Satellite navigation (GNSS)

Also known as GNSS, GPS, Global navigation satellite system, Satnav

Satellite navigation uses timing signals from a constellation of satellites to work out position, velocity and time almost anywhere on Earth, and it is now built into cars, phones and timing equipment. It is a mature commercial service, but its signals are weak at the ground and can be jammed or spoofed.

CommercialDemonstratedReviewed 22 Oct 2026
AI illustration · Illustrative scene, not a photograph of the specific device or experiment.

In plain language

A satellite navigation receiver works out where it is by timing radio signals from several satellites. Each satellite broadcasts its own position and the time it sent the signal. The receiver measures how long each signal took to arrive, turns those delays into distances, and solves for its own position and its own clock error. The first working system of this kind, the US Navy's Transit, entered service in 1964, but it gave a position only every 35 to 100 minutes.

The best-known system today is the US Global Positioning System (GPS). The first GPS satellite launched in 1978, and the US Air Force declared the constellation fully operational in 1995. In 2000 the US switched off Selective Availability, a deliberate blurring of the civil signal, so ordinary users became much more accurate. Europe's Galileo, Russia's GLONASS and China's BeiDou now offer similar services, and cars, phones and timing equipment use them. Because the signals are faint by the time they reach the ground, they can be jammed or faked, and protecting them is an active problem.

Technical explanation

A receiver estimates position from pseudoranges, the apparent travel times of signals from several satellites. Each satellite carries atomic clocks and broadcasts its orbit and clock data, so the receiver can solve for its three position coordinates and its own clock error at the same time. Transit, the first operational system, used the Doppler shift of a single satellite pass instead. Its receivers needed to know their altitude, the system gave two-dimensional fixes only, and it did not provide velocity. It was decommissioned on 31 December 1996 (eoPortal).

Satellite clocks run at a different rate from clocks on the ground for two reasons. Special relativity makes a fast-moving clock run slow, and general relativity makes a clock in weaker gravity run fast. GPS World, which cites Ashby's work and lists his 2003 review in Living Reviews in Relativity among its references, gives a net gain of about 38 microseconds per day for a GPS satellite clock, made up of about 7 microseconds per day lost to motion and about 45 microseconds per day gained from weaker gravity. The review itself was not read in full, so the figures should be checked against Ashby's own text before reuse.

Civil GNSS signals are weak, so local jamming can block a receiver, and spoofing can send false signals that mislead a receiver about its position or time. Galileo's Open Service Navigation Message Authentication (OSNMA) was declared operational on 24 July 2025. It lets a receiver check that navigation data came from Galileo itself. EUSPA says it helps mitigate spoofing, but does not prevent spoofing or protect against jamming.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Demonstrated Shown in a published experiment, test or shipping product.

The first GPS satellite, NAVSTAR-1, launched on 22 February 1978.

Sources: [2] · Block I section · medium confidence

Selective Availability ended a few minutes after midnight EDT at the end of 1 May 2000, and the US government states that it has no intent to use it again.

Sources: [1] · FAQ and main text

The Transit system became operational in 1964 and was decommissioned on 31 December 1996. It gave two-dimensional fixes, required the user to know their altitude, could not determine velocity, and gave a fix only every 35 to 100 minutes.

Sources: [3] · Transit page · medium confidence

Galileo's OSNMA authentication service was declared operational on 24 July 2025. It lets users verify that navigation information comes from Galileo itself.

Sources: [5] · Article body

EUSPA states that OSNMA helps mitigate spoofing but does not prevent spoofing or offer protection against jamming.

Sources: [5] · Article body

Reported, unverified Stated by a company or team but not independently confirmed.

eoPortal gives 27 April 1995 as the date the US Air Force Space Command declared GPS to have full operational capability. The official date has not been checked against US Space Force records.

Sources: [2] · Mission Status section · medium confidence

A GPS satellite clock gains about 38 microseconds per day relative to a clock at rest at mean sea level. About 7 microseconds per day are lost from special relativity and about 45 microseconds per day are gained from general relativity.

Sources: [4] · Article body · medium confidence

Limitations and open problems

  • Civil satellite signals are weak at the ground, so local jamming can stop a receiver getting any position, and the Galileo authentication service does not protect against jamming (EUSPA).
  • Spoofing can mislead a receiver about its location or time, and EUSPA states that the Galileo authentication service does not prevent spoofing.
  • Authentication such as OSNMA only helps receivers that implement the protocol, so existing equipment needs upgrading before it can check signals.
  • Transit gave two-dimensional fixes only, could not provide velocity, and gave a fix only every 35 to 100 minutes, so it is a historical rather than a current benchmark (eoPortal).

Possible future uses

Speculative

Ideas, not predictions. None of these has been demonstrated.

  • Vehicle navigation units that check signed satellite data, as OSNMA allows, so that a spoofed position is flagged before a driver is sent the wrong way.
  • Connected cars that compare satellite positions with their own wheel and gyroscope sensors to spot signals that disagree with the vehicle's motion.

Timeline

  1. 1964The Transit satellite navigation system becomes operational, giving two-dimensional fixes that depend on the user knowing their altitude. Sources: [3]
  2. 22 Feb 1978NAVSTAR-1, the first GPS satellite, launches. Sources: [2]
  3. 27 Apr 1995The US Air Force Space Command declares GPS to have full operational capability, according to eoPortal. Sources: [2]
  4. 2 May 2000The US ends Selective Availability, a few minutes after midnight EDT at the end of 1 May 2000. Sources: [1]
  5. 24 Jul 2025Galileo's OSNMA authentication service is declared operational. Sources: [5]

Glossary

GNSS
Global navigation satellite system: a general term for satellite systems such as GPS, Galileo, GLONASS and BeiDou.
Selective Availability
A deliberate reduction in the accuracy of the civil GPS signal, switched off by the US in May 2000.
Jamming
Blocking or drowning out a satellite signal with a stronger radio signal, so a receiver cannot use it.
Spoofing
Sending false signals that imitate satellite signals, to mislead a receiver about its position or time.

Sources

  1. government report
    Selective Availability
    GPS.gov (US Government), 22 Oct 2026. Accessed 22 Oct 2026.
  2. other
    GPS (Global Positioning System)
    ESA eoPortal Directory, 22 Oct 2026. Accessed 22 Oct 2026.
  3. other
    Transit (NNSS)
    ESA eoPortal Directory, 22 Oct 2026. Accessed 22 Oct 2026.
  4. news
    Inside the box: GPS and relativity
    Pratap Misra. GPS World, 9 Oct 2023. Accessed 22 Oct 2026.
  5. news
    Celebrating one year of Galileo OSNMA: a milestone for trusted positioning
    European Union Agency for the Space Programme (EUSPA), 27 Jul 2026. Accessed 22 Oct 2026.

Revision history

3 changes since this profile was created.

Changes on 22 Oct 2026
Published on owner instruction (car lineage tech tree, 2026-10-22)165371a
Sources and claims re-checked against originals409337d
Initial profile drafted from primary sourceseaac9eb