Navigation Systems

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ILS

The Instrument Landing System (ILS) is made up of a localizer and glideslope and provide guidance to inbound aircraft during low visibility conditions.

The localizer is located at the stop end of the runway on extended runway centerline and provides horizontal guidance. The glide slope is located beside the runway near the touchdown point and provides vertical guidance. Inbound aircraft "fly the needle", i.e. if an aircraft is left of centerline, the aircraft's navigation needle goes right, indicating the pilot must fly right.

The common localizer antenna configuration is made up of 14 log periodic dipole antennas (there are some systems with fewer antennas, however 14 is the norm due to reflections of large buildings along the runway). The composite radiation pattern of 14 antennas create the left and right aircraft needle movement. The glide slope antenna system has three variations depending on terrain: null reference (flat terrain) has two antennas equally spaced; sideband reference (down sloping terrain) has two antennas with the top antenna being three times the height of the lower; and finally the capture effect (uneven terrain) which has three antennas, all equally spaced on the mast. Each of the three systems accomplish the same thing, to move the aircraft needle up and down according to the glide path angle.

Note: ILS localizers and Distance Measuring Equipment (DME) transponders (100 watts) can be co-channel to provide DME information to ILS, non TACAN equipped aircraft.

Equipment configuration and ability to provide acceptable guidance determines what category the ILS will be certified as, and directly effects the pilots decision point (when a pilot must be able to see the runway or call a "missed approach").

Cat I requires 1 transmitter and 1 monitor. Acceptable guidance to 1/2 mile and 200'.

Cat II requires 2 transmitters (1 on cold standby), 2 monitors, an inner marker, and a TACAN/DME (or middle marker). Acceptable guidance to 1/4 mile and 100' above runway threshold. The standard Cat II minima is 100' above TDZE (touchdown zone elevation) and between 1200-2400' RVR (runway visual range).

Cat III requires 2 transmitters (1 on hot standby), 3 monitors, an inner marker, and a TACAN/DME (or middle marker). CAT III minima depend on the category CAT III; IIIa has a DA (decision altitude of 50' above TDZE and an RVR between 800-1200; CAT IIIb minima has no DA and requires an RVR of 600-800; and CAT IIIc is a 0/0 landing and is not authorized at most airports.

ILS Basics

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TACAN

The TACAN (TACtical Air Navigation) is primarily a military long range navigational aid sited with or instead of a VOR. Transponder outputs a 3KW pulsed RF signal between 962 and 1213 MHz (126 separate channels), and provides: azimuth, station identification, and distance information. System is usable up to 200 NM and can supply distance information to 100 aircraft at a time. The system is more accurate than the VOR and can reliably maintain azimuth readings within .3 deg by measuring phase shift of radiated signals upon occurrence of the transponder reference bursts.

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VOR

The VOR (Very high frequency OmniRange) provides magnetic bearing information for long range navigation (up to 175 NM). Transmitter outputs 25-100W continuous RF from 108-118 MHz (160 separate channels), and provides bearing, station identification, and possibly voice.

To provide bearing information the VOR uses two signals: a reference and a variable. The reference is an amplitude modulated 9960 (+/- 480) hz FM sub-carrier which varies at a 30 hz rate. The variable is a double sideband suppressed carrier varying at a 30 hz rate. The maximum reference frequency (10440 hz) occurs when the radiation pattern (clockwise rotating limacon pattern created by the variable space modulating the carrier) is pointing north. To determine bearing, the aircraft extracts the 30 hz FM from the reference, and the 30 hz AM from the variable and measures their phase difference.

Note: bearing is the direction to get to the station, azimuth is your location from the station (aircraft due south of the station will get a bearing of 0 deg., and azimuth of 180 deg.)

Note: a transmitter send info in one direction, a transponder both sends and receives info

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VORTAC

A VORTAC system is ----you guessed it, a co-located (within 50 feet) VOR and TACAN. The two systems are frequency co-channeled and their identifiers are tied together. This way VOR users can detect and use the DME function from the TACAN.

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Marker Beacons

Marker beacons are designed to give a pilot an idea of their distance from runway threshold. The full beacon set has 3 markers: an outer (4-7 miles remaining), a middle (3500' remaining), and an inner (1000' remaining). Each marker outputs a specific tone and code (400, 1300, and 3000 hz respectively) which can be heard by a pilot on course doing an ILS approach. The tones are also used to activate a marker indicator light on the pilots panel.

Note: a full beacon set is seldom used to mark an airfield--an inner and possibly middle is the norm (dependant on ILS category)

Nautel Corp

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DBRITE (Digital Bright Radar Indicator Tower Equipment)

Provides ATC (Air Traffic Control) a graphical display of range and azimuth for aircraft within a 60 mile radius (known as primary RADAR). When combined with a TPX-42 (known as secondary RADAR, IFF/SIF, Beacon RADAR, or Synthetic RADAR) and an automation system, the DBRITE can also display: aircraft ID, beacon codes, altitude, speed, trails, coast/suspend & arrival/departure lists, barometric pressure, and collision avoidance information on up to 128 aircraft all overlaid on up to 5 maps.

Note: The TPX-42 has a range of 200 miles, however the DBRITE can only display up to 60 miles.

Primary RADAR information is received via the ASR (Aircraft Surveillance RADAR) and sent to the DSC (Digital Scan Converter) at the RAPCON (Radar Approach Control). Secondary RADAR is received via the ASR and sent to the automation system (ARTS-Automated Radar Terminal System, or PIDP-Programmable Indicator Data Processor). Data is converted into alpha-numerics and sent to the DSC where it is overlaid on selected maps. Once primary and secondary info are combined in the DSC, they are sent directly to the ATC tower via a combination of dedicated lines, modems, and microwave links. The PS&J (Power Supply and Junction) box in the ATC tower processes bit (Built In Test) information and switch position data. This information is sent to the automation system and DSC, where error codes and/or display data changes are initiated.

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Additional Navigation Systems

Omega -Very low frequency, continuous-wave, global radio navigational system. World wide coverage accurate to four miles. System is Coast Guard maintained. (to be phased out)

Decca -Low frequency, continuous-wave, global system primarily used for ships. System measures time delay between at least two transponders to triangulate current position. System is Coast Guard maintained. (in use)

LORAN-C -(Long Range Navigation) a civil marine radio navigation system used in coastal waters for up to 1200 miles. System measures time delay between at least two transponders to triangulate current position. Absolute accuracy (never been there before) is 1/4 miles. Repeatable accuracy (been there before) is 30-50 meters. System is Coast Guard maintained. (in use)

MLS -Microwave Landing System (in use)

Chayka -Russian version of LORAN (in use)

GLONASS -Russian Global Navigation Satellite System (similar to U.S. GPS) (in use)

NAVSTAR GPS - U.S. Global Positioning System (in use)

EGNOS -European Geostationary Navigation Overlay System. Satellite landing system designed to meet Cat I minimums (European version of WAAS) (under development)

GNSS-2 -A civilian controlled international Global Navigation Satellite System. In simple terms, it is basically a combination of GPS and GLONASS (proposed)

U.S. Coast Guard info on: GPS, DGPS, LORAN-C, & Omega

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New Technologies

The FAA is working towards a single navigation system to augment, then replace, ground based radio navigation aids to include: VOR/DME, LORAN-C, OMEGA, ILS, and TACANs. The new system is based on DGPS (Differential Global Positioning System) technology and it's enhanced accuracy over GPS (DGPS is traditional GPS with the addition of ground based "reference receivers" which provide error correction at surveyed points, particularly airfields) to create an Autonomous Precision Approach and Landing System (APALS). See what Raytheon has to say.

The overall system is made up of WAAS (Wide Area Augmentation System), and LAAS (Local Area Augmentation System). WAAS will be able to provide long range guidance and will also meet Cat I minimums for aircraft landings. The LAAS will provide additional local reference receivers to provide increased guidance information to meet Cat II and III requirements. The end result is a system which provides total guidance from take-off to landing with the premise of world wide coverage (precision landings at other countries depends on their support of reference receiver installation).

The current FAA time line is a useable system by 2005 (recommended by the White House Commission on Aviation Safety and Security)

Additional information:

MITRE Corp Center for Advanced Aviation System Developments

GPS World

GPS Applied to Landing Systems

World Navigation (combo of GPS, communications, navigation, air traffic management, TCAS, and more)

Transponder Landing System (TLS) by Advanced Navigation and Positioning Corp is being tested as a jam resistant augmentation to GPS, and is also available as a tactical system (TTLS).

USAF Flight Standards Agency (AFFSA) Programs covering: Joint Precision Approach and Landing Systems (JPALS), Standard Terminal Automation Replacement System (STARS), and Digital Airport Surveillance Radar (DASR)

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