Reduced visibility makes landing impossible without landing instruments.

THE TESTSET

For this scenario I will use a T-30c ramptester from the TIC company.
With this testset u can perform a system test on the VOR (VHF Omnidirectional Range), the Localizer, the Glideslope and the Marker Beacon systems.
VOR/ILS/MB testset

WHAT IS A INSTRUMENT LANDING SYSTEM


The flightcrew have several landing aids that guide the aircraft down to the runway.
First there is the Instrument Landing system itself, there are several systems available but by far the most common is the localiser/glideslope ILS.
approach

The localiser is a set of antenna's that send out two directional frequency ranges, one aimed slightly to the left of the runway and one aimed slightly to the right.
The aircraft receives these two frequencies, when the frequencies are received equally strong the aircraft is on the centerline of the runway.
localiser

The localiser array is positioned at the far end of the runway so that aircraft still have localiser display (centerline of the runway) after touchdown.
localiser array

The localiser sends the left/right guidance to the aircraft, the glideslope sends up/down guidance to the aircraft.
The principle is the same, two different frequencies are send one slightly higher than the proper glideslope path and one sligthly below.
glideslope

It makes sence that aircraft touch down roughly where the glideslope antenna is because that is the vertical beacon.
Therefore the glideslope antenna is positioned at the beginning of the runway so the aircraft has the rest of the runway available to slow down.
glideslope antenna

Then there is the Marker Beacon (MB), these are three directional radio beacons that send out a signal more or less upward.
These 'cones' are located on the centerline of the runway.
The outer marker wich is roughly between 3.5 and 7 nautical miles from the runway threshold, the inner marker is between 0.5 and 0.8 nautical miles from the runway treshold.
Some airfields incorperate a middle marker, this marker is 3500 feet from the runway threshold, on the right glideslope the altitude at this marker should be 200 feet. This altitude is the decision height for a cat 1 autoland so back in the day this marker was used to identify the decision height point.


SOME OTHER LANDING GUIDANCE

On most airports there are also other landing aids available for the flight crew like the Precision Approach Path Indicator or PAPI.
The flightcrew can use these lights for a visual verification that they are on the correct glideslope.
Half the lights should be red and the other half white (sometimes yellow) to be on the correct glideslope.
If there are more red lights than white lights the aircraft is coming in too low and more white lights than red lights means that the aircraft is too high.
PAPI lights

Then there is the the approach lighting system (ALS).
ALS

The lights in the middle indicate the centerline of the runway and the lines are to indicate the horizon.
ALS

As seen here from a flight deck, the horizon bar lights indicate the correct roll attitude of the aircraft for the landing.
horizon bar lights

The Runway End Identification Lights (REIL) wich are flashing lights that indicate the end of the runway.
Runway End Indication Lights

BACK TO THE TESTSET

So let's do the sytem tests. First we find the 'on' switch.
power on

We start with the Localiser and Glideslope to test the ILS (Instrument Landing System) of the aircraft so we select ILS on the function selector. Also we select the attenuator setting.
A attenuator is basically the opposite of a amplifier, it reduces the output signal. The attenuator is used to set the output signal level to the threshold level.
Attenuator and mode selector


I select a frequency on wich we will start to transmit the LOC/GS. The top one is the localiser frequency and the bottom one is the GS frequency. We only tune the localiser frequency in the aircraft, the glideslope is automatically selected for the localiser frequency.
frequency selected
In this case we select 108.10 for localiser and automatically the GS will be selected at 334.70.

Our company uses Multi Mode Receivers (MMR's) rather than VHF Nav receivers and ILS receivers.
The 737 pg first uses a VOR/LOC antenna on the tail that operates the majority of the time.
VOR/LOC antenna

When a ILS (Instrument Landing System) frequency is tuned and the VOR/LOC mode armed or engaged from the DFCS two ILS transfer relays will switch the inputs to the Multi Mode Receivers from the tail VOR/LOC antenna to the nose LOC antenna.
VOR/LOC logic
The attenuator on the ILS transfer relay is designed to reduce electromagnetic interference.

The antenna's are a fair distance away from each other.
antenna's

Like the Localiser and the Glideslope antenna, the VOR/LOC antenna has two seperate connections for the navigation sets.
VOR/LOC connectors

These are the glideslope and the localiser antenna's.
glideslope localiser

I step inside a 737 next generation for this test and allign the IRS (inertial reference system).
IRS allign

The display that we can see on the PFD (Primary Flight Display) is not in a landing mode yet. For this test we need to be in landing mode to test the Instrument Landing system offcourse.
PFD

First let's look at this display, the display is not in the landing mode because the aircraft is in a VOR mode now, the left navigation control panel (we're looking at the left/captain's PFD) is tuned to a VOR frequency, if the frequency selected is a ILS frequency the aircraft will go into a landing mode. The LOC frequency range is between the 108.10 Mhz and 111.95 Mhz and are the odd tenth's, the even tenth's are VOR frequencies.
DME frequency range is between 328.6 Mhz and 335.4 Mhz but because they are automatically tuned to the localiser frequency we don't need to set this one.
VOR mode
Where it says SPL u would normally get the frequency that u have selected until the navigation receiver recognises the station identifier. In this case the receiver has identified the schiphol airport VOR and changed the frequency 108.40 to SPL.

I select the ILS frequency that we have set on the TIC testset (108.10)
108.10 selected
ILS selected

Now u can see that the aircraft is in a landing mode, the localiser and the glideslope scale can be seen now. One dot represents one degree of deviation from the 'on course' indicator in the middle of the localiser and 0.35 degree's deviation for the glideslope.
landing mode

If u look at the localiser scale (the bottom one) wich gives left or right deviations, u can see the magenta colored diamond telling us that we are 'on course' for the localiser.
But if u look at the glideslope scale u can see that we haven't got a glideslope diamond.
This is because we have selected a approach course on the MCP (mode control panel) that is more then 90 degrees left or 90 degrees right of our current magnetic heading and by that is indicating that we are flying on a backcourse approach.
selected course

A backcourse approach is when u land in the opposite direction as the ILS ground stations have been placed. The aircraft will use the (less powerfull) signals coming from the back of the localiser beacon (the 'backbeam').
Pilots use this when there would be a significant tailwind at landing, this obviously reduces the controlability of the aircraft and increases the landing speed. This is unfavorable and this is why they will circle around and land from the opposite direction.
The localiser transmitter is normally situated at the end of the runway to give pilots runway guidance and therefore there is no runway guidance on a backcourse approach. In a backcourse approach the glideslope is inhibited, this is probably for the best too because the glideslope transmitter is normally located at the beginning of the runway to make sure that the aircraft touches ground there and in the case of a backcourse it would be all the way at the far end of the runway.
backcourse approach

It's also nice to know that when u make a backcourse approach, the guidance directions for the localiser are inversed.
Because the aircraft is flying 'mirrored' to the station, the signals that are received are interpreted inversely as how the pilots should steer.
What's more important for us right now is that the aircraft now thinks that we have selected a backcourse and is inhibiting any glideslope information that it may receive.
Therefore we select a course to suit our magnetic heading.
set course to magnetic heading

And there's our glideslope.
glideslope

U can see that the deviation is now pointing one dot to the left and one dot down.
That is because I've selected this on the testset using the preset settings. It is also possible to select variable and u can turn a knob to move the deviation diamonds.
testset

When u turn on the FD's (flight directors) on the MCP (mode control panel) and select APP (approach). The PFD will show VOR/LOC and GS in the roll and pitch section (the top sections of the PFD are from left to right AT(auto throttle)/ROLL MODE/PITCH MODE).
VOR/LOC is either a VOR (VHF omnidirectional range) or a localiser because it is also possible to do a VOR landing. any landing without a glideslope is called a non-precision approach so a NDB (non directional beacon or 'ADF' (Automatic Direction Finder)) approach, a backcourse approach or a VOR approach are non-precision approaches.
When the VOR/LOC and the G/S are active the localiser scale becomes expanded for a more 'finetuned' approach.
APP engaged

So far the instrument landing. Let's have a look at the VOR test. First I select the VOR frequency that we'll be using.
I select 108.00
108.00

And I set the navigation control panel into a VOR mode by selecting the VOR frequency.
cross select the frequenty

The VOR system is a direction indicating system, the station sends out a Frequency Modulated (FM) signal.
The frequency is highest when the directional beacon points due north.
VOR principle
The highest frequency occurs at due north and the lowest frequency occurs at due south.

Because the strength of the signal rotates around the VOR station the strength of the signal received by the aircraft goes from strongest to weakest also.
Here's what the signal strength would look like from the ground station.
VOR station signal strength

So the signal received by the aircraft would is received like a sinus signal.
With the aircraft DUE NORTH of the station this is what the received signal would look like.
VOR north of station
The received signal strength (amplitude) in this position is highest where the frequency is also highest. We have established that due north was where the frequency is highest.
So the aircraft now knows that the aircraft is positioned due north of the station (radial 0 or 360) so if the flightcrew selects 0 degrees on the course selector the deviation bar would be in line with the course indicator and the TO/FROM indicator would display FROM because when we want to fly north (0 degrees) we are flying FROM the station.

If the aircraft was flying DUE EAST of the VOR station we would receive a signal that looked like this.
VOR east of station
Here we see that the signal is received strongest at the intermediate frequency area (area between the highest frequency and lowest frequency). The VOR receiver follows the frequency changes and can identify this position as being east.

This what a VOR groundstation looks like.
VOR station

I have selected both navigation control panels to 108.00 and u can see that the ND (navigation display) now displays these VOR frequencies.
VOR freq's

Also u can see in that picture that the VOR radial is 0 or 360 degrees wich is what I've set on the test set.
Testset

As u can see in this next picture u can fine tune the signal quite a bit from the 0 degrees. Also note that the VOR is also indicated on the RMI (radio magnetic indicator) to the right of the ND (navigation display).
finetuning

On the testset is also this switch wich will put a 1020 Htz test tone on the VOR signal so that we can test the audio circuit of the receiver.
Test tone

So I select the navigation audio on the ASP (audio select panel).
After this the tone should be audible through the cockpit speakers when the testswitch is pressed on the ramptester.
audio select panel

This is a marker beacon antenna. When the aircraft flies over a marker beacon transmitter, this antenna will pick up the signal.
When testing the MB system u should bring the testset pretty close to this antenna to trigger the MB indication.
marker beacon antenna

Unfortunately there wasn't a 737 next generation available at the time I shot this picture so I used a 737 present generation.
The present generation has three lamps indicating the marker beacon signals and the next generation has this info displayed on the PFD.
Here's the outer marker indicated by using the testset.
The Outer Marker normally identifies the Final Approach and is located in line with the localizer.
outer marker

Next is the middle marker. wich is closer to the runway.
middle marker

And then there is the airways marker wich is even closer to the runway.
airways marker