The most extreme case of "slant range error"occurs when the aircraft passes directly over the station instead of reading zero, theDME shows the altitude of the airplane above the station (in nautical miles).įor example, if the station is at sea level and you’re flying at 9000 feet, the DMEwill show 1.5 NM as you pass directly over the station. This is called "slant range" and is slightlymore than the actual horizontal distance because of the difference in elevation betweenthe aircraft and the station. If you are flying away from the station, groundspeed will be accurate but"time-to-station" will actually show "time-from-station" and willincrease as you get farther and farther from the station.Īnother thing to remember is the DME measures the straight-line distance from theaircraft to the ground station. If you are flying in any otherdirection, you will see groundspeed that is erroneously low and time-to-station that iserroneously high. Beware, however, that DME groundspeed and time-to-station are only accurate whenyou are flying directly to or from the ground station. DME ErrorsĭME displays distance in nautical miles, groundspeed in knots, and time-to-station inminutes. It’s quite possible to have your VOR receiver lock ontoone station and your DME to lock onto a different station that shares the same frequency.This is most likely at high altitude over flat terrain, and it has happened to me morethan once. I stronglyrecommend verifying the DME audio. Most audio panels have a switch that allows you to listen to the DME ident. The DMEident is also higher-pitched: 1350 Hz compared with 1020 Hz for a VOR ident. When a VOR andDME are co-located (as in a VORTAC or VOR-DME station), the DME transmits the same codedident as the VOR, but sends it during the pause between successive VOR idents. Most pilots were taught to listen to and verify the Morse code identification of VORstations, but many pilots don’t realize that DME also has identification. When it finds that, it knows they’re replies to its interrogations. It thenexamines the ground station replies looking for a sequence with the same randomly jitteredsignature. Your DME doesn’t send an equally-spaced stream of interrogations, but randomly jittersthe spacing of its interrogations to create a unique "signature". How can your DME sort out replies toits interrogations from replies to other aircraft? Here’s where some clever magic comesin. The ground station may be replying tohundreds of other airborne DMEs in addition to yours. There’s one little complication, however. Your DMEunit is essentially a little radar interrogator, and the DME ground station is just afancy radar transponder. If this sounds a lot like how secondary radar works, it is. It then displays the computed distance on your DME readout. From this,it can figure out its exact distance from the ground station using simple arithmetic,given the fact that it takes 12.359 microseconds for a signal to go out and return onenautical mile. It subtracts the 50 microseconddelay that the ground station introduced to come up with the round-trip time. Your airborne DME equipment receives the reply and measures the elapsed time from whenit sent the interrogation until it received the reply. Incidentally, the purpose of the 50 microsecond delayis to eliminate the possibility of uncoordinated operation when the aircraft is very closeto the ground station When the groundstation receives the interrogation, it waits for 50 microseconds and then sends a pair ofreply pulses back to the aircraft. Each interrogation is made up of a pair of RF pulses. Once you’ve channeled your DME, it starts transmitting a stream of interrogations tothe ground station. But pilots normally never see the actual DME channel only the paired VORfrequency. So forexample if you tune to the VOR frequency 112.40 Mhz, your DME is automatically channeledto 1010.00 Mhz. There are 200 DME channels which are paired with the 200 VOR/LOC channels. Although you dial in the VHFfrequency of a VOR or localizer in the 108-118 Mhz band, you’re actually tuning the DME totransmit on a "paired" UHF frequency in the 978-1213 Mhz band (also called the"L-band") and to receive on a UHF frequency that is offset from the transmitfrequency by 63 Mhz. You tune your DME by selecting a VOR or localizer frequency, either on the DME itselfor on a VOR/LOC receiver to which the DME is channeled. Other approaches use DME to defineoptional step-down fixes to permit lower minimums. Combined with VOR, DME permits you to determine your exact position from a singleground station VOR tells you what radial you’re on and DME tells how far out on thatradial you are.ĭME is required by the FARs for flight at or above FL240 if VOR navigation is used.Some instrument approach procedures require DMEthese always have "DME" intheir title (e.g., VOR DME RWY 5 or LOC DME BC A).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |