NGA > Office of Geomatics > GPS > Documentation

 

 

NATIONAL GEOSPATIAL-INTELLIGENCE AGENCY

GPS PRECISE EPHEMERIDES, SATELLITE CLOCK PARAMETERS AND SMOOTHED OBSERVATIONS

 

PRECISE EPHEMERIS

Earth-centered, Earth-fixed Coordinate system 

WGS84 (G1762)

Position

x,y,z (km)

Velocity

dx/dt, dy/dt dz/dt (dm/s)

GPS time

year, day, hour, minute

Trajectory interval

5 min.

Standard Trajectory

referenced to satellite center of mass

Optional Trajectory

referenced to satellite antenna phase center

Integration step size

300 seconds, reduced to 10 seconds during eclipse boundary crossings

SATELLITE CLOCK PARAMETERS

NGA Satellite Clock Products

Units

Satellite clock Time/Phase offset

ms

Satellite clock Frequency offset

10-4 ms/s = parts in 1010

Time interval for parameters

5 min.

Satellite clock events

All events processed as reinitializations

SMOOTHED OBSERVATIONS

Smoothed range and range difference observations (km) with corrections applied

NGA Smoothed Products

Units

GPS time of observation

year, day, seconds of day

Standard deviation of observation

km

Coordinate system

WGS84 (G1762)

Station position

x,y,z (m), Epoch 2005.0

Station velocity

dx/dt, dy/dt, dz/dt (m/yr)

Temperature

Celsius

Pressure

millibars

Relative Humidity

percent

Data interval

5 min

The carrier phase is smoothed to the pseudorange for all measurements above 10 deg. These data are collected every 1.5 s at NGA and Air Force monitor stations (collected and smoothed with the same algorithm) and every 30 s at IGS stations.

 

References:

·         Computer Program Development Spec., Master Control Station, Ephemeris/Clock Computer Program, NAVSTAR GPS Operational Control System Segment, CP-MCSEC-302C, Part 1, Appendix A, 7 May 1993.

·         Description of the Smoothing Algorithm in the NGA Monitor Station Network, (MSN29), Applied Research Laboratories, The University of Texas at Austin, GR-SGG-97-1, 3 April 1997.

 

Physical Constants

Earth’s semi-major axis (a)

6378.137 km

Lunar radius

1738 km

Solar radius

696000 km

Solar radiation pressure constant

4.560 X 10-6 N/m2

Astronomical Unit

149597870.691 km

Earth’s Gravitational constant (GM)

398600.4418 km3/s2

Sun’s Gravitational constant (GM)

132712400000 km3/s2

Moon’s Gravitational constant (GM)

4902.799186 km3/s2

Inverse of the Earth’s flattening (1/f)

298.257223563

Earth’s angular velocity

7.2921158553 X 10-5 rad/s

Speed of light

299792.458 km/s

Love’s constant

0.290

 

Corrections Applied to Measurements

Ionospheric delay

2-frequency, 1st order correction

Tropospheric refraction

Saastamoinen hydrostatic and wet zenith delay models and Niell hydrostatic and wet mapping functions

Yaw Bias

JPL yaw bias model for Block II and IIA satellites in eclipse

Periodic relativistic effects

 

 

Satellite antenna offsets

See table below

 

Satellite antenna offset (satellite body centered coordinates) 

 

Dx (m)

Dy (m)

Dz (m)

All Block II satellites

0.2794

0.0000

0.9519

All Block IIA satellites

0.2794

0.0000

0.9519

Block IIR PRN 02

-0.0099

0.0061

-0.0820

Block IIR PRN 11

0.0019

0.0011

1.5141

Block IIR PRN 13

0.0024

0.0025

1.6140

Block IIR PRN 14

0.0018

0.0002

1.6137

Block IIR PRN 16

-0.0098

0.0060

1.6630

Block IIR PRN 18

-0.0098

0.0060

1.5923

Block IIR PRN 19

-0.0079

0.0046

-0.0180

Block IIR PRN 20

0.0022

0.0014

1.6140

Block IIR PRN 21

0.0023

-0.0006

1.5840

Block IIR PRN 22

0.0018

-0.0009

0.0598

Block IIR PRN 23

-0.0088

0.0035

0.0004

Block IIR PRN 28

0.0019

0.0007

1.5131

Block IIR-M PRN 05

0.00292

-0.00005

-0.01671

Block IIR-M PRN 07

0.00127

0.00025

0.00056

Block IIR-M PRN 12

-0.01016

0.00587

-0.09355

Block IIR-M PRN 15

-0.00996

0.00579

-0.01227

Block IIR-M PRN 17

-0.00996

0.00599

-0.10060

Block IIR-M PRN 29

-0.01012

0.00591

-0.01512

Block IIR-M PRN 04

0.01245

-0.00038

-0.02283

Block IIR-M PRN 31

0.00160

0.00033

-0.05750

Block IIF-2  PRN 01

0.39100

0.00000

1.09100

Block IIF-8  PRN 03

0.39500

0.00030

1.09070

Block IIF-6  PRN 06

0.39470

-0.00100

1.09170

Block IIF-10 PRN 08

0.39620

-0.00030

1.08560

Block IIF-7  PRN 09

0.39550

-0.00200

1.09220

Block IIF-11 PRN 10

0.39620

-0.00130

1.08310

Block IIF-3  PRN 24

0.39200

0.00200

1.09300

Block IIF-1  PRN 25

0.39200

0.00200

1.09300

Block IIF-9  PRN 26

0.39340

-0.00110

1.09270

Block IIF-4  PRN 27

0.39140

0.00030

1.09040

Block IIF-5  PRN 30

0.39520

-0.00080

1.09040

Block IIF-12 PRN 32

0.39660

-0.00020

1.08430

 

FORCING MODELS

NGA uses IAU Resolutions on Astronomical Constants, Time Scales, and the Fundamental Reference Frame (1976-1980)

Gravitational

 

 

Normalized EGM08 (2008) Earth gravity model truncated at degree 12 and order 12

 

Solar, Lunar and Planetary gravity using the DE403 ephemerides, J2000 epoch, and IAU Resolutions

 

International Earth Rotation and Reference Systems Service (IERS) Solid Earth tides – Tech Note 36

Non-gravitational

 

 

Radiation Pressure: The JPL TJPLXYZ03 - II/IIA version model for Block II and IIA satellites

                                 The JPL TJPLXYZ03 - IIR version model for Block IIR satellites

                                 The Lockheed-Martin Lookup Table - for Block IIF satellites

Kinematic

 

 

Luni-solar and planetary precession (IAU Resolutions, as above)

 

Nutation (IAU Resolutions, as above)

 

Earth rotation (IAU Resolutions, as above)

 

Polar Motion (using NGA initial values generated the week before the orbit fit) + diurnal and semi-diurnal effects

 

UT1-UTC (using NGA initial values generated the week before the orbit fit) + Zonal tide effects + diurnal and semi-diurnal effects

 

Celestial-to-Terrestrial Reference Translation

Orbit Estimation Method

Kalman Filter/RTS Smoother (Square Root Information implementation)

Initial conditions

Previous fit

Satellite state vector in element form at trajectory epoch

Semi-major axis

Eccentricity * sin(argument of perigee)

Eccentricity * cos(argument of perigee)

Inclination

Mean anomaly + argument of perigee

Right ascension of the ascending node

Satellite clock parameters

Time and frequency offsets

Monitor station clock parameters (excluding master station)

Time and frequency offsets

Polar motion parameters

Pole position and pole rate along Greenwich meridian

Pole position and pole rate along 90o west meridian

Rate of change and acceleration of UT1-UTC

Satellite radiation pressure parameters

Radiation pressure scale, and Y-axis acceleration

Tropospheric refraction

One stochastic zenith delay parameter per station

 

Model uncertainties and Process Noise

Minimum range observation (IGS Stations)

100 cm

Minimum range observation (AF Station 85130)

80 cm

Minimum range observation (Other AF and NGA stations)

40 cm

Minimum range difference observation

1.5 cm

Tropospheric refraction variance rate

2.89 cm2/hr

Radiation pressure (each satellite)

Decorrelation time   14,400 s

Steady state scale 0.05

Y-AXIS  5.0 X 10-13 km/s2

Station clock white noise spectral density: (each station)

Time offset                              1.111 X 10-3 ms2/s

Frequency offset                    1.111 X 10-9 ppm2/s

Satellite clock white noise spectral density: (each satellite)

Time offset                              1.111 X 10-3 ms2/s

Frequency offset                    1.111 X 10-9 ppm2/s

Frequency drift                       0.              (ppm/s)**2/s

 

Satellite Clock Estimation Method

Kalman Filter/RTS Smoother (Square Root Information implementation)

Orbit solutions from above method are held fixed for satellite clock estimation

 

Solution parameters

Satellite clock parameters

Time and Frequency offset

Monitor station clock parameters (excluding master station)

Time and Frequency offset

Tropospheric refraction

One stochastic zenith delay parameter per station

 

Solution uncertainties

Minimum range observation (IGS Stations)

100 cm

Minimum range observation (AF Station 85130)

80 cm

Minimum range observation (Other AF and NGA stations)

40 cm

Minimum range difference observation

15.0 cm

Process noise in Kalman Filter

 

Tropospheric refraction variance rate

2.89 cm2/hr

 

Station clock white noise spectral densities

NGA stations (except USNO) and Air Force stations (except Colorado Springs)

Time offset

1.944 X 10-9  ms2/s

 

Frequency offset

4.444 X 10-20 ppm2/s

NGA station at USNO and Air Force station at Colorado Springs

Time offset

1.380 X 10-10  ms2/s

 

Frequency offset

4.444 X 10-20 ppm2/s

IGS stations

Time offset

3.456 X 10-9  ms2/s

 

Frequency offset

4.444 X 10-20 ppm2/s

                         

Satellite clock white noise spectral densities:

Satellite Block IIR/IIF Rubidium clocks

Time offset

8.640 X 10-10  ms2/s

 

Frequency offset

1.110 X 10-19 ppm2/s

 

Frequency drift

0.0 (ppm/s)2/s

Satellite Block II/IIA Rubidium clocks

Time offset

1.944 X 10-9  ms2/s

 

Frequency offset

1.110 X 10-19 ppm2/s

 

Frequency drift

0.0 (ppm/s)2/s

Satellite Cesium clocks

Time offset

1.3824 X 10-8  ms2/s

 

Frequency offset

1.000 X 10-18 ppm2/s

 

Frequency drift

0.0 (ppm/s)2/s

Satellite ‘Nosiy’ Cesium clocks

Time offset

2.000 X 10-8  ms2/s

 

Frequency offset

1.110 X 10-17 ppm2/s

 

Frequency drift

0.0 (ppm/s)2/s

 

Reference:  Swift, E., Mathematical Description of the GPS Multi-Satellite Filter/Smoother, NSWCDD Report (Oct. 2001).

 

Effective date April 30, 2016