GPS Satellite Clocks Gain 38 Microseconds a Day, the US Space Force Runs the System, and India's NavIC Is Down to Three Navigation Satellites

A phone works out where it is by timing radio signals from satellites about 20,200 km above Earth, and it needs at least four of them in view. Each signal carries the satellite’s position and the moment it was sent. The delay between sending and arrival, multiplied by the speed of light, gives a distance.

NOAA’s ocean service gives the speed as 186,000 miles per second. That works out to about 300 metres of travel in one microsecond, a millionth of a second, so a receiver whose timing is off by a microsecond is off by about 300 metres.

Four satellites, three coordinates and one wrong clock

Distances from three satellites would pin down a point in space if the receiver’s own clock were perfect. It is not, so the clock error becomes a fourth unknown. NOAA says the fourth satellite is there to correct for that error.

GPS.gov, the US government’s official site, describes GPS as a US-owned utility, free and open to use. It says the US Space Force operates and maintains it.

The design is 24 slots: six orbital planes, four slots in each, with every satellite circling Earth twice a day. The US commits to keeping at least 24 satellites operational 95 percent of the time. The last count GPS.gov published, dated 3 July 2023, was 31.

On accuracy, GPS.gov says a smartphone typically fixes its position within 4.9 metres (16 feet) under open sky, citing the Institute of Navigation. Professional receivers that use two frequencies or correction services reach a few centimetres in real time. The government’s formal commitment is narrower and concerns the signal, not the phone: a daily global average range error of 2.0 metres or less, 95 percent of the time. On 20 April 2021 it measured 0.643 metres or better.

The clocks are set wrong on purpose

A satellite clock does not tick at the rate of a clock on the ground, for two reasons that Einstein’s theories predict. Speed slows a clock, by about 7 microseconds a day for a GPS satellite. Weaker gravity at that height speeds it up, by about 45. The net is a gain of about 38 microseconds a day, according to Pratap Misra in GPS World.

Uncorrected, that adds up quickly. Neil Ashby, a physicist at the University of Colorado, Boulder, wrote in Physics Today in 2002 that satellite clock errors building up over a single day would cause navigation errors of more than 11 km.

So the clocks are set wrong before launch. Ashby gives the net correction as negative, 4.4645 parts per ten billion, applied to a base frequency of 10.23 megahertz. Misra says the clock is set to 10.22999999543 megahertz, a hair under that.

Ashby also records that the engineers were not sure of the answer. Before the first satellite flew, he wrote, they knew an offset would be needed but were uncertain of its size and even its sign, so they built in synthesizers that could cover the whole possible range. After that satellite’s clock had run for three weeks, the measured shift was 4.425 parts per ten billion, within 1 percent of the calculation.

Four global systems and two regional ones

GPS.gov lists the others. Russia’s GLONASS is owned and operated by the Russian Federation and has 24 or more satellites. The EU owns and operates Galileo, which declared initial services in 2016. China’s BeiDou was formally commissioned in 2020 and has 35 satellites in its operational system. GPS.gov calls the systems complementary and independent.

Two are regional. India’s NavIC is listed with seven satellites and is designed to cover India and 1,500 km beyond the mainland. Japan’s QZSS, owned by the Japanese government and run by a company called QZS System Service, has seven or more satellites and improves coverage over East Asia and Oceania.

ESA describes Galileo as 30 satellites, three of them spares, flying at 23,222 km, higher than GPS. Each carries four clocks, two of each type. The master clock is a passive hydrogen maser that ESA says keeps time to within 0.45 nanoseconds over 12 hours. The backup is a rubidium clock accurate to within 1.8 nanoseconds over 12 hours.

Jamming drowns the signal, spoofing lies

The European Union Aviation Safety Agency, EASA, defines both. Jamming is intentional radio interference that stops receivers locking onto satellite signals. Spoofing is broadcasting counterfeit satellite signals, so that a receiver computes a wrong position, navigation or timing.

EASA first published its bulletin on the subject on 17 March 2022 and issued its fourth revision on 3 July 2026, after what it called analysis of recent jamming and spoofing occurrences. Its web page names the Mediterranean, the Black Sea, the Middle East, the Baltic Sea and the Arctic as sensitive areas and keeps a list of affected flight information regions that is updated weekly. GPS.gov, for its part, lists radio interference and jamming among the less common causes of poor GPS accuracy, behind blocked views and reflected signals.

A university has shown how easy spoofing at sea can be. In June 2013 a University of Texas at Austin team led by Todd Humphreys used a briefcase-sized device on the 213-foot yacht White Rose of Drachs, about 30 miles off Italy. The false civil GPS signals overpowered the real ones and took over the ship’s navigation without triggering an alarm. The yacht ended up on a parallel track several hundred metres from its intended course. Humphreys said the ship turned and the crew could feel it, but the chart display showed only a straight line.

Planes near Delhi, a plane near Plovdiv

The Associated Press reported on 1 September 2025 that GPS was lost as a plane carrying European Commission President Ursula von der Leyen approached Plovdiv in Bulgaria. Bulgaria’s Civil Aviation Authority told the pilots to land using backup aids, and the aircraft landed safely. A Commission spokesperson, Arianna Podestà, confirmed GPS jamming and said Bulgarian authorities suspected “blatant interference by Russia.” Bulgaria’s own statement said only that the satellite signal had been disrupted.

The attribution is disputed. On 4 September 2025 Russian Foreign Ministry spokeswoman Maria Zakharova called the allegation “preposterous” and “100% false,” and said Flightradar24 data showed the jet reported good GPS signal quality from take-off to landing. The same AP report says Latvia’s Electronic Communications Office had identified at least three jamming hot spots along its Russian border in August 2025.

India has its own record. In a written reply to the Rajya Sabha on 1 December 2025, Civil Aviation Minister K. Rammohan Naidu said flights approaching runway 10 at Delhi’s Indira Gandhi International Airport had reported GPS spoofing and used contingency procedures. Flights on other runway ends, which rely on conventional navigation aids, were not affected. He added that GNSS interference reports were coming in from Kolkata, Amritsar, Mumbai, Hyderabad, Bengaluru and Chennai. The Directorate General of Civil Aviation had issued a procedure the previous month for real-time reporting of such incidents around the Delhi airport. All India Radio’s account of the reply gives no source for the signals.

India’s regional system, and the satellite it is waiting for

ISRO’s programme page calls NavIC an independent regional system being developed by India, with a primary service area reaching 1,500 km beyond India’s boundary. It says the expected position accuracy there is better than 20 metres, and that an open service for all users is joined by an encrypted Restricted Service for authorised users. The Week, reporting on 29 September 2026, presents India’s reliance on foreign-owned satellite navigation during the 1999 Kargil conflict as the lesson behind it.

Two setbacks cut the count. NVS-02, launched on 29 January 2025, did not reach its working orbit: ISRO’s mission page says the valves that admit oxidiser to the orbit-raising thrusters did not open, leaving a healthy satellite in an elliptical orbit. In March 2026 the atomic clock on IRNSS-1F stopped, according to Inside GNSS, which cites ISRO. The satellite remains usable for limited services such as one-way messaging but no longer supports standard navigation that depends on precise onboard timing.

That leaves three usable NavIC satellites: IRNSS-1B, IRNSS-1I and NVS-01, The Week reports. Four are needed for standalone positioning. Girish Linganna, a space and defence analyst quoted by The Week, put it as “Four unknowns demand four satellites.” ISRO has had NVS-03 at its Sriharikota launch site since August 2026, and The Week reports it is planned for launch on a GSLV Mk II in mid-October 2026. NVS-04 and NVS-05, Minister of State for Space Jitendra Singh told the Lok Sabha in July, are in advanced development.

NVS-03 carries rubidium atomic clocks, a mix of Indian-made and imported units, and broadcasts in the L1 band that ordinary GPS receivers already use. India’s first home-built atomic clock flew on NVS-01 in May 2023. Once NVS-03 reaches its slot, The Week counts four working NavIC satellites.