I have often read this.
I can't test it... firstly I do not have LPV and secondly there is just 1 UK airport with LPV which is a little island which I have no pressing desire to visit.
However I thought that the CDI presentation on LPV has a similar vertical and lateral sensitivity to the ILS indication.
Because an LPV describes a geometric slope rather than a track described by a propagated radio signal which can be affected by environmental and other factors.
In addition to what Jason says, I think it's mainly because the deviation on an ILS is an angle from the glide path whereas the LPV's deviation is the distance from the glide path. The ILS deviation gets more sensitive the closer you get, thus the autopilot does increasingly violent corrections whereas it stays stable in an LPV scenario.
There are quite a few LPVs in Germany now but unfortunately all with DHs in the non-precision category. This is due to this technology being new. Given that LPVs have similar obstacle requirements as ILSes, you would be safe (but not legal) if you chose to continue below the DH as it is today.
I think an LPV actually has a period where it operates in an angular mode but closer to the runway ends up at a distance from glideslope sensitivity which prevents the wild swings close to the ground. This is because it needs to funnel you in to higher sensitivity than you have at the FAF. It is obviously different in this respect from all other GPS approaches including LNAV/VNAV.
Basically, what Jason said. Also, you can line yourself up on the final approach using GPSS - giving much better intercepts (automatic wind correction...)
I think an LPV actually has a period where it operates in an angular mode but closer to the runway ends up at a distance from glideslope sensitivity which prevents the wild swings close to the ground ... It is obviously different in this respect from all other GPS approaches including LNAV/VNAV.
That is true for horizontal deviation, but vertical sensitivity is angular down to a certain minimum (then linear) on LNAV/VNAV approaches as presented by GPS units. The same is true of Garmin's LNAV+V indications.
Baro-VNAV systems give a linear deviation signal. I imagine this suffers even more than an ILS from environmental effects - has anyone here ever flown one of them in a suitable equipped aeroplane (airliner)?
I am very suprised there are "environmental and other" factors on an ILS that would be evident to the pilot.
A quick google digs out this which, despite missing images, says
With reference to 12 June 2002 data, it is observed from 9 Sep. 2002 measured data that the electronic centreline of the runway (0 DDM) is shifted towards 150 Hz signal side by approximately 0.3° and the course width is widened by 0.15°.
A 0.3° shift in the centreline of ILS causes a horizontal error of 20.9 m at 3 km distance.
Also the LPV indications are angle-based (at least close-in), just like the ILS. It's not like a plain GPS approach which is 0.3nm FS all the way past the FAF.
Also, you can line yourself up on the final approach using GPSS - giving much better intercepts (automatic wind correction.
Can you explain? Normally one is vectored to the ILS so one is flying a pure heading, all the way to the LOC.
The main reason that an ILS is not as solid as the LPV is that the ILS is a propagated signal that can be interfered with. The ILS localizer antennas are located off the departure end of the runway and are adjusted so that the FSD (full scale deflection) of the CDI is +/- 350 feet at the threshold. The longer the runway, the narrower the localizer. There are ILS hold bars painted on the runway so that an aircraft doesn't distort and reflect the localizer path. Structures and terrain can and do generate ground reflections which distort the path.
The GPS determines its X, Y, and Z position in space. For a WAAS GPS, the vertical is a calculation based on position. As long as the GPS position remains within the integrity requirements for the approach, the calculated path is very stable. The lateral path outside of the FAF is the lesser of +/- 1 NM or angular +/- 2 degrees, such that at the threshold the 2 degrees equals +/- 350 feet, the same as for an ILS. Because there is no actual signal, the point at which the path originates from is a fixed distance from the threshold for all runways, regardless of length (approximately at 10,000 feet from the threshold).
Many ILS installations have a note requiring that the autopilot not be used below a certain altitude, this is determined by a kink in the GS or a fluctuating GS during the flight test phase. The kink or fluctuations are usually caused by ground reflections and secondary paths reflecting off of obstacles. The vertical FSD starts out at a fixed value of +/- 150 meters. This point is typically 6 NM from the threshold. From this point inward, the vertical path is +/- 25% of the GS angle, so a 3 degree slope would be +/- .75 degrees. This compares with the ILS GS of +/- .7 degrees. At approximately .6 NM from the touchdown point, for a LPV the FSD again becomes fixed at +/- 15 meters all the way down to the pavement. If the procedure is a LNAV/VNAV or LNAV+V, everything is the same except that the FSD becomes fixed at +/- 45 meters. Other than this difference, the lateral and vertical guidance is the same when flown by a SBAS GPS system. So unlike a non SBAS aided GPS, the SBAS GPS uses angular CDI of 2 degrees for all the RNAV approach procedures, regardless if it is LPV, LP, LNAV/VNAV, or LNAV. However, if the final approach course is longer than 7 NM, the LNAV/VNAV or LNAV CDI is the lower of .3 NM or +/- 2 degrees.
Some other trivia. The integrity requirements for the various approaches HAL (Horizontal Alarm Limit) and VAL (Vertical Alarm Limit) for the final approach segment is as follows (meters):
LPV (250+ DH) LPV (Below 250 DH) LP LNAV/VNAV LNAV+V LNAV
Note: 556 Meters is .3 NM.
On a 3deg GS, I make it that the +/-15m fixed "corridor" doesn't start until just after the usual 200ft decision height point (3500ft or 0.6nm from the touchdown point).
It thus seems the guidance is basically the same on LPV as on an ILS, all the way to the MAP, in practical situations.
Thus I don't see why the instrument indications should make an LPV especially easier to fly, given that they are almost the same between 6nm and the touchdown point.
I have always found an ILS rock solid when flying it on autopilot. Whether an LPV would be flown more accurately I don't know - this will depend on the gain of the control loop. If there is a strong crosswind, one doesn't get very good convergence on the LOC until fairly close in, and maybe the LPV coupling is tighter.
You're ignoring the interference that the ILS suffers from, Peter. The ILS beam is not a straight beam like the LPV "beam".
I can send you the Oziexplorer track logs of numerous ILS approaches and you can check if they are straight or not. They look totally straight to me, laterally at least.
What I was getting at are pilot reports of the LPV being easier / more stable to fly.