Pt/Sinais de Radio
| This article is currently being translated. |
Você está em pé com os olhos vendados no meio de um campo. A uma certa distância, um amigo está gritando "Eu estou aqui!". Você se vira e começa a caminhar na direção do som que vem do seu amigo. Quando são dois amigos (com vozes distintas) e um mapa, com eles dizendo a você onde eles estão nesse mapa, você pode até descobrir onde você está. Isto descreve como um Non Directional Beacon (NDB) (nota do tradutor: farol não direcional ou sinal (de rádio) não direcional).
Se os amigos não gostam de ficar gritando o tempo todo, eles podem fixar cordas numeradas em torno deles. Cada corda leva diretamente para um amigo. Os números na corda são os mesmos dos graus de uma bússola. Assim, comparando os números nas cordas que cruzam o lugar onde você está você pode saber você está. Isto descreve como um VHF Omnidirectional Range (VOR) funciona.
Mas e se você deseja saber onde você está e você tem apenas um amigo? Então o amiga tem que fazer nós nas cordas dizendo o quão longe você está. Isto descreve como um VOR-Distance Measuring Equipment (VOR-DME) funciona.
Um rádio é um termo genérico para a transmissão de ondas eletromagnéticas. Na comparações acima, um sinal de rádio é quem está gritando ou estendendo as cordas. Um sinal pode incluir informação, ou até mesmo música. Comunicação de duas vias é possível onde o equipamento a bordo questiona o equipamento em solo ou o sinal é usado para ATIS ou comunicação com a Torre.
Ondas de rádio permitem Navegação por rádio e IFR, dispensando a necessidade de pontos de referência visuais. Assim, a navegação noturna e em alta altitude é possível.
NDB
Um Non-Directional Beacon (NDB) é o sinal de rádio de orientação mais básico. O equipamento a bordo da nossa aeronave terá que descobrir onde ele está. Qualquer estação de rádio (de música) também é um NDB. As frequências mais comuns de um NDB podem ser encontradas em AM Medium band (530 kHz to 1700 kHz), mas durante o planejamento do voo nós podemos descobrir NDBs fora dessa faixa.
É possível calcular a distância até uma estação NDB. Para um piloto que voa num ângulo de 90 graus oposto à estação (uma das pontas da asa apontando para a estação). O piloto mede o tempo que leva para voar uma determinada quantidade de graus na bússola. A partir do tempo que leva, o número de graus e a velocidade em relação ao solo, o piloto pode calcular a distância até a estação.
Receptores muito modernos podem estimar a distância até a estação NDB pelo azimute até a estação.
Na América os NDBs de navegação operam entre 530 kHz to 1700 kHz com incrementos de 10 kHz. O restante do mundo usa de 531 kHz a 1602 kHz com incrementos de 9 kHz. NDBs fora desses intervalos, provavelmente, devem server para outros fins, como estações de música ou estações DGPS.
O alcance de um NDB típico é em torno de 75 NM (Milhas Náuticas). Quando utilizando LFR o alcance pode ser maior, mas menos preciso. Quanto mais alta a aeronave mais longo é o alcance.
Um NDB é sensível a influências do tempo e do solo, ele pode ser refletido e distorcido fazendo uma navegação menos confiável. Como uma ferramenta de medição de distância o NDB não é muito adequado. Mas ele é bom o suficiente para encontrar pontos em um mapa e pode ser usado para navegação ponto a ponto. Ele é barato de operar.
LFR
The Low Frequency Radio Range (LFR) beacon is a directional beacon that operates at a frequency of 190 to 535 kHz, the long wave frequency and a bit above. Across the world there are plenty of NDB radio beacons in that range but they do not operate as LFR stations. The long wave frequency is known for it's long range due to sky layer reflections but reflected signals are very unreliable for navigation.
RDF
There are two ways to figure out where the NDB is relative to the aircraft. Rotating an antenna manually until finding the direction of the strongest signal (loudest transmission) or have this done automatically (or electronically). The equipment (or person) to do so is called a Radio Direction Finder (RDF).
ADF
An ADF is not a radio station, is the the equipment on board of the aircraft to perform RDF automatically, the Automatic Direction Finder (ADF). Often NDB stations are called ADF stations but that is technically incorrect.
VOR
A VHF Omnidirectional Range (VOR) is a radio beacon that sends out a special signal making it possible for the receiving equipment to figure out the radial of the beacon. The heading (-line) towards the beacon is called a radial.
If a NDB shouts "I am here!", the VOR shouts "To come to me you would have to fly *this* course!".
The VOR uses frequencies in the the Very High Frequency (VHF) range, it uses channels between 108.0 MHz and 117.95 MHz. It is spaced with 0.05 MHz intervals (so 115.00; 115.05; 115.10 etc). The range 108...112 is shared with ILS frequencies. To differentiate between them VOR has an even number on the 0.1 MHz frequency and the ILS has an uneven number on the 0,1 MHz frequency.
- So 108.0; 108.05; 108.20; 108.25; 108.40; 108.45 would be VOR stations.
- and 108.10; 108.15; 108.30; 108.35; 108.50; 108.55 would be ILS stations.
A VOR station is most often also used for communication (ATC) with the airfield. ATIS, ground, tower etc. The omnidirectional signal is transmitted on a modulated continues wave containing the identifying Morse code. The signal contains a AM signal that can be used for voice or ATC. The signal is FM modulated and the lag (delay) between the AM and FM signal identifies the radial.
Typically the transmitted radial is oriented to True North.
Very modern receivers can estimate the distance towards a VOR only station by the azimuth towards the station.
The range of the VOR signals depend on the type used.
- Terminal (T)
- 1,000 up to and including 12,000 AGL out to 25NM.
- Low Altitude (L)
- 1,000 up to and including 18,000 AGL out to 40NM.
- High Altitude (H)
- 1) 1,000 up to and including 14,500 AGL out to 40NM.
- 2) 14,500 up to and including 60,000 AGL out to 100 nm.
- 3) 18,000 up to and including 45,000 AGL out to 130 nm.
- The higher the aircraft the longer the range.
There has to be a clear line-of-sight with the beacon. If there are mountains or other obstructions the VOR signal can not be received.
The beacon can be received mostly with a clear line of sight, but can also be received if obstructed, via various means: diffraction, trophosperic scatter, tropospheric ducting etc.
The range depicted above is only orientative. In real life situations, radio signal propagation characteristics can expand or reduce the range.
While it is more expensive to operate a VOR station compared with a NDB the benefits are obvious. The signal is less hindered by unwanted reflections and other interferences, when the signal is received it is accurate, it takes away confusion about North, since it's oriented True North, it can be used for automated flight and the receivers are more reliable.
VOR-DME
Distance Measuring Equipment (DME) does not use the same frequencies as a VOR!. See | the wikipedia page for a detailed description. A DME tells the equipment on board of the aircraft the distance towards the station. There is two-way communication between the equipment on-board and the DME station to calculate the result.
While a VOR can be a stand-alone beacon, a DME will be paired with a VOR, a VOR-DME. Both senders will contain information about the other and tuning will be automatic. If the VOR station (of a VOR-DME) fails it defaults to a stand-alone DME.
For position finding only one VOR-DME station is needed since it gives the radial towards the station and the distance. Knowing the position of the VOR-DME station on the map will give the position of the aircraft on that map.
ILS
An Instrument Landing System (ILS) beacon can be used as a NDB beacon (with some receivers) but it's range is limited and depends very much on the position of the aircraft towards the station (and runway). The LOC signal of an ILS transmits only one radial, the heading towards the runway, in one direction. It should only be used as a system for landing and not for navigation. The glideslope component transmits a signal in the 300 MHz band, while the localizer is sharing the lower VHF (108-117.95 MHz) with the VOR's. The localizer can also be used on the backcourse, if approved by local authorities, especially when a go-around maneuver is performed.
TACAN
The military uses a slightly different system as the civilian version. The military uses something named TACAN that operates in the frequency band 960-1215 MHz overlapping the DME frequency range. It combines a VOR-DME and includes an azimuth feature that provides more accurate navigation.
VORTAC
Often the VOR-DME part of the TACAN is also made available for civilian navigation. If so, the beacon is called a VORTAC, but the use is as a normal VOR-DME, in the normal frequency range. The range of the beacon varies. Most have the same range as a VOR-DME but many will have different ranges. Sometimes the VORTAC defaults to a NDB-DME refusing to provide a radial.
- If possible plan a route without a VORTAC since they can be quite unpredictable.
Fix
Not a radio beacon, but a combination of them. On aeronautical maps there are fixes. Fixes are usually identified with a five letter code. Another word for a fix is Intersection. A fix is a place that is crossed by two or more easy radials from a VOR or bearings from a NDB, some fixes are on a radial of a VOR-DME and a specific distance. Typically the easy bearings are 45 degrees intervals on a compass rose, but there are plenty of fixes that use other intervals. It is sometimes a bit of guessing what beacons should used for a fix, most often it's the two nearest ones.
Marker beacons
Runways used to have three vertical directed radio beacons in-line with the runway to aid landing, the marker beacons. It was often combined with the ILS
- Outer marker - Typically placed at 4 NM from runway
- Middle marker - Typically placed at 3500 feet from the runway
- Inner marker - Typically placed at 1000 feet from runway. The aircraft should be at decision altitude/height and the pilot should perform the missed approach procedure if the runway is not visible.
Distance from runway varies a lot. On many airfields the markers have been replaced with an ILS and/or VOR-DME. The outer marker is sometimes combined with a NDB.
Airways
An airway is a predefined flightpath. It is composed like a normal flightplan of legs between VOR, NDB and fixes. Most airways are in controlled airspace. Airways have names and make the creation of a flightplan easier. For a flightplan only the legs towards and from the airway have to be defined.
Further reading
External articles
- http://en.wikipedia.org/wiki/Radio_navigation
- http://en.wikipedia.org/wiki/Radio_beacons
- http://en.wikipedia.org/wiki/Differential_GPS
- http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System
- http://en.wikipedia.org/wiki/Local_Area_Augmentation_System
- http://en.wikipedia.org/wiki/Airway_%28aviation%29
- http://www.dxinfocentre.com/ Providing a collection of stations