How GPS Uses Units: Coordinates, Altitude, and Speed

Latitude and Longitude: Degrees, Minutes, Seconds

Every point on Earth is addressed by two angles: latitude (north–south) and longitude (east–west). Both are measured in degrees, and degrees can be subdivided two ways.

In the traditional Degrees, Minutes, Seconds (DMS) system, one degree equals 60 minutes, and one minute equals 60 seconds. The Eiffel Tower sits at 48°51′29.6″N, 2°17′40.2″E. Military navigation, maritime charts, and older surveying equipment still use this format because it maps cleanly onto the sexagesimal (base-60) arithmetic inherited from Babylonian astronomy.

One arcsecond of latitude equals roughly 30.9 meters on Earth's surface. One arcminute equals about 1,852 meters — which is exactly where the nautical mile comes from.

Decimal Degrees vs DMS Format

Consumer GPS devices and web mapping APIs default to decimal degrees (DD), where the same Eiffel Tower location is 48.858222°N, 2.294500°E. Decimal degrees are easier to store, sort, and compute with. Converting between the two formats:

DD = Degrees + (Minutes / 60) + (Seconds / 3600)

So 48°51′29.6″ becomes 48 + (51/60) + (29.6/3600) = 48.858222°.

Precision matters here. Six decimal places in DD gives about 0.11 m of resolution — enough for most consumer navigation. Five decimal places gives 1.1 m; four gives 11 m. Aviation and surveying applications typically use seven or eight decimal places, reaching centimeter-level precision.

Visit unitfyi.com/angle/ to convert between degrees, radians, arcminutes, and arcseconds.

Altitude Units: Meters vs Feet in Aviation

GPS receivers report altitude in meters by default (the WGS 84 standard), but aviation worldwide still uses feet. The International Civil Aviation Organization (ICAO) standardized feet for altitude measurement in the 1940s, and the installed base of equipment is too vast to convert. A cruising altitude of 35,000 feet is 10,668 meters. Transition altitude — the height below which local QNH pressure settings are used — varies by country but is typically 18,000 ft (5,486 m) in the United States.

Altitude in GPS has two components: ellipsoidal height (distance above the WGS 84 mathematical ellipsoid) and orthometric height (distance above the geoid, or mean sea level). The difference between the two, called geoid undulation, can exceed 100 m in some locations. Consumer apps use orthometric height. Aviation altimeters use barometric pressure corrected to the geoid, not GPS ellipsoidal height.

See unitfyi.com/length/ for meter–foot–inch conversions.

Speed Reporting: km/h, mph, Knots

A GPS receiver calculates speed by differencing successive position fixes and dividing by elapsed time. The raw unit is meters per second, but it gets converted to the display unit the application requires:

• km/h — standard for road navigation in most of the world. 1 m/s = 3.6 km/h.
• mph — used for road navigation in the United States, United Kingdom, and a handful of other countries. 1 m/s = 2.237 mph.
• Knots — used universally in aviation and maritime contexts. 1 knot = 1 nautical mile per hour = 1.852 km/h = 0.5144 m/s.

A typical commercial aircraft cruise speed of 480 knots equals 889 km/h or 552 mph. The knot is preferred in aviation because it ties directly to the nautical mile, which ties directly to arcminutes of latitude — a natural fit for celestial navigation.

Convert between all speed units at unitfyi.com/speed/.

How GPS Satellites Calculate Position: Trilateration

GPS relies on trilateration, not triangulation. Each satellite broadcasts its precise position and the exact time the signal was sent. Your receiver measures how long the signal took to arrive and multiplies by the speed of light (299,792,458 m/s) to get a distance called a pseudorange.

With one satellite, you know you're on a sphere of radius r₁. With two satellites, you're on the intersection of two spheres — a circle. With three satellites, you get two points (one usually nonsensical). A fourth satellite resolves the ambiguity and corrects the receiver's clock error. In practice, modern receivers use 8–12 satellites simultaneously, which lets them compute position, velocity, and time in three dimensions with high redundancy.

The unit of distance in these calculations is meters, and the unit of time is nanoseconds.

Relativistic Corrections: Nanoseconds Matter

GPS is one of the few everyday systems where relativistic effects produce measurable, practically significant errors.

Special relativity: GPS satellites orbit at roughly 14,000 km/h (3.9 km/s). Moving clocks run slow. Each satellite clock loses about 7.2 microseconds per day relative to a ground-based clock.

General relativity: Satellite clocks are 20,200 km farther from Earth's gravitational field. Weaker gravity means clocks run faster. This adds about 45.9 microseconds per day.

Net relativistic drift: +38.4 microseconds per day (general > special). Light travels 30 cm per nanosecond, so a 38,400-nanosecond drift would cause position errors of roughly 11.5 km per day. GPS satellite clocks are pre-adjusted to run at 10.22999999543 MHz instead of 10.23 MHz to compensate, and the ground control segment uploads further corrections continuously.

Accuracy Units: CEP, HDOP, PDOP

GPS accuracy is not a single number — it depends on satellite geometry, atmospheric conditions, and receiver quality. Three metrics matter:

CEP (Circular Error Probable): The radius of a circle within which 50% of position fixes fall. Typical consumer GPS CEP is 3–5 m under open sky. High-precision differential GPS (DGPS) achieves sub-0.1 m CEP.

HDOP (Horizontal Dilution of Precision): A dimensionless multiplier that describes how satellite geometry amplifies horizontal position error. HDOP of 1.0 is ideal; values below 2 are excellent; above 5, accuracy degrades noticeably. Actual horizontal error ≈ HDOP × σ_range, where σ_range is the range error of a single satellite (roughly 3–5 m for civil GPS).

PDOP (Position Dilution of Precision): The three-dimensional equivalent. PDOP combines horizontal and vertical DOP. A low PDOP means satellites are spread across the sky, minimizing geometric amplification of errors.

Good GPS receivers display HDOP and PDOP in their diagnostic outputs, giving engineers a real-time read on whether reported coordinates can be trusted to within a few meters or tens of meters.