Apr. 20-May 9, 1995 (Cycles 95-97). Anti-Spoofing (AS) was off during these time
periods. In addition, GPS data collected from fourteen ground stations located at
Goldstone (California), Fairbanks (Alaska), Hartebeesthoek (S. Africa), Kokee Park
(Hawaii), Kourou (French Guiana), Madrid (Spain), McMurdo (Antarctica), Santiago
(Chile), Pamatai (Tahiti), St. Johns (Newfoundland), Tidbinbilla (Australia), Tromso
(Norway), Usuda (Japan), and Yaragadee (Australia). Ten more stations were added for
the 1995 data. Those were: Ny Alesund (Norway), Penticton (Canada), Richmond
(Florida), Arequipa (Peru), Fortaleza (Brazil), Easter Island (Chile), Guam (USA), Taipei
(Taiwan), Kitab (Uzbekistan), Kerguelen (France). The coordinates of Goldstone were
fixed to apriori values, and the coordinates of all other sites were adjusted in the final
gravity solution.
The GPS preprocessing software, TEXGAP, was employed to perform the following
procedures: 1) The pseudo-range data were used to determine the GPSIDR clock
correction. 2) Five I-sec GPSIDR Ll and L2 carrier phase points were fit using a 2 nd
degree polynomial, and this polynomial was evaluated at the ground station received time
to cancel the Selective Availability (SA) effect. 3) The GPSIDR Ll and L2 carrier phase
data were combined to remove the first-order ionosphere effects. 4) Double-differenced
carrier phase measurements were formed using a pair of GPS satellites, the GPSIDR and
a single ground GPS receiver. 5) Cycle-slips were fixed and the data outliers were edited.
The data were sampled at 30 sec intervals, and a 15° elevation cut-off was used to edit
the data collected by the GPS receiver on the earth's surface, and only GPSIDR data
above the TIP local horizon were used.
SOLUTION STRATEGIES
The TIP standard models were used (Tapley et aI., 1994b; McCarthy, 1992) with the
exceptions of the JGM-3 gravitational model, which was used as the reference
geopotential field (70x70 for TIP; 12x12 for the GPS satellites), and the TIP tuned ocean
tide model (CSR tide model 3.0), which was used as the reference ocean tide model. The
GPS non-gravitational model included the ROCK4T radiation pressure and y-axis force
(Fliegel et aI., 1992). The GPS station coordinates are given in the IERS Terrestrial
Reference Frame (ITRF93) [IERS, 1994]. The SLR-derived series for Earth polar motion
and UTI were used as the Earth orientation model. (Eanes and Watkins, 1993).
Antenna phase center variations determined in pre-launch testing were applied to the
TIP antenna. Phase wind-up correction (Wu et aI., 1993) and Doppler phase bias
correction (Bertiger et al., 1994) were applied to the GPSIDR data.
Arc lengths of 3.3-day were used for the 1992-93 data, while 3-day arcs were employed
for 1995 data. The 3.3-day arc is an attempt to sense long period resonance effects, while
minimizing the effect due to the mismodeling of GPS orbits on the recovery of
geopotential parameters.
The double-differenced data were processed in MSODPI (Multi-Satellite Orbit
Determination Program 1) [Rim, 1992] using a batch least-squares procedure. For each
arc, the positions and velocities of the TIP and GPS satellites were simultaneously
adjusted at the initial epoch. For TIP, I-day constant along-track force parameter (C t ) and
I-day once per orbital revolution (I-cpr) parameters in the along-track (T) and cross-track
(N) directions were estimated. For GPS, a ROCK4 scale factor and I-day y-bias
parameters were estimated to get a converged orbit, and these estimates were fixed to
adjust I-day Ct and I-day I-cpr parameters when the geopotential parameters were
adjusted. Since the TIP satellite orbits at an altitude of 1300 km, only geopotential
parameters up to degree and order 36, along with some higher degree resonant terms,
were adjusted. The double-differenced ambiguity parameters were adjusted, and 2.5 hour
zenith delay parameters using MTT mapping function (Herring, 1992) were adjusted for
43
periods. In addition, GPS data collected from fourteen ground stations located at
Goldstone (California), Fairbanks (Alaska), Hartebeesthoek (S. Africa), Kokee Park
(Hawaii), Kourou (French Guiana), Madrid (Spain), McMurdo (Antarctica), Santiago
(Chile), Pamatai (Tahiti), St. Johns (Newfoundland), Tidbinbilla (Australia), Tromso
(Norway), Usuda (Japan), and Yaragadee (Australia). Ten more stations were added for
the 1995 data. Those were: Ny Alesund (Norway), Penticton (Canada), Richmond
(Florida), Arequipa (Peru), Fortaleza (Brazil), Easter Island (Chile), Guam (USA), Taipei
(Taiwan), Kitab (Uzbekistan), Kerguelen (France). The coordinates of Goldstone were
fixed to apriori values, and the coordinates of all other sites were adjusted in the final
gravity solution.
The GPS preprocessing software, TEXGAP, was employed to perform the following
procedures: 1) The pseudo-range data were used to determine the GPSIDR clock
correction. 2) Five I-sec GPSIDR Ll and L2 carrier phase points were fit using a 2 nd
degree polynomial, and this polynomial was evaluated at the ground station received time
to cancel the Selective Availability (SA) effect. 3) The GPSIDR Ll and L2 carrier phase
data were combined to remove the first-order ionosphere effects. 4) Double-differenced
carrier phase measurements were formed using a pair of GPS satellites, the GPSIDR and
a single ground GPS receiver. 5) Cycle-slips were fixed and the data outliers were edited.
The data were sampled at 30 sec intervals, and a 15° elevation cut-off was used to edit
the data collected by the GPS receiver on the earth's surface, and only GPSIDR data
above the TIP local horizon were used.
SOLUTION STRATEGIES
The TIP standard models were used (Tapley et aI., 1994b; McCarthy, 1992) with the
exceptions of the JGM-3 gravitational model, which was used as the reference
geopotential field (70x70 for TIP; 12x12 for the GPS satellites), and the TIP tuned ocean
tide model (CSR tide model 3.0), which was used as the reference ocean tide model. The
GPS non-gravitational model included the ROCK4T radiation pressure and y-axis force
(Fliegel et aI., 1992). The GPS station coordinates are given in the IERS Terrestrial
Reference Frame (ITRF93) [IERS, 1994]. The SLR-derived series for Earth polar motion
and UTI were used as the Earth orientation model. (Eanes and Watkins, 1993).
Antenna phase center variations determined in pre-launch testing were applied to the
TIP antenna. Phase wind-up correction (Wu et aI., 1993) and Doppler phase bias
correction (Bertiger et al., 1994) were applied to the GPSIDR data.
Arc lengths of 3.3-day were used for the 1992-93 data, while 3-day arcs were employed
for 1995 data. The 3.3-day arc is an attempt to sense long period resonance effects, while
minimizing the effect due to the mismodeling of GPS orbits on the recovery of
geopotential parameters.
The double-differenced data were processed in MSODPI (Multi-Satellite Orbit
Determination Program 1) [Rim, 1992] using a batch least-squares procedure. For each
arc, the positions and velocities of the TIP and GPS satellites were simultaneously
adjusted at the initial epoch. For TIP, I-day constant along-track force parameter (C t ) and
I-day once per orbital revolution (I-cpr) parameters in the along-track (T) and cross-track
(N) directions were estimated. For GPS, a ROCK4 scale factor and I-day y-bias
parameters were estimated to get a converged orbit, and these estimates were fixed to
adjust I-day Ct and I-day I-cpr parameters when the geopotential parameters were
adjusted. Since the TIP satellite orbits at an altitude of 1300 km, only geopotential
parameters up to degree and order 36, along with some higher degree resonant terms,
were adjusted. The double-differenced ambiguity parameters were adjusted, and 2.5 hour
zenith delay parameters using MTT mapping function (Herring, 1992) were adjusted for
43