What is interesting is whether the GPS will provide VNAV guidance all the way to the runway i.e. past the MAP (below the MDA, crucially).
Certainly on my setup* it does - I have played with hand flying down to touchdown (with a safety pilot ofc) and it brings you to exactly the right point. And to reiterate, the display units don't know the difference between the advisory glideslope and a real one (they just see a 10HZ stream of deviations) - so how can the autopilot know not to fly them?
As you pass the MAP (horizontal) the GPS suspends auto-sequencing of waypoints - as a result the last active leg continues to be displayed including both horizontal and vertical deviation.
I also take a GPS approach rather than ILS unless I need the minima. Advantages are:
And I can also confirm that it will fly you all the way into the runway if you let it.
I always thought INS would be a fantastic in-flight backup to GPS.
Completely independent of any outside nav sources; it would seem absolutely ideal.
I wonder if a sub-$10,000 ring laser gyro or fibre optic gyroscope would ever become feasible.
As a very precise and comparatively inexpensive backup, DME is far superior to VOR. Almost every FMS as a "VPU" (VORTAC positioning unit) that derives positioning data from DME distances alone.
The only problem I have is the lack of a FMS and with one KN62a and no INS, it would be a problem to navigate with VORs gone and a GPS outage.
I always thought INS would be a fantastic in-flight backup to GPS.
Yes and no. Precision of INS is nowhere near GPS or DME/DME and requires constant updating by either of these so as a true backup or stand-alone navigation source it is not sufficient.
I wonder if a sub-$10,000 ring laser gyro would ever become feasible.
I think that's too late now. There have been some attempts about two decades ago to bring prices down for mass production (sort of...) but the growing availabilty of GPS brought these to a halt. An OEM receiver for GPS can be bought for 5 Euros ...
I rather think that some ground based GPS substitute (don't remember what it's three-letter-abbreviation was, but China is said to be experimenting with such a system) will come sooner or later. All it needs is some pseudo-satellites on the ground at fixed locations (which don't need the expensive atomic clocks because they can be synchronised via radio) that transmit exactly the same signals as real GPS satellites so that any GPS receiver can use them.
RLGs are very expensive, but solid state IRUs with constant updating from GPS are cheap and readily available. I have one I use for light aircraft flight testing and cost about US$2000. Something like this would be rather cheaper and could probably be integrated into some kind of MMD quite easily...
G
A few random things...
For about €20k you can get a little box which takes in a GPS antenna one end, and outputs NMEA data the other. It contains a triple (x y z) fibre optic gyro so if the GPS dies, the NMEA output remains valid. The accuracy is as high as anything found in an airliner. The catch? €20k, and not TSOd etc so it could be used only to drive a portable GPS. I can't find the URL right now (I did speak to the manufacturer a couple of years ago) but this technology is fairly widespread. These things are used in various military apps where you can expect GPS to be jammed. If it was quite a bit cheaper I would "install" one in my plane - with velcro of course 
Edit: here is one of the well known players.
For $10k, no.
Solid state gyros are a couple of orders of magnitude not good enough for aviation. The drift, IIRC, is somewhere of the order of 1 degree per minute. Everybody and their dog is chasing after this Holy Grail but progress is very very slow as no new technology has emerged for many years and most of the improvements have come from compensating for various errors like temperature effects. It's fine for an artificial horizon, accelerometers in autopilots, etc which is where they are used. I am about to install the Sandel SG102 (to replace the KG102A) which is one of many examples.
The "advisory glideslope" method is available only with a WAAS/EGNOS GPS, AFAIK. In Europe, most of the people who can fly a GPS/RNAV approach are not doing it with one of these. I cannot imagine any of those choosing a GPS approach (on which you have to manage the VNAV) over an ILS.
It's interesting that one can buy a bizjet whose FMS can fly the "advisory glideslope" without a "W" GPS. Did it have DME/DME input also? Most low-end bizjet FMSs are GPS-only, AFAIK. Maybe with some sort of barometric input it is certifiable.
It contains a triple (x y z) fibre optic gyro ...
The fibre-optic gyro is only one part of an INS. The other one - the accelerometers - are at least as expensive as the gyros.
It's interesting that one can buy a bizjet whose FMS can fly the "advisory glideslope" without a "W" GPS.
Yes, indeed :-) Would it have been imported by the official german Cessna representative, they would have disabled this feature immediately... On the other hand, unaugmented GPS has a precision of 20-30m which is widely sufficient for non-precision minima.
Did it have DME/DME input also?
Yes. But during approach this becomes more and more unusable as it requires at least three DME stations in good geometrical positions.
Most low-end bizjet FMSs are GPS-only, AFAIK.
All aeroplanes in our fleet, also the part 23 ones (Citation Jets and Kingair) have FMSs with GPS and VPU.
The "advisory glideslope" method is available only with a WAAS/EGNOS GPS, AFAIK. In Europe, most of the people who can fly a GPS/RNAV approach are not doing it with one of these. I cannot imagine any of those choosing a GPS approach (on which you have to manage the VNAV) over an ILS.
No doubt you're right there, although I think the non WAAS 430/530 units used for IFR will start to disappear quite quickly now that 8.33 has become a requirement for everyone. I reckon the huge level of integration you get with the GTN units combined with an EFIS and AHRS will prompt quite a few upgrades in the next couple of years.
For this to make sense though, people do need both an EFIS (auto-slewing HSI and free-text annunciation) and an AD/AHRS to feed data to the GPS.
I am about to install the Sandel SG102 (to replace the KG102A) which is one of many examples.
Out of interest, will you be installing the Sandel horizon as well (SA4550)?
I agree with Genghis. We need some backup. Lets assume there will be a Carrington event like in 1859 and most satellites will go tits up. Big solar flares happen. >
The latest Garmin GPSs (G1000/GTN) will enter "Dead Reckoning" mode if such an event occurred.
They won't just give up.
It works en-route but not for approaches of course.
