21.8 GPS Satellite Navigation & DGPS

21.8 GPS Satellite Navigation & DGPS

Quick Answer: GPS orbits at 12,554 miles, transmits on 1227.6 MHz and 1575.4 MHz, uses 24 satellites in six orbital planes equally spaced and inclined about 55° to the equator. Four satellites are needed for complete position and time. DGPS provides additional correction factors to improve position accuracy.

Sub-topic 3-P-098 (GPS) is six factual items with no calculation. GPS is also the single most operationally important system in this chapter for a GROL holder, because a DSC distress alert without a position is a fraction as useful — and that position comes from GPS over NMEA (sections 19.8, 20.5).

The constellation

What best describes the GPS satellites' orbits? They are in six orbital planes equally spaced and inclined about 55 degrees to the equator.

The geometry is deliberate. Six planes of four satellites each, inclined at 55°, guarantees that at least four satellites are above the horizon from anywhere on Earth at any time — which is exactly the number a receiver needs. The 55° inclination is a compromise: high enough to give useful coverage at high latitudes, low enough to concentrate satellites where most users are.

Note that GPS is medium earth orbit — well above the LEO constellations of section 21.7 (Iridium at 485 miles) but far below geostationary (22,300 miles). The 12-hour period means the constellation repeats its ground track daily, which is why GPS reception geometry at a fixed site is similar day to day.

The frequencies

The GPS transmitted frequencies are 1227.6 MHz and 1575.4 MHz.

Both are in L-band, alongside Iridium (1616–1626 MHz) and Inmarsat. The choice is deliberate: L-band passes through cloud and rain with minimal attenuation, and works with a small patch antenna that needs no aiming — essential for a system meant to work on a moving vessel, aircraft or handheld.

The reason two frequencies exist is worth knowing: the ionosphere delays the signal by an amount that depends on frequency. A receiver that tracks both L1 and L2 can measure that delay directly and cancel most of it, which is why dual-frequency survey receivers are far more accurate than single-frequency ones.

Why four satellites

How many satellites must be received to provide complete position and time? Four.

The reasoning is genuinely elegant and the pool rewards understanding it.

GPS positioning is trilateration by timing. Each satellite broadcasts its position and a precisely timed signal; the receiver measures the travel time and multiplies by the speed of light to get a range. In principle:

So why not three? Because the range measurements depend on the receiver knowing the exact time. Satellites carry atomic clocks; a receiver carries a cheap crystal. A clock error of one microsecond produces a range error of about 300 metres, so an uncorrected receiver clock ruins the fix.

The fourth satellite supplies the fourth equation needed to solve for the fourth unknown — the receiver clock offset — alongside latitude, longitude and altitude. The receiver effectively calculates the exact time as part of the fix. That is why a GPS receiver is also a superb time standard, and why "position and time" is the pool's phrasing.

DGPS

What is DGPS? A system to provide additional correction factors to improve position accuracy.

Differential GPS works on a simple insight: the dominant GPS errors are common to receivers in the same region. Ionospheric delay, tropospheric delay, satellite clock drift and orbit (ephemeris) error affect a reference station and a nearby vessel almost identically.

So a reference receiver at a precisely surveyed location compares its computed position against its known true position, derives per-satellite corrections, and broadcasts them. Any receiver within range applies the corrections and removes most of the shared error.

In the maritime world the corrections are traditionally broadcast by coastal radiobeacons in the 285–325 kHz band, received by a separate DGPS beacon receiver and fed to the GPS set. Accuracy typically improves from tens of metres to a few metres — which matters for harbour approaches, buoy positioning and cable work. The same principle underlies WAAS, EGNOS and modern RTK surveying, each with a different delivery path but the same idea: measure the shared error somewhere known, then subtract it.