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
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