xgps: Exclude unknown-position sats from skyview (again).

This fix, first added in commit 9af0ab644, was inappropriately removed
by commit 6e2842d3e.  It now has a more detailed comment explaining
why it's not redundant with the other filtering.

TESTED:
Ran xgps against data from three Navika-100 receivers, and verified
that the SBAS satellites are no longer inappropriately shown at the
"north point".

1 files changed, 10 insertions, 0 deletions

diff --git a/xgps b/xgps
index 2853b3fb..bf6369b5 100755
--- a/xgps
+++ b/xgps
@@ -451,6 +451,16 @@ class SkyView(Gtk.DrawingArea):
                 # Bad elevation, skip.  Allow just below horizon
                 continue
 
+            # The Navika-100 reports el/az of 0/0 for SBAS satellites,
+            # causing them to appear inappropriately at the "north point".
+            # Although this value isn't technically illegal (and hence not
+            # filtered above), excluding this one specific case has a very
+            # low probability of excluding legitimate cases, while avoiding
+            # the improper display in this case.
+            # Note that this only excludes them from the map, not the list.
+            if sat.az == 0 and sat.el == 0:
+                continue
+
             (x, y) = self.pol2cart(sat.az, sat.el)
             # colorize by signal to noise ratio
             # RINEX 3 uses 9 steps: 1 to 9.  Corresponding to