INTERNATIONAL GPS SERVICES
NOAA/NGS Analysis Strategy Summary

November 23, 1998
updated by Robert B. Leonard, Jr.

ANALYSIS CENTER	National Oceanic and Atmospheric Administration (NOAA) National Geodetic Survey (NGS) URL site - http://www.ngs.noaa.gov/ Phone: 301-713-3205, Fax: 301-713-4322
CONTACT PERSONS	Operations: William Kass E-mail phone : 301-713-3209 ext.-164 Robert Dulaney E-mail phone : 301-713-2853 ext.-204 William Dillinger E-mail phone : 301-713-2851 ext.-209 Gerald Mader E-mail phone : 301-713-2854 ext.-201
SOFTWARE USED	page5 developed at NOAA
NGS PRECISE PRODUCTS GENERATED FOR GPS WEEK 'wwww' DAY OF WEEK 'n' (n=0,1,...,6) 7 weekly files	NGS0wwwwn.EPH GPS ephemeris/clock files in 7 daily files at 15 min intervals in SP3 format, including accuracy codes computed from the formal orbit errors of the solution NGS0wwwwn.ERP EOP (pole and LOD estimates; a priori UT1-UTC) in 7 daily files NGS0wwwwn.SUM Processing summary in 7 daily files
NGS RAPID PRODUCTS	NGS0wwwwn.SP3 GPS ephemeris/clock files at 15 min intervals in SP3 format. Produced within 15 hours of the preceeding day. NGS0wwwwn.EOP EOP (pole and LOD estimates; a priori UT1-UTC)
PREPARATION DATE	Nov. 6, 1998
EFFECTIVE DATE FOR DATA ANALYSIS	Jul. 26, 1998

MEASUREMENT MODELS
Preprocessing	Phase preprocessing in a baseline by baseline mode using double-differences. In most cases cycle slips are fixed automatically looking simultaneously at different linear combinations of L1 and L2. Manual re-editing is done if any baseline shows larger than normal post-fit RMS statistics in the all base-combined solution. At that time, bad data points are removed or new phase bias ambiguities are set up.
Basic observable	Carrier phase, code only used for receiver clock sync.
	Elevation angle cutoff 15 degrees Sampling rate 30 seconds Weighting Aug. 6, 1995 - Jun. 29, 1996 1 m uniform Jun. 30, 1996 -present 0.10 m uniform Rejection criteria No rejection during parameter estimation. Outliers marked during preprocessing step; manual re-edit done for any baseline with high RMS fit.
Modelled observable	Double-differences, ionosphere-free linear combination.
RHC phase rotation corr.	applied (Wu et al., 1993)
Ground antenna phase centre	Elevation-dependent phase center corrections are applied to all antenna types (Mader & MacKay, 1995)
Troposphere	A priori zenith delay Saastamoinen (1972) "dry" + "wet" zenith delays Mapping function NMF dry and wet mapping functions (Niell, 1995) Met. data input latitude, height, DOY model Estimation wet zenith delay adjusted in 2-hour intervals for all sites
Ionosphere	Not modelled (ionosphere eliminated by forming the ionosphere-free linear combination of L1 and L2)
Plate motions	ITRF96 station velocities fixed
Tidal displacements	Solid earth tidal displacement Cartwright & Taylor (1971) tidal potential complete thru degree 3 with newest Shida and Love numbers (Mathews et al., unpublished) Correction applied to remove permanent tide zero-frequency contribution not removed Nominal Love nos. h2 = 0.6067; el2 = 0.0844 h3 = 0.2900; el3 = 0.0152 dh resonance corrections applied for 6 largest tides Pole tide not applied Ocean loading not applied
Earth orientation models	Tidal UT1 (>5day) not modelled Sub-daily EOP Herring & Dong (1994) model applied
Atmospheric load.	not applied
Satellite center of mass correction	Block I x,y,z 0.2110 , 0.0000 , 0.8540 m Block II/IIA x,y,z 0.2790 , 0.0000 , 1.0230 m Block IIR x,y,z 0.2790 , 0.0000 , 1.0230 m
Satellite phase centre calibrat.	not applied
Relativity	periodic corrections applied
GPS attitude model	not applied

ORBIT MODELS
Geopotential	GEM-T3 model up to degree and order 8; C21, S21 as recommended by IERS (1992) GM = 398600.4418 km**3/sec**2 AE = 6378.1363 km
Third-body	Sun and Moon as point masses Ephemeris: Generated from the MIT PEP program GMsun = 132712440000 km**3/sec**2 GMmoon = 4902.7989 km**3/sec**2
Solar radiation pressure	Direct radiation ROCK4 and ROCK42 models (T10 and T20) for Block I and II satellites, resp. (Fliegel et al., 1992)
	Satellite masses PRN 01 972.9 kg PRN 15 885.9 kg PRN 02 878.2 kg PRN 16-19 883.2 kg PRN 03-07 972.9 kg PRN 20 887.4 kg PRN 09 972.9 kg PRN 21-22 883.9 kg PRN 12 519.8 kg PRN 23-29 972.9 kg PRN 14 887.4 kg PRN 31 972.9 kg
	x,y, and z scale plus once-per-revolution perturbation terms adjusted for each 1-day arc.
	Earth shadow model includes: umbra and penumbra
	Reflection radiation: not included
	New GPS satellite attitude model not applied Geometrical effects not applied Orbit dynamic effects not applied Yaw rates --
Tidal forces	Solid earth tides C20, C21, S21, C22, and S22 as in IERS (1992); n=2 order-dependent Love numbers and frequency-dependent corrections for 6 (2,1) tides from Richard Eanes (private comm., 1995) Ocean tides not applied
Numerical Integration	11th order Adams-Moulton Predictor-corrector Integration step variable Starter procedure Initial conditions taken from the rapid day at 12:00 Arc length 24 hours

ESTIMATED PARAMETERS (APRIORI VALUES & SIGMAS)
Adjustment	Least-squares algorithms using 24 hours of double-differenced phase observations from the global network.
Station coordinates	Aug. 6, 1995 - Jun. 27, 1998 Standard 13 stations fixed to the ITRF93 positions as given in IGSMAIL#819, plus MDO1 and MATE also fixed. The ITRF93 velocities are used for coordinate updates. All remaining stations estimated. Rapid Jun. 28, 1998 - Present Standard 47 stations fixed, minus fiducial site GODE, to the ITRF96 epoch 1997.0 positions as given in the Kouba 1998 IGS Mail message. The ITRF96 velocities are used for coordinate updates. All remaining stations estimated. Precise Jun. 28,1998 - Present Final precise orbits are minimally constrained and adjusted over 1 week to be consistent with the weekly combination.
Satellite clock bias	Satellite clock biases are not estimated but eliminated by forming double-differences.
Receiver clock bias	Receiver clock corrections are estimated during the pre-processing using code measurements.
Orbital parameters	Geocentric position and velocity, solar radiation pressure scales and once-per-revolution perturbation terms along the satellites - sun, body centered Y, and orthogonal third directions estimated as constant offsets for each one-day arc.
GPS attitude parameters	none estimated
Troposphere	Jun. 28, 1998 Wet zenith delay estimated as constant offset for 2-hr intervals at each station.
Ionospheric correction	Not estimated; L1 & L2 used for 1st order corrections
Ambiguity	Estimated as real values with no a priori constraints
EOP	Aug. 6, 1995 - Jun. 29, 1996 x and y pole and LOD estimated as constant offsets for each one-day arc. A priori values taken from IERS Rapid Service Bulletin A. Jun. 30, 1996 - x and y pole estimated in a piece-wise, linear (endpoints of a straight line) with no a priori constraints for each one-day arc. LOD estimated as constant offset for each one-day arc. A priori values continue to be taken from IERS Rapid Service Bulletin A. Jun. 28, 1998 - This product is modified in a weekly combination consistant with orbits, EOP, and troposphere.
Other parameters	None

REFERENCE FRAMES
Inertial	Geocentric; mean equator and equinox of Besselian year 1950 (B1950.0)
Terrestrial	Aug. 6, 1995 - Jun. 29, 1996 ITRF93 reference frame realized through a set of 15 station coordinates and velocities as given in IGSMAIL #819 as well as the antenna offsets for the above stations given in /igscb/station/tie/localtie.tab (available from IGS at igscb.jpl.nasa.gov). Jun. 30, 1996 - Feb. 28, 1998 ITRF94 reference frame realized through a set of 15 station coordinates and velocities as given in Kouba (1996) as well as the antenna offsets for the above stations given in /igscb/station/tie/localtie.tab. Mar. 1, 1998 - Jun 27, 1998 ITRF96, epoch 1997.0 reference frame realized through a set of 46 station coordinates and velocities as given in the Kouba 1998 Mail message, as well as the antenna offsets for the above stations given in /igscb/station/tie/localtie.tab. Jun. 28, 1998 - Present Minimally constrained combination using all available station coordinates; Product is modified to be consistent with the weekly combination.
Interconnection	Precession IAU 1976 Precession Theory
	Nutation IAU 1980 Nutation Theory
	Relationship between UT1 and GMST USNO Circular No. 163 (IAU Res.)
	A priori EOP values from IERS Rapid Service Bull. A
	Tidal UT1 (>5day) not modelled Sub-daily EOP not modelled

REFERENCES:

Cartwright, D.E., and R.J. Taylor, New computations of the tide- generating potential, Geophys. J.R. astr. Soc., 23, 45-74, 1971; Cartwright, D.E., and A.C. Edden, Corrected tables of tidal harmonics, ibid, 33, 253-264, 1973.

Fliegel, H., T. Gallini and E. Swift, Global Positioning System radiation force model for geodetic applications, J. Geophys. Res. 97(B1), 559-568, 1992.

Herring, T.A., and D. Dong, Measurement of diurnal and semidiurnal rotational variations and tidal parameters of Earth, J. Geophys. Res., 99(B9), 18051-18071, 1994.

Kouba, J., Personal communication via email Subject: Comments on ITRF94 (FWD), June 11, 1996.

Kouba, J., Personal communication via email dated Feb. 25 1998. Subject: ITRF96-IGS-RS47.SSC for March 1, 1998.

Mader, G.L. and J.R. MacKay, (1996). "Calibration of GPS Antennas". IGS Workshop Proceedings, Silver Spring, Md. March 19-21, 1996. available from the Jet Propulsion Laboratory, Pasadena, California. JPL. Publication 96-23 10/96. pp.81-105.

McCarthy, D.D. (ed.), IERS Standards (1992), IERS Technical Note 13, Observatoire de Paris, 1992.

Niell, A.E., Global mapping functions for the atmosphere delay at radio wavelengths, submitted to J. Geophys. Res., 1995.

Saastamoinen, J., Atmospheric correction for the troposphere and stratosphere in radio ranging of satellites, in The Use of Artificial Satellites for Geodesy, Geophys. Monogr. Ser. 15 (S.W. Henriksen et al., eds.), AGU, Washington, D.C., pp. 247-251, 1972.

Wu, J.T., S.C. Wu, G.A. Hajj, W.I. Bertiger, and S.M. Lichten, Effects of antenna orientation on GPS carrier phase, Manuscripta Geodaetica, 18, 91-98, 1993.

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