Why VOR, or LOC, or GS?

[RV8R999]

"It is very important to know what equipment is installed
in the aircraft. GPS-based FMS/RNAV units certified to
TSO-C145A or TSO-146A may be used when an alternate
airport is required in the flight plan for the approaches at the
destination and alternate airport if the WAAS is operational.
No other navigation avionics would be required. Units
certified under TSO-C129 are not authorized for alternate
approach requirements. The aircraft must have stand-alone
navigation equipment, such as VOR, and there must be an
approved instrument approach at the alternate airport based
on that equipment. (However, once diverted to the alternate
airport, the pilot could fly a GPS-based approach there, as
long as there is an operational, ground-based navaid and
airborne receiver in the aircraft for use as a backup.)"

This excerpt from the FAA cracks me up. How is a VOR anymore "standalone" than a GPS? They both require an external transmission and are both subject to equipment outages. Then it says you have to have a VOR to file to an alternate which by definition should have better weather than the original destination, yet it was ok to file GPS to the destination. Further, once you get to the alternate you are then authorized to use GPS. huh????

It appears to me the FAA is basing the requirement for a VOR as a means of redundancy but assumes the reason for the missed approach at the destination was what? A GPS failure and not weather? If this is the case then why is it then ok to only have a VOR and not a GPS? Does the redundancy argument work both ways?

I'm still goin with only GPS...maybe it will limit the days I can "FILE" IFR but obviously this reg will need to be updated in a few years when VORs are in fact put out of service.

[Ron Lee]

Ken, one of the major issues is INTEGRITY of the navigation signal. VORs have that. Current standalone GPS signals do not.

[Jamie]

Originally Posted by RV8R999

I'm still goin with only GPS...maybe it will limit the days I can "FILE" IFR but obviously this reg will need to be updated in a few years when VORs are in fact put out of service.

Just when is that going to be? Around the same time they eliminate NDBs?

[bizjet]

When I made the original post I wasn't thinking about the cost, which can be a real deal breaker, of course.

I've been flying GPS for 21 years and have yet to experience an outage. Not one. Just lucky, or is the system that good?

I'm really sold on it and I'm no computer savvy youngster. Just an old guy who likes doing things easily and accurately.

Too accurate? Well, could be. I'm thinking of oceanic operations where we used to sort of know where we were. With this stuff if it says that we're 55N, 35W, and at 35,000 - that's exactly where we are and if someone else thinks the same, he's exactly there, too. We are encouraged to offset from centerline and many do.

Interesting conversation, I think.

Regards,

Bill

[Dan B]

The IFR approach accident rate is significantly higher for the ?dive and drive? type of approaches (VOR, NDB, LOC) compared to the approaches with vertical guidance, such as ILS. General aviation seems to bear the brunt of most of the accidents. Installation of an ILS is about $1.5M per runway end plus ongoing maintenance.

The FAA GPS/RNAV initiative is to provide vertically guided IFR approaches to all qualifying runway ends in the US in order to help mitigate the accident rate. The most economical way to do it is by utilizing the GPS. The FAA has been mandated to develop and publish at least 500 new RNAV LPV WAAS approaches per year. The current number of RNAV LPV approaches is 1975, with about 250 of them having descent minima of 200? and ? mile visibility (same as ILS CAT I). There are just over 1200 ILSs in the US. By this time next year, there will be about twice as many runway ends with RNAV LPV approaches as ILS.

The reliability of ground base NAVAIDS can be debated. Outages for maintenance can sometimes be lengthy. Lighting strikes and other issues take a toll on the facilities. Some of the facilities taken out by Hurricane Katrina were out of service nearly a year and a half. The only IFR access to some of the airports on the Gulf Coast right after the hurricane was with GPS RNAV avionics.

WAAS has been determined to have the integrity and continuity to be used for primary navigation in IFR. We are now seeing lower minimum enroute altitudes published on IFR charts for aircraft equipped with GPS avionics. ?T? Routes and VFR waypoints are published in busy terminal areas for use in GPS avionics. The FAA published the new Instrument Pilot Standards, effective Jan 2010, which now allows the RNAV LPV approach to be flown to demonstrate the precision approach requirements. pg7--?Note: An LPV approach is technically a nonprecision approach, however, due to the precision of its glidepath and localizer-like lateral navigation characteristics, an LPV can be used to demonstrate precision approach proficiency (AOA VI TASK B).?

I see the handwriting on the wall and have invested in my avionics accordingly!

[RV8R999]

Well Jamie no one can predict exactly when but I can tell you the US Navy has budgeted over $2B (yes thats a B) toward ensuring our aircraft can navigate without VOR/TACANs within the next couple of years... we don't spend that kinda money on hunches. How do I know? I'm the guy that put the money in that pot. Take it for what its worth.

Regards,

Ken

[RV8R999]

Hmmm...
RAIM
Receiver Autonomous Integrity Monitoring (RAIM) provides integrity monitoring of GPS for aviation applications. In order for a GPS receiver to perform RAIM or Fault Detection (FD) function, a minimum of five visible satellites with satisfactory geometry must be visible to it. RAIM has various kind of implementations; one of them performs consistency checks between all position solutions obtained with various subsets of the visible satellites. The receiver provides an alert to the pilot if the consistency checks fail. RAIM availability is an important issue when using such kind of algorithm in safety-critical applications (as the aeronautical ones); in fact, because of geometry and satellite service maintenance, RAIM is not always available at all, meaning that the receiver's antenna could have sometimes less than five satellites in view. Availability is also a performance indicator of the RAIM algorithm, as it is, basically, a statistical method. Due to this it has a "power of test" which is a measure of the reliability of this test to detect a failure when it happens. This power of test is its availability and it is a function of the geometry of the constellation which is in view and of other environmental conditions. If availability is seen in this way it is clear that it is not an on?off feature meaning that the algorithm could be available but not with the required performance of detecting a failure when it happens. So availability is a performance factor of the algorithm and characterizes each one of the different kinds of RAIM algorithms and methodologies.

[Bill Dicus]

I've had multiple episodes of loss of a useable signal with GPS through the years, although fewer recently. Loran would often fail near CB - doesn't matter anymore. I agree with the pilot who said "I love ILS's."

[Sparky]

I am an career avionics guy, and I agree that GPS is the greatest thing since sliced bread. However, in addition to GPS (WAAS) I will also have dual COM, Dual VOR, dual LOC and dual GS (in addition to dual EFIS AND a standby airspeed, standby altimeter, and standby ADI) in my RV-7. Actually, I have dual GPS too, if you count the stand-alone GPS receiver driving the APRS. Why? Because I can.

[Beer30?]

It's the law.

I could be wrong on this but, it is my understanding that if you file GPS to your destination on an IFR flight plan and an alternate is required, then the alternate must have other than a GPS approach published and you and the aircraft must be able to perform it. This would imply an ILS, LOC or VOR. Also, I still see many more ILS approaches than precision WAAS approaches, although I fly mainly into large commercial airports.

Beer30?