[pct-l] Question about Halfdmile App

[Robert E. Riess]

What a lot of misinformation is being dispensed on this topic.  To provide 
accurate positioning, the entire constellation of GPS satellites must be 
constantly monitored and refined.  They have an orbital velocity of many 
thousands of feet per second, so their position is rapidly changing.  It's 
true, the satellites broadcast their position and time rapidly and 
accurately, subject to the laws of physics and mathematics, and the level of 
accuracy allowed by the department of defense.  Would you want an enemy to 
launch a raid of hundreds of ICBM's or cruise missiles at us using our own 
GPS system for guidance?  Basically, each missile would need a $300 Garmin. 
That's why the DOD can turn the whole system off instantly with the flick of 
a switch.  Without constant monitoring and updating the accuracy of the 
system rapidly degrades.  An error of a third of a billionth of a second 
will produce a position that is off by 6 miles.  The clock in the handheld 
GPS has to have the same accuracy in measuring time as the satellites. 
Otherwise, a handheld GPS unit couldn't even tell you what state you are in. 
The GPS unit calculates its position by calculating its distance from at 
least 3 satellites, which is determined by how long it takes the satellite's 
signal to reach the handheld GPS.  This signal travels at the speed of 
light.  As time passes and your handheld GPS acquires more satellites, it 
can refine its position and provide you more accuracy.  Not a Timex, not 
physically, not metaphorically.

Ancient mariners could calculate their latitude very easily, just directly 
measure the altitude of Polaris.  When one is at the north pole, Polaris is 
directly overhead.  The north pole is at 90 degrees N.  A PCT hiker could 
approximate his position by measuring the altitude of Polaris to determine 
his latitude, and draw a line of latitude across the PCT.  The resulting 
position would be very rough, though.  Minneapolis is at 45 degrees N 
latitude.  The altitude of Polaris measured in Minneapolis is 45 degrees. 
Ancient navigators had a real problem calculating longitude until accurate 
chronographs became available.  Ships usually carried 3 chronographs, and 
the navigator would average them to get time of day.  An error of 1 minute 
would produce a navigational error in longitude of about 60 miles.  Not good 
if there be rocks and shoals.  Merry Christmas and Happy Hiking.  BR