Satellite Navigation for Digital Earth

T
a
ble
4
.8
Ov
ervie
w
of
GNSS
augmentation
systems
Reference
station
Processing
principle
Broadcast
link
Broadcast
format
Performance
T
ypical
systems
W
ide-area
dif
ferential
technique
Ranges
o
f
se
v
eral
thousand
k
ilometers
with
se
v
eral
dozen
reference
stations
Dif
ferencing
in
the
state
domain,
with
corrections
generated
for
orbit
and
clock
errors,
g
rid
ionospheric
V
TEC
v
alues
based
o
n
p
seudorange
measurements
(assisted
by
the
carrier
phase)
GEO
satellites
or
radio
beacons
RT
C
A
Single-frequenc
y
pseudorange
dif
ferential
positioning,
Precision:
3
m
without
initialization
W
AAS
EGNOS
MSAS
GA
GAN
SDCM
Global
p
recise
positioning
technique
~100
globally
reference
stations
Dif
ferencing
in
the
state
domain,
with
corrections
generated
for
orbit
and
clock
errors
based
o
n
carrier
phase
measurements
GEO
satellites
or
through
the
Internet
SOC
o
r
User
-defined
protocol
Double-frequenc
y
carrier
phase
dif
ferential
positioning,
Precision:
decimeter
-le
v
el
w
ith
initialization
o
f
approximately
20
min
GDGPS
StarFire
OmniStar
SeaST
A
R
V
eripos
RT
X
Local-area
dif
ferential
technique
Range
of
a
fe
w
tens
of
kilometers
with
se
v
eral
reference
stations
Dif
ferencing
in
the
observ
ation
domain,
with
pseudorange
corrections
generated
b
ased
on
pseudorange
measurements
(assisted
by
the
carrier
phase)
U/VHF
data
link
RT
C
A
o
r
RT
C
M
Single-frequenc
y
pseudorange
dif
ferential
positioning
Precision:
submeter
le
v
el
w
ithout
initialization
LAAS
Local-area
precise
positioning
technique
Range
of
a
fe
w
tens
of
kilometers
with
se
v
eral
reference
stations
and
can
be
ex
tended
to
lar
ger
re
gions
with
se
v
eral
thousand
reference
stations
Dif
ferencing
in
the
observ
ation
domain,
with
re
gional
error
correction
p
arameters
generated
in
V
RS,
F
KP
,
M
A
C
and
o
ther
processing
modes
based
o
n
carrier
phase
measurements.
U
sers
in
the
re
gion
can
apply
for
such
correction
information
GPRS/CDMA
communica-
tion
link
broadcasting
RT
C
M
Double-frequenc
y
carrier
phase
dif
ferential
positioning
Precision:
centimeter
le
v
el
w
ith
an
initialization
o
f
approximately
1–2
min
CORS

---

4
Satellite Navigation for Digital Earth
141
using the wide-area differential corrections. With uniform precision over broad cov-
erage, the positioning accuracy of the wide-area differential augmentation system
is independent of the distance between the user and the reference station. Several
wide-area differential augmentation systems have been established worldwide, e.g.,
the Wide Area Augmentation System (WAAS) of the Federal Aviation Administra-
tion (FAA), the European Geostationary Navigation Overlay Service (EGNOS), the
Multi-functional Satellite Augmentation System (MSAS) of Japan, the GPS-Aided
Geo Augmented Navigation (GAGAN) of India and the System Differential and
Correction Monitoring (SDCM) of Russia.
4.3.1.1
WAAS
The GPS SPS could not meet the higher accuracy, integrity, continuity and availability
demands of users in aviation and other fields. As a result, the FAA initiated the WAAS
program. As a satellite-based augmentation system (SBAS) to serve North America,
WAAS is aimed at providing GPS differential correction information through GEO
satellites to augment the GPS SPS. In addition to applications in the field of aviation,
the WAAS has also been widely applied to support PNT services.
As illustrated in Fig.
4.5
, the WAAS currently consists of 3 wide-area master sta-
tions (WMS), 38 wide-area reference stations (WRS), 4 Ground Uplink Stations and
Fig. 4.5
Diagram of WAAS (
source
http://www.faa.gov/about/office_org/headquarters_offices/
ato/service_units/techops/navservices/gnss/waas/
)

---

142
C. Shi and N. Wei
GEO satellites. The WMS is responsible for calculating the differential corrections
and monitoring the system integrity. The WAAS data processing center receives real-
time GPS observations from the WRS and computes differential correction vectors
using the RTG (Real Time GIPSY) software package developed by the JPL based
on GIPSY-OASIS. The corrections include the satellite orbit error, satellite clock
error and the ionospheric error. Since these differential corrections are expressed
as vectors, the precision of positioning within the areas covered by the wide-area
differential system is equivalent, in contrast to the local-area differential system. The
corrections are uploaded to GEO satellites through uplink stations and broadcast to
users in RTCA format to improve positioning accuracy. The nominal positioning
accuracy of WAAS (with 95% reliability) is better than 7.6 m in the horizontal and
vertical directions. The horizontal and vertical positioning accuracy of WAAS are
better than 1.0 m and 1.5 m, respectively, in most regions adjacent to the US, Canada
and Alaska.