[IGSMAIL-5430]: bad antenna mounts

Wed Oct 4 10:22:53 PDT 2006

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IGS Electronic Mail      04 Oct 10:23:31 PDT 2006      Message Number 5430
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Author: Jim Ray

At the last IGS workshop in Darmstadt, I gathered evidence indicating that
pillar-mounted antennas (and similar configurations) are subject to serious
systematic errors caused by back-reflections in the near-field.  A copy of
my presentation is available at:
  ftp://ftp.ngs.noaa.gov/dist/Jimr/igsframe-igs06.pdf
I appealed for general improvements in the IGS reference network and a
specific change (very carefully coordinated) in mounting antennas to
minimize near-field reflections and multipath (e.g., preferably using
tripod-type mounts).  Unfortunately, there has been no summary of the
workshop or its recommendations.

In the meantime, I have had a number of one-on-one discussions with
individual station operators.  The purpose of this message is to broadly
distribute the relevant information in the hopes that stations will begin
to consider possible upgrades.

The basic justifications for my views are not at all new.  Elosegui et al.
(JGR, 100, 9921-9934, 1995) demonstrated convincingly that when an antenna
is mounted over a pillar top or any other similar surface, then one should
expect serious difficulties with near-field multipath if the separation
distance is of the order of the GPS wavelength (~20 cm) or so.

Unfortunately, by that time it had already become conventional wisdom that
massive concrete pillars must be among the most stable and therefore best
mounting structures possible for GPS antennas.  Entire networks were built
on that idea and those engendered copies around the world, something that
continues to this day.

Meanwhile it is also worth noting one of the conclusions of Williams et al.
(JGR, 109, B03412, 2004) in their definitive study of GPS times series:
"... Bearing in mind the possible above influences, the evidence does
appear to point to an increased stability at sites with the deep drilled
brace monuments compared to other monument types.  It is also of interest
that the sites situated on buildings should be ranked second in this test
as intuitively we would have suspected these sites to be one of the worst.
On the contrary, we would have suspected that concrete piers would have
performed better, which was note the case since this type of monument
ranked only better than oil platforms."

When we consider also the evidence from the RMS agreement of GPS clock
(H-maser) estimates at the boundaries between daily processing arcs, which
is very sensitive to near-field code multipath conditions, the arguments
for avoiding such configurations would seem to be overwhelming; in addition
to my Darmstadt presentation above see our review paper at:
  ftp://ftp.ngs.noaa.gov/dist/Jimr/metrologia-2.pdf

Now, conclusively, colleagues at Geo++ have actually measured the effect
of near-field multipath using their robot system and shown that the usual
antenna phase center variations (PCVs) are modified by the presence of
near-field surfaces.  Ignoring the near-field multipath, just as ignoring
the PCVs altogether, can cause biases in the position estimates, mostly
in their heights.  A copy of their results, which were presented in a
poster at Darmstadt, is available at:
  http://www.geopp.de/media/docs/pdf/gppigs06_pnf_g.pdf
and a presentation on the observed bias effects in the Netherland's Kadaster
network is available at:
  http://www.euref-iag.net/symposia/2006Riga/05-03.pdf

My own personal view is that tripod-type mounts are probably far superior,
and probably that drilled brace monuments (such as being used by PBO) are
among the best considering both physical and electrical stability.  Our
experiences with a new station at Fortaleza, Brazil have been very
encouraging; see:
  ftp://ftp.ngs.noaa.gov/dist/Jimr/brft.pdf
though I would definitely not argue that this is the ideal mount.

Considering that the IGS network, and its reference frame, is dominated by
antenna mounts of the inferior type, it is completely absurd to speak of
such things as measuring local ties to GPS points at the 1 mm level, or
even realizing a global reference frame at that level.  Considering also
that impending changes in GNSS infrastructure will likely bring about many
station upgrades in the near future, it becomes urgent to alert station
operators to these considerations and to encourage reconsideration of the
antenna mounts.  Ideally, any new mounting structures should be operated
in parallel with the old antenna for some months (or longer) in order to
determine the local GPS-GPS tie, as we have done at Fortaleza.

Despite my having made this and related reference frame appeals many
times, the IGS management continues to fail to exercise coordination or
leadership.  Basic information, as the type above, is not even shared.
Meanwhile, there are many other serious infrastructure issues that continue,
after years, to be left unaddressed.  It should now be clear to everybody
that the IGS is simply not suited to deal with such matters.  As excellent
as the IGS analysis products continue to be, it must be recognized that
data analysis alone cannot solve all problems.

Sincerely,
--Jim

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This message strictly reflects the personal views of the author alone.