Glideslope Operation & Testing

gs_path.jpg


Operation


Basic Navigation Principles


Glideslope is a part of the Instrument Landing System (ILS). Its purpose is to provide the ability to land an aircraft in poor visibility or in good conditions. Using Localizer paired with Glideslope is considered a precision approach. Glideslope is a system that is paired with the Localizer. It is a navigational aid which can be used in IFR flight and is designed to give the pilot a glide path (GP) centered at 2.5 - 3 degrees from the horizontal plane of the runway. The beam is only 1.4 degrees deep: 0.7 degrees below the GS centerline and 0.7 degrees above.
GS_illustration.jpg

It operates in the UHF spectrum and requires its own UHF antenna, mounted in the nose or between the main wings of an aircraft or with a combination VOR / localizer / GS antenna. This antenna is connected to a NAV receiver in the cockpit which is connected to an HSI or CDI instrument which displays the glideslope positioning of the aircraft in relation to the runway when tuned into the runways localizer frequency.
gs.gif

Glideslope information is displayed by either a horizontal pointer or GS "bug" which displays high when the aircraft is below the GP or it will display on the low side of center when the aircraft is above the GP. When there is no signal the GS bug is hidden from view or a GS flag will display.
hsi.jpg

The typical range that a GS is usable from the runway is 10NM. Ground equipment consists of a GS aerial antenna situated between 800 to 1400 feet (typically 1000 ft) from the front end of the runway, mounted just to the side and has focusing hardware consisting of metal plates that ensure that the beam is precisely angled with no possibility of erroneous readings. There is no backcourse for GS.
gs300.jpgws_gs2.jpg
The glideslope transmitter by the runway transmits an AM (DSBFC) modulated at 40% for each side band. The upper signal beam is modulated at 90Hz and the lower signal beam is modulated at 150Hz. Both signals are designed to overlap in the middle which has the effect of both signals being received at varying strength when an aircraft is inside the overlap zone. Each signal is at the same carrier frequency and is interpreted by the GS receiver as one DSBFC signal with a 150Hz and 90Hz sideband.



LOCALIZER / GLIDESLOPE FREQUENCY:

Glideslope frequencies are paired to localizer frequencies internally. Glideslope frequencies do not follow a logical sequence. This pairing enables the pilot to be able to tune in the localizer and glideslope frequencies using only the localizer frequency of the selected runway and does not require them to know the glideslope frequency. The glideslope frequencies allocated are in the UHF band and in the Aeronautical Radionavigation section of 328.6MHz to 335.4MHz. There are a total of 40 glideslope frequencies. Each channel with its designated localizer and glideslope frequencies are in the following table:

Channel
LOC
G/S
Channel
LOC
G/S
Channel
LOC
G/S
Channel
LOC
G/S
18X
108.10
334.70
28X
109.10
331.40
38X
110.10
334.40
48X
111.10
331.70
18Y
108.15
334.55
28Y
109.15
331.25
38Y
110.15
334.25
48Y
111.15
331.55
20X
108.30
334.10
30X
109.30
332.00
40X
110.30
335.00
50X
111.30
332.30
20Y
108.35
333.95
30Y
109.35
331.85
40Y
110.35
334.85
50Y
111.35
332.15
22X
108.50
329.90
32X
109.50
332.60
42X
110.50
329.60
52X
111.50
332.90
22Y
108.55
329.75
32Y
109.55
332.45
42Y
110.55
329.45
52Y
111.55
332.75
24X
108.70
330.50
34X
109.70
333.20
44X
110.70
330.20
54X
111.70
333.50
24Y
108.75
330.35
34Y
109.75
333.05
44Y
110.75
330.05
54Y
111.75
333.35
26X
108.90
329.30
36X
109.90
333.80
46X
110.90
330.80
56X
111.90
331.10
26Y
108.95
329.15
36Y
109.95
333.65
46Y
110.95
330.65
56Y
111.95
330.95


Testing


Glideslope testing is primarily done on the ramp. Bench testing of the system would be very complex with the GS antenna, navigation receiver and instrument connected together, or tested individually and is generally not done. RAMP testing typically fleshes out which part of the system is at fault and from there the suspected hardware can be removed from the aircraft and further tested on the bench.

To test a glideslope system on the ramp, a testing instrument such as an IFR NAV-402A is required along with a BNC type RG-58 (50 ohm) coax cable and a 2-3ft long test antenna with ground plane plate and adjustable tripod.

There are four basic tests required:
1) Standard deviation (up/down)
2) Full deviation (up/down)
3) Center
4) GS flag

These tests are conducted at two separate channels with the IFR NAV402. They are designated as HI and LO. The LO channel tunes to 42X, or 329.60MHz. The HI channel tunes to 18X, or 334.70MHz. Note that testing of other frequencies is possible by manually setting the frequency.
nav402.jpg

Ramp Test Procedure

1. Set up the NAV-402A on an appropriate workbench near the aircraft GS antenna.
2. Connect the test antenna cable to the RF OUT connector (15) and start with an output level of -80dBm.
3. Place the antenna near the aircraft.
4. Set the MODE switch (5) to G/S fixed.
5. Set the G/S DDM switch (6) to 0.
6. Set the XTAL HI/LO switch (7) to HI.
7. IMPORTANT: The IFR NAV-402A requires a 15 minute idle-on warmup period for accurate testing.
8. Power on the aircraft avionics bus and turn on the Navigation receiver(s) under test.
9. Tune the Nav Rx to 108.10.
10. The selected CDI or HSI should have the GS indication visible and centered. Adjust output power if the signal is erratic or the flag does not pull on the GS indicator.
11. Set the G/S switch to .091 on the 150 side. The indicator should be on the 1st mark ABOVE 0.
12. Set the G/S switch to .175 on the 150 side. The indicator should be on the 5th mark ABOVE 0.
13. Set the G/S switch to .400 on the 150 side. The indicator should be at its MAXIMUM level ABOVE 0.
14. Set the G/S switch to ~150, the GS indication should disappear.
15. Repeat steps 11 to 14 but set to the 90 side. All indicator positions should be BELOW 0.
16. Tune the Nav Rx to channel frequency 110.50.
17. Set the XTAL HI/LO switch to LO.
18. Repeat steps 10 to 15.



References:
http://www.navfltsm.addr.com/ils.htm
http://en.wikipedia.org/wiki/Instrument_landing_system
http://www.dbsant.com/GlideS.php
IFR NAV-402AP User Manual
Avionics class notes and handouts