With the help of electronic warfare (EW), Russia has learned to throw off the course of American missiles used by the Ukrainian military.
Source. CNN writes about this with reference to sources.
The HIMARS multiple launch rocket systems (MLRS), which the United States began supplying to Ukraine last summer, have radically changed the situation at the front. They are used to destroy supply bases in the Russian rear and played an important role in the success of the fall counteroffensive.
But in recent months, the effectiveness of HIMARS missiles has declined, five representatives of the United States, Britain, and Ukraine told CNN. Russian troops use electronic warfare to disrupt GPS targeting systems.
Russian state media reported in December that Russian air defense forces had received new software that allows them to shoot down HIMARS missiles, citing a unit commander in Zaporizhzhia.
Although the missile can hit a target even with a jammed GPS signal (also because it has significant inertia), HIMARS have recently begun to miss more often, a Ukrainian source told CNN, based on information from drone operators on the front line.
One of them called the failures due to Russian interference "significant" – he had not seen this before November. According to another operator, the effectiveness of HIMARS has significantly decreased.
Ukrainian military experts, with the help of American ones, are looking for ways to circumvent Russian interference, "it's a constant cat-and-mouse game," said a Pentagon official. Such ways include redesigning the guidance system software and the missile itself.
In addition, the United States is helping Ukrainian forces locate and destroy Russian electronic warfare assets. This activity is a "high priority," according to one of the Pentagon documents that became publicly available through a leak.
In total, the United States has supplied Ukraine with 18 HIMARS systems and promised 20 more. Other NATO countries have provided 10 MLRS.