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een switched off. L1 and L2 are therefore both currently used for precise position
determination.
The ITU has recently allocated two extra frequencies to GPS and Galileo; the L5 and E5
frequency bands both fall within the L-band currently allocated to ARNS (in particular,
DME). Note that this is also applicable to the future allocation for Galileo E6.
Power flux density limits are placed on GNSS, to protect the current ARNS applications
(i.e. in the E5, E6 and L2/G2 bands): WRC-03 placed an aggregate EPFD of -
121.5dBW/m 2 /MHz on all RNSS systems in the band 1164 – 1215 MHz.
To ensure sufficient availability, accuracy and integrity for critical aviation applications,
GNSS is supplemented by augmentation systems – these can be wide area (WAAS),
regional area (RAAS), or local area (LAAS).
Wide area augmentation systems tend to be space-based, to provide the greatest
possible coverage. In Europe, EGNOS is being developed to provide the space-based
augmentation service from 2005. Whether this will support safety-of-life applications (such
as civil aviation) has yet to be determined (through the production of the safety case).
EGNOS (geostationary satellites supported by ground stations) will operate over the
same frequencies as GNSS (L1, L5 and E5 being in current plans). Similar systems exist
in the USA (WAAS), Japan (MSAS) and China (SNAS).
Local area augmentation systems are ground-based, and are commonly known as GBAS
(Ground-Based Augmentation Systems). These consist of data transmissions of
navigation information from ground stations. They operate over the VHF frequencies
(108-117.975MHz) with 25 kHz channel spacing, and therefore compete with ILS and
VOR for spectrum allocation.
AIRPORT-ONLY NAVIGATION AIDS
3.4.3.6 ILS
The ILS is an approach system that provides horizontal and vertical guidance, and
distance information. ILS consists of transmitter stations on the ground and receivers onboard.
Technical Characteristics
The system is composed of several subsystems:
VOR/ILS Localizer: is used to provide lateral guidance to the aircraft and thus allows for
tracking the extended runway centreline. The localizer information is typically displayed
on a course deviation indicator (CDI) which is used by the pilot until visual contact is
made and the landing completed. The localizer radiates on a carrier frequency between
108 to 112 MHz with 50 kHz channel spacing. This carrier is modulated with audio tones
of 90 Hz, 150 Hz, and 1020 Hz. The 1020 Hz tone is used for facility identification.
Sectorised antennae are used to ensure the primary strength of the signal is aligned with
the runway centre-line (within 10 degrees of the on-track signal, the reception range is
approximately 18nm). The effective radiated power is approximately 50W.
ILS Glide slope: provides the pilot with vertical guidance. This signal gives the pilot
information on the horizontal needle of the CDI to allow the aircraft to descend at the
proper angle to the runway touchdown point. The glide slope radiates on a carrier
frequency between 329 and 335 MHz and is also modulated with 90 Hz and 150 Hz
tones. The glide slope frequencies are frequency paired with the localizer, meaning the
pilot has to tune only one receiver control. The glide slope is normally between 2.5 and 3
degrees so that it intersects the middle marker at an altitude of about 200 ft and the outer
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