The Waves of the Radio
All navigation instruments in our airplanes work off of radio waves (VOR, ILS, GPS, NDB, etc.) Let’s go ahead and talk about the different types of radio waves, and what each one is good for.
- Ground Wave – This radio wave travels across the Earth’s surface, bounded by the ground and the ionosphere (the ionosphere is that layer of the atmosphere that acts as a mirror to most radio signals due to the high level of free electrons and ionized particles, simply stated, its a mirror about 30 ish miles about the Earth). So when a “ground wave” is transmitted, it bounces back and forth between the ground and the ionosphere making it’s way across the Earth’s surface, but of course, loses it’s strength as it travels. Think AM radio and NDB navigation.
- Sky Wave – Great for very long distances, the signal is “bent” by the ionosphere back towards Earth. Think HF radios used by aircraft to communicate across oceans. Not the best for navigation since the “bend” is highly variable and not reliable. However, you can use this to send a semi-clear signal for thousands of miles. Remember Amelia Earhart’s signal from a HF radio supposedly being heard in random places around the globe shortly after she went missing? Not a super reliable way to send a signal, but it can sure travel a long way to somewhere. Sky wave radios are being replaced by more reliable satellite communication (Space Wave).
- Space Wave – Very high frequency, these waves are able to penetrate the ionosphere with little to no distortion. Because it is so high frequency, it is considered to be mostly line of sight (the waves reflect off of hard objects). Errors caused by the ionosphere are corrected by the receiver (GPS receiver) and ground-based stations (WAAS, which we’ll talk about more here soon).
Here’s what you really need to know….
Higher the frequency of the signal, more line of sight it is, lower the frequency, more it will bend to and follow the surface of the Earth. Remember it this way, Navigation aids (VOR, ILS, etc) work from 108-117.95 Mhz, while Communication radios work from 118-136 Mhz, meaning someone once upon a time determined it was more important for an airplane to be able to receive a Navigational Signal than a Communication Signal.
How GPS Works
30 Satellites are in orbit around the Earth. These Satellites are always in a predetermined position (your GPS receiver has a “map” stored on it knowing where each satellite should be at any given time). The GPS satellites broadcast a signal. This signal can be picked up by an infinite number of receivers just like an infinite number of people can listen to the same radio station at the same time. This is a one-way signal, there is absolutely NO communication between your GPS Receiver and the GPS satellites. The signal the GPS satellites broadcast is just one simple message…..what time is it?
Knowing that the signal from each satellite moves at the speed of light (a constant speed), your GPS receiver looks at the timestamp from each satellite down to the millisecond. Since all the satellites have their clocks “synced up”, if one signal received from satellite A says its noon, and satellite B says its 12:00:00:03 (noon, zero minutes, zero seconds, and 3 milliseconds), then your receiver can start to paint a picture of how far you are from each satellite, and with at least:
- 3 satellites, can give you a 2 dimensional position of where you are on the Earth’s surface
- 4 satellites can give you a 3 dimensional fix
- 5 satellites can give you RAIM (Receiver Autonomous Integrity Monitoring) or basically check the signal and make sure it’s accurate within certain limits.
Remember, the signals move at a constant (the speed of light), yet the ionosphere varies based on solar strength and other factors, and that creates very small positional errors. This is why to fly a GPS approach or navigate legally with GPS you need a receiver with RAIM ability.
How WAAS Works
What is WAAS? Wide Area Augmination System. It’s what makes your GPS receiver more precise (assuming the receiver itself is WAAS capable). The idea here is to use ground-based stations that know exactly where they are on the Earth’s surface and have them receive the radio signal from the GPS satellites. They then compare the error in the position to where the station really knows it is at on the ground, and rebroadcasts a signal that is “cleaned up” to the aircraft to help make the aircraft’s position much more accurate. See the diagram below for how the process works. Note: if you are planning on doing a GPS approach using vertical guidance, you will most likely need an airplane equipped not only with an approved GPS for navigation, but also WAAS approved (the ability of the unit to receive this extra signal and hone in its position). I.e. a Garmin 430 vs a Garmin 430W (the W means it has WAAS).