INTRODUCTION - GPS Satellite Surveying - page 7

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During 1991 and 1992, the geodetic community embarked on major efforts to
explore the limits of GPS on a global scale. The efforts began with the GIG91 cam-
paign and continued the following year with the International GPS Service (IGS)
campaign. GIG91 (GPS experiment for International Earth Rotation Service [IERS]
and Geodynamics) resulted in very accurate polar motion coordinates and earth rota-
tion parameters. Geocentric coordinates were obtained that agreed with those derived
from satellite laser ranging within 10 to 15 cm, and ambiguities could be fixed on a
global scale providing daily repeatability of about 1 part in 10 9 . Such results are
possible because of the truly global distribution of the tracking stations. The primary
purpose of the IGS campaign was to prove that the scientific community is able to
produce high-accuracy orbits on an operational basis. The campaign was successful
beyond all expectations, confirming that the concept of IGS is possible. The IGS
service formally began January 1, 1994.
For many years, users worried about what impact antispoofing (AS) would have
on the practical uses of GPS. AS implies switching from the known P-code to the
encrypted Y-code, expressed by the notation P(Y)-code. The purpose of AS is to make
the P-codes available only to authorized (military) users. The anxiety about AS was
considerably relieved when Hatch et al. (1992) reported on the code-aided squaring
technique to be used when AS is active. Most manufacturers developed proprietary
solutions for dealing with AS. When AS was actually implemented on January 31,
19 94, it presented no insurmountable hindrance to the continued use of GPS and,
pa rticularly, the use of modern techniques such as OTF. GPS users became even less
de pendent on AS with the introduction of accurate narrow correlator spacing C/A-
co de receivers (van Dierendonck et al., 1992), since the C/A-code is not subject to
AS measures. By providing a second civil code on L2, and eventually a third one
on L5, and adding new military codes, GPS modernization will make the P(Y)-
co de encryption a nonissue for civilian applications, and at the same time, provide
en hanced performance to civilian and military users.
A major milestone in the development of GPS was achieved on December 8, 1993,
wh en the initial operational capability (IOC) was declared when twenty-four satellites
(B locks I, II, IIA) became successfully operational. The implication of IOC was
th at commercial, national, and international civil users could henceforth rely on the
av ailability of the SPS. Full operational capability (FOC) would be declared July 17,
19 95, when twenty-four satellites of the type Blocks II and IIA became operational.
Te unissen (1993) introduced the least-squares ambiguity decorrelation adjustment
(L AMBDA), which is now widely used.
The determination of attitude/orientation using GPS has drawn attention for quite
so me time. Qin et al. (1992) report on a commercial product for attitude determi-
na tion. Talbot (1993) reports on a real-time kinematic centimeter-accuracy survey-
in g system. Lachapelle et al. (1994) experiment with multiple (single-frequency)
re ceiver configurations, in order to accelerate the on-the-fly ambiguity resolution
by means of imposing length constraints and conditions between the ambiguities.
While much attention was given to monitoring the ionosphere with dual-frequency
and single-frequency code or carrier phase observations, Kursinski (1994) discusses
the applicability of radio occultation techniques to use GPS in a general earth's at-

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