Ever since its inception, India’s regional navigation system has been plagued by problems affecting its positioning data. The Indian Space Research Organisation (ISRO) on Friday (March 13) informed that the atomic clock of one of the five remaining satellites that were still providing this data stopped working — meaning positioning data from the satellite IRNSS-1F was also lost. “IRNSS-1F satellite launched in March 2016 has completed its design mission life of 10 years…On 13th March 2026, procured on-board Atomic clock stopped functioning. However, the satellite will continue to function in-orbit for various societal applications to provide one way broadcast messaging services,” the space agency said.Atomic clocks are key for satellites being able to provide positioning data that is used for applications such as navigation of vehicles, mapping and surveying, and even planning large constructions with accuracy. Importantly, the NVS-02 — launched in January 2025 to replace one of the older satellites in the constellation — was also not able to reach its final orbit and thus provide positioning data.What is the IRNSS — also known as NavIC — and how has the satellite navigation system fared so far? We explain.The Indian Regional Navigation Satellite System (IRNSS) — also referred to with the operational name of Navigation with Indian Constellation (NavIC) — was planned to be a seven-satellite system to provide positioning data over the Indian subcontinent and 1500 km around it.It was designed to be a regional system similar to the American GPS. With all its satellites functioning, the system was designed to provide location accuracy of around 10 metres over the Indian landmass and surrounding countries. The NavIC system is designed to be more accurate over India, with the satellites placed directly above the region. This ensures better availability of signals even in difficult geographical locations than GPS, whose signals are received in India at an angle making it difficult to access in certain areas like valleys and forests.Also in Explained | What ISRO is doing to boost the use of NavIC, India’s version of GPSHowever, the system has been running into troubles since its inception.Story continues below this adHow many satellites in the constellation continue to provide positioning data?After the successful 2023 launch, the constellation had five satellites that could provide the positioning data: IRNSS-1B, 1C, 1F, 1I, and NVS-01, which is the new generation of NavIC satellites. Now, the atomic clock on board the IRNSS-1F has also been lost. Atomic clocks on board some of the initial satellites started failing early on, with replacement satellites planned to keep the system running. Besides the failing atomic clocks, some of the initial satellites are also aging out. IRNSS-1A was launched into orbit on July 1, 2013, and the 1B and 1C satellites were launched in the following year. 1A is almost defunct — the failed 1H mission of 2018 was intended to replace this satellite — and the other two are also past their 10-year mission lives.The last of the first-generation IRNSS satellites was 1I — a replacement for the failed 1H launch — which was launched in 2018. IRNSS-1H was launched in 2017 but did not reach orbit after the heat shield protecting the payload failed to open.What happened to NVS-02?Story continues below this adNVS-02 was the second of the new-generation satellites meant for the navigation constellation. It was successfully placed in a highly elliptical transfer orbit by the GSLV-F15 in ISRO’s 100th launch in January last year. But the satellite failed to move to its final orbit due to an electrical failure.A committee reviewing this found after a thorough analysis of simulation data that the main reason for the failure was a signal not reaching the pyro valve of the oxidiser line of the engine — meaning, the engine could not be ignited. This, the committee felt, was most likely because of the disengagement of at least one of the contacts to the connector — meaning the circuit to deliver the signal could not be completed.More in Explained | Isro’s 100th launch: why this is significant, the road aheadAnother criticism faced by ISRO over the navigation constellation is the delay in developing the user segment. A 2018 report by the Comptroller and Auditor General (CAG) of India said that even though the Cabinet cleared funding of Rs 200 crore to develop user receivers in 2006, work on the project started only in March 2017, by which time seven launches had already taken place, wasting the mission life of several of the satellites.Now, data from NavIC is already in use for aviation, shipping, and railways. And, several of the newer cell phones carry chipsets that can use data from NavIC, along with American GPS and Russian GLONASS.Story continues below this adWhat have been the advancements made to the new-generation satellites?The most important change to the new-generation satellite for the constellation was an indigenously developed atomic clock. ISRO decided to develop its own atomic clocks after several on board the previous generation satellites failed.A satellite-based positioning system determines the location by accurately measuring the time it takes for a signal to travel to and from an object using the atomic clocks on board. Failure of clocks means the satellites are no longer able to provide accurate locations.Also in Explained | Why a regional navigation system matters to IndiaThe newer generation satellites also have a longer mission life of 12 years, instead of the 10 for previous satellites.Story continues below this adImportantly, the new-generation satellites send signals in a third frequency, L1, besides the L5 and S frequency signals that the existing satellites provide. This improves interoperability with other positioning systems as L1 frequency is most commonly used by GPS. This band also helps in using the NavIC data in wearable devices such as smartwatches that use low-power, single-frequency chips.Which other countries in the world have such systems?There are four satellite systems in the world that provide global navigation data — the US Global Positioning System (GPS), the Russian GLONASS (GLObalnaya NAvigatsionnaya Sputnikovaya Sistema), the European Galileo, and the Chinese Beidou. Japan has a four-satellite system called Quasi-Zenith Satellite System (QZSS) that can augment GPS signals over the country.GPS, GLONASS, and Galileo all have over 20 satellites placed in medium-earth orbit at a distance of around 20,000 km from the Earth. Beidou has over 40 satellites in a mix of medium-earth orbit and higher geosynchronous orbits of over 35,000 km. India and Japan’s systems, on the other hand, have fewer satellites — seven and four — all placed in higher geosynchronous orbits.