How does a localizer work?

[Roy Smith]

How does a localizer work? I don't mean "how do you use one?", but rather
"how is it built?" Is the principle (i.e. modulation) the same as a VOR
except for the azimuthal scale, or is the only similarity that you use the
same indicator instrument on the panel?

A VOR xmitter is usually pretty obvious from a visual standpoint (a big
round building with a pointy thing sticking up), but even though we've got
a localizer where I fly (CDW), I havn't seen anything on the field which
obviously looks like it might be the transmitter antenna. What should I be
looking for? I would imagine that for a relatively narrow beam width, some
sort of phased array might do the trick.

--
Roy Smith <r...@nyu.edu>
Hippocrates Project, Department of Microbiology, Coles 202
NYU School of Medicine, 550 First Avenue, New York, NY 10016
"This never happened to Bart Simpson."

[Lars-Henrik Eriksson]

In article <roy-2111...@mchip8.med.nyu.edu> r...@nyu.edu (Roy Smith) writes:
How does a localizer work? I don't mean "how do you use one?", but rather
"how is it built?" Is the principle (i.e. modulation) the same as a VOR
except for the azimuthal scale, or is the only similarity that you use the
same indicator instrument on the panel?

The modulation is entirely different. In principle, the LLZ station
transmits two signals with narrow lobes, both on the LLZ
frequency. One is modulated with 90 Hz, the other with 150 Hz. The
lobes are oriented so that on the LLZ course, the two signals have
equal strength. To the side, one signal will be stronger and this will
cause a CDI deflection.

Because of this arrangement the LLZ is reliable only close to the
final approach course. Typically it is valid within +-35 degrees or
+-10 degrees, depending on the distance from the runway. Outside this
area it might give you completely weird indications even without
flagging.

The glideslope signal uses exactly the same principle. With the
glideslope there is also the problem of false indications outside the
relatively narrow service area.

A VOR xmitter is usually pretty obvious from a visual standpoint (a big
round building with a pointy thing sticking up), but even though we've got
a localizer where I fly (CDW), I havn't seen anything on the field which
obviously looks like it might be the transmitter antenna. What should I be
looking for? I would imagine that for a relatively narrow beam width, some
sort of phased array might do the trick.

I don't know what a "traditional" localiser station looks like, but
high quality LLZs (e.g. CAT II) are usually a row of low antennas
oriented perpendicular to the runway.

--
Lars-Henrik Eriksson Internet: l...@sics.se
Swedish Institute of Computer Science Phone (intn'l): +46 8 752 15 09
Box 1263 Telefon (nat'l): 08 - 752 15 09
S-164 28 KISTA, SWEDEN Fax: +46 8 751 72 30

[Bob Furtaw]

In article <roy-2111...@mchip8.med.nyu.edu>, r...@nyu.edu (Roy Smith)
wrote:

There are in effect, two transmitters on the same radio frequency, in phase
with eachother. One has is 90 Hz modulated and the other 150Hz. They are
"angled" (and somewhat everlapped, so no gap) so that the 90 Hz is to the
left as you approach the runway. On the runway centerline extention, the
two are calibrated to be equal, thus "needle center". If you move to the
left, the 90 Hz appears stronger driving the needle right, and visa versa.
BTW, the glide slope uses the same technology (90 Hz above).

In the receiver, the front end (RF amp, mixer, and IF) is the same for VOR
and ILS technologies. When you switch to ILS (LOC) frequencies the proper
circuit is switched in to detect the scheme stated above properly. (and
switch out the OBS).

Bob Furtaw - W8IL fur...@comm.mot.com
CFI-A/G/I/MEI, CGI-A/I
All disclaimers ever written by anybody apply. :-)

[Ron Natalie]

Roy Smith (r...@nyu.edu) wrote:
: How does a localizer work? I don't mean "how do you use one?", but rather

: "how is it built?" Is the principle (i.e. modulation) the same as a VOR
: except for the azimuthal scale, or is the only similarity that you use the
: same indicator instrument on the panel?

The latter. Though the localizer shares lots of parts of the receiver and
the indicator, the method of modulation is different. The OBS is ignored,
for the loc is not omnidirectional. It uses two modulated (90 Hz left and
150 Hz right) signals beamed to the left and right of the runway
centerline. You indicator displays the differential between the two.
These are colored orange and blue (note your indicator frequently
are colored this way) and they are marked that way on the chart.

Note that the usable area is thus only about 35 degrees left and right of
the centerline (when you're close in). Also note that a dots deflection
indicates twice the angle that the VOR does.

The glideslope operates similarly on a paired frequency (up around 330
MHz).

Marker beacons all transmit on 75 MHz. The modulation are differ in
tone and pattern depending on whether it's an inner, middle, or outer
marker.

: A VOR xmitter is usually pretty obvious from a visual standpoint (a big

: round building with a pointy thing sticking up), but even though we've got
: a localizer where I fly (CDW), I havn't seen anything on the field which
: obviously looks like it might be the transmitter antenna. What should I be
: looking for? I would imagine that for a relatively narrow beam width, some
: sort of phased array might do the trick.

It looks like six or eight low, usually painted red, T shape antennas
oriented in a row perpendicular to the runway centerline just off the
DEPARTURE end of the runway. The glideslope antennas are on a short
tower located just to the side of the runway at the point where the
glideslope intersects with the ground.

Marker beacon antennas look like short Y shape towers with the elements
space along the V part of the Y. Again, they're ususually painted red.
They're located along the approach path.

-Ron

[Paul Kube]

In article <3at8bd$p...@topaz.sensor.com>, r...@topaz.sensor.com (Ron Natalie) writes:
|> Also note that a dots deflection
|> indicates twice the angle that the VOR does.

Nope. Degree-per-dot deflection on a localizer varies
from site to site. What doesn't vary is that full-scale
deflection is 350' either side of centerline at runway
threshold.

(From the AIM.)

--Paul

[SCOTT HANKEN]

You've read all the answers thus far, and they are all correct. Here is
a little more information.
Localizer
Function: Provide horizontal guidance
Antenna: Optimum: 1000 FT from End of Runway and on Centerline.
Horizontal polarization.
Building: Transmitter building is offset 2000 FT minimum from the center
of the Antenna Array and within 90 to 120 degrees from the approach end.
Frequency: 108.10 to 111.95 MHz
Modulation: Navigation Modulation depth on Course 20% for 90Hz and for
150Hz. Code identification. 1020Hz at 5%.
Course width: course width varies, tailored to provide 700FT at
Threshold or 6 degrees (Full scale Limits)