. 1
How does differential GPS eliminate errors?
Errors common to both base and rover (satellite clock, ephemeris, atmosphere) are nearly equal and cancel when differencing observations: Single difference: Between receivers Double difference: Between receivers and satellites Triple difference: Add time differencing — GEOG 862, Lesson 4
What is GDOP composed of?
GDOP² = PDOP² + TDOP² Where: PDOP² = HDOP² + VDOP² GDOP = Geometric DOP (position + time) PDOP = Position DOP (3D) HDOP = Horizontal DOP VDOP = Vertical DOP TDOP = Time DOP — Caltrans Unit 10
Why is L5 important for modernized GPS?
L5 (1176.45 MHz) provides: • Third frequency for better ambiguity resolution • Wider bandwidth for improved tracking • Safety-of-life signal for aviation • Better ionospheric correction with triple-frequency — GEOG 862, Lesson 2
What are the key differences between GPS and GLONASS?
GPS: • CDMA (same frequency, different codes) • 31+ satellites, 6 planes, 55° inclination • 20,200 km altitude GLONASS: • FDMA (different frequency per satellite) • 24 satellites, 3 planes, 64.8° inclination • 19,100 km altitude • Better high-latitude coverage — Van Sickle, Ch. 1
How do you convert ellipsoid height to orthometric height?
H = h - N Where: H = Orthometric height (elevation) h = Ellipsoid height (from GPS) N = Geoid undulation (from GEOID18) Example: If h = 325.42m and N = -28.15m H = 325.42 - (-28.15) = 353.57m — GEOG 862, Lesson 9
What causes multipath, and how is it mitigated?
Causes: Signal reflection from surfaces (buildings, vehicles, water, ground) Mitigation: • Careful site selection (avoid reflective surfaces) • Choke ring or ground plane antennas • Longer observation sessions (multipath repeats) • Carrier smoothing algorithms • Elevation mask (15°) — Mahun, GPS Surveying