GPS Vs Other IFR Approaches

Sat Sep 30, 2000 11:45 pm

To all the pilots out there.
Do you prefer GPS to the other types of IFR you can fly?

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Joined: Tue Nov 09, 1999 6:20 pm

RE: GPS Vs Other IFR Approaches

Sun Oct 01, 2000 2:14 am

No. As of right now, other types are more accurate in the approach phase.
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RE: GPS Vs Other IFR Approaches

Sun Oct 01, 2000 11:32 am

Essentially there is no difference between a GPS approach and other IFR approaches with the exception of where the information originates from.

Once in "final approach" phase, GPS requires a local ground based transmitter for "fine tuning" the information so IMHO it is no more accurate than an ILS.

Remember too that the accuracy of cockpit displays plays a very important role in how accurate information is presented to pilots. Even with more accurate presentation, trying to maintain such precise accuracy requires a less stable and comfortable ride for the passengers. Is such precision THAT necessary? Not IMHO.  
*NO CARRIER* -- A Naval Aviator's worst nightmare!

RE: GPS Vs Other IFR Approaches

Tue Oct 03, 2000 12:04 am

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.
aaron atp
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Joined: Sun Mar 19, 2000 1:17 pm


Tue Oct 03, 2000 4:23 am

>>>Great system. On paper.

I installed a RayChart 320 GPS chartplotter in my fathers boat late this summer, and we've been very impressed with WAAS accuracy.

WAAS has been operational for a while, but the FAA and Raytheon are still working on the certification process. A test signal is being transmitted, but it is the same signal as the future "type 2" signal. I believe the transmitted signal is identified as "Type 0" to prevent WAAS capable aviation GPS's from depending on the signal.

WAAS signal-in-space


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