Global Positioning System Reference
In-Depth Information
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
The GLONASS broadcast ephemeris provides the satellite positions in the PZ90
coordinates system. Much effort has been made to compute accurate transformation
parameters between PZ90 and WGS84. For example, see Bazlov et al. (1999a,b) and
references therein. For small baselines any residual error due to the inaccurate trans-
formation is likely to cancel when single differencing. When performing point posi-
tioning with pseudoranges it might be important to keep in mind that the GLONASS
clocks are steered according to UTC(SU) whereas GPS time follows UTC(USNO)
within prescribed margins. In the case of the precise ephemeris computed by the IGS
the reference frame is ITRF for both systems and time refers to a common standard.
Of course, the GLONASS observables can be used to form popular functions, such
as the ionospheric, the ionospheric-free, the wide-lane ambiguity, and the multipath
functions. Using the definition of the frequencies (3.91) and (3.92), we obtain the
expressions that correspond to (5.13) to (5.16) for GLONASS
α
GLO
=
f
1
/f
2
2
[27
(
9
/
7
)
2
=
=
81
/
49
(7.129)
Lin
—
0.0
——
Nor
PgE
β
GLO
= α
GLO
/(
α
GLO
−
1
)
=
81
/
32
(7.130)
γ
GLO
=
1
/(
α
GLO
−
1
)
=
49
/
32
(7.131)
δ
GLO
=
√
α
GLO
/(
α
GLO
−
1
)
=
63
/
32
(7.132)
GLONASS has attracted a lot of attention, not only because of its potential to
increase the number of usable satellites but also because of the fact that its satellites
transmit at different carrier frequencies. The following is a sample of relevant litera-
ture: Raby and Daly (1993), Leick et al. (1995, 1998), Gourevitch et al. (1996), Pov-
alyaev (1997), Pratt et al. (1997), Rapoport (1997), Kozlov and Tkachenko (1998),
Roßbach (2001), and Wang et al. (2001).
[27
7.
7.7 Relative Positioning within CORS
The National Geodetic Survey (NGS) is the lead agency in establishing and operating
the continuously operating reference station (CORS) system (NGS, 2002). The coop-
eration includes academic, commercial, and private organizations. While NGS does
not guarantee that a particular site is operating at any given time, it tries to expand
the system so that all points in the contiguous United States will be within a specified
distance from an operational site. See Figure 7.19 for a map of CORS sites; at this
writing there are more than 420 participating sites.
Tying to CORS stations is a practical way of connecting points determined by GPS
to the geodetic frame. All CORS sites are known with centimeter accuracy in either
the ITRF reference system or the NAD83 geodetic datum. The dual-frequency carrier
phase and pseudorange observations of any CORS site, plus other ancillary station
data and the precise ephemeris can be readily downloaded for postprocessing by the
user. NGS also offers a processing service, online positioning user service (OPUS),
via the Internet. The user simply uploads the observation files to NGS and receives
