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McCaskill’s work cleared a big hurdle in development of GPS
by Chuck Gordon

Tom McCaskill ’58’s eureka moment was, he implies, divinely inspired.

It was the late 1960s, and McCaskill was a young mathematician early in his career at the Naval Research Laboratory in Washington, D.C., across the Potomac from Alexandria, Virginia. McCaskill worked in the space applications branch for Roger Easton, an NRL physicist whose vision led to the development of Global Positioning System technology.

McCaskill was still in his 20s but proving useful to Easton as the Cold War simmered. McCaskill, Easton and other scientists and math whizzes were civilians drafted in to provide the military with some of the weapons needed to keep potential enemies at bay. In the 1960s, in the wake of the Bay of Pigs invasion, the primary adversary was the Soviet Union.

Tom McCaskill holds a book about GPS

GPS had massive potential to help protect Americans from the Soviet threat. Knowing a ship’s precise whereabouts at all times could be a huge advantage for the world’s largest navy, which had cruisers and destroyers scattered across the globe. “One of my uncles, Commander James Robert MacKenzie, said that in the Navy there’s only three things you need to know: You’ve got to know where you are, what time it is and how much money you’ve got,” McCaskill says. “GPS will take care of two of the three.”

Easton was intent on developing the technology. In the 1950s, he had designed the Vanguard satellite, the U.S.’s answer to Sputnik, and envisioned a space surveillance system. GPS was a key to making it work. A decade after developing Vanguard, Easton assigned McCaskill the task of calculating a ground receiver’s position, instantaneously, based on two types of data from a satellite. It was a vital step in the development of GPS, a technology most of us use every day, but it was an extremely complicated problem at the time.

At this point, in 1968, GPS was still about a decade from deployability. But Easton and McCaskill were closing in on several key components of the technology – a huge one being instantaneous positioning. The atomic clocks in the satellites that were orbiting the Earth ran faster than their counterparts in receivers on the ground. Unless those times synched up, there would be no way to tell a ground receiver’s precise position.

McCaskill had been wrestling with the problem for six months when an idea came to him one day – “all of a sudden” – while he was in church. “You’ve heard about inspiration from God,” he says. “When I got back to the lab, I tried it and it worked.”

Using the Theory of Relativity, McCaskill devised an intricate mathematical solution to correct for the time difference, thus clearing a sizable hurdle in the development of GPS technology. It’s why, today, your GPS can pinpoint your location so accurately.

“It’s literally the key to GPS, what makes it work,” says Tom’s wife, Helen McCaskill ’59.

Without McCaskill’s contributions, lots of things we take for granted today might not be possible.

Doing the math

In the past couple of decades, Global Positioning System technology has changed the world. Pilots use it to navigate the airways. Farmers use it to determine which parts of their fields need fertilizing. The military, of course, puts it to a variety of uses. And Uber and Lyft would be shells of themselves without it.

As convenient and functional as it is, GPS isn’t perfect. It rankles the McCaskills a bit when visitors use GPS to find their house in Unionville. The technology doesn’t quite work correctly.

“The GPS wants you to turn into the woods,” Helen says.

Tom quickly clarifies: “That depends on the database here on earth, not up there in the satellite.”

McCaskill should know. After all, he did the calculations for much of the NRL’s satellite-navigation activities. “I was the principal one that did the math,” he says.

McCaskill, who spent his entire 42-year career at the NRL, did more than just complicated computations. He and other NRL scientists, mathematicians and engineers traveled all over the world, testing atomic clocks and helping explain the concepts behind the technology as it evolved. Easton was the visionary, but it was McCaskill who most often talked about their work at conferences.

“Roger was not a speech-giver, so Tom gave all the speeches that needed to be given,” Helen says.

"In the Navy there’s only three things you need to know: You’ve got to know where you are, what time it is and how much money you’ve got. GPS will take care of two of the three.”

In 1966, McCaskill found himself at a frequency-control symposium sponsored by the IEEE (Institute of Electrical and Electronics Engineers), of which he is now a life member. He was scheduled to give his first-ever professional speech, about the atomic clocks in the NRL’s satellites. “Our paper that I presented was on the last day, Friday afternoon, after the coffee break, and I figured nobody was going to come,” McCaskill says.

Instead, it was standing-room only. “I found out what the podium’s for,” McCaskill says. “It’s to hang onto.”

It wasn’t just military personnel and scientists who were interested in what McCaskill had to say. Even then, a large variety of companies understood what pinpoint navigation could mean for the world. It is estimated that, as of 2017, GPS had benefited the private sector to the tune of $1.4 trillion, the bulk of it in the preceding five years. “I don’t get a cent of that,” McCaskill says with a cackle.

After solving the instantaneous-positioning problem, McCaskill continued to work alongside Easton as the Navy went full steam ahead on Easton’s Timation (time + navigation) system, which is the basis for GPS. But it took years for Easton and his team to get full credit. They were pulled off GPS in the early 1970s after the Air Force took over the program. 

Brad Parkinson, an Air Force colonel and head of a joint program at the Pentagon that ultimately oversaw the GPS program, and Ivan Getting, founding president of the Aerospace Corporation, were for years considered the key inventors. “You wouldn’t believe the competition between the Navy and the Air Force,” Helen says, “like they’re on the football field.”

Not until 2010 did the National Inventors Hall of Fame recognize Easton’s efforts. For his work, Easton eventually received the National Medal of Technology from the second Bush Administration.

McCaskill is justifiably proud of the work he and the rest of the NRL’s space applications branch did to bring GPS to reality.

“I feel it’s worthy of a Nobel physics prize for relativity,” McCaskill says of the team’s work on Timation.

Big contributions

McCaskill grew up in tiny Carthage, N.C., which he describes as a pretty rough place in the 1940s. On the first day of first grade, on his way to the playground, he was ambushed. “I went down the stairs, made a left-hand turn, and ‘Wham!’ There was a guy who gave me a bloody nose,” he says. “I got a bloody nose on the first day. And the bottom line, things did not improve.”

More interested in getting an education than nursing his wounds every day, McCaskill spent a good deal of his childhood at the Carthage public library, where he could quench his intellectual thirst. His diligence made an impression on the local librarian. “Miss Dewey said she had a vision that I was going to do great things,” McCaskill says. “I didn’t know what the great things were, but she was right.”

Tom McCaskill's yearbook photo, sophomore year

It took Wingate Junior College, and its math and chemistry professors, to set him on the proper course. Although the McCaskill family was relatively poor, Thomas and his five siblings all managed to graduate from college. McCaskill followed his older brother Hugh McCaskill ’57 to Wingate. For McCaskill, Wingate offered an opportunity to bone up on some science and math courses before he transferred to N.C. State University.

He loved every minute of it. McCaskill sharpened his academic interests by participating in the chemistry and math clubs and the Sophomore Engineering Society, and he was on the honor council and was in Who’s Who.

Also, his social life improved. “I got more than an education,” he says. “I got a wife!”

McCaskill met Helen Jane Latham during his sophomore year. They began dating and continued dating (by mail) while Tom went to N.C. State and Helen, a year later, moved on to Appalachian State Teachers College. After graduating from State (with honors) with a degree in engineering mathematics, McCaskill faced a decision. He had a few job offers, none of which was in North Carolina and therefore near his beloved Helen.

“My best two offers, one came from Phillips 66 Petroleum out in Oklahoma, and the other from the Naval Research Laboratory,” he says. “I knew if I went to Oklahoma I’d never see her again.”

The NRL turned out to be a good fit. “Roger, there’s no question he’s a very smart man, a visionary and everything, but he wasn’t all that good at mathematics,” McCaskill says. “He hired seven mathematicians.”

It didn’t take long for McCaskill to make his mark. Using data from the Echo I and Echo II satellites, McCaskill discovered and analyzed an effect related to calibrating the U.S. Navy Space Surveillance System that led to a co-patent, alongside Easton.

Over time, McCaskill became Easton’s go-to guy for figuring out the equations necessary to turn Easton’s visions into reality. Among McCaskill’s other contributions to GPS, he derived all of the “dilution of precision” mathematics that were used to evaluate the performance of satellite constellations under consideration for GPS. He also determined the optimal inclination for the constellation that was later adopted for GPS.

In 1974, McCaskill was the first to derive and program a technique for measuring the frequency stability of the orbiting Navigation Technology Satellite-1 spacecraft clock. His title was ultimately upgraded to “research physicist.”

In 2005, having recently retired from the NRL, McCaskill felt compelled to again work for a branch of the government: the legislative branch. Running a campaign that focused primarily on eradicating domestic violence, separating church and state, and promoting universal health care, McCaskill ran for the U.S. Senate, trying for the Maryland seat formerly held by Paul Sarbanes.

Although McCaskill was unsuccessful in his attempt to get into Congress, his impact on the world continues to be felt today. Every time a fire or rescue truck reaches its destination quickly, a plane lands safely, or (in the near future) your self-driving car delivers you to your destination without incident, you can thank GPS – and McCaskill.

“The most important benefit is that GPS saves lives,” McCaskill says.


Shortly after the Winter 2019 issue of Wingate magazine went to press, Tom McCaskill succumbed to a battle with cancer. He was 81.