IN ACTION
around 12 hours. It’s at this altitude we find our GNSS satellites. In recent years the COSPAS SARSAT system, which receives and delivers distress messages from EPIRBs and PLBs, has also been using more MEO satellites. Once fully functional this system will mean complete global coverage and almost real-time beacon detection.
LEO (Low Earth Orbit)
These satellites are the closest to the earth’s surface, with an altitude of around 300–500km. This is where we find the International Space Station. LEO satellites are mainly used for communication and have the smallest footprint on the earth’s surface at any one time.
GNSS satellites
The GNSS satellites are made up of four main constellation systems. Historically we’d have had to buy a different receiver for each system, limiting the number of satellites we could use. However, in recent years, we’ve been able to use multiple GNSS constellations, which increases the likelihood of having enough satellites in view to create an accurate position. The four systems are operated by different countries – GPS by USA, Glonass by Russia, Galileo by Europe and BeiDou operated by China.
Pinpoint location
Each satellite transmits a signal to a receiver which calculates its distance from that satellite. This gives a large circle of possible locations. With more satellites it reduces the number of possibilities. To explain this further, we can think of the satellite transmission as a sound signal, and the satellite as a lighthouse making the sound. We could calculate a distance from the lighthouse if we knew the time it made the sound and the time we heard the sound. But the satellite is also moving, so we need to know where it was when it transmitted. The signal broadcast by the satellite contains very accurate information about both. In the panel opposite we illustrate how the communications work.
Fig. 2
Satellite communication explained Fig. 1
Working from the lighthouse analogy, at 1200 a sound is made from the blue lighthouse (the star in Fig. 1). We hear the sound 15 seconds later as sound travels approximately one mile every five seconds. This means we could be anywhere three miles from the lighthouse – the blue circle.
In Fig. 2 the red lighthouse also makes a sound, which takes 20 seconds to reach us. This gives us a range of four miles from the red lighthouse – the red circle. The two points where the blue and red circles cross are our two possible locations.
Fig. 3
The green lighthouse makes a sound that takes ten seconds to arrive; this puts us two miles away from the green lighthouse – the green circle. Where all three circles cross is where we are. GNSS uses this general principle with electro- magnetic signals of around 1.6GHz to create the position circles.
We need at least four satellites in clear view of our receiver to create a horizontal position, i.e. latitude and longitude. The more widely spread these satellites are, the more accurate the position fix will likely be.
Next issue we look at ways this position might be degraded and how to guard against it.
Imray charts withdrawal
Imray has announced that it will be phasing out its paper chart publishing due to the increase in digital navigation. It will prioritise its pilot books, cruising guides and Explore with Imray digital series, but will continue to print charts and support Imray Notices to Mariners throughout the 2025 season. The UKHO, which announced in 2023 it intended to phase out paper charts, has committed to continuing whilst there is a need, aiming to help with a safe, compliant transition to digital.
rya.org.uk WINTER 2024 45
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