Aircraft Navigation
For centuries, explorers the world over, have found ways to navigate to points unknown and return to their own homelands. The earliest navigators recognized the stability of the heavens and used the stars to navigate. This use of the stars, celestial navigation, enabled the Egyptians to find their way throughout the Mediterranean Sea, Leif Erickson to navigate the North Atlantic Ocean, Christopher Columbus to find the West Indies (he was actually looking for India), and Magellan to circumnavigate the earth.
Early explorers used the magnetic compass and the sextant. The compass is a relatively simple device. A simple compass has a bar magnet that sits atop a pivot point. The south end of the magnet will always point to the magnetic North Pole. A course can be maintained by keeping the compass pointer at the same relative position to the vessel’s direction of travel. More complex compasses have compass cards with the cardinal points (north, south, east, and west) attached above the magnet. The compass card with magnet are allowed to float in a bowl of water or oil thereby reducing friction, dampening the rolling movements of the vessel and allowing the free movement of the card. The top of the compass housing is aligned with the ship or aircraft. The pilot of the vessel only needs to keep the selected course at the top of the compass to maintain that direction.
The sextant was the other necessary tool used by early navigators. The sextant allowed the navigator to take sight on a known star or the sun. Knowing the time of day and then measuring the angle between the star or sun and the horizon can make a plot of latitude and longitude with a fair degree of accuracy.
These tools of the early explorers were necessary to orient the explorers to their position. This orientation is no more complex than what we do when we orient ourselves in a dark room. If a person wakes in the middle of the night and sees the glow of a digital clock across the room, the person, through knowledge of the room, knows the location of doors and furniture.
A chart or map is another necessity for successful navigation. We have all seen the early maps used by Columbus. These maps were of the known world at the time and only included representations, as those explorers were able to describe those lands they had previously found. Modern maps show all known points of land and give accurate representations of where things are on the earth.
Celestial navigation is still in use today. During World War II and later, navigators on ships and long-range bombers used the technique of "shooting the stars" to fix their positions on the globe.
During the early days of aviation, pilots did not fly very high above the ground. Therefore their technique of piloting aircraft by following roads and flying from town to town was sufficiently accurate to enable the pilot to reach a chosen destination. This technique is known as pilotage. Many towns during the early days of aviation had the town name painted on large water towers or on the tops of barns. Pilots flew by using the same maps that were used by automobile drivers. There were several drawbacks to this most basic form of navigation. During night flight or during periods of low visibility the checkpoints perhaps were not visible. When flying over sparsely populated areas with few towns, roads or other easily identifiable landmarks the pilot stood a good chance of getting lost.
In order for the pilot to successfully navigate over greater distances the use of “dead reckoning” was put into use. Dead reckoning (short for deductive reckoning) is simple. First a course line is drawn on a map or aeronautical chart from the point of departure to the first checkpoint or the destination airport. That course line direction is computed relative to true north (true north lines are drawn on aeronautical charts and plotted with a protractor). The course line is then marked with checkpoints easily identifiable from altitude about every 10 to 15 miles. The course heading is expressed by degrees i.e. 150 degrees true, and then corrected for variation (variation is the difference in degrees between true north and magnetic north). This computation gives the course to be flown on the magnetic compass in the aircraft.
Next in the process of dead reckoning is the computation of how long to fly. If the aircraft cruises at a speed of 120 mph ground is covered at a rate of two miles-per-minute. If the destination airport is 150 miles distant, the estimated time of arrival should be about an hour and 15 minutes after takeoff. The use of groundspeed computation will enable the pilot to very accurately estimate arrival at a given destination.
The checkpoints are used to verify the groundspeed of the aircraft and to correct for the effects of wind. The wind aloft will either add (tailwind), or subtract (headwind) from the ground speed. A wind from the side will necessitate the need for a wind correction angle being applied to the course heading to maintain the course line. Think of it this way, if a sailor is on one side of a river with a rowboat and want to cross the river to a dock on the other side, the flow of the river will carry the boat downstream unless the boat is aimed at a point upstream of the dock.
Modern electronic radio navigation came into use before World War II. Worldwide pilots use Very High Frequency Omni Range Radio Beacons (VOR) to navigate. These VORs radiate a signal that can be received by aircraft equipped with VOR receivers. These receivers allow the pilot to select any of 360 course lines radiating from the VOR station and to navigate to or from the station. The indicator in the aircraft will show deviation from the center of the course line selected on the dial of the instrument. The pilot can interpret the instrument indications and know if the aircraft is to the left or right of the course line and if he/she is flying to or from the station.
The VOR can be visualized by thinking of the station as the hub of a bicycle wheel. The course lines are the spokes and the aircraft pilot can choose which spoke to fly on.
The most popular and accurate system in use today is the Global Positioning System (GPS). The GPS system is made up of satellites in earth’s orbit and a GPS receiver on an aircraft. The receiver can receive signals from the satellites and give the aircrafts position in latitude and longitude as well as the aircrafts altitude. The GPS is under control of the U.S. military and accuracy to civilian users is limited. However this limitation still gives astounding accuracy to within only a few inches!
Modern electronic navigation instruments are capable of guiding a properly equipped aircraft and pilot to a landing exactly on the spot of the runway marked as the destination from thousands of miles away.
Airport Authority Meets Thursday
The September meeting of the Santa Ynez Valley Airport Authority will take place at 6 p.m. on Sep. 6, at the airport administration building. The authority will be conducting business, including information on the hanger waiting list, the closures during the upcoming resealing of the taxiways and runway and the progress on the conditional use permits pending before the county planning commission.
