Measuring The Miles

From star-gazing to satellite navigation systems

Navigation has undergone many changes over the years. It all started with early world travelers learning to read the celestial guidelines. In fact several early airliners were provisioned with a dedicated navigator’s station complete with a star-gazing window.

In more recent years looking to the stars for navigation has taken on a whole new persona. The commissioning of satellite navigation systems makes getting from point A to point B virtually a nonchallenge, providing of course, the data base is up to date. Radio navigation has played a significant role in aviation for more than 80 years and will no doubt continue to be an integral part of our industry. Systems such as automatic direction finding (ADF) and very high omnidirectional range (VOR) employ principles to determine a course to a radio transmitter plus compare signal phase relationships to enable an aircraft to fly a precise track to or from the transmitter station.

Distance factor
One of the key elements to any navigation query is the distance factor. Unfortunately the basic radio navigation principles employed by ADF and VOR do not easily lend themselves to this calculation.

The concept of distance measuring equipment (DME) is a bit different than distance provided by a satellite-based system. Rather than displaying distance over the ground, this device will show slant angle distance, or in other words true distance to the ground-based transponder. In the world of aviation this means if an aircraft is flying at an altitude of 6,076 feet above the ground and flew directly over a DME station the displayed distance would be 1 nautical mile.

In the years following World War II, aviation technology was in a boom period with research abounding on many fronts including navigation. In the early 1950s in a land down under (Australia) Edward Bowen while in the employ of the Commonwealth came up with the concept of today’s DME.

The principle was based on the physical law that if you know the speed of an object and the amount of travel time then the distance covered can easily and accurately be calculated.

Although the principle is relatively basic, the means to accomplish was a bit more complex. There would need to be a combined radio transmitter and receiver on the aircraft more commonly referred to as an Interrogator and when connected through an “L” Band antenna (short and stubby) that will communicate with a transponder on the ground.

In fact the DME system utilized the same concept as Air Traffic Control (ATC) with the exception that the roles are reversed. In this case the aircraft is requesting information and the ground-based equipment is responding. Most frequently, DME is collocated with precision navigation devices such as VOR stations and instrument landing systems (ILS). The DME Interrogator is automatically tuned to a DME station that will coincide with a specific VOR frequency.

UHF and VHF bands
DME utilizes the ultra high frequency (UHF) band. There are certain advantages over conventional very high frequencies (VHF). Even though the VOR utilizes the VHF portion of the radio spectrum, the aircraft will most often pick up the DME signal well ahead of the VOR. This is due in part to the physical characteristics of the UHF wave.

Although both frequencies are referred to as “line of sight” transmissions the UHF portion being a shorter wave can be bounced off the charged particles of the ionosphere creating a “skip” effect enabling a bit more range. Another factor influencing signal quality is the heating and cooling of the atmosphere. “Troposheric Ducting” is a term used to relate to radio wave transmission enhancement or degradation based on thermal conditions.

Most current DME systems are calibrated for a maximum range of 300 nautical miles. Ground-based equipment can vary based on usage. Where a device used in conjunction with an ILS will have significantly less output power than a unit associated with high altitude navigation. The calibration of the unit connected to ILS will be set to display zero just as the aircraft comes into the touchdown zone of the runway.

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