Development of the world’s first global satellite navigation system began in 1958; the US Navy’s navigation satellite system known as Transit, which became fully operational in 1967.
Transit 1B, launched April 13, 1960, was the first successful satellite to support the Transit system, following the unsuccessful launch of Transit 1A in September 1959.
The US Navy utilized the Transit satellite system to assist submarines in navigating their locations by observing changes in satellite signals, similar to the way a sound from a siren changes as it approaches and then moves away.
The first GPS prototype satellite, Navigation Technology Satellite 2 (NTS-2), was launched in 1974 to evaluate GPS functionality.
The first fully operational satellite of the Navigation System with Timing and Ranging (NAVSTAR) GPS, Navstar 1 (also known as GPS-1), was launched in 1978 as part of the Block I series.
In April 1995, the GPS NAVSTAR satellite constellation, consisting of 24 satellites primarily comprised of Block II and IIA models in precisely arranged orbits, became fully operational.
Each of the GPS satellites in the NAVSTAR constellation orbits Earth twice daily at roughly 12,550 miles above the surface.
A GPS receiver calculates its two-dimensional (2D) position (latitude and longitude) using signals from at least three satellites.
If equipped, the receiver can also determine altitude with a signal from a fourth satellite, providing a precise three-dimensional (3D) location fix.
The GPS satellites use solar panels to charge batteries that power their electronics; they are also equipped with small rocket boosters to maintain their trajectory.
Civilian GPS primarily uses the L1 (1575.42 MHz) and L5 (1176.45 MHz) frequencies, with L2 (1227.60 MHz) also available for civilian use.
The military also uses these frequencies, along with the encrypted Precise (P) code on the L1 and L2 frequencies, for enhanced accuracy and protection against GPS spoofing.
GPS spoofing poses a threat by manipulating receiver locations with fake signals, potentially hijacking drones or autonomous vehicles.
Before 2000, civilians had limited GPS access due to intentionally degraded accuracy from Selective Availability, which the US government discontinued in 2000.
In 2020, the Federal Aviation Administration reported that high-quality GPS devices can determine a location on the ground within 16 feet of its actual position 95% of the time, with further improvement to 11.5 feet or less using wide area augmentation system technology.
Lockheed Martin, a global aerospace and defense company, is under contract with the US Space Force and is responsible for constructing the GPS III and future GPS IIIF satellites, with the first projected to launch at the end of 2026.
GPS provides precise positioning and navigation for various applications, including ground travel, aviation, maritime industries, and land surveying.
It also provides accurate timing, essential for transaction verification in financial networks and for use in synchronizing call routing in telecommunication networks.
Additionally, GPS precise timing and location data is used for synchronizing electrical grids, supporting emergency services’ navigation and tracking capabilities.
Most GPS satellites rely on multiple highly accurate rubidium atomic clocks for precise timing and accurate navigation. These clocks feature exceptionally long-term accuracy, losing only about one second every 300 years.
The GPS Block III system incorporates the L1C civilian signal (1575.42 MHz) to improve positioning accuracy. It is compatible with other global navigation satellite systems, such as China’s BeiDou and the European Union’s Galileo.
Suitable for civilian applications like navigation and precision positioning, the L1C signal transmits data at 1,023 bits per second using advanced modulation techniques.
The US government’s GPS website highlights that GPS III technology enhances navigational accuracy and signal reliability, benefiting numerous applications, including precision agriculture tools like GPS-guided farm tractors.
GPS III satellites SV01 to SV05 are in orbit, while GPS III-A (advanced) SV06 was launched Jan. 18, 2023, and GPS III-A SV07 will launch no earlier than January 2025.
GPS III-A satellites contain cesium atomic clocks, which use the vibrations of cesium-133 atoms to generate the clock’s signal, providing incredible precision with an estimated error of less than one second every 300,000 years.
GPS signals, while generally accurate within a few feet in clear weather, can be disrupted by the ionosphere, a layer of charged particles in Earth’s atmosphere.
This interference can cause errors and signal loss, especially during severe space weather events like geomagnetic storms and solar flares, potentially resulting in inaccuracies of tens or even hundreds of feet.
Experts are exploring how artificial intelligence (AI) could improve GPS accuracy, especially in areas with signal interference or tall buildings.
The US Space Force and the National Institute of Standards and Technology are already using AI to strengthen GPS signals and timing.
AI-GPS technology also has potential for use with real-time traffic updates and self-driving cars.
The GPS Control Segment at Schriever Space Force Base in Colorado manages a global network of ground stations that monitor, maintain, and update the current GPS satellite constellation consisting of 31 operational satellites.
The Department of Defense has requested $1.59 billion in the 2025 budget to enhance GPS capabilities.
The official US government GPS website is http://www.gps.gov/.
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| A GPS III-A satellite in orbit. ( Image-Photo by gps.gov) |