Practical Navigation Course
Unit 3 · Electronic & Radionavigation

Chapter 11: Satellite Navigation and GNSS

GNSS gives continuous global position from ranging to satellites. Understanding how it fails matters more than knowing how it works.

Principles and constellations

A receiver measures pseudoranges to at least four satellites, solving for latitude, longitude, height and clock offset. GPS (US), GLONASS (Russia), Galileo (EU) and BeiDou (China) are the four global systems; QZSS and NavIC are regional. Multi-constellation receivers improve availability and geometry, particularly in harbours where buildings mask the sky.

Error sources and augmentation

Ionospheric and tropospheric delay, satellite clock and ephemeris error, multipath and receiver noise combine into a few metres of error. Differential GPS broadcasts corrections from a fixed reference station on MF beacons, reducing error to 1–3 m. SBAS systems (WAAS, EGNOS, MSAS) broadcast corrections and integrity data from geostationary satellites. RTK reaches centimetre accuracy over short baselines.

  • DGPS — MF beacon, local, metre-level
  • SBAS/WAAS — satellite broadcast, wide area, with integrity messages
  • RAIM — receiver autonomous integrity monitoring warns of a bad satellite

Dilution of precision and vulnerability

DOP expresses how satellite geometry amplifies ranging error: GDOP, PDOP, HDOP, VDOP and TDOP. Satellites spread widely across the sky give low HDOP (good); satellites clustered together give high HDOP even with strong signals. GNSS is a weak signal easily jammed and spoofable; a position that looks precise can be badly wrong. Cross-check against radar ranges, visual bearings and the DR plot continuously.

Formula cards

Position error

σ_position = HDOP × σ_range

Geometry multiplies ranging error; this is why DOP matters.

Key takeaways

  • • Four satellites minimum: three dimensions plus receiver clock.
  • • Low HDOP means good geometry; high HDOP degrades the fix regardless of signal strength.
  • • GNSS precision is not integrity — always cross-check independently.

Knowledge check

Immediate feedback with rationales. Answer each question to see the explanation.

Chapter 11 quiz

0 of 3 answered

  1. 1.The minimum number of satellites required for a three-dimensional GNSS fix is:

  2. 2.An HDOP of 6.0 indicates:

  3. 3.WAAS is best described as: