Delivered-To: igsmail@igscb.jpl.nasa.gov From: Jim Ray "(NGS" 301-713-2850 "x112)" Message-Id: <200602151811.NAA24033@ness.ngs.noaa.gov> Subject: [IGSMAIL-5303]: Local BRFT ties at Fortaleza, Brazil To: igsmail@igscb.jpl.nasa.gov Date: Wed, 15 Feb 2006 13:11:41 -0500 (EST) Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-igsmail Precedence: bulk ****************************************************************************** IGS Electronic Mail 15 Feb 10:12:19 PST 2006 Message Number 5303 ****************************************************************************** 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.