GNSS: Errors, DGPS and DOP

How GNSS finds your position

Read first: position lines and fixes and chart datums.

In short: each satellite says “this is where I am and this is the time”; the receiver turns travel time into a range, and several ranges cross at your position.

• GNSS = Global Navigation Satellite System. GPS is the US system; the word GNSS covers all of them.
• Ships must carry a GNSS receiver or a terrestrial radio-navigation receiver that updates the position automatically (SOLAS V/19.2.1.6; MGN 379 (M+F) Amd 1 §4.1). More on the full list in SOLAS V/19 bridge equipment.

The principle, step by step

1. Each satellite transmits its own known position and the precise time the signal left it.
2. The receiver picks up the signal and notes when it arrived.
3. The receiver knows the start time, so it works out the time difference.
4. Time difference × speed of light = range from the satellite. That range is a position circle (in 3D, a sphere) around the satellite’s known position.
5. Where the ranges cross is the fix.

• 3 satellite ranges → 2D fix (latitude and longitude).
• 4 satellite ranges → 3D fix (adds height). The extra range also lets the receiver solve its own clock error.
• On the bridge: the signal travels at about 300,000 km per second. A timing error of 1 microsecond ≈ 300 m of range; 1 nanosecond ≈ 30 cm. Tiny clock errors matter.

GPS is not the only system

Source: official US GPS website (Other GNSS page); EUSPA (Galileo). MGN 379 Amd 1 §4.2 lists the same systems.

• A multi-constellation receiver sees more satellites, so it gets better geometry and more redundancy.
• Check the receiver settings on start-up and regularly: the defaults may not suit your area (MGN 379 Amd 1 §4.2).

The three segments of GPS

In short: satellites in space, a control network on the ground, and every receiver that listens.

Space segment

• 24 satellites in the baseline constellation.
• 6 orbital planes, 4 satellites per plane, plus spares.
• This is the minimum for global coverage.
• Medium Earth orbit, about 20,200 km up; each satellite goes round the Earth twice a day (official US GPS website, Space Segment).
• In July 2023 there were 31 operational satellites (official US GPS website — page not updated since February 2025).

Ground (control) segment

• Its job: constantly monitor and maintain the system. It tracks the satellites, corrects their orbits and clocks, and uploads fresh navigation data.
• Two versions of the numbers are in circulation:
  • Older version: 1 master station in Colorado (USA) and 5 monitoring stations near the equator.
  • Current version (official US GPS website, Control Segment): Master Control Station at Schriever SFB, Colorado; alternate at Vandenberg, California; 16 monitor sites (6 US Air Force / Space Force + 10 NGA); 11 command-and-control antennas (4 GPS ground antennas + 7 Air Force Satellite Control Network stations).
• In the exam, give the current version and say the older figure is out of date.

User segment

• Anyone with a GPS receiver: ships, aircraft, phones. The receiver calculates position and time.

Datum: WGS 84 and the chart

In short: a GNSS position is only as good as the match between its datum and your chart’s datum.

• Datum = the reference a position is measured from.
• GPS uses WGS 84, a model of the average shape of the whole globe.
• Many older paper charts use a local datum. The same lat/long can sit in a different place on the chart.
• Can you use a chart on a different datum? Yes, if you know the difference between the two datums. Apply the shift given in the chart note.
• No shift given? Treat the plotted position with caution (MGN 379 Amd 1 §4.7).
• Receivers can convert datums, but use this with care. Trap: set the receiver to output what ECDIS expects (usually WGS 84), or the correction may be applied twice — once by the receiver and once by ECDIS (MGN 379 Amd 1 §5.11).

GNSS errors and their causes

In short: errors come from the satellite, the path through the atmosphere, the receiver, and the ship’s surroundings — plus deliberate interference.

The classic list

• Mnemonic: work top to bottom — satellite (clock, orbit) → sky (ionosphere, troposphere) → receiver (clock, noise) → surroundings (multipath).
• MGN 379 Amd 1 §4.6 adds: obstructions such as cranes or steep terrain, and a poor cut of angle between satellites (see DOP below).

Jamming and spoofing

• Jamming = intentional interference: the signals are blocked or degraded, so you lose or degrade the fix (MGN 719 (M+F) §1.2).
• Spoofing = false position information is presented to the receiver (MGN 719 §1.2). It can look perfectly normal.
• Wrong positions go straight into the chart system or ECDIS with little warning. ECDIS alarms only if the position it is fed shows danger — it cannot tell the feed is wrong (MGN 379 Amd 1 §4.5).

Accuracy you can expect

• GNSS usually gives horizontal accuracy of about 5–25 m, 95% of the time; with augmentation, 1 m or better (MGN 379 Amd 1 §4.2).
• Navigate to the real accuracy, not the number of decimals on the display.
• The displayed position may be the antenna’s, not the ship’s reference point. Check the antenna offsets, especially on long ships.

Dilution of Precision (DOP)

In short: DOP is a number for how good the satellite geometry is. Low = good spread of satellites = reliable fix.

• Definition: the reliability of the position due to the geometry (azimuth and elevation) of the satellites in view of the receiver.
• It is the satellite version of the angle of cut in a cross-bearing fix: well-spread satellites give a sharp fix; bunched satellites give a long, thin area of uncertainty.
• Types: HDOP (horizontal) — the one mariners use; VDOP (vertical); PDOP (position, 3D); TDOP (time); GDOP (geometric, overall).

What the values mean

Two scales are in use. Both agree: lower is better.

When and where HDOP gets high

• Satellites bunched in one part of the sky, or all high overhead.
• Part of the sky blocked: close under cliffs, in a fjord, alongside high buildings or cranes, under a bridge.
• High latitudes: poorer satellite geometry can reduce accuracy or lose the fix (MGN 379 Amd 1 §4.2, §4.5).
• Few satellites in view (receiver set to one constellation only, or some signals lost).

DGPS and augmentation systems

In short: a station at a known position measures the GPS error and tells nearby receivers how to correct it.

How DGPS works

• DGPS = Differential GPS: a system to improve GPS performance (accuracy and integrity).

1. A base (reference) station receives the satellite signals.
2. It knows its own position accurately.
3. It compares the range it measures to each satellite with the range calculated from its known position.
4. The difference gives the error (mostly timing).
5. The correction is transmitted to users in the area.
6. The user’s receiver applies the correction → a more accurate position.

• Works best close to the station: the further away, the less the errors match.

GBAS and SBAS

Source: official US GPS website (Augmentation Systems); MGN 379 Amd 1 §4.3.

• The receiver shows its correction status, often as a traffic light (MGN 379 Amd 1 §4.3).

DGPS today

• UK and Ireland: the lighthouse authorities’ marine DGPS stopped after 31 March 2022. It had run since 1997 (Trinity House; MIN 665 (M+F), now withdrawn).
• USA: all NDGPS service ended on 30 June 2020 (official US GPS website).
• Why: with Selective Availability gone (2000), plain GPS gives about 3–5 m; DGPS gave about 1–2 m. When Selective Availability was on, DGPS took GPS from about 50 m to about 5 m (Trinity House).
• SBAS is still active (MGN 379 Amd 1 §4.3). The lighthouse authorities advised mariners to fit type-approved SBAS receivers.

RAIM — the receiver checks itself

• RAIM = Receiver Autonomous Integrity Monitoring. The receiver compares expected and measured ranges and flags a bad fix, usually green / amber / red.
• Needs at least 5 satellites (MGN 379 Amd 1 §4.6.2). Amber = poor geometry.

Terrestrial backup: LORAN and eLORAN

In short: a land-based, low-frequency radio system that fixes by time differences — no satellites needed.

How LORAN works

• Frequency: 100 kHz, low frequency (LF).
• Hyperbolic system: the receiver measures the time difference between pulses from 2 stations → 1 line of position.
• A second pair of stations → a second line. Two lines = a fix.
• Best accuracy near the centre of the chain, close to the stations: better angle of cut.
• Range: ground wave 600–800 NM (depends on atmospheric conditions); sky wave 1,200–2,000 NM (depends on ionisation).

LORAN errors

• Fixed errors:
  • Land error — land slows the pulse.
  • Baseline extension error — beyond the ends of the line joining two stations the hyperbolae bend sharply and accuracy collapses.
• Dynamic errors:
  • Clock error.
  • Sky-wave contamination — reflection from the ionosphere mixes with the ground wave.
• General errors of any terrestrial radio system: atmospheric attenuation, effect of the ionosphere, land effect.

eLORAN and LORAN versus GPS

• eLORAN = enhanced LORAN: modern technology that corrects for the known errors and raises accuracy.
• Pros: does not rely on satellites; land-based; can be set up locally (no need to be global); a good backup to GPS; you are not left with only the sextant.
• Cons: not always accurate; high maintenance cost; no worldwide coverage; largely discontinued.
• UK today: no terrestrial system is operational in UK waters (MGN 379 Amd 1 §4.1). A UK eLORAN is planned: timing by 2028, full positioning by 2030 (MGN 719 §6.1).

Using GNSS safely on the bridge

In short: GNSS is the main position source, so check it all the time by independent means.

• Never rely on one positioning device. Accidents have come from over-reliance, including faults passed on to radar or auto-tracking (MGN 379 Amd 1 §4.5).
• GNSS feeds ECDIS, AIS, radar, autopilot, DP and GMDSS (MGN 719 §2.2). One bad position spreads everywhere — including your AIS report to other ships.
• GNSS is one of the three required ECDIS inputs, with gyro and log (MGN 379 Amd 1 §5.2).
• If the speed log fails and both feed the radar, you are left with speed over ground only (MGN 379 Amd 1 §3.9).
• Independent checks (MGN 719 §4.2):
  • Radar ranges and bearings and parallel indexing.
  • Radar overlay on ECDIS (MGN 379 Amd 1 §3.8, §5.8).
  • Visual bearings, celestial navigation, DR and EP.
• Warning signs: no fix or a sudden jump; a position on land; SOG that does not match the log; HDOP rising; RAIM going amber or red.
• Practise GNSS-loss procedures in drills (MGN 719 §3.2.6).
• Cross-checking the position is part of a proper lookout (COLREG Rule 5; MGN 379 Amd 1 §7).