What ways do we have of expressing frequencies?

[Morten Olsen Lysgaard]

Hi, I'm working on the python django database framework.
The first task I'm working on is parsing OSM-XML so that we can use
the existing data we have created from the OSM stack. This is going
quite well and on the way I'm learing a lot about how our database
must look to make sense.

In this task I've encountered a question. What ways do we have of
representing communication/navigation frequencies.
I started by saying that a frequency can be a number, suffixed by a
si-prefix, eg 128kHz
This works well but in the existing dataset we have some navaids
especially this one:

(47.4443611,
19.2618611,
{'height': '155.0',
'height:class': 'amsl',
'height:unit': 'm',
'name': 'ILS 13L (CAT II) PDME',
'navaid': 'yes',
'navaid:activetime': 'H24',
'navaid:dme': '28Y',
'navaid:ident': 'BPL',
'navaid:type': 'DME'})

here you can se that the navaid:dme, the DME frequency of this navaid
is 28Y, is that a common way of writing frequencies? I've only
encountered different versions of kHz, mHz etc.
What does this unit mean. Does it have a direct translation to normal
Hz. If so, can we convert it to Hz and store it in our database as
that which would be nice for consistency, but maybe bad for
readability.
Any suggestions are welcome :)

[Glen Wilson]

28Y is a channel, not a frequency.  This table describing ILS, TACAN, and VOR frequencies shows how they relate to channels.  ILS Localizer, Glide-slope, and DME frequencies are paired.  Same is true for VOR/DME.  The DME in your example is almost certainly part of an ILS (I wrote that before I saw the name field).

A pilot only has to set one Localizer frequency (e.g. 109.15MHz), the glide-slope frequency will automatically be set (in this case to 331.25MHz).  DME frequency will also be set automatically (1052MHz/1115MHz).

DMEs work on two frequencies (interrogate and reply), but instead of using two database fields to store them, I propose you just store the channel (28Y in this case).  You could even reuse the frequency field instead of creating a new one just for DMEs (although the purists will disagree ;)).  People who actually care about the frequencies can either look it up from the table, or the OAM software can return the values for them.

You should still store individual frequencies for ILS LOC, ILS GS, and VOR facilities (e.g. 109.15MHz, 331.25MHz, and 116.2MHz respectively; I made that last frequency up - VOR is not related to ILS like that).

Because "slant range" is an issue with DME facilities, they should always have an elevation associated with them (you'll get a different distance reading depending on whether you're 30' or 30000' above the facility).

Hope this helps -- Glen

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[Ákos Maróy]

Morten,

As Glen is pointing out as well, DME's are defined as 'channels', and there is a particular conversion table from channel to frequency. this channel info is published in the AIP as a definition of the DME. DME is a sort of active navaid, as an an aircraft would send an interrogation signal to the DME, which will return a response on a frequency offset from the original one. from the time difference, the distance can be calculated, see more here:

http://en.wikipedia.org/wiki/Distance_measuring_equipment#Radio_frequency_and_modulation_data


the postfix 'X' or 'Y' is not a unit of measurement, but signals the channel characteristic (it's related to spacing)


of course as DME's are co-located with VOR or ILS, one never sets the DME channel manually. there is a table which maps VOR / ILS frequencies to DME channels, and the onboard equipment will try to find the corresponding DME.


for a data modeling purpose, the channel info is part of the DME's definition, and is in effect equivalent to the frequency info.


Akos