Re: Why so expensive (flight recorders)

First, let me echo what Bill Daniels said - the GFAC are doing great
work on igc recorders, all of us appreciate not having to use cameras
at the turnpoint. Second, like him, I loan my Volkslogger to anyone
who wants to use it when I'm not in the search for my third diamond.

My problem with COTS gps systems is more practical. The Dilution of
Precision (DOP) in the z-axis is latitude-dependant; this is due to
the orbits of the satellites (see, Electronic Navigation Systems,
Bjorie Forsell - simply, DOP is how "good" the gps 3-D fix is). For
glider pilots in northern climes, this is a problem. Having to obtain
a more-expensive system just because you live in a winter wonderland
seems unfair. Straight precision (geometric DOP) of the GPS system is
also variable (and that's if you don't fly in the US, where the Dept
of Defense is practicing gps denial many weekdays).

First, let's get some toys to play with; a good, free gps analyzer
from Trimble is their planning software, at http://www.trimble.com/planningsoftware_ts.asp.
Download it. Start it. You have to put in an almanac, from Trimble
gps data resources, further down the page (download in ssf format,
import it to the planning software using the 'import' feature). Now
you're set. Let's assume I'm doing a 5 hr today at Alert, Nunavut,
Canada, 8228N 06230W (a Canadian Forces Station, "we supply Santa").
Input today's date - Feb 24, 2008, 24 period. Input the appropriate
coordinates in 'station' pulldown.
Take a look at the pull-down chart for vertical DOP (wiki has a good
discussion of DOP that's not mathematically intense). Note that the Z-
precision of the gps signals is not useable around 0745 and, to a
lesser extent, around 1130. So, a altitude badge at this place and
time would not be possible. Having a system that predictably doesn't
work sometimes for z-precision (altitude) as our method of doing
altitude claims seems optimistic (other words also occur). Pull-down
the station to Munich, and see that at various times today, it's also
not great. Try Omarama, NZ, approx 4430S, 17000E - look at 1800
today - not useable. It would be interesting to look at olc traces
today from around there to see if there was an effect...

Now, put in YOUR lat/long and take a look at the V-DOP for a 24 hr
period at your home station; you may be lucky, and see no problems;
extend the period, and have another look. GPS geometry is pretty
tricky. As an Official Observer, I use this software to take a look
at igc files when the system has x-y problems - and most times, the x
or y-DOP sucks at the time. Note, US Coast Guard GPS status and NANUs
(Notice to Navstar Users) are also available from the Trimble gps data
sources page; it should be checked if you intend to use GPS on a
flight (good info for Contest Directors). Good one to bookmark.

Use of COTS for horizontal fixing is a great idea, though as an OO I'm
not wild about having to learn a bunch of new systems. Using gps for
altitude is a big mistake, and the GFAC is to commended for not going
that way. Those who say that COTS gps seems to be the way to go are
correct; it SEEMS to be. Take a closer look at the details
(apparently, the devil is in them) and it's not so clear anymore. The
US military does move the satellites to improve precision at times
(Gulf Wars), and the accuracy back in North America is degraded
somewhat, and Selective Availability is currently set to zero, and
could come back in certain circumstances.

I have experience in testing electronic navigation systems (incl GPS)
on a large fixed wing aircraft as an operational test and evaluation
project officer (if you have one system, you know where you are; with
three, you're never quite sure...), have a graduate degree including
work in this area, some software background, and a number of years
flying in the school of hard knocks. I've been playing with
electronic data recorders since 1997.

I hope everyone has fun with the software; I have my flame-proof suit
on; what do you think?

Dan
Yes, it's cold, and in lieu of flying, I use GPS analysis software...
<sigh>
.

Relevant Pages

  • Re: 1m Accuracy - Do you believe it?
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  • Re: The Commodore Combo
    ... GPS is a pretty simple system. ... However, how many surface meters, from a 2D ... We have 32-Bit Single Precision Floating Point capability. ... > decimal digits when the units is for example - meters. ...
    (comp.sys.cbm)
  • Re: The Commodore Combo
    ... GPS provides accuracy to that level. ... However, how many surface meters, from a 2D ... We have 32-Bit Single Precision Floating Point capability. ... just to calculate one satellite location given that they are pretty ...
    (comp.sys.cbm)
  • Re: Clarification about the term "GPS Shutdown"
    ... > terrorists. ... Precision is completely unnecessary. ... It depends on the nature of the target and the nature of the attack. ... A GPS ...
    (sci.geo.satellite-nav)
  • Barometric vs GPS altitude
    ... I have read all I have found about GPS for mountain biking, ... get altitude profiles, with good precision. ... friends carried handheld GPS units (two eTrex Vista, one eTrex Legend, ... We were talking about altitude precision, ...
    (sci.geo.satellite-nav)