[IGSMAIL-1359] ALBH, DRAO Advisory

Michael Michael
Wed Jun 19 17:57:34 PDT 1996

IGS Electronic Mail      Wed Jun 19 17:57:34 PDT 1996      Message Number 1359

Author: Michael Schmidt
Subject: ALBH, DRAO Advisory


The following information is meant as a "heads up" advisory and concerns the
effect of changes to the antenna and antenna environment at ALBH and DRAO
(as well as all other WCDA sites). This information is preliminary and is
meant to advise users of our data of some recent findings. It is also hoped
that this information might generate further discussions of what might be a
wide-spread problem stemming from (undocumented) antenna changes.

A re-analysis of GPS data from our WCDA sites is currently underway in order
to quantify the apparent positional changes due to changes in antenna types
(Dorne Margolin B to Dorne Margolin T), antenna heights, the addition of
antenna domes, and the addition of antenna RF skirting. To date we have
identified apparent changes in the mean phase centre heights at all WCDA
sites ranging from 4 to 18 mm that are coincident with the physical antenna
changes such as the ones listed above. At one site (ALBH), there appears to
be a significant horizontal offset (~ 5 mm) of the phase centre introduced
by a change of a Type B to a Type T DM antenna, which, incidentally, also
necessitated  a change in the way the antenna was mounted on the
forced-centre monument. It should be noted that since the apparent changes
in the phase-centre positions are a near-field/ scattering effect, the size
of the step as determined from the analysis of our data will be dependent on
the elevation cut-off angle used.


DRAO: The AOA Dorne Margolin Type-T (Turbo Rogue) antenna is centre-mounted
directly on an anodized, solid aluminum cylindrical base, 10 cm in height
and 10 cm in diameter, which in turn is centre-mounted on a standard brass
forced-centre plate embedded in the top of the concrete pier. The aluminum
base permits orientation of the antenna while maintaining forced centring.
Consequently, the HI of the antenna,i.e. the distance between the reference
surface of the monument and the antenna reference point is 10 cm (0.100 m).
A RF skirt consisting of aluminum screening has been clamped around the
outer perimeter of the antenna ground-plane and draped over the upper end of
the concrete pier thereby enclosing the open space between the bottom of the
antenna and the top of the pier with a conducting screen. At this point, the
screening has been tied tightly to the pier with wire and no additional
electrical grounding of this mesh has been attempted.

             DRAO: DORNE MARGOLIN T (not to scale)

                     /     +     \                        <-- 0.128  L2
                    |      +      |                       <-- 0.110  L1
   +--------------------------------------------------+   <-- 0.102  TCR
   |                                                  |
   |                                                  |
   |                                                  |
   |                                                  |
|+-+--------------------------------------------------+-|+  <-- 0.038
|+-------------------+-------------+--------------------|+  <-- 0.035  BCR
|                    |             |                    |
|                    +------x------+     <----          |  <-- 0.000  ARP
|                    *             *         |          |
|                    *             *         |          |
|                    *             *         |          |
|                    *             *       10cm         |< metal screening 
|                    *             *         |          |  (RF skirt)
|                    *             *         |          |
|--------------------*------*------*--------------------| TOP OF CONCRETE PIER
|                                                       |
|                                                       |

ALBH: The antenna installation at ALBH (and all other WCDA sites) is
identical to the one described above for DRAO except that an aluminum flange
ring has been installed on the bottom of the antenna ground plane and
extends about 3 cm beyond the outer perimeter of the ground plane. An
acrylic, hemispherical dome is attached to this flange ring and the aluminum
skirting is clamped to the outer perimeter of the flange ring as opposed to
the antenna ground plane.  The acrylic dome covers the entire antenna /
choke ring assembly.

              ALBH: DORNE MARGOLIN T (not to scale)
                  .      .      .       .     .      .    
           .                                              .<-- ACRYLIC DOME
       .                          -----                       .
     .                        /     +     \                     .  <--0.128  L2
   .                         |      +      |                      .<--0.110  L1
  .      +--------------------------------------------------+      .<--0.102
 .       |                                                  |      .
 .       |                                                  |      .
 .       |                                                  |      .
 .       |                                                  |      .
 .     +------------------------------------------------------+    . <-- 0.038
 .     +-------------------+-------------+--------------------+    .
<--0.035  BCR
|==========                |             |             ============| FLANGE RING
|                          |             |                         |
|                          +------x------+     <----               |
|<--0.000  ARP
|                          *             *         |               |
|                          *             *         |               |
|                          *             *         |               |
|                          *             *       10cm              |< metal
|                          *             *         |               |(RF skirt)
|                          *             *         |               |
|                                                                  |PIER
|                                                                  |
|                                                                  |

Note that the use of the "RF skirt" was first suggested by Tom Clarke as a
means to reduce multipath / near-field effects from the cavity between the
top of the pier and the bottom of the antenna ground plane and such skirts
have now been installed at all WCDA sites. 


As outlined above, preliminary analysis has identified apparent changes in
the mean position of antenna phase centres at the following times:

a) change in antenna types and cavity height: ALBH @ 94.104
b) change in cavity height: ALBH @ 95.011; DRAO at 95.102
c) addition of RF skirts: ALBH@ 95.202; DRAO at 96.010

Note that any physical changes in HI's have been correctly logged in the
site information files for both ALBH and DRAO and the observed changes in
the phase centres are in addition to the recorded HI changes. The additions
of the RF skirts have NOT been logged in the site information files. In
retrospect, it now appears that any changes in the near-field of the GPS
antennas should be logged in the IGS site-information files.

The maximum change in the vertical that we have observed coincident with any
one (or a combination of) the noted antenna changes approaches 2 cm. Such
steps have been identified from day-to-day baseline analyses as well as from
the weekly SINEX analyses produced by NRCan. Our conclusion is that any
regression analyses on DRAO or ALBH data must allow for step functions at
the noted dates or resulting estimates of long term linear trends will be
biased. As soon as our analyses are complete, we will post our best
estimates of apparent changes in the phase centres of the affected sites.
This preliminary note is to indicate to the IGS community that we are aware
of the problem and to suggest caution in the interpretation of secular
trends at GPS sites where such antenna changes have taken place.

Please address any comments or questions directly to us.


Herb Dragert                      Mike Schmidt
dragert at pgc.emr.ca           schmidt at pgc.emr.ca

Michael Schmidt                          Tel  (604) 363-6760
Pacific Geoscience Centre                FAX (604) 363-6565
Geological Survey of Canada              internet:  schmidt at pgc.emr.ca
P.O. Box 6000
9860 West Saanich Rd.
Sidney, B.C.  Canada  V8L 4B2

[Mailed From: schmidt at pgc.emr.ca (Michael Schmidt)]

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