Now, here's the catch for you as a civilian pilot — Page 316, Lesson 315

We're in the middle of the error budget for satellite navigation, and I want to pick up with the ionospheric error, because that's the one that dominates everything else. The ionosphere is the layer of the atmosphere that contains free electrons, and it slows down the radio signal. The key point here is that the delay is proportional to the number of electrons the signal passes through, and that's proportional to the square of the frequency. So if you transmit on two different frequencies, you can measure the difference in delay between them, and from that you can calculate the total delay experienced by the signal. Once you know that total delay, you subtract it out, and you're left with a very accurate range measurement. That's the whole reason the military uses two frequencies — it lets them remove this error almost completely. Now, here's the catch for you as a civilian pilot. Only the C/A code is available to civilian users. The C/A code is the coarse acquisition code, the standard positioning signal. And because you only have that single frequency, you cannot do that two-frequency correction. So the ionospheric error is the most significant of the errors in satellite navigation systems. For single frequency operation, the maximum error is 5 metres. That's the number I want you to hold onto — 5 metres, single frequency, ionospheric error, and it's the biggest one in the whole budget. Let's move on to the tropospheric propagation error. Now, the troposphere is the lowest layer of the atmosphere, where we live and fly. Because satellite navigation systems are so inherently accurate, the effect of variations in tropospheric conditions on the passage of radio waves has become significant. What varies? Pressure, temperature, density, and humidity. And here's the relationship: increased density and increased absolute humidity both reduce the speed of propagation. So denser, more humid air slows the signal down more. To give you a feel for the scale of this, a change in transit time of one nanosecond — that's one billionth of a second — would give an error of 0.3 metres. So every nanosecond of delay costs you 0.3 metres of range error. And just like the ionospheric error, this is minimized with the use of two frequencies. Next is receiver noise error. Every radio receiver generates internal noise — that's just the electronics inside the box. In the case of a GNS receiver, that internal noise can cause errors in the measurement of the time difference. Remember, the whole ranging principle is based on measuring the time difference between when the signal was sent and when it was received. If the receiver's own noise corrupts that timing measurement, you get a range error. The maximum here is 0.3 metres. Finally, multipath reception. This happens when the signal doesn't just arrive by the direct path. Reflections from the ground and from parts of the aircraft itself cause the signal to arrive by multiple paths — hence the name multipath. The reflected signals arrive later and can corrupt the direct signal. This can be minimized in two ways: by careful siting of the aerial, so you place the antenna where it's least likely to pick up reflections, and by internal processing techniques inside the receiver. The maximum error here is 0.6 metres. So let me put the whole budget together for you, because this is the picture you need. Ionospheric, single frequency — 5 metres, the big one. Tropospheric — that's the one where one nanosecond costs you 0.3 metres. Receiver noise — 0.3 metres. Multipath — 0.6 metres. And the common thread running through the ionospheric and tropospheric errors is that using two frequencies minimizes both. That's why the military, with access to both frequencies, gets a much tighter error budget than you do as a civilian on the single C/A code.