GPS BIIR-13 (PRN 02)

GPS BIIR-13 (PRN 02), catalogued by NORAD as object 28474 and carrying the international designator 2004-045A, is an American navigation satellite operated by the United States Air Force as part of the Global Positioning System constellation. Launched on 5 November 2004, it is also known by its alternate designations USA-180 and GPS SVN-61. The satellite marked a notable milestone within the Block IIR generation: it was the thirteenth of twenty-one such spacecraft to reach orbit and the final example built in the original, unmodified Block IIR configuration before subsequent vehicles incorporated modernized signal capabilities. As of the time of writing, the satellite remains in orbit and continues to function as a constituent element of the GPS network.

Mission and Purpose

The Global Positioning System is a satellite-based radionavigation network maintained by the United States government and made available worldwide, free of charge, for civilian and military users. By continuously broadcasting precise timing and ranging signals, GPS satellites allow receivers on the ground, at sea, or in the air to calculate their position, velocity, and time with high accuracy. The constellation is organized so that at any given moment, a sufficient number of satellites are above the horizon from virtually any point on Earth, providing continuous global coverage.

GPS BIIR-13 (PRN 02) forms one node in this global web. Its pseudo-random noise code identifier, PRN 02, is the numeric label by which GPS receivers and signal-processing systems identify and lock onto its transmissions. Each satellite in the constellation carries atomic clocks of exceptional stability, and the integrity of the timing signal these clocks provide is fundamental to the sub-meter or meter-level positioning that the system achieves. The Block IIR series to which this satellite belongs was designed with improved onboard autonomy compared to earlier generations, giving these satellites the ability to maintain navigation accuracy for limited periods even in the absence of ground contact — a characteristic that enhanced the resilience of the overall system.

The satellite's operator, the United States Air Force, is responsible for the command, control, and maintenance of the GPS space segment, coordinating through the dedicated ground infrastructure that monitors satellite health, uploads corrected ephemeris data, and manages the constellation as a whole. While the Air Force oversees the space and control segments, the system itself is a dual-use resource, supporting both military operations and the vast global ecosystem of civilian navigation, timing, and positioning services that has grown up around GPS over several decades.

No specific public catalog record details the precise operational mission status of this individual satellite at present, and its current service condition is not confirmed in publicly available tracking databases. However, given its launch date and the typical operational design life of Block IIR satellites, it represents one of the more mature vehicles in the GPS fleet.

Orbit and Tracking

GPS BIIR-13 (PRN 02) operates in medium Earth orbit (MEO), the orbital regime specifically chosen for navigation constellations because it offers a practical balance between coverage geometry and signal propagation delay. At medium Earth altitudes, a relatively small number of satellites — arranged in carefully designed orbital planes — can provide continuous global coverage, whereas achieving the same from low Earth orbit would require many more vehicles.

The satellite's current tracked orbital parameters place its apogee at 20,641 km and its perigee at 19,739 km above Earth's surface, indicating a nearly circular orbit with only a modest difference between its highest and lowest points. This near-circularity is characteristic of GPS satellites and is important for maintaining consistent signal geometry and predictable coverage patterns. The orbital inclination is 55.1°, meaning the satellite's ground track spans latitudes from approximately 55° north to 55° south, ensuring useful coverage across the densely populated mid-latitudes as well as significant portions of the polar regions. An inclination of this magnitude, combined with the altitude, is standard across the GPS Block IIR fleet and is part of the deliberate constellation design that keeps the satellite geometry favorable for users worldwide.

The orbital period is approximately 718.0 minutes, or roughly twelve hours. This is not coincidental: GPS satellites are designed to complete almost exactly two orbits per sidereal day, which causes the ground track — the path traced over Earth's surface — to repeat from one day to the next. This repeatability was an intentional feature of GPS orbital design, as it simplifies the predictability of satellite visibility for ground-based operators and users alike, and it means that any given location on Earth sees the same satellites pass overhead at the same local times each day.

NORAD catalog number 28474 is the unique identifier assigned to this object in the Two-Line Element (TLE) catalog maintained by the United States Space Force's 18th Space Control Squadron. This catalog entry is the basis for the orbital elements published on tracking platforms, and it is updated regularly as new observations refine the satellite's state vector. Observers and analysts using satellite-tracking software can use this NORAD ID to retrieve current predictions for the satellite's position at any given time.

Design and Operator

GPS BIIR-13 (PRN 02) was built by Lockheed Martin, the American aerospace and defense company that served as the prime contractor for the entire Block IIR series. The satellite is based on the AS-4000 satellite bus, a platform developed by Lockheed Martin (drawing on heritage from the earlier Astro Space division) that provides the structural, power, thermal, and attitude-control infrastructure onto which the GPS payload is mounted. The AS-4000 bus was selected for the Block IIR program because of its ability to support the power requirements of GPS payloads, its radiation-hardened electronics suitable for the medium Earth orbit radiation environment, and its design for extended operational life.

As the thirteenth launched Block IIR satellite and the last in the original configuration, GPS BIIR-13 occupies a specific place in the production and launch history of the series. The subsequent Block IIR satellites — designated Block IIR-M — incorporated new civil and military signals not present on the original IIR vehicles, including a second civil frequency (L2C) and a new military M-code signal. By launching before this transition, USA-180 completed the original IIR manifest while the modernization effort was already underway, representing the end of one design chapter and the beginning of another within the same broad family of spacecraft.

The United States Air Force — now institutionally succeeded in its space roles by the United States Space Force, which was established in December 2019 — has historically been the operating authority for the GPS constellation. The Space Force inherited responsibility for GPS satellite operations, though the Air Force designation remains associated with assets procured and launched before the organizational transition.

The satellite's mass is not confirmed in the publicly available catalog record for this object.

Significance and Legacy

GPS BIIR-13 (PRN 02) occupies a specific niche in the history of satellite navigation as the concluding member of the unmodified Block IIR line. The Block IIR generation as a whole represented an important step in GPS evolution: these satellites introduced autonomous navigation capability, improved accuracy, and greater operational robustness compared to the original Block II and Block IIA vehicles they were designed to supplement and eventually replace.

The GPS constellation that users around the world depend upon for navigation, timing synchronization, and a wide range of derived applications is not a static infrastructure but rather a continuously managed and gradually renewed fleet. Satellites from multiple generations coexist in the constellation at any given time, and the contribution of any individual vehicle is determined not only by its technical specifications but by its position in the orbital geometry. A single GPS satellite failure or degradation can affect coverage quality in certain regions, which is why constellation management — including decisions about when to maneuver, retire, or replace individual satellites — is a continuous operational concern.

As one of the older assets remaining in the GPS constellation based on its 2004 launch date, GPS BIIR-13 represents the kind of long-lived operational asset that characterizes well-engineered government spacecraft programs. Whether it remains actively contributing to the navigation solution or has been placed in a reserve status is not confirmed in the publicly available tracking data. Regardless of its precise current role, its two decades in orbit reflect the durability of the Block IIR design and the careful stewardship of the GPS fleet by its operators. Its presence in the NORAD catalog as an active, tracked payload confirms that it has not decayed or reentered the atmosphere, and it continues to be monitored as part of the broader space object surveillance mission that supports both spaceflight safety and operational continuity of the GPS system.