Question: The IERS Reference Meridian (IRM), is the prime meridian (0° longitude) maintained by which organization, it is also the reference meridian of the Global Positioning System (GPS), originally Navstar GPS, is a space-based radionavigation system owned by the United States government and operated by the United States Air Force?
Context: Longitude ( or , Australian and British also ), is a geographic coordinate that specifies the east-west position of a point on the Earth's surface.  It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda (λ).  Meridians (lines running from the North Pole to the South Pole) connect points with the same longitude.  By convention, one of these, the Prime Meridian, which passes through the Royal Observatory, Greenwich, England, was allocated the position of zero degrees longitude.  The longitude of other places is measured as the angle east or west from the Prime Meridian, ranging from 0° at the Prime Meridian to +180° eastward and −180° westward.  Specifically, it is the angle between a plane containing the Prime Meridian and a plane containing the North Pole, South Pole and the location in question.  (This forms a right-handed coordinate system with the z axis (right hand thumb) pointing from the Earth's center toward the North Pole and the x axis (right hand index finger) extending from Earth's center through the equator at the Prime Meridian.) A prime meridian, based at the Royal Observatory, Greenwich, in London, was established by Sir George Airy in 1851.  By 1884, over two-thirds of all ships and tonnage used it as the reference meridian on their charts and maps.  In October of that year, at the behest of U.S. President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., United States, for the International Meridian Conference.  This conference selected the meridian passing through Greenwich as the official prime meridian due to its popularity.  However, France abstained from the vote, and French maps continued to use the Paris meridian for several decades.  In the 18th century, London lexicographer Malachy Postlethwayt published his African maps showing the "Meridian of London" intersecting the Equator a few degrees west of the later meridian and Accra, Ghana. A prime meridian is a meridian (a line of longitude) in a geographical coordinate system at which longitude is defined to be 0°.  Together, a prime meridian and its antimeridian (the 180th meridian in a 360°-system) form a great circle.  This great circle divides the sphere, e.g., the Earth, into two hemispheres.  If one uses directions of East and West from a defined prime meridian, then they can be called Eastern Hemisphere and Western Hemisphere. The Tenerife meridian was the prime meridian of choice for Dutch cartographers and navigators from the 1640s until the beginning of the 19th century.  It lies at 16° 38′ 22″ W of the current international prime meridian, the IERS Reference Meridian through Greenwich. The IERS Reference Meridian (IRM), also called the International Reference Meridian, is the prime meridian (0° longitude) maintained by the International Earth Rotation and Reference Systems Service (IERS).  It passes about 5.3 arcseconds east of George Biddell Airy's 1851 transit circle or 102.478 m at the latitude of the Royal Observatory, Greenwich.  It is also the reference meridian of the Global Positioning System (GPS) operated by the United States Department of Defense, and of WGS84 and its two formal versions, the ideal International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF). The Global Positioning System (GPS), originally Navstar GPS, is a space-based radionavigation system owned by the United States government and operated by the United States Air Force.  It is a global navigation satellite system that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. A GPS satellite is a satellite used by the NAVSTAR Global Positioning System (GPS).  The first satellite in the system, Navstar 1, was launched February 22, 1978.  The GPS satellite constellation is operated by the 50th Space Wing of the United States Air Force. USA-213, also known as GPS SVN-62, GPS IIF SV-1 and NAVSTAR 65, is the first satellite in the Block IIF series of Global Positioning System navigation satellites.  It will be used to relay signals for the United States Air Force Navstar Global Positioning System (GPS).  The satellite was launched at 03:00 UTC on 28 May 2010.  It will be placed into plane B of the GPS constellation, and will transmit the PRN-25 signal.  PRN-25 was previously broadcast by USA-79, which was retired in late 2009 after almost eighteen years of service. GPS Block IIIA, or GPS III is the next generation of GPS satellites, which will be used to keep the Navstar Global Positioning System operational.  Lockheed Martin is the contractor for the design, development and production of the GPS III Non-Flight Satellite Testbed (GNST) and the first eight GPS III satellites.  The United States Air Force plans to purchase up to 32 GPS III satellites.  GPS IIIA-1, the first satellite in the series, was projected to launch in 2014, but significant delays have pushed the initial launch to the spring of 2018. GPS Block IIF, or GPS IIF is an interim class of GPS (satellite), which are used to keep the Navstar Global Positioning System operational until the GPS Block IIIA satellites become operational.  They are being built by Boeing, and will be operated by the United States Air Force following their launch by United Launch Alliance, using Evolved Expendable Launch Vehicles.  They are the final component of the Block II GPS constellation to be launched.  On February 5, 2016, the final satellite in the IIF-block was successfully launched, completing the block.
Answer: International Earth Rotation and Reference Systems Service (IERS)