limitations of the df517

[starshippe]

. . the operation of the df517 in scan mode is not, to me, intuitively obvious.  because of this, it is easily possible, while in the scan mode, to fly directly over any number of active 121.5 and/or 243.0 elt's, with no indication whatsoever from the unit. while the thought behind the unit's activities in scan makes sense, the aircrew must understand what the assumptions are, and what the limitations are, and how to get around them. the software assumes that there is a 406 data burst, and that it will be heard first because it is a much stronger signal than the 121.5 or 243.0 homing transmitter. this makes sense. it may not, however, be the case.   

. . the sequence of events, during a search for a 406 elt/epirb, would start with the aircraft initially detecting the 406 data burst. the display would indicate the relative bearing to the target, assuming that the burst is strong enough to break the squelch. (the squelch should be set slightly above the background noise.) the observer notifies the mp of the relative bearing, who then brings the aircraft to bear on the new heading. the mo also notes the time, relative bearing, aircraft course when the bearing was received, and aircraft position. he passes this information to mission base, who plots this lop from the aircraft position. (ok some of this stuff is never actually done, but it still seems like a good idea.) the mo can also check the 406 data page display to see if there is a lat and lon included with the data. there are cautions on the accuracy of this position, which are noted later. also, the time that the elt, or the data burst, is first heard should be recorded, and passed to the ic. the bearing information presented on the unit is now old information, having only been good information at the instant that it was received. 

. . yes, the lat and lon should have already been received, but it wouldn't hurt to check. u never know.

. . the 517 should now do a number of things. it should start the time since last signal received timer. it should start scanning the two vhf frequencies, spending about two seconds on each. if it hears a signal, in other words if the squelch is broken, on a frequency, it remains on that frequency, and displays active real time relative bearing information on the display. if it does not hear a signal, in 45 seconds it switches back to 406, for the next data burst. the squelch has to have been previously set up properly on each band. if the squelch is set too low, the unit will remain on that band, thinking that there is a signal there. if the squelch is set to high, the unit will not hear the signal.

. . as the 406 burst should put the aircraft on course to fly directly over the elt, at some point the unit will detect the homing beacon, usually on 121.5, and spend the rest of the mission on that frequency. a successful find can usually be accomplished by simply following the dot. station passage will as usual be indicated by a quickly strengthening, then diminishing, signal, and reversal of the dot. the aircrew passes this info to mission base, and pretty soon everyone is home.

. . all this works just fine as long as there is a 406 data burst to detect. if the unit is in scan, and it does not receive a data burst on 406, it will never, and i do mean never, move off of 406. it will never check 121.5 or 243.0. i recommend that the aircrew every two minutes, manually break the squelch on the unit. this causes it to think that it has received a signal, and it obediently starts scanning the two vhf freqs. after 45 seconds it dutifully returns to 406. (yes, i'm using the civilian definition of vhf, where uhf frequencies require a wavelength of less than a meter.)

. . there is one very good reason that the unit will not receive the data burst on 406 mhz. the df517 was only designed to monitor the first two 406 elt frequencies, 406.025 and 406.028. as the industry filled up the available slots, other frequencies were used. cessna, for instance, has been using frequencies, other than the only two receivable with the 517, since at least the middle of last year. the cap knew the limitations of this unit when they were purchased, and replacements of the df517 with the rt600 were planned, and tooled up for by rho theta. we are now told that they are not. we are to be left with equipment that can not monitor the frequencies being used by newly manufactured elts and epirbs. this makes me wonder.

. . having one of the aircraft radios tuned to 121.5 will obviously alert the aircrew also. they can then manually switch the df unit to 121.5 and proceed as usual.

. . part of the problem with using the df517, at least in this area, is the lack of suitable training elts. there simply aren't any. one possibility is to find a military buddy with access to a handheld that will operate on, say, 409 mhz, and put this in as the training freq for the 406 band. obviously u would have to have the proper permissions, etc., but we have found that this does provide a method by which the 406 data burst can be simulated from the ground. the fact that there is no data modulating the signal is of no concern to the df unit. it will still present the proper relative bearing.

. . i continue to wonder why there is no concern for training equipment for 406 elts.

. . another problem with both of the rho theta units, the df517 and the newer rt600, is their susceptibility to fm broadcast transmissions. the aircrew should have the df unit enabled into the audio system, and verify that the signal has a valid elt sweep. 

. . more and more of these elts are going to have embedded lat and lon encoded into the data burst. i have a personal locator beacon, and offer the following cautions based on its operating manual. the gps is only turned on every twenty minutes to get a position. also, if no position is resolved, it keeps transmitting the old position for four hours. so, the position reported can be up to four hours and twenty minutes old. if the epirb is on a boat, it could be many miles from that last transmitted position. i'm not saying that the information should be ignored, as it is probably good information, just that it should  be realized that it may not be current. and, if the position gets the aircraft close enough to be able to hear the homing beacon, it has done its job.

. . i hope this has helped folks understand the operation of the df517. i will try to put together something on the rt600 once i get one to try out. i will also try to get my df ppt presentation put on line, so that i can explain other things that i wonder about.   

thanks for listening,

bill

[sardak]

Since you started a new thread to post your DF PowerPoint, I'll provide comments over there.  http://captalk.net/index.php?topic=13951.0

Mike