Ron Jensen > > Hi Vivian > > > I'm more than willing to be corrected on this one, but my understanding > is > > as follows: > > > > A VORTAC station has 3 frequencies in RL - TACAN, VOR, and DME. > > This is my understanding, too. Further A TACAN "channel" includes 3 > frequencies, Two for TACAN/DME and one for VOR/ILS. [1]
I'd like to see where this idea comes from - not [1], which is a very simplistic description. A TACAN channel includes just 2 frequencies - "airborne" (or "interrogation") and "ground" (or "reply"). If you have a TACAN receiver (military), the response frequency contains both the bearing and range information. If you have a DME instrument (civilian) you will get range information only. VOR is described by its frequency. Thus a VOR/DME station should have a frequency and a channel associated with it and a DME just a channel. AKAIKS DAFIF does just this. If Navaids are collocated as defined by AFMAN33-120_USAFESUP1_I 17 DECEMBER 2004, then frequency pairing is mandated. However, not all TACAN channels have an associated VOR frequency assigned. Nav.dat seems to assume that TACAN _uses_ the associated VOR assigned frequency. Where there is no VOR frequency / TACAN channel pairing, then nav.dat, in a fit of madness, seems to use the mandated Localizer / TACAN channel pairing. What it does for TACAN channels which have neither, I can't find out for now. Perhaps there are none. We have a GIGO situation here. > > In FG nav.dat only the VOR and DME are held. > > Actually, nav.dat only holds the VOR freq. The DME uses the associated > VOR frequency. Yes - incorrectly - should be the frequency indicated by the DME Channel > > Thus it is a fix in the case of VORTAC to misalign the TACAN > channel/frequency > > pairing to the VOR frequency in stead of the TACAN frequency. > > There is not a mis-alignment. The TACAN spec includes the VOR freq > pairing, as well as the DME freq pairing. Not for all TACAN channels, at least not in the reference which I am using (AFMAN33-120_USAFESUP1_I 17 DECEMBER 2004). It is simply incorrect to use the paired VOR frequency for a stand alone TACAN ... > It is possible in real life > to tune a VOR/DME reciever to (most?) TACAN stations and receive DME > information. > > Unfortunately, there are other stations in > > the world which are TACAN only, which seem to have the correct TACAN > > frequency assigned: Flesland TACAN is one such example. > > Flesland TACAN (FLE) uses channel 092X [2] > Flesland VOR/DME (FLS) uses channel 102Y (115.55 MHz) [3] > > zgrep Flesland nav.dat.gz: > 12 60.300036 005.213589 185 11450 130 0.000 FLE Flesland TACAN > 12 60.311261 005.212194 213 11555 130 0.000 FLS Flesland VOR-DME > I'm wrong here - the nav.dat data is incorrect, well rubbish really. The only correct bit is that the TACAN and VOR-DME are separate. (Well, and the positions) > Under the current version of TACAN.dat FLE is tuned using 058Y and FLS > is not tuneable. > > FLS should give DME only to the onboard TACAN system, currently the > TACAN system in FG would ignore it, if it were tuneable, I think. > > > Thus we fix the VORTAC at the expense of the TACAN. > > I don't see this. With a corrected TACAN.dat and the current nav.dat > both VORTAC and TACAN work fairly well. I use HIF TACAN all the time: > > 12 41.120503 -111.963681 4806 11120 40 0.000 HIF Hill TACAN > > The correct fix is to modify nav.dat to show all 3 frequencies for the > VORTAC stations. > > If I understand correctly, you are concerned because the current TACAN > uses the name of the DME station to determine whether or not to provide > TACAN service. Adding a section for TACAN Azimuth that mirrors the DME > entries for VORTAC and TACAN stations makes sense, the TACAN DME would > respond to all DME entries (12/13) in nav.dat and TACAN azimuth would > respond only to the new (number??) TACAN entries in nav.dat. > > Overall, I don't see a way to proceed without first replacing TACAN.dat. > I would very much like get nav.dat fixed so that it corresponds to the RL situation, and we can provide proper VOR, TACAN and DME facilities. I'm not clear how practical this is: probably cannot be done, in the short term at least. (If you feel inclined to take that on, I for one would be delighted.) It would also probably require a rewrite of some of the DME code. In the interests of expediency, I think the short term (and probably long term as well) is to use the TACAN channel / VOR frequency pairing which you produced, and just accept that there are blocks of channels which cannot be used, and hope that nav.dat copes with that situation. We at FG pride ourselves on our accuracy and realism. This is all far from that. I really, really, don't like to do this. As I said before, it's a hack to fix a hack, and I'm sure will lead to misunderstandings in the future. Overall, this has forced me to research this properly, instead of relying on memory. I hope that we are all clear on this now. I suppose we ought to record all this somewhere. Vivian _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
