FACTBOX: Five facts about Russian military intelligence

(Reuters) - Russia's President Dmitry Medvedev on Friday sacked Valentin Korabelnikov, the head of Russia's powerful military intelligence service.

Following are some key facts about the service:

* Russian military intelligence service is known by its Russian acronym GRU, which stands for Main Intelligence Directorate. Moscow's other, better-known Foreign Intelligence Service (SVR) is the successor to the KGB's First Chief Directorate.

Unlike the KGB, GRU was not split up when the Soviet Union collapsed. It has a special status and answers directly to the chief of the general staff, one of the three people who control Russia's portable nuclear control. GRU chiefs are picked by the president.

* Russian military intelligence has a spy network abroad that is believed by espionage experts to be several times bigger than that of Russia's Foreign Intelligence Service.

Its experts decipher and analyze espionage information gathered by dozens of Russian military space satellites.

GRU also has several elite special forces units that fought in many post-World War Two conflicts including Afghanistan and Chechnya.

Sulim Yamadayev, who was shot dead last month in Dubai, was commander of GRU's Vostok battalion which fought in Chechnya and Georgia.

* GRU, whose official emblem features a bat hovering above the globe, was founded as the Registration Directorate in 1918 after the Bolshevik Revolution. Revolutionary leader Vladimir Lenin insisted on its independence from other secret services, and ever since GRU has been seen as a rival by other Soviet secret services.

* GRU has confirmed or tacitly accepted it was behind some major spy operations abroad. But it has also suffered several humiliating blows to its reputation when some of its top agents defected to the West.

One such defector was Oleg Penkovsky, a friend of the then GRU chief. He informed Washington of a Moscow operation to place nuclear missiles in Cuba. The scandal led to the Cuban crisis and the world balanced on the brink of a full-blown nuclear war for several days. Penkovsky was arrested in 1962 and executed in 1963 after being found guilty of high treason and espionage.

* Vladimir Rezun, a GRU officer who defected to the West in the 1970s, published a partly autobiographical book about the spy service under an assumed name of Viktor Suvorov. He called the then GRU headquarters the "aquarium," the nickname given to the Moscow compound by those working in it.