The use of geolocation, in one form or another, is integral to every part of aviation. Furthermore, the need for accuracy and reliability far outstrips that required for navigation on land, or even maritime navigation (whereas a ship will float without fuel, and airplane will remain airborne for a relatively short time). Positional accuracy is also of great importance if any sort of aerial remote sensing is to be done; after all, data is only as accurate as its geospatial reference.
The first, and still a fundamental means of geo-location is termed VFR (Visual Flight Rules) navigation. In essence, this involves flying within sight of the ground at all times. The pilot then uses a VFR navigation chart (also known as a VNC, or a sectional chart), which shows ground elevation, roads, rivers, lakes, power lines, and other such features, to determine where the aircraft is based on what can be seen on the ground in the immediate area. Although it might seem wildly inaccurate, this system has a number of advantages: a paper chart will never malfunction, and it’s often possible to determine your position within several hundred metres. The disadvantages are severe, though: in cases where you need greater positional accuracy, there is no way to obtain it, and it requires that you be able to see the ground at all times. It would also prove quite useless if you wished, for example, to truth the location of any feature displayed on the chart. Clearly, something better is needed.
The NDB (Non-Directional Beacon) is a step in the right direction. It consists of little more than an AM radio station broadcasting a morse-code identifier. The receiver in the aircraft cockpit, termed an ADF (Automatic Direction Finder) uses this signal to determine where the broadcast location (the beacon) is located and shows the heading to the station. This is an extremely common form of instrument navigation, especially in Canada, because of its low cost and long range (like an AM radio station, an NDB signal does not require line-of-sight to the transmitter), but it is still not ideal because of a number of dangerous flaws. Perhaps the most dangerous is that the ADF does not change if the beacon’s signal is lost, requiring the pilot to listen to the morse-code identifier at all times to ensure that the station is still broadcasting. Another danger of NDB navigation is the possibility of drift: assuming there is a crosswind, following the ADF blindly will lead to a curved path, which could cause any number of problems, not limited to controlled-flight-into terrain or the possibility of a mid-air collision.
Based on the signals from two NDBs, it is possible to uniquely determine one’s position within the level of error in an NDB signal (which can be fairly large, depending on atmosphere conditions, and topography). By drawing a line inbound to the two NDBs on the given headings, one can determine that they are located where the lines cross one another. Thus, navigation by NDB assures reasonably high positional accuracy, whatever its other disadvantages may be. It should be noted that any positional or directional determination is done entirely based on the ADF receiver, and does not rely on the data transmitted by the beacon, but rather the transmission itself.
The VOR (VHF Omnidirectional Radio Range) system was designed to compensate for many of the flaws of the NDB. Unlike the NDB, which only transmits a single signal, the VOR beacon transmits two signals: a non-directional reference signal, and a directional signal the rotates at a constant rate of 30 times per second, with the two signals being transmitted at the same time precisely at magnetic north. The VOR receiver in the aircraft uses the phase difference between the two signals to determine the heading to the station. This allows more accurate navigation, because not only does it allow the pilot to determine the heading to the transmitter, it also allows the pilot to determine their deviation from a desired course to the station, making wind drift much more noticeable and easier to correct. The important distinction between the NDB and the VOR is that the signal from an NDB does not contain any inherent positional data, whereas the VOR signal does.
The price paid for this extra capability is high: VOR stations are much more expensive to build and maintain, and are therefore much less common. Additionally, since they operate on VHF frequencies, the receiver and transmitter must be in line-of-sight. It is a nice tool for navigation, but it doesn’t allow much more certainty than the NDB in terms of absolute position. At best, it would allow one to reach a desired absolute position easier than an NDB.
Although you would still require two conventional VORs to determine your position absolutely, some VORs are equipped with a DME (Distance-Measuring Equipment) transmitter that allows a suitable receiver to determine their distance from the beacon. Knowing the distance from a VOR, and the direction to or from the VOR allows one to quickly and accurately determine their position, or locate a desired position, with only one beacon.
Of much more interest to data collectors, and of equal interest to pilots, are Area Navigation (RNAV) systems, which are designed to accurately determine the position of the aircraft at any given time. These are something of a recent innovation, and tend to be much more expensive and/or error-prone than traditional forms of aerial navigation.
The precursor to GPS, called LORAN-C, was the first major attempt at using a such a system for aerial navigation. Like GPS, LORAN-C used signal differences between a number of stations to determine position. Unlike GPS, these transmitters are land-based, meaning that many more stations are required to maintain availability over a given area. Although LORAN-C was not without its problems, it worked reasonably well at its task. It is not, however, accurate enough to allow for precision instrument approaches, with accuracy between 0.1 and 0.25 nm.
The next system is one that everyone knows and loves: GPS. Although one may simply bring a handheld GPS aboard an aircraft to determine their position, a GPS approved for aircraft navigation, although more expensive, is the preferable option, as, by law, it must use some sort of augmentation to achieve the necessary precision to perform a Category 1 Precision Approach (in layman’s terms, being able to guide a plane well enough that a landing is possible with a 200-foot cloud ceiling). This means that, for the vast majority of aerial surveying work, such a GPS will provide an incredibly accurate georeference.
Although this solution provides excellent utility for aerial surveying, it’s still the subject of some debates when in comes to aeronautical navigation. First of all, like any electronic system, it is subject to failure. Additionally, the US military can arbitrarily disrupt service and/or provide erroneous data if they feel it necessary, which is not a particularly comforting thought when you’re flying through a mountain valley using only the position reference from your GPS. For this reason, it’s preferable to plan flights based on conventional navigational aids, using the GPS to augment whatever data you receive through the conventional instruments. There have also been accusations that GPSs make novice pilots lazy, to the point of not having paper charts or reference materials (which is a contravention of the applicable laws), but this can hardly be blamed on a flaw of the instrument itself.
Last, but certainly not least, is INS, or Inertial Navigation System. As they are the most expensive option, they are typically not found on anything smaller than commercial jets. The premise is simple: initialize the system in a known location, and keep track of movement via a system of gyroscopes. As long as the gyroscopes are powered, the Inertial Navigation System is able to provide the location of the aircraft. This system has the advantage of being completely self-contained, and thus not susceptible to disruption of service like GPS; however, it’s also much less likely to be installed on any given aircraft.
So, next time you look at data collected from an airplane, just think about the time and effort spent getting to the location, and figuring out exactly where it is: geo-informational science is at work before the sensors are even turned on!
Canada. Transport Canada. Aeronautical Information Manual. TP 14371E, 2006.
From the Ground Up. Millenium Edition. Ottawa: Aviation Publishers, 2000