MAGELLAN COLORTRACK'S "ALTIMETER"

 This  information was passed to me by a GPS Engineer who  thought 
 the  newsgroup  might be interested in how  Magellan  ColorTrak's 
 altimeter  feature  works.  It looks as if Magellan  may  have  a 
 substantial enhancement in  GPS altimeter function.
 Joe Mehaffey
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 Joe et al -
 
 I  got curious about how exactly Magellan does  their  altimeter.  
 After a comment I got from an Avocet engineer about Avocet's  new 
 replacement  for their altimeter watch,  I decided to go  to  the 
 source  and ask. The Magellan tech reps were a bit  confused  but 
 eventually I got to an engineer type and found out the following.
 
 The "altimeter" in the ColorTrak is not really an altimeter,  but 
 is  a  pressure sensor that is used to help the  software  decide 
 whether  the change in GPS altitude is a real change in  altitude 
 or  is  just due to SA or other physical  effects.  The  software 
 includes a pressure-altitude curve (I surmise it is the  standard 
 ICAO curve). The pressure measurement is examined for changes and 
 if the GPS jump is large and varying (say jumping up and down  as 
 frequently  happens with SA), then the 3D solution is  damped  to 
 reduce  the motion in altitude. On the other hand, if the  change 
 in altitude continues (they didn't tell me the time interval, but 
 I gather it is several seconds), then the altitude is allowed  to 
 continue  in the direction indicated. I asked specifically  about 
 the case of a pressurized airplane and was told that they  indeed 
 did  consider  that and that there is no problem there.  The  GPS 
 will  indicate the 35,000 feet that your 747 is at,  despite  the 
 cabin  pressure  of  6000 ft. The  GPS  computed  altitude  takes 
 precedence,  just that the variations are damped. It is not  like 
 an  Avocet, Casio, or Thommens electronic altimeter in  that  you 
 can set it to a known altitude or get a barometric pressure.
  
 I also took the opportunity to ask about something that has  been 
 mentioned from time to time on the newsgroup about the 2/3/4000XL 
 (12  channel  only?),  namely the  minimum  altitude  indication. 
 People have reported that they never see sub-sea level altitudes, 
 whereas  older  units  (and Garmins  and  Eagles)  show  negative 
 altitudes because of the SA jumps. Magellan said that indeed they 
 did  put a sea level cutoff for the altitude because  users  were 
 complaining so much (boaters particularly) about seeing  negative 
 altitudes  (hey, guys, just set it to 2D mode and set your  known 
 altitude). There are only 2 areas in North America and one or two 
 other  areas  in the world where you find land below  sea  level. 
 Death Valley and the Salton Sea area in California, the Dead  Sea 
 area in the Middle East, and parts of the Netherlands. Even then, 
 the  maximum  depression is less than 300 feet, so the  error  is 
 still  within  current SA limits. Seems reasonable to  me  for  a 
 consumer toy. Real GPSs (survey units, for example, and  aircraft  
 units) do not have this cutoff.