Re: [Flightgear-devel] Re: DME bias question
Erik Hofman <[EMAIL PROTECTED]> wrote: > Alex Perry wrote: > > From: "Curtis L. Olson" <[EMAIL PROTECTED]> > > > It is instrument specific as to what it does when the computed distance goes > > below zero. Some of them just report zero distance, while others go > negative. > > Often, the ones that clamp at zero will still be showing a non-zero speed. > > > > From: David Culp <[EMAIL PROTECTED]> > > > >>I've never seen a negative DME, and I don't think it's possible to see one. > > > > The physical implementation of the bias is that the transmitter on the > > ground responds to the incoming pulse slightly sooner than it 'should' > > so that the speed of light round trip time is reported as a bit shorter. > > It is equally feasible to respond slightly later than the standard > > specifies, so that the reported distance is somewhat on the long side. > > Thus, the bias value in the database of DME stations can have either sign. > > Are you sure about this? > From a technical point of few the distance will increase again after > passing the locater because dt never can become negative. > > Erik > The DME ground station employs a nominal delay of 50 uS ( microseconds) on both X and Y channels, therefore an aircraft will send out a pulse pair and upon receipt of its reply will subtract 50 uS . The remaining delay is the round trip time taken by the radio signal in free space. >From this delay the distance to the dme beacon can be calculated. In order that a single dme transponder can be used at both ends of a runway the dme is normally sited at an equidistant point from both thresholds. The apparant 50 uS delay point is then adjusted such that it passess through both threesholds. Most airborne dme equipment does not expect to see a number less than zero after subtracting 50 uS. This will happen once the aircraft has passed through the zero point at either threshold. Upon seeing this less than zero value most airborne equipment displays either zero or gibberish. There is no signed arithmetic within the airborne dme equipment therefor "negative numbers" can appear as very large positive ones. HTH Tracy ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Re: DME bias question
Alex Perry wrote: From: "Curtis L. Olson" <[EMAIL PROTECTED]> It is instrument specific as to what it does when the computed distance goes below zero. Some of them just report zero distance, while others go negative. Often, the ones that clamp at zero will still be showing a non-zero speed. From: David Culp <[EMAIL PROTECTED]> I've never seen a negative DME, and I don't think it's possible to see one. The physical implementation of the bias is that the transmitter on the ground responds to the incoming pulse slightly sooner than it 'should' so that the speed of light round trip time is reported as a bit shorter. It is equally feasible to respond slightly later than the standard specifies, so that the reported distance is somewhat on the long side. Thus, the bias value in the database of DME stations can have either sign. Are you sure about this? From a technical point of few the distance will increase again after passing the locater because dt never can become negative. Erik ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
Re: [Flightgear-devel] Re: DME bias question
Alex Perry wrote: Yes, I believe, without having the plate handy, that LAX does this. As I recall, the reason is that they use the same ILS frequency for the two ends of each runway. Since the ILS frequency is associated with the DME channel, they either make life hard for the crews or they use the same DME channel for both ends. To use the same channel, you either build two transmitters or build one with a bias applied. Since LAX have four runways with this issue, bias saves a lot of money. We'll have to figure out where Robin got this information from. Is it present in the FAA database? I double-checked the DAFIF, and the DME_BIAS field for each of the 8 ILS approaches as KLAX is exactly equal to the DME distance on the NACO plates. In Robin's database, the DME bias numbers I've found so far are all whole nautical miles, with no fractional part. I am not certain yet, but so far, it looks like he's taken the DAFIF number and simply stripped off the decimal part, so that 2.1 becomes 2.000, 1.8 becomes 1.000, etc. In any case, a quick sanity check throws some doubt on the KLAX numbers. For example, Peel gives a DME bias of 2.0 for the ILS 7L and the ILS 25R. According to the IAPs, the DME reading at the threshold of 7L should be 2.1 nm and the DME reading at the threshold of 25R should be 2.0 nm -- that means that the actual distance is either 0.1 and 0.0 nm or 4.1 and 4.0 nm depending on how the sign works, putting the DME transmitter roughly halfway down the runway in either case -- that leaves us with a runway that's either 600 ft long or 49,000 ft long. In fact, runway 7L/25R is just under 9,000 ft long -- that suggests that a DME transmitter about 3000 ft past the far end in each direction would give the correct result. Can DME be broadcast directionally (i.e. shielded in one direction?)? The other alternative is that there is a single DME transmitter about halfway down the runway (i.e. 0.75 nm from each end) with a bias of about 1.25 nm. Peel's database also has 8 (not 4) different lat/lon positions for DME transmitters. So Peel's numbers seem to be wrong, whether KLAX's DMEs use a bias or not: the numbers we currently have look like they're just the DAFIF numbers with the decimal parts zeroed, but the DAFIF DME bias field means something different. We should probably back this out until we have time to figure out what's going on. All the best, David ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
[Flightgear-devel] Re: DME bias question
From: "Curtis L. Olson" <[EMAIL PROTECTED]> > Recently I added support for adjusting the DME readout based on an > optional per transmitter bias that is part of Robin's nav data. > [...] so it reads 0.00 at the touch down point. It is only in specific countries where the goal is to have it read zero. More generally, the idea is to have it read the same for multiple runways so that it is easier for pilots to fly various approaches w/out surprises. Curt continued: > My question is, in real life, what happens to the dme readout when you > get inside of the bias range. It is instrument specific as to what it does when the computed distance goes below zero. Some of them just report zero distance, while others go negative. Often, the ones that clamp at zero will still be showing a non-zero speed. From: David Culp <[EMAIL PROTECTED]> > I've never seen a negative DME, and I don't think it's possible to see one. The physical implementation of the bias is that the transmitter on the ground responds to the incoming pulse slightly sooner than it 'should' so that the speed of light round trip time is reported as a bit shorter. It is equally feasible to respond slightly later than the standard specifies, so that the reported distance is somewhat on the long side. Thus, the bias value in the database of DME stations can have either sign. From: David Megginson <[EMAIL PROTECTED]> > I've never heard of "DME bias" either -- that's not to say that it does not > exist, but it's certainly not common. I know at least one of the London airports does this, for example. From: "Curtis L. Olson" <[EMAIL PROTECTED]> > All the approaches at KLAX have a 1 or 2 nm bias for the DME. Like you > say, there is no *requirement* for this, but there are installations > that have a bias set up. Yes, I believe, without having the plate handy, that LAX does this. As I recall, the reason is that they use the same ILS frequency for the two ends of each runway. Since the ILS frequency is associated with the DME channel, they either make life hard for the crews or they use the same DME channel for both ends. To use the same channel, you either build two transmitters or build one with a bias applied. Since LAX have four runways with this issue, bias saves a lot of money. ___ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
