GPS BIIF-4 (PRN 27)

GPS BIIF-4, catalogued by NORAD under identifier 39166 and carrying the international designator 2013-023A, is an American navigation satellite operated by the United States Air Force. Better known by its operational designations — among them USA-242, GPS IIF-4, GPS IIF SV-5, and Navstar-68 — this spacecraft is the fourth member of the Block IIF generation of Global Positioning System satellites. Launched in May 2013 and subsequently brought into operational service, it continues to function as one node in the constellation that underpins precision navigation, timing, and positioning services used across the world every day.

Mission and Purpose

The Global Positioning System is a satellite-based radionavigation network maintained by the United States government. It provides continuous, all-weather positioning and timing data to an enormous variety of users — from commercial aviation and maritime shipping to personal smartphones and precision agricultural equipment. The constellation is structured so that a sufficient number of satellites remain visible above the horizon from virtually any point on Earth at any given moment, allowing receivers to triangulate their position with high accuracy.

GPS BIIF-4 belongs to the Block IIF series, a transitional generation of GPS satellites that bridged earlier legacy designs and the more capable Block III vehicles that followed. The Block IIF satellites were procured to replenish and sustain the constellation during a period when demand for GPS services was growing rapidly and the older satellites they replaced were approaching or exceeding their design lifespans. Each Block IIF satellite broadcasts navigation signals on multiple frequencies, including the civilian L2C signal that earlier GPS generations could not transmit, as well as the military M-code signal intended for use by armed forces with more robust anti-jamming characteristics. This multifrequency capability allows civilian receivers to correct for a significant source of positioning error caused by the ionosphere, improving accuracy meaningfully for users with compatible equipment.

The spacecraft received the informal launch campaign name "Vega" — a practice common within GPS launch operations — before settling into its operational role within the constellation. Once positioned in its assigned orbital slot and verified through a checkout period, it was declared operational and began contributing to the global navigation service. The satellite is assigned Pseudo-Random Noise code 27, or PRN 27, which is the identifier that GPS receivers use to distinguish its signal from those broadcast by other satellites in the constellation.

Orbit and Tracking

GPS BIIF-4 occupies a Medium Earth Orbit, the orbital regime characteristic of the entire GPS constellation. Current tracking data places its apogee at approximately 20,565 km and its perigee at approximately 19,813 km above Earth's surface, yielding a nearly circular orbit with only modest eccentricity. The orbital inclination is 54.5 degrees relative to the equatorial plane, a value deliberately chosen for the GPS constellation to ensure adequate coverage across most populated latitudes while also providing useful coverage at higher latitudes important for military and polar navigation.

The satellite completes one full orbit approximately every 718 minutes, which works out to roughly eleven hours and fifty-eight minutes per revolution. This period is very close to exactly half a sidereal day — a defining characteristic of the GPS orbital design. Because each satellite completes almost exactly two orbits for every rotation of the Earth, a given GPS satellite traces nearly the same ground track each day, making the geometry of the constellation highly predictable and repeatable. Users and system operators can rely on knowing, with considerable confidence, which satellites will be visible from a given location and at what times.

The spacecraft is tracked by the United States Space Force's Space Surveillance Network and listed in the publicly accessible catalog maintained by Space-Track.org. Its NORAD catalog number, 39166, is the identifier used by tracking databases worldwide to distinguish it from the thousands of other objects in Earth orbit. As of the time of publication, the satellite remains in orbit and has not undergone any reentry or decay event.

Design and Operator

GPS BIIF-4 was manufactured by Boeing, which held the contract for the Block IIF series under an agreement with the United States Air Force. Boeing's satellite manufacturing heritage in the GPS program extends back through multiple generations, and the Block IIF design drew on considerable accumulated experience in building spacecraft intended for long-duration operation in the demanding medium Earth orbit environment. MEO subjects satellites to repeated passages through the Van Allen radiation belts, making radiation hardening of onboard electronics a critical engineering requirement. The Block IIF satellites were designed with this environment in mind, incorporating radiation-tolerant components and systems intended to sustain reliable operation over an extended service life.

The United States Air Force served as the operator of GPS BIIF-4 at launch and through most of its operational history, with Space Force subsequently assuming responsibility for the GPS constellation following the establishment of that service branch in 2019. The 2nd Space Operations Squadron, based at Schriever Space Force Base in Colorado, is the unit responsible for day-to-day command and control of GPS satellites, including the upload of navigation message data that satellites broadcast to users. The ground control architecture — known as the Operational Control Segment — maintains contact with each satellite in the constellation through a global network of ground antennas and monitoring stations.

The satellite was launched on May 14, 2013, with liftoff occurring during the evening hours in the Eastern time zone. It was carried to orbit aboard a United Launch Alliance Atlas V rocket from Cape Canaveral, which is the standard launch vehicle used for the Block IIF series. The mass of this particular spacecraft is not publicly recorded in the available catalog data. Following launch and insertion into its target orbital regime, the satellite underwent a period of on-orbit checkout and testing before being declared operational on June 21, 2013, approximately five weeks after launch.

Significance and Current Status

The Block IIF series as a whole represented an important step forward for the GPS program. Compared with the Block IIA and Block IIR satellites they supplemented, the Block IIF vehicles introduced enhanced signal capabilities that served both civil and military users. The addition of the L5 signal — a third civilian frequency — on Block IIF satellites was particularly significant for safety-of-life applications such as aviation, where redundant frequency measurements provide more reliable and precise positioning under challenging ionospheric conditions. Civil aviation authorities and standards bodies invested considerable effort in certifying GPS receivers capable of using L5, anticipating a future in which the full constellation would offer this capability.

As the fourth Block IIF satellite launched, GPS BIIF-4 contributed to the gradual expansion of L5 coverage as successive satellites joined the constellation. With each additional Block IIF and, later, Block III satellite, the probability of a receiver having simultaneous access to multiple L5-capable satellites increased, moving the program closer to the threshold at which L5 could be formally declared operational for aviation use.

More broadly, GPS BIIF-4 is one element in a constellation that has become foundational infrastructure for the modern global economy. The system's timing signals underpin financial transaction networks, telecommunications synchronization, and power grid management in ways that are often invisible to end users but are nonetheless critical. Positioning services derived from GPS inform logistics, emergency response, precision agriculture, surveying, and a vast ecosystem of consumer applications. Each satellite in the constellation, including GPS BIIF-4, thus carries a significance that extends well beyond its individual specifications.

The satellite continues to orbit Earth as of the most recent catalog update, operating in the medium Earth orbit band alongside its Block IIF siblings and the newer Block III satellites that have joined the constellation in subsequent years. Its orbital parameters remain consistent with the GPS constellation design, and no decay or reentry event has been recorded for this object.

Observing GPS BIIF-4

GPS BIIF-4 is not a candidate for casual naked-eye observation under normal circumstances. At an orbital altitude of roughly 20,000 km, the spacecraft is far more distant than the low Earth orbit satellites — such as the International Space Station — that are commonly observed by amateur skygazers. At that range, even a satellite of reasonable physical size reflects insufficient sunlight to appear as a readily visible object without optical aid. Dedicated amateur astronomers equipped with telescopes and appropriate tracking software may, under favorable geometry and lighting conditions, detect GPS satellites as faint moving points, but this requires both equipment and planning beyond standard satellite watching.

For those interested in tracking the satellite's position computationally, its NORAD ID 39166 can be used to retrieve current two-line element sets from public sources such as Space-Track.org or CelesTrak. These elements, updated regularly, allow prediction of the satellite's position and visibility windows using widely available satellite tracking applications.