Know the Earth…Show the Way NATIONAL GEOSPATIAL-INTELLIGENCE AGENCY Approved for Public Release NGA Case # 06-364 NGA’s Role in GPS Barbara Wiley

Know the Earth…Show the Way

NATIONAL GEOSPATIAL-INTELLIGENCE AGENCYApproved for Public ReleaseNGA Case # 06-364

NGA’s Role in GPSNGA’s Role in GPS

Barbara Wiley

NATIONAL GEOSPATIAL-INTELLIGENCE AGENCY


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Approved for Public Release NGA Case # 06-364

Coauthors:

• David Craig

• Dennis Manning

• John Novak

• Randall Taylor

• Leonard Weingarth



3


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]



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Documents

Know the Earth…Show the Way NATIONAL GEOSPATIAL-INTELLIGENCE AGENCY Approved for Public Release NGA Case # 06-364 NGA’s Role in GPS Barbara Wiley