A recent post about DRDF in particular saw some interesting discussion on the differences between differerent aids to air navigation. I thought it might be useful to start a thread looking at the different types of aids developed over the years and their respective uses and capabilities. In future items I plan to extend the article to cover Ground Based Radio Aids, ATC assisted navigation and Landing Aids.
There's are plenty to discuss and so I thought it might be an idea to start off with first principles to provide some context.
The primary requirements for navigating an aircraft from one point to another is the human eyeball, a map and a magnetic compass.
All aircraft are fitted with at least one compass.(and in the days before drones a pilot with a map)
Unfortunately the magnetic compass relies on a weak magnetic force to work which means (essentially) that it's constantly affected by stronger external forces such as acceleration, deceleration and changes of aircraft attitude. It's only when the aircraft is in a steady state of flight that it is stable enough to use accurately.
For that reason most aircraft are fitted with gyro stabilised and/or gyro driven direction indicators. These are known variously as Direction Indicators (DI) or gyro compasses etc. These may need to be manually synchronised at intervals with the master compass to give correct readings.
Additionally all aircraft are fitted with timepieces (ideally with sweep second hands and stopwatch functionality)
So at its simplest all you need to navigate from one point to another is a map, compass and a watch.
This assumes of course that you know in which direction to fly and how far it is from your departure point to your destination or waypoint.
To determine these variables you need a map or more correctly a chart. There are a few different types and scales but the simplest is known as the Topographical chart - 1: 250 000 or 1:500 000 scale typically. These are designed for visual navigation and have airfields, airspace boundaries, danger areas, ATC radio frequencies and obstructions marked on them. They also (as the name suggests) display ground countours and spot heights and elevations. These charts are also overlaid with lat and long information which allows you to reference all measurements relative to true or magnetic north.
To measure direction and distance you would normally use a protractor overlaid with reference marks for aligning the protractor with the grid lines on the chart, thus referencing all directional measurements to magnetic north.
Bearings, headings, tracks etc are always expressed in 3 digit form. Conventionally 360 is North, 180 is South etc.
To measure distance you would use a graduated rule marked in nautical miles. (6080 feet or 1 minute of latitude)
So to fly from say Wittering to Shawbury you would draw a line on the chart with a marker and use the protractor to measure the angle between the two points relative to true north. This is known as the True TRACK Tr(T).
(The chart will normally reference true north) - you must then factor in Magentic Variation (the angular difference between true north and magnetic north) to give you something to reference the compass to - helpfully this difference is marked on the chart. (Around these parts it's significant even if currently it's only a few degrees, but it's not a constant over time and needs checking.)
You would then measure the length of the track line with your graduated rule.
So (and I'm guessing here) the Magnetic Track required to fly from Wittering to Shawbury is measured as 280 (just north of west) and the distance is 60 nautical miles as the crow flies.
As a handy rule of thumb aircraft fly at 1,2,3,4,or 5 miles per minute. So I'd hop into my aircraft which flies at say 180 knots (1 knot = 1 nautical mile per minute) at Wittering and steer a Heading of 280 on my compass or DI safe in the knowledge that after 20 minutes I'd be overhead Shawbury. (pilots steer HEADINGS on the compass or DI to maintain their required TRACK over the ground)
Sadly most of us couldn`t maintain an exact heading for 2 minutes let alone 20 - and the chances are that there will be some wind blowing at altitude which will result in us DRIFTING off track and speeding us up or slowing us down relative to the ground. Fortunately it's easy to access wind vectors from the met people so allowances can be made in our calculations, and drift corrections refined as the flight progresses.
Even so it's prudent to make a note of features along your route to check your progress as you go along and modify headings and timings accordingly.
The above is the most basic form of air navigation and is known as Dead Reckoning. DR.
Dead reckoning is OK when you can see where you are going to keep an eye on things, but is most definitely not OK in poor visibility or if you are not in visual contact with the ground and need to know where you are relative to the terrain and any obstructions. - (Sadly the UK is littered with wrecks of aircraft that were forced to descend below cloud without accurately knowing their position.)
Furthermore Dead Reckoning navigation can place a heavy workload on the pilot of high performance aircraft especially in complex airspace regions where precise point to point navigation is required.
To this end an array of "Radio Aids" have been developed over time to assist pilots to navigate accurately irrespective of the prevailing weather conditions.
Some of these aids (or navigation beacons) are sited in key locations such as airfields or along pre defined aerial corridors (Airways) and don`t require any ground based assistance - Others require active participation from air traffic control. These will be discussed in a later post.
Ideally and at its simplest a pilot needs to know his position at all times. Technically speaking in three dimensions. Position can be expressed in a number of ways...eg
* 5 miles south of Wittering at altitude 3000 feet
* 52 25 North 01 27 East at Flight Level 250
* Overhead Shawbury at height 1500 feet
* 5 miles south east of Cottesmore climbing Flight Level 120
* 23 miles from Seaford on the 120 radial at altitude 5000 feet.
Notice that in each case that three elements are required. (we need only discuss two as altitude/height or flight level can be read off the aircraft altimeter)
That leaves two - distance and bearing to or from a point.
At this point it may help to use an analogy.
Imagine you telephoned a friend and asked him where he was - If the reply (from his hands free mobile) was "I'm on the M1" you'd effectively know his bearing to or from London for example, but you wouldn`t know his position.
If however the reply was "I'm on the M1 Northbound just about to pull into Leicester Forest East Services" you'd know his precise position; a) because it was expressed as a known point and b) because the bearing and distance from London (for example) is known.
It is on the above principles that many ground based (electronic) navigation aids are employed........
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