GPS Error Sources & Corrections

What is ionospheric delay?

Signal delay from free electrons in ionosphere (50-1000 km altitude). Frequency-dependent, correctable with dual-frequency. Can cause 2-50 meter errors.

How is ionospheric delay corrected?

1. Dual-frequency observations (delay proportional to 1/f²) 2. Ionospheric models (Klobuchar, IONEX) 3. Differential corrections from nearby base

What is tropospheric delay?

Signal delay from lower atmosphere (0-40 km). Frequency-independent. Typically 2-25 meters at zenith. Modeled using temperature, pressure, humidity.

Why can't tropospheric delay be eliminated with dual-frequency?

Tropospheric delay is frequency-independent (affects all frequencies equally), unlike ionospheric delay which varies with frequency.

What is multipath error?

Signal reflections from surfaces before reaching receiver. Causes range errors up to several meters. Reduced by antenna design and site selection.

How is multipath mitigated?

• Choke ring or ground plane antenna • Good site selection • Long observation sessions (averages out) • Multipath-resistant receiver technology

What is satellite clock bias?

Difference between satellite clock and GPS system time. Corrected using clock parameters in navigation message.

What is receiver clock bias?

Difference between receiver clock and GPS time. Estimated as unknown parameter, requiring minimum of 4 satellites.

What is ephemeris error?

Difference between broadcast and actual satellite position. Broadcast: 1-2m. Precise ephemeris: centimeters.

What is a cycle slip?

Discontinuity in carrier phase from signal interruption, receiver malfunction, or low SNR. Must be detected and repaired for accurate positioning.

What is integer ambiguity?

Unknown number of complete carrier wavelengths between satellite and receiver at initial lock. Must be resolved for centimeter-level accuracy.

What is the antenna phase center?

Electronic center where signal reception is referenced. May vary with satellite elevation and azimuth, requiring calibration for precise work.

What is phase center variation?

Changes in effective antenna phase center with signal direction. Causes systematic errors requiring modeling or calibration.

What is UERE?

User Equivalent Range Error - combined effect of all range error sources as standard deviation. Typical standalone GPS: 5-8 meters.

What is GDOP?

Geometric Dilution of Precision - factor relating range errors to position errors based on satellite geometry. Lower = better geometry.

What are the DOP components?

GDOP = √(HDOP² + VDOP² + TDOP²) HDOP: Horizontal VDOP: Vertical PDOP: Position (3D) TDOP: Time

What is good PDOP for surveying?

PDOP < 3 is good PDOP < 2 is excellent PDOP > 6 may degrade results

Why is VDOP usually larger than HDOP?

Satellites are distributed around the horizon but none below. This horizontal spread is better than vertical spread, improving HDOP over VDOP.

What weather conditions affect GPS accuracy?

• Ionospheric storms • Frontal systems (tropospheric) • Temperature gradients • Heavy precipitation (signal attenuation)

How does baseline length affect differential GPS?

Longer baselines mean atmospheric conditions differ more between stations, reducing error cancellation effectiveness.

What is the typical ionospheric delay at zenith?

Day: 15-50 ns (5-15 meters) Night: 5-15 ns (1.5-5 meters) Maximum during solar maximum: 100+ meters

What is the tropospheric delay formula?

Delay ≈ 2.3/cos(z) meters at sea level, where z is zenith angle. Increases to ~25 meters at 5° elevation.

What causes cycle slips?

• Signal obstruction (trees, buildings) • Low signal-to-noise ratio • Receiver malfunction • High dynamics exceeding tracking loop bandwidth

How are cycle slips detected?

• Triple difference analysis • Widelane combinations • Doppler check • Geometry-based detection • Ionosphere-free combinations