The People Who Built GPS and Its Predecessors

Modeling the Earth beneath the orbit West worked on mathematical models of Earth’s shape and on satellite geodesy at the Navy’s research facility in Dahlgren. Models from that work were later incorporated into GPS. Her part of the story starts with the ground under the receiver. To use satellite measurements for navigation, you need a model of the planet those measurements are tied to.

Putting precise time in orbit Easton’s Timation experiments at the Naval Research Laboratory tested navigation using time-synchronized satellites. Timation I went up in 1967. A receiver could use the timed signal to estimate how far away the satellite was, an approach that carried into GPS.

Making separate ideas work as one system Parkinson led the program that brought earlier approaches together into the GPS design. The job included developing and testing the pieces that had to work together, from spacecraft to receivers. He also pushed for equipment people could use without making each receiver prohibitively expensive.

Letting satellites share a frequency Spilker helped design the ranging signal for civilian GPS receivers. Different codes let a receiver tell satellites apart even when they use the same frequency. His team also built an early receiver to process the signal, so this story is about what a listener could actually pick out of the broadcast.

Making the case for a positioning system Getting advocated using satellites to locate moving vehicles. His contribution was in advancing the idea and pressing for a program capable of delivering it. Read his role alongside Parkinson’s to see the difference between making the case for a system and directing its design and tests.

Making the early GPS satellites last Schwartz managed Rockwell’s GPS satellite program. His team had to protect the spacecraft against radiation in the upper Van Allen belt. The satellites had a three-year design life, and some of the early spacecraft operated for more than nine. That durability mattered for a navigation service that depended on keeping multiple working satellites in orbit.

Shrinking the clock An atomic clock has to fit on the satellite and survive there. Fruehauf helped develop a radiation-hardened rubidium clock and later worked on a miniature rubidium oscillator that met GPS accuracy requirements. His work shows why proving a clock in the lab and making one that belongs on a spacecraft are different jobs.

Turning the change in pitch into an orbit Guier and George Weiffenbach used the changing frequency of Sputnik’s signal to work out its orbit. Guier later helped derive a model of Earth’s gravity field from tracking data for Transit. Both steps show what careful analysis can recover from signals a satellite is already sending.

Getting a signal worth measuring Weiffenbach had the receiver and frequency reference that let him and William Guier listen to Sputnik. He found a way to turn their recorded signals into digital data for analysis and continued working on Doppler tracking for Transit. The mathematical idea needed observations that were stable enough to test it.

Reversing the tracking problem McClure saw a different use for the method his colleagues had tested. If the satellite’s orbit was already known, the same kind of signal could tell an observer where they were on Earth. That reversal became the basis of Transit.

Making Transit work in practice Kershner led the Johns Hopkins team that turned the idea into a working navigation system. A receiver needed both the changing frequency it measured during a pass and the orbital data the satellite broadcast. The system depended on satellites, tracking stations, computing, and receivers all working together.