Question: Grand Admiral a naval rank in one of the highest in the several European navies that use this system, what system are they using? 

Context paragraphs:
Erich Raeder during World War II: Erich Johann Albert Raeder (24 April 1876 – 6 November 1960) was a naval leader in Germany who played a major role in the Naval history of World War II.  Raeder attained the highest possible naval rank—that of "Großadmiral" (Grand Admiral) — in 1939, becoming the first person to hold that rank since Alfred von Tirpitz.  Raeder led the "Kriegsmarine" (German War Navy) for the first half of the war; he resigned in 1943 and was replaced by Karl Dönitz.  He was sentenced to life in prison at the Nuremberg Trials, but was released early due to failing health.  Raeder is also well known for dismissing Reinhard Heydrich from the Reichsmarine in April 1931 for "conduct unbecoming to an officer and a gentleman".

Commodore (rank): Commodore is a naval rank used in many navies that is superior to a navy captain, but below a rear admiral.  Non-English-speaking nations often use the rank of flotilla admiral or counter admiral or senior captain as an equivalent, although counter admiral may also correspond to rear admiral.

Erich Raeder: Erich Raeder (24 April 1876 – 6 November 1960) was a naval leader in Germany who played a major role in the naval history of World War II.  Raeder attained the highest possible naval rank—that of Grand Admiral — in 1939, becoming the first person to hold that rank since Alfred von Tirpitz.  Raeder led the "Kriegsmarine" for the first half of the war; he resigned in 1943 and was replaced by Karl Dönitz.  He was sentenced to life in prison at the Nuremberg Trials, but was released early due to failing health.

Grand admiral: Grand admiral is a historic naval rank, the highest rank in the several European navies that used it.  It is best known for its use in Germany as "Großadmiral".  A comparable rank in other navies is that of fleet admiral.

Admiral of the fleet: An admiral of the fleet or fleet admiral (sometimes also known as admiral of the navy or grand admiral) is a military naval officer of the highest rank.  In many nations the rank is reserved for wartime or ceremonial appointments.  It is usually a rank above admiral (which is now usually the highest rank in peace-time for officers in active service), and is often held by the most senior admiral of an entire naval service.

Admiral of the fleet (Russia): Admiral of the fleet or fleet admiral (Russian: aдмирал флoта , "admiral flota") is the highest naval (deck) rank of the Russian Federation.  It is the equivalent of the Soviet naval rank of admiral of the fleet and the present Russian rank of general of the army.  The rank is roughly equivalent to 4-star admiral ranks of other nations.  Marshal of the Russian Federation is the only superior rank in the Russian armed forces.

Interwar naval service of Erich Raeder: Erich Johann Albert Raeder (24 April 1876 – 6 November 1960) was a naval leader in Germany before and during World War II.  Raeder attained the highest possible naval rank—that of "Großadmiral" (Grand Admiral) — in 1939, becoming the first person to hold that rank since Alfred von Tirpitz.  Raeder led the "Kriegsmarine" (German War Navy) for the first half of the war; he resigned in 1943 and was replaced by Karl Dönitz.  He was sentenced to life in prison at the Nuremberg Trials, but was released early due to failing health.  Raeder is also well known for dismissing Reinhard Heydrich from the Reichsmarine in April 1931 for "conduct unbecoming to an officer and a gentleman".

Erich Raeder resignation and later: Erich Johann Albert Raeder (24 April 1876 – 6 November 1960) was a naval leader in Germany before and during World War II.  Raeder attained the highest possible naval rank—that of "Großadmiral" (Grand Admiral) — in 1939, becoming the first person to hold that rank since Alfred von Tirpitz.  Raeder led the "Kriegsmarine" (German War Navy) for the first half of the war; he resigned in 1943 and was replaced by Karl Dönitz.  He was sentenced to life in prison at the Nuremberg Trials, but was released early due to failing health.

Admiral: Admiral is one of the highest naval ranks in some navies, and in many navies is the highest rank.  It is usually abbreviated to "Adm" or "ADM".  The rank is generally thought to have originated in Sicily from a conflation of Arabic: أمير البحر‎ ‎ , "amīr al-baḥr", "commander of the sea", with Latin "admirabilis" ("admirable") or "admiratus" ("admired"), the first was admiral Lumu Farouk, although alternative etymologies derive the word directly from Latin, or from the Turkish military and naval rank miralay.

Military rank: Military ranks are a system of hierarchical relationships in armed forces, police, intelligence agencies or other institutions organized along military lines.  Usually, uniforms denote the bearer's rank by particular insignia affixed to the uniforms.  Ranking systems have been known for most of military history to be advantageous for military operations, in particular with regards to logistics, command, and coordination; as time went on and military operations became larger and more complex, military ranks increased and ranking systems themselves became more complex.

Supporting paragraphs: Grand admiral Military rank
Answer: Military ranks