From: Michael Schmidt Subject: [IGSMAIL-1359] ALBH, DRAO Advisory Date: Wed, 19 Jun 1996 17:57:34 PDT ****************************************************************************** IGS Electronic Mail Wed Jun 19 17:57:34 PDT 1996 Message Number 1359 ****************************************************************************** Author: Michael Schmidt Subject: ALBH, DRAO Advisory UPDATE ON WCDA SITES (ALBH & DRAO) 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. PRESENT ANTENNA INSTALLATIONS: 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 TCR . | | . . | | . . | | . . | | . . +------------------------------------------------------+ . <-- 0.038 . +-------------------+-------------+--------------------+ . <--0.035 BCR |========== | | ============| FLANGE RING | | | | | +------x------+ <---- | |<--0.000 ARP | * * | | | * * | | | * * | | | * * 10cm |< metal screening | * * | |(RF skirt) | * * | | |--------------------------*------*------*-------------------------|TOP CONCRETE | |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. APPARENT STEP FUNCTIONS: 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@95.102 c) addition of RF skirts: ALBH@ 95.202; DRAO@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. Regards Herb Dragert Mike Schmidt dragert@pgc.emr.ca schmidt@pgc.emr.ca ******************************************************************************** Michael Schmidt Tel (604) 363-6760 Pacific Geoscience Centre FAX (604) 363-6565 Geological Survey of Canada internet: schmidt@pgc.emr.ca P.O. Box 6000 9860 West Saanich Rd. Sidney, B.C. Canada V8L 4B2 ******************************************************************************** [Mailed From: schmidt@pgc.emr.ca (Michael Schmidt)]