GS aerial Glideslope 150 Hz 90 Hz -1,000 feet 50 feet 11-51 Figure 11-113.  A traditional course deviation indicator is shown on the left. The horizontal white line is the deviation indicator for the  glideslope. The vertical line is for the localizer. On the right, a Garmin G-1000 PFD illustrates an aircraft during an ILS approach. The  narrow vertical scale on the right of the attitude indicator with the “G” at the top is the deviation scale for the glideslope. The green  diamond moves up and down to reflect the aircraft being above or below the glidepath. The diamond is shown centered indicating the  aircraft is on course vertically. The localizer CDI can be seen at the bottom center of the display. It is the center section of the vertical  green course indicator. LOC1 is displayed to the left of it.  Figure 11-111.  An ILS localizer antenna.  Figure 11-112.  A glideslope antenna broadcasts radio signals to  guide an aircraft vertically to the runway.  Compass Locators The UHF antenna for aircraft reception of the glideslope  signals comes in many forms. A single dipole antenna  mounted inside the nose of the aircraft is a common option.  Antenna manufacturers have also incorporated glideslope  reception into the same dipole antenna used for the VHS  VOR/ILS localizer reception. Blade type antennas are also  used.  [Figures 11-114] Figure 11-115  shows a VOR and a  glideslope receiver for a GA aircraft ILS. runway approximately 1,000 feet from the threshold. It  transmits in a wedge-like pattern with the field narrowing  as it approaches the runway.  [Figure 11-112]  The glideslope transmitter antenna is horizontally polarized.  The transmitting frequency range is UHF between 329.3  MHz and 335.0 MHz. The frequency is paired to the localizer  frequency of the ILS. When the VOR/ILS receiver is tuned  for the approach, the glideslope receiver is automatically  tuned. Like the localizer, the glideslope transmits two signals,  one modulated at 90 Hz and the other modulated at 150  Hz. The aircraft’s glideslope receiver deciphers the signals  similar to the method of the localizer receiver. It drives a  vertical course deviation indicator known as the glideslope  indicator. The glideslope indicator operates identically to the  localizer CDI only 90° to it. The VOR/ILS localizer CDI and  the glideslope are displayed together on whichever kind of  instrumentation is in the aircraft.  [Figure 11-113] It is imperative that a pilot be able to intercept the ILS to  enable its use. A compass locator is a transmitter designed  for this purpose. There is typically one located at the outer  marker beacon 4–7 miles from the runway threshold. Another  may be located at the middle marker beacon about 3,500 feet  from the threshold. The outer marker compass locator is a  25 watt NDB with a range of about 15 miles. It transmits  omnidirectional LF radio waves (190 Hz to 535 Hz) keyed  with the first two letters of the ILS identifier. The ADF  receiver is used to intercept the locator so no additional 