SAFA Skysailor Magazine

37 November | December 2021 SKY SAILOR So how does it work? Each satellite in the constellation carries a number of atomic clocks and radio transmitters and receivers. The satellite knows exactly where it is in space at any given point in time, receiving regular orbit information from ground stations. The satellite transmits several streams of information over a number of frequencies. Depending on the receiver, some or all of the frequencies are received, and the code and time information is compared with the receiver’s time and data. The receiver calculates the time and code differences to determine the distance the signal has travelled from the satellite to the receiver. By doing this calculation for all signals from all of the satellites the receiver can ‘see’, a fix is taken of the point on the Earth where the re- ceiver is located, within a circle of error (Deteri- oration of Position or DOP). To get a 2D fix, you need to get locked fixes from three satellites; to get a 3D fix you need 4 satellites. Signals are affected by obstructions and can reflect from objects on the ground. Modern GPS receivers will be accurate to within a couple of metres. Surveying rated equipment will be accurate within a couple of centimetres. Scientific grade equipment is accurate within millimetres. This is a really simplistic overview of how the system works. If you want to dive really deep, get hold of ‘GPS for Land Surveyors’ by Jan van Sickle (ISBN: 978 0 8493 9195 8). While GNSS were being developed, in 1979, the COSPAS-SARSAT initiative was being launched by the US, Canada, France and the USSR. This is a constellation of satellites that detects and locates radio distress beacons. Signals received are forwarded to search and rescue authorities’ ground stations in over 200 nations, at no cost to users or nations. Beacons transmit on 121.5 MHz, 243 MHz and 406 MHz, but only 406 MHz signals are received by the satellites – the older two frequencies are only picked up by nearby aircraft or rescue personnel. Finally, we turn our attention to satellite communication networks. There are lots of them but the two of interest for our purposes are Globalstar and Iridium. Globalstar launched their first satellites in 1998 and completed the constellation of 52 satellites in 2000. In 2007 they launched eight more, and between 2010 and 2013 launched 24 second-generation satellites. The satellites orbit at an inclination of 52 degrees (similar to the ISS) at an altitude of 1414km, which means they do not cover polar regions. Find out more here: www.globalstar.com . The Iridium network was originally devel- oped by Motorola and became available in 1998. It currently consists of 66 satellites in polar orbits inclined at 86.4 degrees and a low earth orbit altitude of 781km. This ensures complete world-wide coverage. Find out more here: www.iridium.com . So now we have looked at the infrastructure and technologies behind the scenes let’s turn our attention next to the products available.