Navigation Related Activities, using navigation satellites
Many Scouts will already be familiar with satellite navigation, either on their smartphone, vehicle 'SatNav' or a dedicated navigation receiver.
This facility is frequently referred to as GPS, which stands for Global Positioning System. However, other satellite navigation constellations have since been put into orbit and they all sit under the more general name of GNSS, which stands for Global Navigation Satellite System.
A modern receiver can use multiple constellations, providing more reliable and accurate position information.
Each satellite contains an atomic clock, broadcasting orbit and timing data. A receiver knowing where the satellites are, together with the relative arrival time of the signals can calculate a navigation solution, as well as providing very accurate time.
Satellite navigation receivers work best when they have a clear view of the sky down to horizon level. Urban built up areas reduce the visibility of the satellites and can also result in signals reflecting off buildings, referred to as multi-pathing. This causes timing errors, which result in poorer navigation solutions. There are also other factors that can reduce accuracy. Many satellite navigation receivers provide an estimate of the position accuracy, although this is usually derived from the geometry of the visible satellites and generally unable to detect the effects of multi-pathing or other radio propagation effects.
A simple activity to demonstrate this is to take a position reading in latitude and longitude and then plot it on Google Maps. This can also be done using Ordnance Survey Grid References, with the receiver set to 1 metre resolution, i.e. 10 figure references. Try this several times and observe that the position plotted will change.
Many navigation receivers will display a calculated satellite position map, showing which ones are visible. Satellites widely spaced apart are the best ones for the receiver to use, in the same way that land based navigation works best with a long baseline.
Newer dual band (L1 + L5) navigation receivers can correct for some of these effects, providing more accurate positioning.