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  • Please see the most recent updates in the "Where did the .com name go?" thread. Posts number 16 and 17.

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ILS and VOR questions

PRB

PRB

Administrator
Staff member
I know that ILS and VOR share the same frequency range, 108 – 117.95 MHz, but they don't share the exact same frequencies. ILS frequencies are between 108 and 111.95, with numbers after the decimal point starting with odd numbers, and ending in 0 or 5, like 108.15, 108.30, etc. VOR stations have frequencies with numbers after the decimal point starting with odd numbers, and everything between 112 and 117.95. I got that. What I don't get is why, when I tune the ILS frequency for Fresno runway 29 (111.30), my VOR needle points to it. Do ILS and VOL share the same physical radio unit in the plane? That would make sense, given they share the same frequency band, and could explain why the VOR needle points to ILS stations. But I thought ILS transmitters were totally different “things” than VOR stations, pointing only straight down the runway, etc.
 
The same NAV radio you use for receiving VOR's is the same radio you use for an ILS.
 
The VOR and ILS signals are detected by the same receiver. The VOR signals are omnidirectional and provide relative bearing to the station. ILS signals are highly directional along the runway heading, providing altitude and heading references to the landing aircraft. With an ILS frequncy selected no point to the station is provided. And yes, ILS frequencies are in the 108 Mhz to 111 Mhz range and odd tenths selected.
I have noticed that FSX gives points with ILS, but this is not correct.
 
The direction to the VOR, ILS or NDB will be displayed on the ADF gauge if the freq is tuned correctly and the proper radio is connected to the ADF. The VOR is also able to provide a radial TO or FROM the VOR antenna on the CDI. You can select the radial you want to fly on with the Course selection knob. The ILS works similar except that it will only provide a radial TO and FROM the runway angle. It is not necessary to select a radial for the ILS as it is fixed but most pilots will dial in the radial that the ILS is set to as an aid to the approach. The NDB is simply a beacon and does not have the ability to provide a radial TO or FROM the antenna which makes it a little more complicated to align on a radial FROM or TO the NDB.

There are many tutorials available on the internet that describe in detail exactly how the navaids work and will give you a clearer understanding of navigating using them.

Dave
 
jojohnson9 said:
The VOR and ILS signals are detected by the same receiver. The VOR signals are omnidirectional and provide relative bearing to the station. ILS signals are highly directional along the runway heading, providing altitude and heading references to the landing aircraft. With an ILS frequncy selected no point to the station is provided. And yes, ILS frequencies are in the 108 Mhz to 111 Mhz range and odd tenths selected.
I have noticed that FSX gives points with ILS, but this is not correct.
Click to expand...
Ah! This has been the source of my confusion over this. It never made sense to me that my VOR needle should point to the ILS station, as though it was just another VOR signal. I suppose the localizer part of the ILS signal could be a VOR transmitter, but I always assumed the localizer provided more precise deviation info than a VOR station.

The glide slope must transmit in a different frequency than the localizer, of the receiver wouldn't be able to distinguish it from the localizer signal, no? I guess the nav radio “knows”, when it's tuned to an ILS frequencies, to look for the associated glide slope signal, at whatever frequency it's transmitting on, probably at a standard offset from the one you tune.
 
An ILS consists of two independent sub-systems, one providing lateral guidance (localizer), the other vertical guidance (glide slope or glide path) to aircraft approaching a runway. Aircraft guidance is provided by the ILS receivers in the aircraft by performing a modulation depth comparison.
The emission patterns of the localizer and glideslope signals. Note that the glide slope beams are partly formed by the reflection of the glideslope aerial in the ground plane.

A localizer (LOC, or LLZ until ICAO designated LOC as the official acronym)[1] antenna array is normally located beyond the departure end of the runway and generally consists of several pairs of directional antennas. Two signals are transmitted on one out of 40 ILS channels in the carrier frequency range between 108.10 MHz and 111.95 MHz (with the 100 kHz first decimal digit always odd, so 108.10, 108.15, 108.30, and so on are LOC frequencies but 108.20, 108.25, 108.40, and so on are not). One is modulated at 90 Hz, the other at 150 Hz and these are transmitted from separate but co-located antennas. Each antenna transmits a narrow beam, one slightly to the left of the runway centerline, the other to the right.

The localizer receiver on the aircraft measures the difference in the depth of modulation (DDM) of the 90 Hz and 150 Hz signals. For the localizer, the depth of modulation for each of the modulating frequencies is 20 percent. The difference between the two signals varies depending on the position of the approaching aircraft from the centerline.

If there is a predominance of either 90 Hz or 150 Hz modulation, the aircraft is off the centerline. In the cockpit, the needle on the horizontal situation indicator (HSI, the instrument part of the ILS), or course deviation indicator (CDI), will show that the aircraft needs to fly left or right to correct the error to fly down the center of the runway. If the DDM is zero, the aircraft is on the centerline of the localizer coinciding with the physical runway centerline.

A glide slope (GS) or glide path (GP) antenna array is sited to one side of the runway touchdown zone. The GP signal is transmitted on a carrier frequency between 328.6 and 335.4 MHz using a technique similar to that of the localizer. The centerline of the glide slope signal is arranged to define a glide slope of approximately 3° above horizontal (ground level). The beam is 1.4° deep; 0.7° below the glideslope centerline and 0.7° above the glideslope centerline.

These signals are displayed on an indicator in the instrument panel. This instrument is generally called the omni-bearing indicator or nav indicator. The pilot controls the aircraft so that the indications on the instrument (i.e., the course deviation indicator) remain centered on the display. This ensures the aircraft is following the ILS centreline (i.e., it provides lateral guidance). Vertical guidance, shown on the instrument by the glideslope indicator, aids the pilot in reaching the runway at the proper touchdown point. Many aircraft possess the ability to route signals into the autopilot, allowing the approach to be flown automatically by the autopilot.
http://en.wikipedia.org/wiki/Instrument_landing_system

A VOR ground station sends out a master signal, and a highly directional second signal that varies in phase 30 times a second compared to the master. This signal is timed so that the phase varies as the secondary antenna spins, such that when the antenna is 90 degrees from north, the signal is 90 degrees out of phase of the master. By comparing the phase of the secondary signal to the master, the angle (bearing) to the station can be determined. This bearing is then displayed in the cockpit of the aircraft, and can be used to take a fix as in earlier radio direction finding (RDF) systems, although it is, in theory, easier to use and more accurate. This line of position is called the "radial" from the VOR. The intersection of two radials from different VOR stations on a chart provides the position of the aircraft. VOR stations are fairly short range, the signals have a range of about 200 miles.
http://en.wikipedia.org/wiki/VHF_omnidirectional_range

The VOR article also explains the types of cockpit instruments.
"This information is then fed to one of four common types of indicators:
An Omni-Bearing Indicator (OBI) is the typical light-airplane VOR indicator...
A Horizontal Situation Indicator (HSI) is considerably more expensive and complex...
A Radio Magnetic Indicator (RMI), developed previous to the HSI,...
An Area Navigation (RNAV) system is an onboard computer, with display...

Note that the information is NOT displayed on an "ADF gauge" (ADF uses an entirely different system and frequency range) but both may be displayed on a RMI (sometimes selectable to show 2 VOR signals, two LF (NDB or other) or one of each).
 
Sort of too bad you can't create a maintenance feature in the ILS within FSX. Many times one side of the system will be down, so that you only have directional control, and not altitude. You can determine that either through a NOTAM or through listening to the Morse Code identifier.

You can still fly the approach, but it's no longer precision. In that case, your minimums will be higher, as indicated on the approach plate.

During my IFR test I was asked to fly the ILS with no vertical control. Unfortunately the configuration of my airplane is such that I have only one head which provides ILS control so could not use one needle only. So I was just to ignore the vertical needle. Halfway through the approach I got so focused that I forgot I was doing that, and flew the normal precision approach. As I approached the precision approach minimums I suddenly remembered and just looked at the guy grading me and said "Dave, I'm sorry, I forgot I wasn't to use vertical guidance and I've busted the minimums". He looked at me and said, "well that's okay, because I forgot too, and didn't remember until you just said so."

I passed with "flying" colors....so to speak.
 
000rick000 said:
If you really want to grasp this subject or any topic really grab a copy of the FAR/AIM (in the United States) from your local FBO. Also try this...I came across this website during my own commercial pilot training. They have interactive topics for most things. No, I did not attend here. I did my training in the SW USA.

http://selair.selkirk.ca/Training/instrument-rating/IFR-simulations/How-VOR-works.html
Click to expand...

I have a 2011 edition of the FAR/AIM, forgot all about that valuable resource. Dug it out today and it does indeed clear up any confusion anyone might have on this and a great many other things! :icon_lol:
 
srgalahad said:
Note that the information is NOT displayed on an "ADF gauge" (ADF uses an entirely different system and frequency range) but both may be displayed on a RMI (sometimes selectable to show 2 VOR signals, two LF (NDB or other) or one of each).
Click to expand...

My Bad, ADF and RMI are similar appearance but the RMI is infinitely more versatile as it will perform all the tasks of the old ADF with superior ease while including the ability to point to any navaid not just the NDB.

Dave
 
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