© Mark Ollig
Today’s Global Positioning System (GPS) had its roots with the US Navy developing the first satellite-based navigation system, known as Transit, in the late 1950s.
The Transit system’s initial purpose was to provide navigation for US Navy submarines.
The system was named “Transit” because it tracked the time it took for satellites to move into and out of view, helping to pinpoint locations on the ground or water.
The US Navy launched the first experimental navigation satellite, Transit 1A, using a three-stage Thor-Able rocket.
The third stage failed to ignite, and the satellite payload fell back to Earth after reaching an altitude of 400 miles.
Although the Transit 1A mission was unsuccessful, it marked the initial attempt at a space-based satellite navigational system.
Launched April 13, 1960, Transit 1B became the first successful US navigation satellite, demonstrating the feasibility and potential accuracy of satellite navigation in determining positions.
Although its orbit around Earth was not perfectly circular, it successfully transmitted radio signals that laid the foundation for today’s GPS.
Transit satellites, operational from 1960 to 1967, were primarily solar-powered. However, early models Transit 4A and 4B used nuclear battery power via a radioisotope thermoelectric generator.
These satellites weighed between 264 and 308 pounds, and they communicated using radio signals at 150 MHz.
Their navigation data is encoded using a pseudorandom noise (PRN) binary code, which serves as a unique identifier for each satellite.
Devices on the ground use this code to calculate their position in relation to the satellites.
A ground-based receiver detects, decodes, and processes signals from orbiting navigational satellites to determine its exact location.
The Transit satellite system, although designed for military navigation, became valuable for scientific research and civilian use.
In 1973, the US Department of Defense began the Navigation System with Timing and Ranging (NAVSTAR) program.
The program officially became known as the Global Positioning System (GPS) in 1978, the same year the first NAVSTAR satellite (OPS-5111/NAVSTAR 1) was launched Feb. 22 using an Atlas F rocket.
The satellite, weighing 845 pounds and measuring 5.3 feet in diameter by 6.7 feet tall, was the first of eleven Block I experimental GPS satellites.
NAVSTAR was powered by solar panels and batteries, and it communicated via L1 (1575.42 MHz) and L2 (1227.60 MHz) radio signals, using PRN codes.
The L1 frequency (1575.42 MHz) transmitted both the Coarse/Acquisition (C/A) code for civilian use and the Precision (P) code.
The P-code, also available on the L2 frequency (1227.60 MHz), was restricted to authorized users.
Due to Selective Availability (SA), a US government policy implemented for national security reasons, the civilian C/A code’s accuracy was intentionally degraded to approximately 328 feet.
The military had exclusive access to an encrypted version of the P-code, known as the Y-code, providing enhanced security and accuracy.
GPS satellites transmit navigation messages containing essential timing and orbital information (ephemeris, almanac, clock corrections, status messages) at a constant rate of 50-bps (bits per second).
This standardized transmission rate ensures compatibility across all subsequent generations of GPS satellites and allows older GPS receivers to function seamlessly with newer satellites.
The frequency at which a GPS receiver updates its position varies depending on the application.
Consumer-receiving devices typically update their position once per second (1 Hz), which aligns with the 50-bps transmission rate.
Pilotless aerial vehicles (UAVs) and autonomous vehicles require much faster update rates, sometimes reaching ten times per second (10 Hz) or even faster, to ensure precise tracking and navigation.
The GPS Block I satellite’s Precise Positioning Service (PPS) signal offered military users location accuracy within a range of approximately 10 to 20 feet.
In comparison, the civilian Standard Positioning Service (SPS) signal accuracy was intentionally degraded to 328 feet due to Selective Availability (SA).
SA, a US government policy, intentionally degraded civilian GPS accuracy by manipulating satellite data, including timing and location information.
While the military retained access to a more precise signal, civilian accuracy via SA was limited to about 328 feet.
The launch of this NAVSTAR satellite marked an essential step in developing the modern GPS.
In 1983, President Ronald Reagan approved the development of GPS as a dual-use (military and civilian).
The first Block II satellite, a NAVSTAR satellite, was successfully launched into orbit using a Delta II rocket Feb. 14, 1989. It represented the second generation of GPS satellites. It used solar panels and batteries for power and communicated via radio signals at 1227.60 MHz (L2 frequency) and 1575.42 MHz (L1 frequency), transmitting timing and orbital information.
At the outset, only the US military had access to the more accurate PPS signal on both the L1 and L2 frequencies.
In May 2000, the US government ended Selective Availability, improving GPS accuracy and reliability for civilian use worldwide.
Today, the Global Positioning System is owned by the US government and operated by the US Space Force.
Although I use the GPS in my car, I still keep a folded paper highway map in the glove compartment – just in case.
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