Introduction to GNSS & GPS
GNSS provides precise positioning using satellite constellations orbiting at ~20,200 km, enabling everything from phone navigation to centimeter-accurate surveying.
Theory & Foundations
Receivers calculate position by measuring signal travel time from multiple satellites. Four ranges solve for latitude, longitude, altitude, and clock bias via trilateration.
GNSS satellites broadcast precise time signals from atomic clocks. Range = speed_of_light × time_delay. Four satellites minimum: 3 for XYZ position + 1 for clock error.
Four constellations: GPS (USA, 31 satellites), GLONASS (Russia, 24), Galileo (EU, 30), BeiDou (China, 35+). Multi-constellation receivers use all simultaneously for better accuracy.
Error sources: Ionospheric delay (±5m), tropospheric delay (±0.5m), multipath (±1m), orbital errors (±2m), clock errors (±2m). Total standalone accuracy: 3–5m.
Correction methods by accuracy: Standard (3–5m) → SBAS (1–3m) → DGPS (0.5–1m) → PPP (5–10cm) → RTK (1–2cm) → Static survey (sub-mm).
In-Depth Coverage
GNSS Constellations
Four global systems provide worldwide coverage.
- GPS (USA): 31 satellites, operational since 1995
- GLONASS (Russia): 24 satellites, 1993
- Galileo (EU): 30 satellites, full service 2020
- BeiDou (China): 35+ satellites, global 2020
- Regional: QZSS (Japan), NavIC (India)
Accuracy Levels
Different techniques achieve different precision.
- Smartphone GPS: 3–5m (sufficient for navigation)
- SBAS (WAAS/EGNOS): 1–3m (aviation approaches)
- DGPS: 0.5–1m (GIS field data collection)
- RTK: 1–2cm (surveying, construction)
- Static observation: Sub-millimeter (geodetic control)
Error Sources
Understanding errors is key to achieving required accuracy.
- Ionosphere: Charged particles delay signals (largest error)
- Troposphere: Water vapor causes signal bending
- Multipath: Signals bounce off buildings/surfaces
- Satellite geometry: PDOP measures constellation quality
- Clock bias: Receiver clock less precise than atomic clocks
Key Techniques
Data Visualization
- 📱Smartphone3–5 meters accuracy
- 📡DGPS0.5–1 meter accuracy
- 🎯PPP5–10 centimeter accuracy
- 📐RTK1–2 centimeter accuracy
Practical Workflow
- Define accuracy requirements
- Select GNSS method (RTK, PPK, static)
- Plan survey: datum, coordinate system
- Configure receiver: constellations, elevation mask
- Establish base or connect to CORS
- Collect observations
- Post-process if needed
- Export and assess quality (PDOP, residuals)
Software & Tools
Video Tutorials
Real-World Application
Real-World Projects
National CORS network for RTK corrections.
Case Study
Problem-Based Learning
Rural Bangladesh, no existing CORS network nearby, centimeter accuracy required.
Solution: Deploy RTK base station on known control point. Survey with rover collecting 10-second observations per parcel corner. Process with quality checks.