assessing the compatibility of microwave geodetic …tah/herr_g21a_06.pdf6 05/28/02 agu spring 2002...

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1 05/28/02 AGU Spring 2002 1 Assessing the Compatibility of Microwave Geodetic Systems Prof. Thomas Herring Room 54-611; 253-5941 [email protected] http://bowie.mit.edu/~tah 05/28/02 AGU Spring 2002 2 Overview • Examine the compatibility of VLBI and GPS systems by comparing clock and atmospheric delay estimates from the two systems • Data Analyzed: – End of the CONT96 VLBI experiments – Common stations: Fairbanks, Wettzell, Kokee, Nyales20. No GPS at NRAO station. – Additional 20 global stations used in GPS analysis

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Page 1: Assessing the Compatibility of Microwave Geodetic …tah/Herr_G21A_06.pdf6 05/28/02 AGU Spring 2002 11 Wettzell Atmosphere 0.000 0.050 0.100 0.150 0.200 0.250 260 265 270 275 280 Wettzell

1

05/28/02 AGU Spring 2002 1

Assessing the Compatibilityof Microwave Geodetic

SystemsProf. Thomas Herring

Room 54-611; [email protected]

http://bowie.mit.edu/~tah

05/28/02 AGU Spring 2002 2

Overview

• Examine the compatibility of VLBI and GPSsystems by comparing clock and atmosphericdelay estimates from the two systems

• Data Analyzed:– End of the CONT96 VLBI experiments– Common stations: Fairbanks, Wettzell, Kokee,

Nyales20. No GPS at NRAO station.– Additional 20 global stations used in GPS analysis

Page 2: Assessing the Compatibility of Microwave Geodetic …tah/Herr_G21A_06.pdf6 05/28/02 AGU Spring 2002 11 Wettzell Atmosphere 0.000 0.050 0.100 0.150 0.200 0.250 260 265 270 275 280 Wettzell

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05/28/02 AGU Spring 2002 3

VLBI and GPS Network

05/28/02 AGU Spring 2002 4

Analysis Technique

• VLBI: CALC+SOLVK Kalman filter– Clock process noise consistent with 10-14 Alan

standard deviation at 1000 seconds; Quadraticterms removed apriori.

– Atmospheric process noise: Derived from delay-ratevariance: Typical random walk values 10mm/sqrt(hr); variations of factor of three about thisvalue

– Constant zenith delay based on station height andzero water vapor

Page 3: Assessing the Compatibility of Microwave Geodetic …tah/Herr_G21A_06.pdf6 05/28/02 AGU Spring 2002 11 Wettzell Atmosphere 0.000 0.050 0.100 0.150 0.200 0.250 260 265 270 275 280 Wettzell

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05/28/02 AGU Spring 2002 5

Analysis Technique

• GPS: GAMIT program– Clocks estimated epoch-by-epoch relative to

Fairbanks/Gilcreek clock– Phase clocks aligned with clocks from pseudo-

range estimates– Atmospheric delay estimated with piece-wise linear

function with nodes every 2-hours. Stochasticnoise 20 mm/sqrt(hr)

– Constant apriori zenith delay.

05/28/02 AGU Spring 2002 6

Some effects that might be expected:Clock effects

• GPS is controlled by 10.23 MHz oscillators• On the Earth’s surface these oscillators are set to

10.23x(1-4.4647x10-10) MHz (39,000 ns/day ratedifference)

• This offset accounts for the change in potential andaverage velocity once the satellite is launched.

• The first GPS satellites had a switch to turn this effecton. They were launched with “Newtonian” clocks

Page 4: Assessing the Compatibility of Microwave Geodetic …tah/Herr_G21A_06.pdf6 05/28/02 AGU Spring 2002 11 Wettzell Atmosphere 0.000 0.050 0.100 0.150 0.200 0.250 260 265 270 275 280 Wettzell

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05/28/02 AGU Spring 2002 7

Corrections terms

• Propagation path curvature due to Earth’s potential (a fewcentimeters)

• Clock effects due to changing potential

• For e=0.02 effect is 47 ns (14 m)

Dt =2GM

c3 ln Rr + Rs + rRr + Rs - r

Ê

Ë Á

ˆ

¯ ˜

Dt =- GM

c2 e a sin E

05/28/02 AGU Spring 2002 8

Relativistic Effects (SA off)

-50

-25

0

25

50

0 4 8 12 16 20 24

PRN 03 Detrended; e=0.02Clock - trend (ns)GR Effect (ns)

Cloc

k er

ror (

ns)

Time (hrs)

50 ns=15m

Page 5: Assessing the Compatibility of Microwave Geodetic …tah/Herr_G21A_06.pdf6 05/28/02 AGU Spring 2002 11 Wettzell Atmosphere 0.000 0.050 0.100 0.150 0.200 0.250 260 265 270 275 280 Wettzell

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05/28/02 AGU Spring 2002 9

Comparisons

• Atmospheric delays:– Easiest comparison because apriori zenith delay

chosen as constant and the same apriori used forVLBI and GPS

• Clocks:– Clocks are more difficult to compare because:

• No absolute standard for either system (show clocksrelative to Fairbanks/Gilcreek)

• VLBI has different quadratic removed each day. Clocksrelative to ensemble

• GPS also referred to Fairbanks. Only linear trend removedeach day.

05/28/02 AGU Spring 2002 10

Fairbanks Atmospheric delays

-0.100

-0.050

0.000

0.050

0.100

0.150

260 265 270 275 280

Fairb

anks

Atm

osph

ere

(m)

1996 Day of year

Page 6: Assessing the Compatibility of Microwave Geodetic …tah/Herr_G21A_06.pdf6 05/28/02 AGU Spring 2002 11 Wettzell Atmosphere 0.000 0.050 0.100 0.150 0.200 0.250 260 265 270 275 280 Wettzell

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05/28/02 AGU Spring 2002 11

Wettzell Atmosphere

0.000

0.050

0.100

0.150

0.200

0.250

260 265 270 275 280

Wet

tzel

l Atm

osph

ere

(m)

1996 Day of year

05/28/02 AGU Spring 2002 12

Zoom of Wettzell (Sep 19-20)

0.040

0.060

0.080

0.100

0.120

0.140

0.160

0.180

263 264 264 264 265 266 266

Wet

tzel

l Atm

osph

ere

(m)

1996 Day of year

GPS Day boundaries

Page 7: Assessing the Compatibility of Microwave Geodetic …tah/Herr_G21A_06.pdf6 05/28/02 AGU Spring 2002 11 Wettzell Atmosphere 0.000 0.050 0.100 0.150 0.200 0.250 260 265 270 275 280 Wettzell

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05/28/02 AGU Spring 2002 13

Zoom Wettzell (Sept 30-Oct 2)

0.040

0.060

0.080

0.100

0.120

0.140

0.160

0.180

274 275 276 277 278 279

Wet

tzel

l Atm

osph

ere

(m)

1996 Day of year

GPS Day boundaries

Notice ~2cmdeviations

05/28/02 AGU Spring 2002 14

KOKEE Atmosphere

0.050

0.100

0.150

0.200

0.250

0.300

260 265 270 275 280

Koke

e At

mos

pher

e (m

)

1996 Day of year

Page 8: Assessing the Compatibility of Microwave Geodetic …tah/Herr_G21A_06.pdf6 05/28/02 AGU Spring 2002 11 Wettzell Atmosphere 0.000 0.050 0.100 0.150 0.200 0.250 260 265 270 275 280 Wettzell

8

05/28/02 AGU Spring 2002 15

NyAlison Atmosphere

0.000

0.050

0.100

0.150

0.200

0.250

260 265 270 275 280

NyAl

ison

atm

osph

ere

(m)

1996 Day of Year

05/28/02 AGU Spring 2002 16

Zoom of NyAlison (Sep 30-Oct 2)

0.000

0.020

0.040

0.060

0.080

0.100

274 275 276 277 278 279 280

NyAl

ison

atm

osph

ere

(m)

1996 Day of Year

Page 9: Assessing the Compatibility of Microwave Geodetic …tah/Herr_G21A_06.pdf6 05/28/02 AGU Spring 2002 11 Wettzell Atmosphere 0.000 0.050 0.100 0.150 0.200 0.250 260 265 270 275 280 Wettzell

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05/28/02 AGU Spring 2002 17

Clock comparisons

• Clock comparison are “complicated” by theremoval of trends from VLBI results (can becorrected but currently time-consuming).

• Absolute offsets can not be comparedbecause of VLBI group delay ambiguities; theGPS alignment of phase and range; unknowndelays in VLBI and GPS electronics

• Examine difference of Wettzell and Fairbanksclocks

05/28/02 AGU Spring 2002 18

Wettzell-Fairbanks comparison

-0.60

-0.40

-0.20

0.00

0.20

0.40

264 266 268 270 272 274 276 278

Cloc

k of

fset

(m)

1996 Day of year

Green GPS; Brown VLBI

"Quadratic" in VLBI probably due to apriori quadratic in VLBI analysis

Linear trends removed each GPS day;Linear and quadratic (apriori) from VLBI each day

Page 10: Assessing the Compatibility of Microwave Geodetic …tah/Herr_G21A_06.pdf6 05/28/02 AGU Spring 2002 11 Wettzell Atmosphere 0.000 0.050 0.100 0.150 0.200 0.250 260 265 270 275 280 Wettzell

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05/28/02 AGU Spring 2002 19

Wettzell-Fairbanks with VLBI quadraticremoved

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

274 275 276 277 278 279

Wet

tzell-F

airba

nks c

lock (

m)

1996 Day of year

Likely phase connectionproblem in GPS

Zoom next figure

05/28/02 AGU Spring 2002 20

Zoom area for Wettzell

-0.10

-0.05

0.00

0.05

0.10

278 278 278 279 279 279

Wet

tzell-F

airba

nks c

lock (

m)

1996 Day of year

Not clear why difference in behavior;GPS shows atmospheric delay fluctuation

not seen in VLBI estimates (separation problem?)

Page 11: Assessing the Compatibility of Microwave Geodetic …tah/Herr_G21A_06.pdf6 05/28/02 AGU Spring 2002 11 Wettzell Atmosphere 0.000 0.050 0.100 0.150 0.200 0.250 260 265 270 275 280 Wettzell

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05/28/02 AGU Spring 2002 21

Conclusions• Initial study of comparison between VLBI and GPS

clock parameters and atmospheric delay estimates.• Initial results indicate that clocks are comparable

quality (no smoothing in GPS)• Next steps:

– Accounting for clock rates and accelerations applied to VLBI– More consistency in mapping functions and gradients– Effects of model changes in GPS (antenna phase centers) on

clock comparisons– Time dependent process for GPS atmospheres (validation

using VLBI data)• Possible use of GPS atmospheres and clocks in the

analysis of VLBI?