VGS diagram

[ssobol]

Recently when developing the VGS diagram for a simulator I encountered a discrepency in determining the position of the GS transmitter antenna.

Using the Jepp charts, the distance from the runway threshold can be determined from the runway length and the published "usable length beyond the glideslope".

Another way of getting the distance from the runway threshold of the GS transmitter is using the published threshold crossing height (TCH) and the tangent of the glideslope angle. This was discussed in a presentation at the 2004 FSEMC and was shown to be a more accurate way of determining the GS transmitter position. At the time the UKCAA said using the TCH was an acceptable method of determining the GS transmitter position.

On the simulator I am working on I was having trouble getting the VGS to work out. Looking at the information on the Jepp charts I determined that there can be a difference in the GS position between these two methods of calculating it. I sampled a number of CAT 2 runways and found while the difference between the two methods is less than 50 feet about half the time, there are instances where the difference is significantly more than that. At MSP 12L the difference is 320 feet, at DFW 35R the difference is 287 feet. There are several runways where the difference exceeds 100'.

Depending on the numbers used to generate the VGS diagram, there can be significant differences in what is expected to be seen. Which way is the correct one?

Depending on the aircraft simulated this difference may be close to or even exceed the allowed tolerance for the VGS test.

Has anyone else encountered this problem? Have there been any issues raised by the FAA?

Thanks.

[reprob8]

Recently when developing the VGS diagram for a simulator I encountered a discrepency in determining the position of the GS transmitter antenna.

Using the Jepp charts, the distance from the runway threshold can be determined from the runway length and the published "usable length beyond the glideslope".

Another way of getting the distance from the runway threshold of the GS transmitter is using the published threshold crossing height (TCH) and the tangent of the glideslope angle. This was discussed in a presentation at the 2004 FSEMC and was shown to be a more accurate way of determining the GS transmitter position. At the time the UKCAA said using the TCH was an acceptable method of determining the GS transmitter position.

On the simulator I am working on I was having trouble getting the VGS to work out. Looking at the information on the Jepp charts I determined that there can be a difference in the GS position between these two methods of calculating it. I sampled a number of CAT 2 runways and found while the difference between the two methods is less than 50 feet about half the time, there are instances where the difference is significantly more than that. At MSP 12L the difference is 320 feet, at DFW 35R the difference is 287 feet. There are several runways where the difference exceeds 100'.

Depending on the numbers used to generate the VGS diagram, there can be significant differences in what is expected to be seen. Which way is the correct one?

Depending on the aircraft simulated this difference may be close to or even exceed the allowed tolerance for the VGS test.

Has anyone else encountered this problem? Have there been any issues raised by the FAA?

Thanks.

Go to the following link:

http://www.airnav.com/

It should have fairly well up to date info on US airports, their ILS systems, localizer antenna, and glideslope antenna locations and positions relative to their runways.

[ukvizeng]

From my experience, The TCH method works best for the FAA/CAA. Always get your data from the same Jepp plate that the Inspectors use as thats what they will reference it too.

If you were getting a large error or discrepancy in the Segment I would suspect there is an error in one of the calculations.

[bluesman]

I recall some years ago that there was a discrepancy between what the FAA wanted and what the JAA wanted.
The JAA insisted that the Shack Offset be off the runway (as in the real world) and the FAA believed that a test airport should be used with the GS antenna down the middle. I think the reason was for simplicity sake.
Does this ring a bell with anyone?

I'll review our method and post back to this next week. I believe we are using the TAN of the GS also.

[ukvizeng]

The Segments have been agreed with the regulators now , FAA/CAA/JAA DAGC whom ever. that there will be no offset in the calculations. Jepp plate should give the distance down the runway and the Threshold crossing height. The aircraft data is available in the simulator documentation package or you can find it in the VGS calculation in the Visual QTG/ATM manuals.

[ssobol]

My point is that using the Jepp chart information, if you use the TCH to determine the position of the GS transmitter you get a different number than is listed on the runway lengths page (usable distance beyond GS, 10-9A). The difference can be over 300 ft. It has nothing to do with whether the GS is offset to the side of the runway or not.

[ukvizeng]

The GS tx Location can not be used to locate the glide slope. The beam radiated is flat at a predefined angle which crosses the runway threshold at the TCH (jepp value). So there will be a difference in the real location of the ILS tx , but this has no bearing on the radiated beam.

The calculation therefore of the VGS has been amended to be based on the TCH and a straight line and should be in the RAES Simulator Evaluation Handbook. Section 4C.

If you send me your data i can produce a segment which you can compare to yours and determine which is better.
It is worth checking how the visual model is aligned. put the sim on gs at 0 pitch and wheel height to the TCH-wheels to a/c ils rx. Slew the airfield model so the threshold is directly below the eye point and on centre line.

hope this helps. if not message me crossfing

[LarryM]

Looking at the published data for these navaids linked to the FAA flight inspection internet site (http://avnwww.jccbi.gov/datasheet/), it appears that in both cases (KMSP 12L and KDFW 35R), the glideslope RPI (runway point of intercept) to threshold distance and the GPI (ground point of intercept) to threshold distance differs substantially. Note that the GPI is defined relative to the threshold elevation where the RPI is defined relative to the actual runway elevation where the extended glideslope is calculated to intercept the runway. In cases such as this, I would have to assume that the runway has some type of slope (down in both cases) between the threshold and the glideslope intercept point. There is an FAA Order (6750.16), Siting Criteria for Instrument Landing Systems, that details much of the factors that go into installing these systems (including the dreaded transmitter offset question) and can help with problems such as this.

For the purposes of the VGS, if the GPI number is used, the trig works out with the threshold crossing height and G/S angle. The problem is that the approach plate uses the RPI number and that is what appears to be causing the differences you are encountering. Another problem here is that if the glideslope is aligned with the published TCH(RDH) and the runway slope is not modeled in the visual database, then the RPI will not be correct in the sim. So the TCH and G/S angle method should be the correct way to do the VGS calculations and align the glideslope, but the issue about the differing intercept point due to runway slope should be addressed for any impact to training.

In situations such as this, it is advisable to send any changes to the VGS to the NSP beforehand so that the engineering section can review the changes before an inspector is on site.