[teqc] qc -dn and +dn lines in ASCII time plot

Lou Estey lou at unavco.org
Fri Dec 2 08:49:41 MST 2005


Lately I've been getting an unusual number of queries about the
meaning of the '-dn' and '+dn' lines in the ASCII time plot from
teqc's qc.  So here's an attempt to explain them better.  From
the tutorial:

"The discrepancy between reality and theory is recorded in the
"-dn" and "+dn" lines, which are the SV tracking discrepancy counts,
and record the two bounds of the discrepancy. They can be thought
of as the "good new/bad news" to the number of SVs not tracked. The
line "-dn" records the minimum discrepancy of all observation epochs
for that time bin while the line "+dn" records the maximum discrepancy
of all observation epochs for that time bin. The discrepancy count
is also shown in hexidecimal notation, with (blank) for 0."

Example: let's suppose you are using the default width for the ASCII
time plot of 72 characters, and you are looking at 24-hrs worth of
data at 15-sec sampling.  That means that each ASCII column in the
plot represents 80 epochs (== 5760/72).

Now let's say you see in some column a value of 1 on the -dn line and
3 on the +dn line.  This means that there is 1 SV in that column that
was not tracked by the receiver for all 40 epochs, but there were 3 SVs
in that column that were not tracked for 1-40 epochs.

The -dn/+dn lines are right above the '+10' (or whatever the cutoff angle
is) line for a reason.  The '+10' (or whatever) line shows the max SV count
for that column of what could have been tracked.  Here's a recent example
from site ALUT 2005/332:

-dn|                            1  1 1                          1           |-dn
+dn| 12 1112 121   111 1  11211111221121  111  1 2    2 1 21  1123   1  2 12|+dn
00:00:00.000                                                        23:59:45.000

On the '+10' line you can see the variability in the number of SVs that could
have been tracked; this is the theoretical maximum based on teqc's point-position
of the antenna and the SVs using the supplied broadcast navigation messages,
taking into account the horizon and cutoff angles (which are 0 and 10 degrees
by default).  Above 9, 'a' here means 10 SVs, 'b' means 11 SVs, and so on.

On the '+dn' and '-dn' lines, you see lots of blank, which means '0' SVs -- or
there are many columns where the rx is tracking in one of the 80 epochs of that
column all of the SVs that are possible to be tracked out of those 80 epochs.

In the '+dn' line: A '1' means there is one SV somewhere in the 80 epochs of
that column that could have been theoretically been tracked (again, based on
the computed positions of the antenna and SVs, and the horizon and cutoff
angles), but for some reason wasn't tracked, a '2' means there were two
such SVs, a '3' means there were three such SVs, and so on.  So this is
the "bad news" end of the tracking spectrum.

In the '-dn' line: A '1' means there is one SV that was not tracked for all
80 of the epochs, a '2' would mean there were two such SVs, and so on.

(Always remember: for all the qc computations, masking of sky due to azimuthally
varying obstructions is not currently taken into account.)

Consequently, in the same column the '-dn' value <= the '+dn' value <= the '+10'
(or whatever angle) value.  The three lines form a crude ASCII histogram.

I'm pretty sure that the '-dn' and '+dn' lines take in account the tracking
capabilities of the receiver -- if they are known  (The '+10' (or whatever angle)
line doesn't; this is supposed to show really show you the number of SVs that
could have been tracked.)  For site KMTR, which has a Turborogue only capable
of tracking a maximum of 8 SVs, we have (for 2005/332):

-dn|               1222222121   11                 1 1111   11 13      11   |-dn
+dn|          2 1123333423333111111 122 1    1132111122232111123441 111211 1|+dn

You can see there are plenty of ASCII columns with more than 8 SVs "visible",
but, for example, the '-dn' line often has a blank in those columns, which means
the receiver is tracking all the SVs it possibly can for at least one epoch in
each column with a blank in the '-dn'.

I realize this was a long winded explanation, but I hope you agree that these
three lines provide a very interesting summary of the gross tracking stats as
a function of time -- once you understand what they mean.


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