NAV-06 · Lesson 02

The Constellations Above You

Four governments, roughly 130 satellites, and two receiver capabilities that almost everyone confuses with each other.

Reading: about 15 minutes  ·  Field drill: about 20 minutes

1. Why there is more than one system

The United States built GPS for the Department of Defense in the 1970s and opened it to civilian use in stages through the 1990s. Every other global system that followed exists for the same reason, and it is not a technical reason. A country whose military, banking timestamps, power grid synchronization, and emergency services all depend on a foreign navigation signal has handed control of its critical infrastructure to a government its citizens cannot vote out. Russia, the European Union, China, India, and Japan each reached that conclusion independently and built their own.

The civilian payoff is accidental but substantial. A receiver that can hear several systems sees more satellites at any given moment, fixes faster from cold, and holds a position under tree cover, between buildings, and at high latitude where any single system thins out. Of the roughly 130 active satellites across the four global systems, somewhere between 25 and 35 are above your horizon at any moment, and a receiver standing in the open will typically be tracking 15 to 25 of them.

Figure 2.1 All four global systems sit inside a band about 4,000 kilometers deep, roughly 20,000 km up. They are peers rather than tiers, and none is inherently better placed than the others — what differs is constellation size, orbit design and clock generation. Earth’s surface is another 18,000 km below the bottom of the chart.

Two regional systems fill in rather than compete. Japan’s QZSS keeps a satellite nearly overhead the Japanese islands, which is exactly what a receiver needs in a dense urban canyon, and most receivers simply treat it as additional GPS satellites. India’s NavIC covers India and about 900 miles beyond, and as of early 2026 it is operating below design strength after a series of atomic clock failures dropped it to three working satellites. That last detail is a useful reminder: constellations are maintained infrastructure, and infrastructure degrades.

2. Two axes, not one

Before the distinction makes sense, correct one common picture. A multi-system receiver is not switching between systems, choosing GPS in one place and Galileo in another. It listens to everything it can hear simultaneously and treats each visible satellite as one more measurement feeding a single calculation. The arithmetic does not know or care which government launched a given satellite — only how precisely its signal can be measured and how well its orbit is known.

That diagram is also the cleanest way to hold the next distinction, because the two capabilities people confuse act on two different steps. When someone says a receiver is “multi-GNSS” or “multi-band” or “dual-frequency,” they may be describing either. Both improve a fix. They improve different steps of it.

Axis one: how many systems

A single-system receiver hears GPS only. In open country that is fine. Under canopy, in a valley, or between buildings it degrades quickly, because the receiver needs four satellites minimum and wants considerably more than four for good geometry — and restricted sky is exactly the condition that starves it. A multi-system receiver draws from a pool of twenty or more, which means it can discard the ones with poor geometry or weak signal and still have plenty left. More systems improves Step 3. It buys you geometry, which is what Lesson 01 called the first of the four degradation conditions.

Axis two: how many frequencies

The ionosphere bends satellite signals slightly on the way down, and the amount of bending changes with solar activity. A single-frequency receiver has to estimate that delay from a model and simply accepts a few meters of error it cannot remove. A dual-frequency receiver listens on two widely separated frequencies at once, measures how differently each was delayed, and solves the atmospheric error directly instead of guessing at it. More frequencies improves Step 2. It buys accuracy from a source no number of extra satellites can address.

That last combination is the one that changed what consumer gear can do. It is also the one that costs you: running all systems can shorten battery life by roughly twenty to forty percent compared with a single-system, single-frequency mode, and adding the second frequency costs more again. Manufacturers name these modes differently, and some now offer an automatic mode that shifts between them based on conditions. Lesson 03 covers choosing among them for a specific trip; what matters here is knowing what each one is actually buying.

3. SBAS: the setting labeled WAAS or EGNOS

Most handhelds carry a setting labeled WAAS/EGNOS, and most owners either leave it on forever or switch it off on the strength of something they read in a forum. Neither is a decision. The setting enables a satellite-based augmentation system, almost always written SBAS, and it is worth thirty seconds of understanding. Some devices label the menu item SBAS outright; others name two of the regional systems and leave you to work out that they mean the same thing.

SBAS works like this. A network of ground stations at precisely surveyed locations measures how far off the satellite signals arriving at those stations actually are. Those measured errors get uplinked to a satellite parked over the region, which broadcasts the corrections back down on a frequency your existing antenna already receives. A receiver listening for it applies the corrections in real time. In open country on a handheld the improvement is real but modest — on the order of a few feet.

The part worth more than the accuracy is integrity. If one of the satellites overhead starts broadcasting bad data, SBAS flags it to receivers within about six seconds so they stop trusting it. That is a warning about a specific failure the receiver could not otherwise detect on its own, which is exactly the class of problem Lesson 01 was about.

Coverage is regional and the names are local. Every system in the table below is an SBAS, and they are interoperable by agreement, so a receiver supporting one generally picks up whichever is overhead — which is why the menu label naming two of them is misleading about what the setting actually does.

Russia and China operate their own equivalents, still in deployment. Enabling the setting does not commit you to the American system; it commits the receiver to listening for whichever regional correction is reaching it.

4. What the military gets, and what changed in 2000

GPS was built for the Department of Defense, and the civilian signal is the second product of that system rather than the first. Two pieces of that history still shape what people believe about their gear.

From the early 1990s until May 2000, the civilian signal was deliberately degraded. The technique injected pseudo-random timing errors into the civilian code, and it typically cost about 164 feet horizontally and more than 300 feet vertically. The degradation was global, not aimed — every civilian receiver in the world, allied militaries included, saw the same corrupted signal. It was switched off on May 1, 2000, and civilian accuracy improved overnight from a few hundred feet to a few tens of feet with no hardware change whatsoever. In 2007 the United States committed to building future satellites without the capability at all. It is not coming back, and any advice written on the assumption that civilian GPS is accurate only to a hundred meters is a quarter-century out of date.

What did not go away is the military’s encrypted signals, which resist both jamming and spoofing in ways the civilian signals do not. But the gap is no longer where most people assume it is:

5. What this does and does not buy you

Everything in this lesson makes a fix better. None of it makes a fix self-verifying, and that distinction is the spine of this course.

More systems means better geometry, which means fewer of the failures Lesson 01 described. More frequencies means the atmospheric error is measured instead of guessed. Augmentation means a modest correction plus a genuine warning when a satellite misbehaves. All three raise the floor. None of them tells you that the coordinate on your screen is written in the format the person receiving it expects, that the datum matches the map in your hand, or that the reflection off the wall behind you is being counted as a real signal. Those failures survive every improvement in this lesson intact.

There is also a hard ceiling. Multiple constellations raise the bar for anyone trying to deny you a position, because jamming one frequency band does not necessarily reach all four systems equally. It does not remove the possibility. An adversary willing to spend the power can cover everything at once, and spoofing — broadcasting false signals convincing enough that the receiver computes a wrong position and reports it confidently — is not solved by having more satellites to be lied to by.