« Reading List: Code of Conduct | Main | Reading List: Superman: Red Son »

Monday, July 27, 2015

Reading List: GPS Declassified

Easton, Richard D. and Eric F. Frazier. GPS Declassified. Lincoln, NE: Potomac Books, 2013. ISBN 978-1-61234-408-9.
At the dawn of the space age, as the United States planned to launch its Vanguard satellites during the International Geophysical Year (1957–1958), the need to track the orbit of the satellites became apparent. Optical and radar tracking were considered (and eventually used for various applications), but for the first very small satellites would have been difficult. The Naval Research Laboratory proposed a system, Minitrack, which would use the radio beacon of the satellite, received by multiple ground stations on the Earth, which by interferometry would determine the position and velocity of a satellite with great precision. For the scheme to work, a “fence” of receiving stations would have to be laid out which the satellite would regularly cross in its orbit, the positions of each of the receiving stations would have to be known very accurately, and clocks at all of the receiving stations would have to be precisely synchronised with a master clock at the control station which calculated the satellite's orbit.

The technical challenges were overcome, and Minitrack stations were placed into operation at locations within the United States and as far flung as Cuba, Panama, Ecuador, Peru, Chile, Australia, and in the Caribbean. Although designed to track the U.S. Vanguard satellites, after the unexpected launch of Sputnik, receivers were hastily modified to receive the frequency on which it transmitted its beeps, and the system successfully proved itself tracking the first Earth satellite. Minitrack was used to track subsequent U.S. and Soviet satellites until it was supplanted in 1962 by the more capable Spacecraft Tracking and Data Acquisition Network.

An important part of creative engineering is discovering that once you've solved one problem, you may now have the tools at hand to address other tasks, sometimes more important that the one which motivated the development of the enabling technologies in the first place. It didn't take long for a group of engineers at the Naval Research Laboratory (NRL) to realise that if you could determine the precise position and velocity of a satellite in orbit by receiving signals simultaneously at multiple stations on the ground with precisely-synchronised clocks, you could invert the problem and, by receiving signals from multiple satellites in known orbits, each with an accurate and synchronised clock on board, it would be possible to determine the position, altitude, and velocity of the receiver on or above the Earth (and, in addition, provide a precise time signal). With a sufficiently extensive constellation of satellites, precision navigation and time signals could be extended to the entire planet. This was the genesis of the Global Positioning System (GPS) which has become a ubiquitous part of our lives today.

At the start, this concept was “exploratory engineering”: envisioning what could be done (violating no known law of physics) if and when technology advanced to a stage which permitted it. The timing accuracy required for precision navigation could be achieved by atomic clocks (quartz frequency standards were insufficiently stable and subject to drift due to temperature, pressure, and age of the crystal), but in the 1950s and early '60s, atomic clocks were large, heavy, and delicate laboratory apparatus which nobody imagined could be put on top of a rocket and shot into Earth orbit. Just launching single satellites into low Earth orbit was a challenge, with dramatic launch failures and in-orbit malfunctions all too common. The thought of operating a constellation of dozens of satellites in precisely-specified high orbits seemed like science fiction. And even if the satellites with atomic clocks could somehow be launched, the radio technology to receive the faint signals from space and computation required to extract position and velocity information from the signal was something which might take a room full of equipment: hardly practical for a large aircraft or even a small ship.

But the funny thing about an exponentially growing technology is if something seems completely infeasible today, just wait a few years. Often, it will move from impossible to difficult to practical for limited applications to something in everybody's pocket. So it has been with GPS, as this excellent book recounts. In 1964, engineers at NRL (including author Easton's father, Roger L. Easton) proposed a system called Timation, in which miniaturised and ruggedised atomic clocks on board satellites would provide time signals which could be used for navigation on land, sea, and air. After ground based tests and using aircraft to simulate the satellite signal, in 1967 the Timation I satellite was launched to demonstrate the operation of an atomic clock in orbit and use of its signals on the ground. With a single satellite in a relatively low orbit, the satellite would only be visible from a given location for thirteen minutes at a time, but this was sufficient to demonstrate the feasibility of the concept.

As the Timation concept was evolving (a second satellite test was launched in 1969, demonstrating improved accuracy), it was not without competition. The U.S. had long been operating the LORAN system for coarse-grained marine and aircraft navigation, and had beacons marking airways across the country. Starting in 1964, the U.S. Navy's Transit satellite navigation system (which used a Doppler measurement system and did not require a precise clock on the satellites) provided periodic position fixes for Navy submarines and surface ships, but was inadequate for aircraft navigation. In the search for a more capable system, Timation competed with an Air Force proposal for regional satellite constellations including geosynchronous and inclined elliptical orbit satellites.

The development of GPS began in earnest in 1973, with the Air Force designated as the lead service. This project launch occurred in the midst of an inter-service rivalry over navigation systems which did not abate with the official launch of the project. Indeed, even in retrospect, participants in the program dispute how much the eventually deployed system owes to its various precursors. Throughout the 1970s the design of the system was refined and pathfinder technology development missions launched, with the first launch of an experimental satellite in February 1978. One satellite is a stunt, but by 1985 a constellation of 10 experimental satellites were in orbit, allowing the performance of the system to be evaluated, constellation management tools to be developed and tested, and receiver hardware to be checked out. Starting in 1989 operational satellites began to be launched, but it was not until 1993 that worldwide, round-the clock coverage was available, and the high-precision military signal was not declared operational until 1995.

Even though GPS coverage was spotty and not continuous, GPS played an important part in the first Gulf War of 1990–1991. Because the military had lagged in procuring GPS receivers for the troops, large numbers of commercial GPS units were purchased and pressed into service for navigating in the desert. A few GPS-guided weapons were used in the conflict, but their importance was insignificant compared to other precision-guided munitions.

Prior to May 2000 the civilian GPS signal was deliberately degraded in accuracy (can't allow the taxpayers who paid for it to have the same quality of navigation as costumed minions of the state!) This so-called “selective availability” was finally discontinued, making GPS practical for vehicle and non-precision air navigation. GPS units began to appear on the consumer market, and like other electronic gadgets got smaller, lighter, less expensive, and more capable with every passing year. Adoption of GPS for tracking of fleets of trucks, marine navigation, and aircraft use became widespread.

Now that GPS is commonplace and hundreds of millions of people are walking around with GPS receivers in their smartphones, there is a great deal of misunderstanding about precisely what GPS entails. GPS—the Global Positioning System—is precisely that: a system which allows anybody with a compatible receiver and a view of the sky which allows them to see four or more satellites to determine their state vector (latitude, longitude, and altitude, plus velocity in each of those three directions) in a specified co-ordinate system (where much additional complexity lurks, which I'll gloss over here), along with the precise time of the measurement. That's all it does. GPS is entirely passive: the GPS receiver sends nothing back to the satellite, and hence the satellite system is able to accommodate an unlimited number of GPS receivers simultaneously. There is no such thing as a “GPS tracker” which can monitor the position of something via satellite. Trackers use GPS to determine their position, but then report the position by other means (for example, the mobile phone network). When people speak of “their GPS” giving directions, GPS is only telling them where they are and where they're going at each instant. All the rest: map display, turn-by-turn directions, etc. is a “big data” application running either locally on the GPS receiver or using resources in the “cloud”: GPS itself plays no part in this (and shouldn't be blamed when “your GPS” sends you the wrong way down a one-way street).

So successful has GPS been, and so deeply has it become embedded in our technological society and economy, that there are legitimate worries about such a system being under the sole control of the U.S. Air Force which could, if ordered, shut down the civilian GPS signals worldwide or regionally (because of the altitude of the satellites, fine-grained denial of GPS availability would not be possible). Also, the U.S. does not have the best record of maintaining vital infrastructure and has often depended upon weather satellites well beyond their expected lifetimes due to budget crunches. Consequently, other players have entered the global positioning market, with the Soviet/Russian GLONASS, European Galileo, and Chinese BeiDou systems operational or under construction. Other countries, including Japan, India, and Iran, are said to be developing their own regional navigation systems. So far, cooperation among these operators has been relatively smooth, reducing the likelihood of interference and making it possible for future receivers to use multiple constellations for better coverage and precision.

This is a comprehensive history of navigation systems and GPS from inception to the present day, with a look into the future. Extensive source citations are given (almost 40% of the book is end notes), and in the Kindle edition the notes, Web documents cited within them, and the index are all properly linked. There are abundant technical details about the design and operation of the system, but the book is entirely accessible to the intelligent layman. In the lifetimes of all but the youngest people on Earth, GPS has transformed our world into a place where nobody need ever be lost. We are just beginning to see the ramifications of this technology on the economy and how we live our day-to-day lives (for example, the emerging technology of self-driving cars would be impossible without GPS). This book is an essential history of how this technology came to be, how it works, and where it may be going in the future.

Posted at July 27, 2015 23:53