How American Soldiers Invented GPS On Labor Day Weekend In 1973

From The Federalist.com (Aug. 31, 2023):

With the letters GPS, we instantly recognize an innovation that has revolutionized our lives. The concept was born half a century ago in a sweltering room at the Pentagon over Labor Day weekend in 1973.

That’s the genesis of the concept for a constellation of platforms orbiting the Earth, transmitting radio signals to determine location. Many years of calculation, experiment, and miniaturization led to the Navigation Signal Timing and Ranging (NAVSTAR) satellites that became known as the Global Positioning System (GPS).

The dedication and intellect of the engineers, scientists, and technicians involved led the way to worldwide freedom from getting lost — and governments’ ability to always know where you are if you’re carrying a phone.

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The Beginnings of Satellite Positioning

So how did that all change? A few days after the Soviet Union launched Sputnik a few hundred miles above the Earth’s surface, scientists at the Johns Hopkins Applied Physics Laboratory realized the satellite could be tracked from the ground merely by the radio signals its transmitters emitted by exploiting the Doppler shift as it passed.

In August 1966, the U.S. Air Force submitted proposal 621B to engage these principles in reverse: using the satellite to locate a ground position, such as for search-and-rescue operations. Although useful for military use, that was not economically scalable. It also required an enormous quantity of satellites for global coverage from low-Earth orbit.

Then over Labor Day weekend in 1973, a group of U.S. Air Force officers led by Col. Bradford Parkinson met in empty Pentagon rooms to bring this idea to fruition. Ground receivers had to be passive, with no feedback to the transmitters in orbit. The system had to operate worldwide. Intersection from four separate signal emitters within the line-of-sight would correspond to a single receiver location.

Measuring the time elapsed can calculate the distance between the satellite and the receiver. Depending on these time delays, the intersection of the spheres corresponds to the receiver’s location, allowing trilateration plus time offset to compensate for the receiver’s temporal inaccuracy. The convergence in this example points to a location in northern Europe.

Configuring GPS

The Atlas booster then available constrained the payload mass for reaching medium orbit. The teams chose a constellation of at least 24 platforms with 12-hour orbital passes at 12,500-mile altitude to satisfy payload requirements, mapped into six orbital planes with four or five satellites per plane.

To distinguish multiple satellites operating on the same frequency, James Spilker at the company Aerospace created code division multiple access, or CDMA. This technique assigns a particular identifier to each transmitter.

Richard Schwartz and Hugo Fruehauf at Rockwell miniaturized and radiation-hardened rubidium vapor oscillators to serve as the NAVSTAR atomic clocks. These achievements from Air Force, Rockwell, and Aerospace teams are summarized in the Lonely Halls documentary.

Fixing What Went Wrong with the Concept

Sources of errors for this plan abounded. Among these included space-time dilation, revealed by Albert Einstein in 1905 and 1915. Due to special relativity, the NAVSTAR satellite’s orbital speed of 2.4 miles per second decelerates the clocks by seven microseconds per day.

By contrast, general relativity speeds up those same clocks by 45 microseconds per day due to the GPS satellite’s greater distance from the Earth’s core. That reduces that gravitational influence compared to the surface.

Together, failure to compensate for these effects induces an error of more than six miles per day. So before launch, the satellite clocks are slowed down by 0.000000045 percent to synchronize with ground stations.

Additional sources of error include free electrons in the ionosphere, Sagnac distortion, and Earth’s axis perturbations called the Chandler wobble. GPS compensates for these by incorporating two separate transmit frequencies, calibrated against each other, as well as atmospheric modeling and coordinate transformation between transmitter and receiver.

GPS Finally Launches in the 1970s

Led by Roger Easton, Naval Research Laboratory developed a pair of proof-of-concept satellites to test time-based navigation. These were launched in 1974 and 1977.

Starting in February 1978 and continuing until October 1985, the Air Force launched 10 prototype Block-I NAVSTAR satellites aboard Atlas F rockets for testing. From February 1989 through November 1997, it launched nine Block-II and 19 Block-IIA by Rockwell aboard Delta-II rockets (after the exclusion of the Space Shuttle). This formed the initial GPS constellation.

Today, 31 GPS satellites remain operational, not counting four reserve or spare platforms. Various improvements have been incorporated.

Throughout their development, GPS satellites have shared common features. Shown below is the Block-II configuration, weighing about one ton (about double the prototype Block-I). Louvers on the side dissipate excess heat via thermal radiation to maintain consistent internal temperature. Attitude thrusters adjust satellite orientation. Photovoltaic solar arrays convert sunlight into electrical power. [read more]

Interesting military history.