I have an Apollo GX-50 in my Cherokee 140.
It is a good piece of gear and is a very nice gadget for en-route navigation. That is it. A nice gadget.
Problem is the GX-50, nor any other GPS device TODAY can be used, universally, for precision approaches. While GPS systems are able to give pretty good guidance, there is no ability to provide the glide scope (in addition to lateral deviation) within the parameters that the FAA dictates.
In the paragraph above, I mentioned "universally" which means that there are exceptions to the rule. I believe the FAA will grant waivers on a case by case basis for precision GPS navigation. They have done this in Alaska for AS approaches into Juneau. The FAA has also allowed GPS navigation and approaches for medical helicopters in Colorado.
Keep in mind that the FAA is planning to implement a GPS system of NAVAIDS within the next several years. The system will fall into two categories. The Wide Area Augmentation System (WAAS) and the Local Area Augmentation System (LAAS).
Here is where we go into a little detail. There are 24 satellites in our GPS constellation system designed to cover the entire globe. Based on your position and the amount of time it takes for you to receive a signal--your position is calculated and triangulated--often to within several meters of accuracy. Those meters that your position is off by, can be thought of as system error. The errors in the GPS system are intentionally induced in the "p" code or precision code that some of you may have heard about. The self induced errors are so that civilians can use a military designed system--without the accuracy needed, simply put, for use of national implication. Along with the "p" code ( which I think is in terms of milli-seconds, perhaps---VERY small, and I could be wrong) errors can be induced by sattellites going down for various reasons, etc.
There are ways around the errors. For the Local Area Augmentation System (LAAS) all airports will have the ability for precision approaches from all runway ends. The way they go about this is by setting up a ground station--which is about the size of a generator. This ground station must be set up on a surveyed, known point. This system will take the errors from the GPS constellation and re-transmit the corrected signal to the aircraft. "They" say that this will allow CATIIIC to any airport which has the LAAS--and from what I have read, the FAA wants to make it universal.
An ILS system (as we know it today--markers, localizer, glideslope) cannot be used from all runway ends. Things such as obstructions, freq interference, etc can make the ILS impossible to use. In Europe, FM frequency congestion is also becoming a major problem. A friend of mine at ARINC mentioned that a conventional ILS costs $1.2 mil per runway end to set up! It is expected that the one GPS ground station box will cost far less than that (you also don't need inner, middle and outer marker stations anymore).
The WAAS is simply a corrected, enroute nav system that operates on the same principle as LAAS--your positional accuracy is off by a little more.
Great system. On paper. The FAA isn't there yet (Remember in the 80s when they wanted MLS to be the universal system of CHOICE?!). Until it is, I'll use my RNAV systems and the GPS as a handy back-up.