So I was thinking about WAAS and how, being a type of precision navigation, it provides vertical guidance. My question is what is the actual method for determining altitude and thus relative position to the glide path?

Does the GPS receiver in the aircraft actually compute altitude (Based on GPS and WAAS signals) and then compare it to an altitude the aircraft should be on (based on the approach and lateral position)? Or is the altitude information derived from the actual altimeter on the aircraft and then a glide slope readout is provided based on altitude and lateral position?

 

PFM....

I think the glidepath is a series of waypoints in the database, but I have to check.

 

Precision vertical guidance is not really based on altitude, it is based on your being at a certain point in space.

When doing GPS vertical guidance, the GPS is used to guide the airplane laterally and vertically so as to arrive at the correct point at the right time. The aircraft altimeter is used for situational awareness and minimums, just like with an ILS GS.

A certain number of satellites(3-4) are required to get a fix, plus one additional satellite for RAIM. Some GPS installations use baro-aiding, where data from the aircraft's altimeter is fed to the GPS box for use as an additional fix plane. This can allow the use of one less satellite without losing RAIM.

There are many errors which can affect the accuracy of the GPS signal, and these vary over time. The WAAS system corrects for some of this by sampling the signal at various fixed ground stations (where the actual position is known for certain), determining the error, and then transmitting the correction via satellite to the user receivers which automatically apply the correction.

WAAS has a limitation, in that it uses only a handful of ground stations, so the error at the nearest ground station is probably not the same as the location where you want to shoot an approach.

LAAS will solve this problem by sampling the GPS signal at or near each airport, and provide a custom error correction for that location and time. This will allow lower GPS minimums in the future.

 

Yeah thats how I figured it. It knows you are 2.34567 nm from the missed approach point and AT 2.34567nm you need to be at 784 feet and that is how it determines the glide path information. But my question was how does it compute deviance? But lets say you are at 850 feet when you should be at 784, how does the GPS know your actual altitude and thus give you a deviance indication (fly down needle)?

LAAS will be cool when it starts to become implemented - am I correct that there is no civilian LAAS approaches in use publicly?

 

Like any other FMS approach, the waypoints are in the Db. It compares your position to the Db, compares your GPS altitude with where you SHOULD be at the point on the course, and generates a deviation signal for the needle.

GPS can determine your altitude above the plane of the earth's surface, just like it can determine your lateral position. In fact GPS could be used in space also as long as you are within range of the signal. You would probably need some special software for that though, normal receivers automatically throw out position solutions which show you to be in outer space (this is the equivalent of the old ADF reciprocal bearing).

I think they have some in place for testing in Alaska, not sure if they restricted to certain operators.

 

this might help a bit:

FAASTeam - FAA Safety Team - Learning Center Library Contents

here's a quote from the above:

Perhaps this summary will make it easier. Every IFR-certified and installed GPS unit allows the pilot to descend to LNAV (or Straight-in) and circling approaches. Baro-VNAV-equipped GPS systems can also descend to LNAV/VNAV minima. WAAS receivers can descend to LNAV, LNAV/VNAV, and LPV minima. Need another hint? Look for the DA designation versus the Minimum Descent Altitude (MDA) abbreviation on the minima line. Only procedures with vertical guidance have DAs. A descent angle may be provided on procedures which have only LNAV minima, to aid in a stabilized descent, but the MDA must still be respected.

 

So the old answer of "It just does" doesn't work? It's all in the reciever. It knows your lateral position, and it knows your vertical position due to the decrease in inaccuracy's from the WAAS signals. The GPS unit generates the deviation laterally and vertically in the same regards. It knows where you're at, where you're supposed to be, and gives the difference.

 

I'll just go with PFM computing the glide slope!