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[IGSMAIL-5303]: Local BRFT ties at Fortaleza, Brazil
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IGS Electronic Mail 15 Feb 10:12:19 PST 2006 Message Number 5303
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Author: Jim Ray
As announced by Miranda Chin (IGS Mail #5213, 21 Sept. 2005), a new GPS
tracking station, BRFT, was installed in Sept. 2005 at the Radio-
Observatorio Espacial do Nordeste (ROEN) facility outside Eusebio, Brazil
(about 23 km southeast of Fortaleza). The site is situated almost 4 degrees
south of the equator. It is planned that BRFT will soon replace the
original GPS station, FORT, which has operated since May 1993. Both systems
are co-located with the 14.2-m Fortaleza VLBI telescope, which as operated
since July 1993.
The configuration of the two GPS stations is briefly summarized below:
FORT BRFT
---------- --------------------------- ---------------------------
DOMES no. 41602M001 41602M002
monument dimpled steel disk on dimpled point within SCIGN
roof of one-storey bldg leveling mount atop 1.5-m
tall steel tripod on rooftop
receiver ROGUE SNR-8000 LEICA GRX1200PRO
firmware 3.2.32.11 2.12
antenna AOAD/M_TA_NGS with LEIAT504
aluminum backplane
radome conical (uncalibrated) none
hgt offset 0.643 m 0.0083 m
clock external Sigma Tau H-maser external Sigma Tau H-maser
--------------------------------------------------------------------
Both stations are located near different corners on the rooftop of the
one-storey ROEN control building.
Using all the available simultaneous 24-hr data involving FORT and BRFT
between 27 Sept. 2005 and 28 Jan. 2006 (93 days), the following tie between
their reference points has been determined:
FORT (41602M001) -> BRFT (41602M002) GPS vector tie
---------------------------------------------------
dX: Wtd mean = 6.9259 Wtd RMS = 0.0021 m
dY: Wtd mean = 5.1912 Wtd RMS = 0.0017 m
dZ: Wtd mean = -0.3336 Wtd RMS = 0.0003 m
---------------------------------------------------
means for 93 days from 27 Sept. 2005 - 28 Jan. 2006
The analyses used L3 ionosphere-free carrier phase data and were weighted
daily by a factor proportional to the number of available data (although
this has negligible effect compared to equal weighting). Relative antenna
phase center offsets and elevation-dependent variations have been applied
in the analysis according to the IGS standard file:
ftp://igscb.jpl.nasa.gov/igscb/station/general/igs_01.pcv
The statistical standard errors for the mean components would be much
smaller than 1 mm if all days were fully independent, which is known to be
highly optimistic. So formal errors of about 1 mm for each component are
recommended as conservative.
Rotated to a local topographic frame, the dX, dY, dZ tie vector corresponds
to the dN, dE, dU offsets: -0.1841, 8.3713, 2.2171 m. In other words, the
BRFT station is located mostly east and up from FORT.
The FORT -> BRFT tie vector, together with the published IGb00 geocentric
coordinates for FORT (R. Ferland, IGS00 (v2) final, IGS Mail #4666, 29 Oct.
2003), can be used to infer the equivalent position of BRFT in the IGb00
frame (aligned to ITRF2000), as shown below:
BRFT (41602M002) coordinates in IGb00 frame
(epoch 1998.0)
-------------------------------------------
X = 4985393.5481 +/- 0.00413 m
Y = -3954993.3972 +/- 0.00367 m
Z = -428426.8034 +/- 0.00221 m
vel X = -0.001887 +/- 0.001378 m/y
vel Y = -0.003463 +/- 0.001189 m/y
vel Z = 0.010888 +/- 0.000530 m/y
-------------------------------------------
The IGb00 velocity for BRFT is assumed to be identical to FORT. The formal
errors for the BRFT position are those of FORT with 1 mm per component added
quadratically. Note that the FORT velocity was relatively poorly determined
in IGb00, especially in the X component, which makes propagation to recent
epochs a bit problematic. The accuracy of the FORT (and by inference BRFT)
position and velocity could be significantly improved using the longer
history now available.
The importance of having an accurate local tie measured between the reference
points of the co-located VLBI and GPS systems was recognized from the very
beginning. This is indispensable for tying together the technique frames
forming the ITRF. There are only about 30 such VLBI-GPS tie sites around
the world and only a handful of those have accurate locally surveyed ties.
NOAA personnel, under the direction of W.E. Carter, led the survey campaign
to measure this tie during 11-30 Sept. 1993, with assistance from Brazilian
colleagues. An NGS report of the survey results was prepared by C.C. Glover,
O.W. Murray, and M. Chin in Mar. 1994.
As reported by Glover et al. (1994), the vector tie from the FORT GPS to
the Fortaleza VLBI reference point is given below:
FORT (41602M001) -> Fortaleza VLBI (41602S001)
vector tie (m) (epoch 24 Sept. 1993)
----------------------------------------------
dX = -16.5977 m
dY = -21.7320 m
dZ = -45.8047 m
----------------------------------------------
The objective of the survey was to yield the highest accuracy tie possible,
estimated as 2 to 3 mm.
This survey tie, together with the FORT -> BRFT GPS tie from above, can be
used to infer the tie betwen the new BRFT GPS and the Fortaleza VLBI
reference point:
BRFT (41602M002) -> Fortaleza VLBI (41602S001)
vector tie (m)
----------------------------------------------
dX = -23.5236 m
dY = -26.9232 m
dZ = -45.4711 m
----------------------------------------------
based on 1993 survey + GPS tie
The formal error of this tie is unlikely to be better than ~3 mm per
component. Indeed, in a comparison between local ties and the global VLBI
and GPS frames, J. Ray and Z. Altamimi (Evaluation of co-location ties
relating the VLBI and GPS frames, J. Geodesy, doi: 10.1007/s00190-005-0456-z,
79(4-5), 189-195, 2005) found a discrepancy for FORT (41602M001) ->
Fortaleza VLBI (41602S001) of 3.0, 0.6, and 22.7 mm in the dN, dE, dU local
frame. The comparatively large error in dU is probably due, in large
measure, to the presence of the uncalibrated conical radome over the FORT
antenna. It is well established that such radomes can modify the phase
response of the antenna in such a way as to cause apparent height offsets
up to several cm. Consequently, both GPS -> VLBI ties should be used with
caution.
It is planned to return the FORT antenna + radome to NGS for calibration
measurements after the FORT station is decommissioned. Hopefully, those
results will clarify the role of radome effects on the local tie. A new
site survey is also under consideration.
Early geodetic results indicate that the new BRFT station performs very
well. As was hoped, the new antenna mount design (placed well above any
nearby reflecting surface) seems to have reduced the effects of code
multipath (and presumably phase multipath) by a large amount, judging
from the very low level of clock discontinuities seen at day boundaries.
The performance so far is among the best of any H-maser station in the
IGS network.