The time pieces available to them also proved much too inaccurate to be dependable over the long-term. Harrison did actually make some fairly significant strides with his friction-free marine chronometer, but that still resulted in imprecise timekeeping and a short shelf-life, being that seawater and its varying temperatures are quick to corrode most everything it comes into contact with. As a result, even though the larger problem remained unsolved, these early attempts to determine longitude would lay the foundation for making precise GPS location & navigation a present day reality.
Now, fast-forward to the 1950’s, and the basic fundamentals of navigation hadn’t changed. It was still necessary to have a stationary reference point and also a reliable means of calculating precise time. The difference in this instance was that mankind was much better prepared, both technologically and intellectually, for the undertaking. We had long since solved the problem of determining longitude, and from 1884 had been using the Greenwich Meridian, or Greenwich Mean Time as 0° Longitude. The U.S. and the Soviet Union were mired in the beginnings of the cold-war, and the Space Race was motoring along at a rapid pace. After the surprise launch of Sputnik in 1957, two American physicists with the John’s Hopkins Applied Physics Laboratory (APL), William Guier and George Weiffenbach, decided to show some initiative and monitor the satellite and its’ initial activities.
In short order, they noticed that they could get a fix on the satellites’ location along its orbit by changes in the microwave signals it was transmitting. Basically, they could establish the proximity of the satellite due to differences in its radio signal. Being able to accurately predict the satellites’ position they simply…well, maybe not so simply, reversed their calculations to determine the location of the earthbound receiving station in relation to the satellite itself. This is called Doppler Navigation, and is essentially the basic architecture of GPS as we know it today, with the space-based satellite constellation communicating with earthbound GPS receivers. This early discovery would pave the way for the first satellite navigation system and the first predecessor of GPS, called Transit.
Transit was a joint APL/DARPA effort for the U.S. Navy that began development in 1958. The first prototype launch attempt in 1959, Transit 1A, failed to reach orbit. However the second attempt, Transit 1B, successfully entered orbit and tested in 1960, subsequently beginning naval service in 1964. And so, after the dust had settled, Transit stood as mankind’s very first satellite navigation system. The seed was planted and the roots of today’s GPS were beginning to grow!
As much of a pioneering breakthrough as Transit was, it still had its’ shortcomings. The 6 satellites that Transit employed gave a stable point of reference for navigation, with 3 active satellites and 3 being utilized as spares. However, even though considerable improvements in timing had been made, the instrument being used, the stable quartz oscillator, still lacked perfect precision. You might be asking yourself, “Why is this guy so fixated on this precise time nonsense”? If I may, I’ll give a brief explanation.
First, a minimum of 4 satellites is needed to convey accurate results. It’s important to know that the satellites DO NOT rotate with the earth; rather their orbits are “fixed” on the center of the earth. The orbits are also designed to ensure that at least 6 satellites are always in view. This gives GPS receivers the stationary reference points they need, with the 2 remaining satellites used to ensure absolute accuracy. The reason that precise timing is so critical is due to the fact that a GPS receiver calculates its position based on the exact time that each satellite transmits its signal message. Then, by using the speed of light as measurement, the duration of each signal’s travel time between the satellites and the receiver is determined using navigation equations. Imagine that each time, distance, and satellite location determination represents a sphere. That gives us 4 spheres, each corresponding to 1 of the 4 satellites involved. The point where each of the 4 spheres meets and intersects is the location of the GPS receiver. Simultaneously and in nanoseconds, calculations are also made to account for various possible sources of signal interference and to make necessary error corrections. Therefore, considering the vast distances involved between the satellites and the receivers, even the slightest error in timing can translate to a position location that’s hundreds of miles away from where a receiver is actually located. To illustrate the point, your GPS device could think you were in Chicago when you were actually in Indianapolis. How would you like those driving directions??
Speaking of driving directions, you know how your GPS device tells you which way to go and your estimated time of arrival, right? Well, there are 4 satellites in orbit calculating those things for you based on the position changes of your GPS device. The satellites pretty much say, “Their receiver was there at this time, and it’s here now….so we know they’re headed in this direction, at this speed, and we estimate that they should get to their destination by this time. Your GPS device then relays this message to you in the voice of a sophisticated English woman”. 😉 Pretty cool, right. Did you ever think you would be getting driving directions from outer space?? One giant leap for mankind indeed! So you see, that’s why “precise synchronized time” is so critically important to the accuracy and reliability of GPS, and why it was such a vast improvement over each of its predecessors. In addition, GPS satellites are in continuous operation while Transit only provided a “fix” approximately every hour or so. Thankfully, even though Transit was doing its job and was serving a good purpose, we were about to enter into the next evolution in satellite navigation technology. (In the late 1980s, Transit peaked at roughly 100,000 military and commercial users. The system was fully decommissioned in 1996)
Part 2 With Infographic Coming Soon! Stay Tuned!
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