Global Positioning System Reference
In-Depth Information
p
km,
IF
− ρ
p
km
R
km
=
T
km
+
d
km,
IF
,
Φ
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
Φ
−
d
t
km
−
(7.232)
− λ
1
N
km,
1
+ λ
2
N
km,
2
=
1
− α
f
I
km,
1
,P
(t)
p
km,I
(t)
d
km,I,
Φ
Φ
+
(7.233)
The user at the roving station is free to use any modeling and interpolation model to
compute the respective tropospheric and ionospheric corrections.
The message formats for data exchange between a single base station and a sin-
gle rover generally follow the standards set by the Radio Technical Commission for
Maritime Services (RTCM). RTCM is a nonprofit scientific and educational organi-
zation consisting of international member organizations that include manufacturers,
marketing, service providers, maritime user entities representing interests from small
recreational craft to deep-sea shipping, educational institutions, labor unions, and
government agencies (RTCM, 2002). RTCM special committees address in-depth
radiocommunication and radionavigation areas of concern to the RTCM members.
The reports prepared by these committees are usually published as RTCM recom-
mendations. The RTCM special committee 104 deals with global navigation satellite
systems. It has issued Standards for Differential GNSS (currently version 2.3) and
Standards for Differential Navstar GNSS Reference Stations (currently Version 1.1).
It is expected that RTCM standards will be available in the near future, including all
message types needed for real-time RTK within multiple reference station networks.
[29
Lin
—
-
——
Lon
PgE
7.
9.3 WADGPS
The modeling in (7.227) and (7.228) and the achievable accuracy for the corrections
T
kn
and
I
kn,
1
,P
usually determine the size of the area over which real-time RTK is
possible, unless accurate corrections are available from other sources. As the area in-
creases, the ambiguities cannot be fixed and the carrier phases are used to smooth the
pseudoranges. The tropospheric and ionospheric corrections are typically parameter-
ized by latitude and longitude and transmitted to the user via geostationary satellites
for such wide area differential GPS (WADGPS) networks. Also, the tropospheric cor-
rections and the orbital satellite errors are typically dealt with separately. Early work
on WADGPS is found in Brown (1989), Kee et al. (1991), and Ashkenazi et al. (1992).
Several WADGPS systems have been implemented around the world; e.g., White-
head et al. (1998) describe a system that is privately operated to support precision
agriculture. The Federal Aviation Administration (FAA) is developing a WADGPS
called WAAS (wide area augmentation system). WAAS is a satellite-based augmen-
tation system (SBAS), meaning that the differential corrections and other relevant
data important for enhancing reliability and integrity of the system are transmitted
via satellites. WAAS will provide guidance to aircraft at thousands of airports and
airstrips where there has previously been no precision landing capability (Loh et al.,
1995). Other SBASs have been developed in Europe and Japan under the names Eu-
ropean Geostationary Navigation Overlay Service (EGNOS) and MTSAT Satellite-
based Augmentation System (MSAS). Several U.S. agencies, such as the Federal
Railroad Administration, the U.S. Coast Guard, the Federal Highway Administration,
and the Office of the Secretary of Transportation are developing the nationwide differ-
ential global positioning system (NDGPS). The system began as an expansion of the
[29
