DME means “Distance Measuring Equipment”. It is a navigation instrument used in airplanes. DME predates the era of GPS, but it still used. To understand what DME is, it helps to first understand what a VOR is, because DME facilitates navigation by VOR. I assume the reader is already familiar with VOR.<\/p>\n
DME operates at frequencies in the 1 GHz range (960 to 1215 MHz), nearly 10x higher than VOR (108 – 117.95 MHz). DME ground stations are virtually always colocated with a VOR. To use the DME you tune it to the same frequency as the VOR. Yet how can that be, if the DME operates at a higher frequency? Every VOR frequency has a specific designated DME frequency. When you tune the VOR frequency into a DME, it looks up the corresponding DME frequency and uses that instead of what you tuned.<\/p>\n
Why do things this way? Since VOR and DME stations are virtually always colocated, tuning the same frequency for both the VOR and the DME is convenient and reduces pilot workload.<\/p>\n
Unlike a VOR, which is receive-only, a DME both receives and transmits. The DME in the aircraft transmits a pulse, the DME ground station receives this pulse and transmits a reply. The DME in the aircraft receives this signal from the ground station and uses the time elapsed (the speed of light, plus a fixed latency) to compute the distance to the ground station.<\/p>\n
The astute reader might wonder, what if several aircraft are all pinging the same DME ground station at the same time? How does each aircraft’s DME receiver know which transmission from the ground station is for itself? The answer, in a word, is jitter. The DME in the aircraft transmits multiple pulses uniquely jittered, or spaced pseudo-randomly in time. The DME ground station transmits the reply to each aircraft with pulses using that aircraft’s jitter. The DME in the aircraft listens only to replies that have its unique jitter, ignoring the rest.<\/p>\n
One consequence of this is that each DME ground station can support only a limited number of aircraft. That limit is around 100.<\/p>\n
Distance to the ground station<\/strong>. This is “slant distance” which includes your altitude. A nautical mile is 6,076 feet, so if you fly directly over the DME ground station at 6,076 of altitude it will show a distance of 1.0, not zero.<\/p>\n Ground speed<\/strong>. The DME computes this from the distance changing over time, so it is the speed relative to the station. But it’s based on slant distance. If you are flying directly to or from the station, it is your actual ground speed if you are far away from the station. Yet as you get closer to the station, when flying directly overhead it will show a ground speed near zero since you’re moving almost tangent to the station directly below you. If you are flying around the station in a circle, the DME speed will be zero, since your distance to the station is not changing.<\/p>\n Time to the station<\/strong>. Some DMEs compute and display how long it will take you to reach the station, based on your distance and speed, if you fly directly toward it.<\/p>\n Because the DME in an aircraft must transmit (unlike VOR which is receive-only), the transmitter power is limited. The power of a DME transmitter varies by model. I’ve seen it as low as 25 watts and as high as 100 watts.<\/p>\n When it comes to electrical devices and circuits, receivers usually last longer than transmitters. Transmitters use more power, which is current and heat, and over time those heat cycles take their toll. Thus, while many VORs still work when 40+ years old, this is less common for DMEs.<\/p>\n DME frequencies are in the GHz range which is direct line of sight. And the DME antenna on the airplane is usually on the bottom of the fuselage. Thus if you don’t have a direct line of sight to the station (you’re at a low altitude, there are mountains between you and the station) it won’t work.<\/p>\n ADS-B Out transmits on 978 or 1090 MHz, both of which are in the same range as DME. Thus, ADS-B Out may cause conflicts and prevent DME from working.<\/p>\n Each DME ground station can only support a limited number of airplanes at a time. If the station is in busy airspace, it may ignore your airplane’s DME signal and your DME won’t work. Having a low powered DME transmitter exacerbates this problem, since over-saturated DME ground stations prioritize the strongest signals and ignore weaker ones.<\/p>\n","protected":false},"excerpt":{"rendered":" What It is DME means “Distance Measuring Equipment”. It is a navigation instrument used in airplanes. DME predates the era of GPS, but it still used. To understand what DME is, it helps to first understand what a VOR is, because DME facilitates navigation by VOR. I assume the reader is already familiar with VOR. … Continue reading DME and its Limitations<\/span> Limitations, or Ways DME can Fail<\/h3>\n