There was news recently that an issue with the attitude control system caused the end of life of the Megha-Tropiques mission. The satellite had completed more than ten years in orbit. This reminded me of a comment we had submitted to the Parliamentary Standing Committee.
One of the comments I had written as part of The Takshashila Institution’s comments to the Parliamentary Standing Committee on Science and Technology, Environment, Forests, and Climate Change addressed said:
The Department of Space must aim to put in orbit replacements for operational satellites before they reach end-of-mission life and not of end-of-design life.
pg 7, Comments to the Parliamentary Standing Committee on Science and Technology, Environment, Forests and Climate Change on Demand for Grants (DFGs) in the Union Budget for FY 2022-23
With reference to Megha-Tropiques, which had a mission life of three years, I meant that we seek to replace them before the end of three years and not the ten years it eventually served because its parts were designed to last this long.
Perhaps, replacing Megha-Tropiques was not top of mind at ISRO or CNES, the French space agency. But, consider a satellite that we plan to launch in the near future, Oceansat-3.
Oceansat-1 was launched in 1999 and had a mission life of 5 years. Oceansat-2, the replacement for Oceansat-1, was launched in September 2009, six months after 10 years of Oceansat-1’s launch anniversary. Oceansat-1 survived for 11 years.
Oceansat-2 had a mission life of 5 years. The scanning scatterometer (SCAT) on board the Oceansat-2 failed after 4.5 years. In 2016, India launched a satellite, SCATSat-1, to replace the functionality. Oceansat-2 has now been functional for 12.5 years. Oceansat-3 has been plagued with delays. The new launch date for Oceansat-3 is now August-September 2022.
The pandemic played a role in the delay. If Oceansat-2 had failed after 11 years like it’s predecessor, we would not have a Oceansat. What would that mean? The Wikipedia page says:
Oceansat satellites facilitate a range of applications including documenting chlorophyll concentration, phytoplankton blooms, atmospheric aerosols and particulate matter as well as marine weather forecast to predict cyclones.
Oceansat Wikipedia page
Forget if the others don’t make sense to you but imagine not being able to predict cyclones with accuracy. Remember the claims of being able to predict cyclones much earlier and hence being able to save more lives? What happens if that function goes kaput?
It was that functionality that was lost and was replaced by SCATSat-1 in 2016. That instrument has now been running for 5.5 years. It has crossed six months since end of it’s five year mission life. Maybe it will survive eleven years like other satellites in the series. Maybe not. Hence, replace satellites before they die.
Update on April 10 @ 2126 hrs IST: @zingaroo on Twitter had an update on SCATSat-1:
Many other things could also go wrong. This is why redundancy is good. But, there are constraints. There are budgetary and people constraints. I think that is why we must work towards replacing the satellite as close as possible to the end of mission life.
This is not the first time I am making this point. I had written an article for The Wire Science in 2019 where this was one of the issues that I had raised. In the article, I applauded ISRO because it was doing much better at replacing satellites of the CARTOSAT series compared to other remote sensing satellites just after mission life.
I thought of putting the suggestion again in the comments to the Parliamentary Committee because I thought the pandemic reinforced the lesson. In case of an unforeseen incident because of which we are not able to replace a satellite before mission life, we have some tolerance before if we launch it before end of design life. But, it we launch it close to end of design life, there is a possibility that there may be a loss of the satellite before we can launch the replacement.
Parallel Spirals 
