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NATIONAL GEOSPATIAL-INTELLIGENCE AGENCYApproved for Public ReleaseNGA Case # 06-364
NGA’s Role in GPSNGA’s Role in GPS
Barbara Wiley
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Approved for Public Release NGA Case # 06-364
Coauthors:
• David Craig
• Dennis Manning
• John Novak
• Randall Taylor
• Leonard Weingarth
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Overview
• NGA’s GPS Mission• WGS 84• GPS Monitor Station Network• Network Assessment• Clock Analysis• Legacy Accuracy Improvement Initiative (L-Aii)• Precise Ephemeris• Earth Orientation• Summary
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NGA’s GPS Mission
• 30+ years of satellite tracking• Precise orbits needed to produce precise
positions• From the Master Positioning, Navigation and
Timing Plan:– NGA is responsible for:
• Services to support DoD navigation systems• Maintenance of GPS fixed site operations • Generation and distribution of GPS precise ephemerides
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WGS 84 Reference Frame
Reference Frame: Global network of control stations that binds an Earth-centered, Earth-fixed 3-D coordinate system to the earth
Control Station Position Accuracy
Transit (1 - 2 m) Jan 1987
G730 (~10 cm) Jun 1994
G873 (~5 cm) Jun 1997
G1150 (~1 cm) Jan 2002
Ensure the WGS 84 Reference Frame errors are negligible in the GPS ephemeris error budget
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GPS Monitor Station Network
Tahiti
Alaska
Austin, TX
St. Louis, MO
USNO
South Africa
Bahrain
United Kingdom
Australia
New Zealand
Korea
Ecuador
Argentina
GPS Monitor Stations NGA Site (11) NGA Test Site (2) USAF Site (6)
Hawaii
ColoradoSprings
Ascension Diego Garcia
KwajaleinCape Canaveral
• GPS Wing (formerly Joint Program Office) designs and acquires the GPS space systems
• USAF operates and maintains the GPS constellation
• NGA provides data and analysis to USAF on GPS satellites referenced to WGS 84
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Network Assessment• High performance: Dual Redundancy
Data Availability by Month 1/2003-8/2006 (6 Stations)
90.00
91.00
92.00
93.00
94.00
95.00
96.00
97.00
98.00
99.00
100.00
Jan-
03
Mar
-03
May
-03
Jul-0
3
Sep-0
3
Nov-0
3
Jan-
04
Mar
-04
May
-04
Jul-0
4
Sep-0
4
Nov-0
4
Jan-
05
Mar
-05
May
-05
Jul-0
5
Sep-0
5
Nov-0
5
Jan-
06
Mar
-06
May
-06
Jul-0
6Tota
l
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Clock Analysis
NGA Station Clock Stability For The Year 2005
0.0
0.5
1.0
1.5
2.0
2.5
3.0
85402 85403 85404 85405 85406 85407 85410 85411 85412 85413a 85413b 85414
Station Number
nan
ose
con
ds
1 Hour Stability
1/2 Day Stability
1 Day Stability
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Legacy Accuracy Improvement Initiative (L-Aii)
• L-Aii included:– Initial addition of 6 of NGA’s GPS monitor
stations to the OCS Kalman filter– Model improvement to the OCS Kalman
filter
• Results:– OCS has visibility of all satellites at all times– 44% improvement to the ZAOD orbits– 10% improvement in SIS URE
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NGA GPS Network Tracking Data
• Currently 8 stations sent in near-real-time to the GPS Master Control Station at Schriever AFB– Improves integrity monitoring
• All satellites in view of at least two stations > 99% of the time
– Improves accuracy• Most users accuracy on the ground is 3-10m depending on
equipment and environment• Most users received 5-10% accuracy improvement
• Remaining 3 NGA stations will be incorporated for more robustness
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Coverage by GPS Monitor Stations
Percent Coverage
0
20
40
60
80
100
0 >=1 >=2 >=3 >=4 >=5 >=6 >=7 >=8
Number of stations viewing each satellite
OCS only
OCS + 6 NGA
OCS + 11 NGA
Assumes a 10 degree elevation mask
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NGA Precise Ephemeris for GPS is DoD Truth- Precise determination of where the satellites were- Precise GPS clock solutions- Used for precise positioning and system analysis
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Overall RMS Comparison
0.0000
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
1994 1996 1998 2000 2002 2004 2006 2008
Year
Av
era
ge
va
lue
(m
)
NGA Precise Ephemeris comparisons
Retuning Retuning and new tropo model
Range diff data from Phase Obs added
Additional Tracking Stations Deployed
L-Aii begins
G1150 implemented
URE (m)
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
51910 52410 52910 53410 53910
MJD
ME
TE
RS
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Mean Axisof Rotation
Axis ofFigure
North Pole
Earth Orientation Prediction
Earth Orientation is used by the GPS Master Control Station to transform between Earth fixed coordinates to Earth inertial coordinates. The transformation accuracy directly affects GPS accuracy
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Polar Motion
XY Polar Motion (~15 Years)
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
-0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
seconds of arc (X)
se
co
nd
s o
f a
rc (
Y)
XY Polar
2005/2006
2006 preds
1st Pt 1992
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UT1-UTC
UT1-UTC (with last two applied leap-seconds)
-0.90
-0.80
-0.70
-0.60
-0.50
-0.40
-0.30
-0.20
-0.10
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
Year
se
co
nd
s
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Summary
• NGA supports the GPS MCS
• With L-Aii, NGA’s role in GPS has changed
• Now an integral partner in real-time GPS accuracy and integrity
• Real-time operations• NGA’s GPS Monitor Station Network maintained
at a high standard• GPS products available online
– www.nga.mil
• Email: [email protected]