No RMI on a localizer

BrewMaster

BrewMaster

Well-Known Member
Why is that? I assume because the localizer is shooting out only 1 radial? Which means when in range(35 degrees either side out to 10NM and 10 out to 18NM) the arrow will only point to that radial, otherwise it will just lay down. Or not....

Any II's know?
 
It's because when you look at the radio signal emitted by the LOC, it is completely different from the one emitted by a VOR. For one thing, your LOC does not transmit the direction of the course, just "left of course, right of course". There is a lot more electrical engineering stuff that I kind of remember from my avionics course, but that is the essence of it. Completely different makeup of the signals.
 
Ok, but how is that different from a CDI or HSI which picks up VOR and LOC? Or is that what you mean, a RMI is different?
 
BrewMaster said:
Why is that? I assume because the localizer is shooting out only 1 radial? Which means when in range(35 degrees either side out to 10NM and 10 out to 18NM) the arrow will only point to that radial, otherwise it will just lay down. Or not....

Any II's know?
Click to expand...

Because ur suppose to flip up the OM dummy :sarcasm::)

The LOC doesn't have a "big blip" and "swinging blip". VOR's have a omnidirection transmission and then the sweeping transmission (lighthouse analogy). RMI takes that info + your heading (technically the heading off the other guys side) and pops up a needle. RMI doesn't interpret the LOC freq that way.

Been awhile since my II, forgive mistakes.
 
Here is my attempt at a better explanation.

The VOR signal is read as a phase difference between the 2 different signals, the variable and the reference. Setting the OBS just adjusts what your CDI sees as the zero-in other words, if you set the OBS to 180, and you are on the 180 radial, your CDI gets a signal that both phases are in sync. With me so far?

Ok, good. The important thing to realize is that no matter what you twist on the OBS, the signal coming from your VOR reciever into the VOR head is exactly the same. If you are on the 180 radial, the variable and the reference phase signal coming from the antenna will always be 180* out of sync.

When we hook up an RMI, all we do is steal the VOR signal BEFORE you put any input from the OBS into it. The RMI is set up so that whenever the raw VOR signal (not processed or "resolved" by the circuitry connected to the OBS) indicates a 180* out-of-phase condition, the RMI will point at 360, putting the tail of the needle on 180. This cues you, the pilot, that you are on the 180 radial.

Now compare the Localizer. First, you must realize that dialing in a localizer frequency activates a separate set of circuitry in your nav radio. This works because, as you probably know, certain frequencies are reserved for localizers, others for VORs, and ne'er the twain shall meet.

When recieving a localizer, all your nav radio is doing is comparing the strength of two different tones transmitted on the carrier frequency-150 Hz and 90 Hz, IIRC (also labeled in pilot diagrams as "blue" and "yellow"). When your nav radio senses too much blue, it transmits a "fly to yellow" signal to the CDI, causing a needle deflection. Notice that there is no input of any actual magnetic course into this system. This is why when you move the OBS when navigating with a localizer, it does nothing except confuse your student. Therefore, there is no way for this signal to drive a needle pointing to the station. Even if you did set up a system that had OBS input, your RMI needle would only be accurate as long as you set your OBS correctly. If you set your OBS to 360 on a localizer whose course was 130, your RMI would point at 180 as long as you were on course.

Finally, there is no direct degree measurement on a localizer. This is because of the infamous "700 feet at the threshold" standard. Your RMI would not know how many degrees to deflect the needle, because your nav radio does not know how many degrees wide your particular LOC course is, just how strong the "fly right" or "fly left" signal is.
 
Roger said:
When we hook up an RMI, all we do is steal the VOR signal BEFORE you put any input from the OBS into it. The RMI is set up so that whenever the raw VOR signal (not processed or "resolved" by the circuitry connected to the OBS) indicates a 180* out-of-phase condition, the RMI will point at 360, putting the tail of the needle on 180. This cues you, the pilot, that you are on the 180 radial.
Click to expand...

Are you sure an RMI works this way? It doesn't need to. As long as it points to the source of the radio signal, the tail will point to the appropriate bearing from, just as it does for an ADF.
 
tgrayson said:
Are you sure an RMI works this way? It doesn't need to. As long as it points to the source of the radio signal, the tail will point to the appropriate bearing from, just as it does for an ADF.
Click to expand...
Taylor-
Are you suggesting that perhaps the RMI uses a phase relationship from two antennae (similar to the loop/sense setup) and determines direction that way?

I don't think that's the case, but I guess it is possible. I'll see what I can find. We didn't talk extensively about RMIs in my 2-years-ago avionics class.

*edit* part of the reason I think that is that the KX155 was able to give you a digital readout of radial to the station, with no heading input whatsoever. If the nav radio is doing that already, it seems like it would be easy to simply output that information to an instrument and move a bearing pointer.
 
Roger said:
Taylor-
Are you suggesting that perhaps the RMI uses a phase relationship from two antennae (similar to the loop/sense setup) and determines direction that way?
Click to expand...

No, I'm saying that the phase relationship is unnecessary, just as it's unnecessary for an NDB. If I have a rotating compass card beneath the pointer, the fact that the needle merely and stupidly points to the station is enough to give me bearing to and from, without any fancy circuitry.

I don't *know* this is the case, but it's always been my assumption.
 
Here's a way to test it-
Get someone with an RMI to go somewhere with a VOT. Tune up the VOT. If the VOR RMI works like an ADF, the needle will point to the station. If it works based off the VOR phase relationship, it should point to 360. Does that sound correct to you?

*edit* from the Instrument Flying Handbook, page 7-16:

To use the VOT service, tune in the VOT frequency 108.0
MHz on the VOR receiver. With the CDI centered, the
OBS should read 0° with the TO/FROM indication showing
FROM or the OBS should read 180° with the TO/FROM
indication showing TO. Should the VOR receiver operate an
RMI, it would indicate 180° on any OBS setting.
Click to expand...
 
*edit* from the Instrument Flying Handbook, page 7-16: Should the VOR receiver operate an RMI, it would indicate 180° on any OBS setting
Click to expand...

Hmmm. Interesting. I'd still like to see it in a real airplane. It seems to suggest that the RMI knows where the 180 is located on the dial. That's not the way my simulator behaves, but that may well be a misunderstanding on the part of the programmer. So lose the gyro and the arrow no longer points to the station?
 
tgrayson said:
So lose the gyro and the arrow no longer points to the station?
Click to expand...
I'm guessing that it will still point to the correct number on the dial, however the dial will be frozen. That is just a guess though.
 
Ya'll kinda speaking over my head and maybe I completely missed the intent of the thread. I guess practically why you couldn't use an RMI for a loc could be a good question. But why would you want to use an RMI? Wouldn't that be harder as the RMI is more raw than a mostly graphical OBS CDI?

Again, maybe the wither-tos and the why-fors were the nature of the OP.
 
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