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Re: Is this group about amateur radio in the UK?

In article <h2s6u1$hk2$1@aioe.org>, Jeff <jeff@local.host> writes
>
>"Walt Davidson" <g3nyy@invalid.invalid> wrote in message
>news:22n155taerlgdka15ihurioeun3bkn5h40@4ax.com...
>> On Sun, 5 Jul 2009 17:42:09 +0100, "Jeff" <jeff@local.host>
wrote:
>>
>>>However, there is no relationship between NDBs and DME, they can exist
>>>entirely separately and are totally different systems. DME is more usually
>>>co-located with a VOR beacon.
>>
>> I know they can exist separately, but it is nonsense to say that there
>> is no relationship between NDB and DME.  At many airfields they are
>> used together as the basis of a standard instrument procedure for
>> approach to the runway.
>>
>> Definition.  NDB/DME Procedure: An instrument approach procedure for
>> use by aircraft landing on runways which do not have an ILS or where
>> the ILS is not available. The pilot follows a bearing towards the
>> Non-Directional Beacon and is provided with information on the
>> distance of the aircraft from the airfield by the Distance Measuring
>> Equipment.
>
>You are missing the point Walt, the original contention was that NDBs were
>"crude nav aids". In fact they are, just simple CW beacons, with pretty
poor
>accuracy due to the limitations of the DF receiver in the aircraft. It is
>just like a fox hunt on top band with very similar equipment.
>
>Although they may be used in conjunction with DME they are not linked to DME
>in any way and were around many years before DME was invented. You could
>just as easily associate a railway line with DME, which you follow towards
>an airfield looking at the DME to give you range.

Jeff is essentially correct, Walt in parts.

MF beacons have been around a *VERY* long time and used to be the only
aid pre-WWII; they were called RANGE in the USA.   Some beacons are/were
primarily used for en-route navigation for obtaining the usual LOP for
fixing position, but most beacons are used as a legacy aid for recovery
to a field on an NDB APPROACH (see below).  The receiving equipment is
fairly basic in the classical use, DF/ADF.  The NDB's may be also used
by RNAV systems where the beacon data (along with GPS, DME, VOR, LORAN,
DOPPLER, AIR DATA UNIT and etc) for a region, say North Europe, are
stored in its library and where all takes place automatically whilst
transiting through an area (it pick & mixes to its hearts content).
NDB's have *NO* inherent accuracy - it is just a simple beacon and the
accuracy is YOURS (or not).  The CW bit disappeared some considerable
time ago and all NDB's are now AM.

Distance Measuring Equipment is very much post WWII and is classed as a
secondary radar system.  It operates around 1 GHz, depending upon the
channel selected.  The aircraft sends a pulse pair which is received
and, after a calibrated delay, is returned to the aircraft on a separate
frequency.  DME is accurate to 0.1 of a nautical mile or 1% of distance,
whichever is the greater.

NDB's and DME, VOR, or ILS (the Instrument Landing System) are *UTTERLY*
separate from one another, except that the VOR may be combined with the
DME to make it a VOR/DME.  The failure of one has *ZERO* effect upon the
others.  The *ONLY* change it would make is to the combination of aids
used.

*The important part to grasp is that the linkage of NDB to DME is as a
*PROCEDURE ONLY*  there is no technical linkage of equipment.*

An NDB may be used in one of 3 ways on recovery to an airfield viz:

1) To establish a HOLD, if required.
2) To "gate" a feed in to an APPROACH of some type, eg NDB to RADAR.
3) An approach aid in its own right!

Look at the NDB chart for EGBJ, (not far from you Walt):

http://www.nats-uk.ead-it.com/public/index.php%3Foption=com_content&task
=blogcategory&ids&Itemid2.html

Last chart on the list.  It shows the NDB HOLD and the NDB PROCEDURE for
rwy 09.  The hold is the racetrack around the beacon at a safe height.
They are 2 quite separate entities.  That procedure is as good as your
ADF (+/- 5 degrees and you are lucky), your timings on the stopclock and
your last estimates of the wind; hence the MISSED APPROACH HEIGHT of
745/644 feet  QNH/QFE cos you might not be just where you hope you are.

The second chart from bottom is for rwy 27 and is an NDB HOLD to
NDB/DME.   Because of the high ground to the east, there is no permitted
NDB pure option - you *must* have a working DME - your heading may be
off the centreline, but ranging is good enough for terrain avoidance.
It is still not a precision approach and hence the high missed approach
as before.

Third from bottom, once again, you may have an NDB HOLD or go for the
approach as required, the radar type is still non precision but better
than the previous, hence a better chance at the bottom.....

South of you EGTG - has an ILS as well - more options to pick and mix!

EGLL and good luck :D



John

--
JC Morrice
john@pentode.demon.co.uk

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