First, let me finish the thought on the ionosphere — Page 316, Lesson 315

Right, so we’ve just dealt with the ionospheric propagation error and how using two frequencies cancels most of it. Now I want to take you through the remaining error sources in a satellite navigation system, because each one has a specific magnitude and a specific way we minimise it. First, let me finish the thought on the ionosphere. The whole point of using two frequencies is to reduce the total delay the signal experiences, and that directly minimises the error, letting the receiver calculate a very accurate range. And I want to stress this: the ionospheric error is the most significant of all the errors in satellite navigation systems. For single-frequency operation, the maximum error is 5 metres. That’s the headline number to remember — 5 metres, single frequency. Now, the next error source is the tropospheric propagation error. Here’s the key idea: because satellite navigation systems are so inherently accurate, the effect of variations in tropospheric conditions on the passage of radio waves has become significant. So we can’t ignore the atmosphere anymore. What varies? Pressure, temperature, density, and humidity. These all affect the speed of propagation. Specifically, increased density and increased absolute humidity reduce the speed of propagation. So the signal slows down, which means it takes longer to arrive, which the receiver interprets as a longer range. Let me give you a feel for the magnitude. A change in transit time of one nanosecond — that’s one billionth of a second — would give an error of 0.3 metres. So you can see how sensitive this is. And just like the ionospheric error, the tropospheric error is minimised with the use of two frequencies. Same remedy. Next, receiver noise error. Every radio receiver generates internal noise. In the case of a GNS receiver — that’s your Global Navigation Satellite receiver — this internal noise can cause errors in the measurement of the time difference. The maximum error here is 0.3 metres. So it’s small, but it’s there, and it’s a direct consequence of the receiver’s own electronics. Finally, multipath reception. This happens when the signal reflects off the ground and off parts of the aircraft. So instead of just the direct signal, the receiver also picks up reflected copies, which arrive later and corrupt the measurement. How do we deal with it? Two ways: careful siting of the aerial — so where we physically mount the antenna on the aircraft — and internal processing techniques inside the receiver. The maximum error from multipath is 0.6 metres. So let me line these up for you, because the numbers matter. Ionospheric, single frequency: 5 metres maximum — the biggest one. Tropospheric: tied to a 0.3 metre error per nanosecond of transit time change. Receiver noise: 0.3 metres maximum. Multipath: 0.6 metres maximum. And the two-frequency technique is your main weapon against both the ionospheric and tropospheric errors, while multipath is fought with antenna siting and internal processing. That’s the full error budget from this part of the chapter.