Guest Post by Professor David Last
Satellite navigation may be the most successful innovation of recent decades among science-based industries. As the number of receivers world-wide soars past 2 billion, GPS has been joined by Russia’s GLONASS, China’s Compass-Beidou, Japan’s QZSS, India’s IRNSS and Europe’s Galileo. The latest smart-phones already mix-and-match multiple systems. Throw in the WAAS-like augmentations and we’re heading for 150 satellites. But is more necessarily better?
To ensure compatibility with GPS, the established standard, the new systems are versions of the same technology, with just a hint of garlic here, a whiff of curry there. They have had to squeeze alongside GPS into the few narrow radio bands allocated to navigation. There they share the vulnerabilities of GPS to solar weather and radio interference. Jam one and you jam them all.
And each new satellite increases the noise level in those radio bands, so finding and locking onto the extremely weak signals gets harder. Guenther Hein has shown that when the number of satellites passes 70, the noise they transmit exceeds the cosmic noise and reception deteriorates. Too many satellites, and you’d pick up none at all!
In urban canyons, tall buildings block signals from most directions. Satellite geometry is poor and position errors high. Adding satellites to blocked areas of sky is pointless. Adding a few where they are visible will improve geometry and accuracy, but it’s a rapidly diminishing return.
Sure, if one constellation develops a system fault, as GPS did last January and GLONASS in April 2014, you’ll be thankful for a fall-back – provided your receiver is designed to detect and exclude the failing constellation. If not, having more satellite systems may make you more vulnerable, not less.
Professor David Last is a Consultant Engineer and Expert Witness specialising in Radio Navigation and Communications Systems. He is a Professor Emeritus at the University of Bangor, Wales and Past-President of the Royal Institute of Navigation. He acts as a consultant on radio-navigation and communications to companies and to governmental and international organisations and is active as an Expert Witness, especially in forensic matters concerning GPS.
