An Introduction to GNSS

They all laughed at Christopher Columbus when he said the world was round.

Christopher Columbus wasn’t the first person to propose the world was round. Far from it. By the fifth century BCE, many Greek scholars had accepted a spherical earth as fact. Around 240 BCE, Eratosthenes, a Greek mathematician, poet, athlete, geographer and astronomer—a Renaissance man—ingeniously calculated the radius of the earth with surprising accuracy. 

Although Columbus knew the earth was round, he had obviously not read or agreed with Eratosthenes, for he significantly underestimated its size. He projected that heading west, the distance from the Canary Islands to Japan was 3,700 km, not 19,600 km as we now know it to be. Had Columbus known the true distance, he may have lost heart. He would certainly have had trouble convincing others to fund his first voyage or to sail with him. 

Columbus navigated to the New World using dead reckoning, the technique of estimating one’s current position based on a previously determined one. For example if I head west from a known location at 10 km/hr then, in two hours, I will be 20 km west of my starting point. The challenge in dead reckoning was the accurate and regular estimation of speed and heading. 

A great deal of exploration was carried out at a time when positioning was not a very exact science, sometimes with dire consequences. In 1707, several ships of the Royal Navy struck the rocks near the Isles of Scilly, southwest of Cornwall, with a loss of four ships and 1,400 men. Navigational error was blamed. Although it is not certain whether the error was in the determination of longitude or latitude, the tragedy led to the Longitude Act of 1714. Through this act, the British government offered prizes for people Who could solve or advance the problem of accurately determining longitude, including one of £10,000 for a method that could determine longitude to within 60 nautical miles, about 111 kilometres. 

Although none of the larger prizes offered by the Longitude Act were ever awarded, the initiative led to the development of many navigation techniques and equipment, including significant improvements in shipborne chronometers (clocks), then critical to the accurate determination of longitude. With the advent of radio in the early 1900s, time signals were sent to ships, which could use the signal to regularly adjust their chronometers. In the 1940s, LORAN (LOng-RANge navigation system) was introduced. This allowed ships to triangulate their position using radio signals from LORAN stations at known shore-based locations. The first satellite, Sputnik, was launched in 1957 and it was not long before scientists contemplated working back from a known satellite orbit to determine a position on earth. Many of the problems faced by earlier navigators were quickly becoming historical footnotes. 

What has most significantly changed navigation techniques is the advent of Global Navigation Satellite Systems (GNSS), which started with the launch of the U.S. Department of Defense Global Positioning System (GPS) in the late 1970s. Early applications of GNSS were developed for the military and soon expanded to the survey and mapping industries—driven largely by the tremendous advances in accuracy, efficiency as well as cost reductions. Now, vehicles, whether on land, in the air or at sea—routinely rely on the precise positioning information provided by GNSS technology. In fact, the ready adoption of the technology, from mining to unmanned, and the increasingly complex requirements for positioning, anywhere, anytime, is driving innovation in the industry that includes the integration of GNSS technology with a variety of other sensors and methodologies. This multifusion approach is sure to drive innovation in the industry for many years to come. 

The goal of this book is to present complex GNSS concepts and applications in a manner that informs without overwhelming. By the end of the book, you will understand the basics of GNSS and will have a solid foundation for further study or application.

We hope you enjoy the read.

(published in 2015)

 


Chapter 1 - GNSS Overview


Chapter 2 - Basic GNSS Concepts


Chapter 3 - Satellite Systems


Chapter 4 - GNSS Error Sources


Chapter 5 - Resolving Errors


Chapter 6 - GNSS + INS


Chapter 7 - GNSS Denial


Chapter 8 - GNSS Applications and Equipment


GNSS-Book-Cover-Second-Edition