Space Weather influence on satellite based navigation and precise positioning

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Space Weather influence on

satellite based navigation and

precise positioning

R. Warnant, S. Lejeune, M. Bavier

R. Warnant, S. Lejeune, M. Bavier Royal Observatory of Belgium

Royal Observatory of Belgium

Avenue Circulaire, 3

B-1180 Brussels (Belgium)

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Summary

 _{What this talk is NOT :}_{What this talk is NOT :}

– Detailled statistics about all possible ionospheric (SW) effects on all _{Detailled statistics about all possible ionospheric (SW) effects on all}

possible GNSS applications

 Effect of the ionosphere on 2 differential applications : _{Effect of the ionosphere on 2 differential applications :}

DGPS and Real Time Kinematic

– Illustrate the fact that the type of ionospheric phenomena which play _{Illustrate the fact that the type of ionospheric phenomena which play}

a role in the error budget depends very much on the application

 There is no simple relationship between a given ionospheric _{There is no simple relationship between a given ionospheric}

activity (TEC) and the positioning error.

– A given ionospheric activity (TEC) will not always result in the same _{A given ionospheric activity (TEC) will not always result in the same}

positioning error

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From navigation to high accuracy geodesy (1/2)

 GNSS signals are used in the frame of many different _{GNSS signals are used in the frame of many different}

positioning techniques :

– Abolute or differential_{Abolute or differential}

– Code and/or carrier phase measurements_{Code and/or carrier phase measurements}

– Real time or post-processing_{Real time or post-processing}

– The accuracy ranges from a few mm (high accuracy geodesy) to a _{The accuracy ranges from a few mm (high accuracy geodesy) to a}

few m (navigation).

 _{The effect of the ionosphere (SW) on GNSS signal}_{The effect of the ionosphere (SW) on GNSS signal}

propagation remains one of the main error sources for most

of the positioning techniques

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From navigation to high accuracy geodesy (2/2)

 The data processing algorithms strongly depend on the _{The data processing algorithms strongly depend on the}

positioning technique used.



 « residual » ionosphere effect which affects the position « residual » ionosphere effect which affects the position depends on the technique used

depends on the technique used

 Four categories of applications :_{Four categories of applications :}

– Absolute navigation (5 – 20 m)_{Absolute navigation (5 – 20 m)}

– Differential navigation (DGPS, 1-5 m)_{Differential navigation (DGPS, 1-5 m)}

– Field geodesy (Real Time Kinematic or RTK, few cm)_{Field geodesy (Real Time Kinematic or RTK, few cm)}

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SW and Absolute Positioning

 The ionosphere is the _{The ionosphere is the}

origin of a delay in GNSS

radio signal propagation

 This effect of this delay on _{This effect of this delay on}

(apparent) signal path, I

(m) : (m) : 2 40.3 cos( ) IP IP VTEC I f Z 

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SW and differential positioning (1/2)

 Combines the measurements made by a minimum of 2 _{Combines the measurements made by a minimum of 2}

receivers (stations) to remove common error sources

 _{Based on the assumption that measurements made by 2}_{Based on the assumption that measurements made by 2}

«neighbour» receivers are affected in the same way by the

different error sources (in particular ionospheric errors)

 The ionosphere residual effect on differential positioning :_{The ionosphere residual effect on differential positioning :}

1 2 12 ₂ 1 2 40.3 ( ) cos( ) cos( ) IP IP IP IP VTEC VTEC I f Z Z  

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SW and differential positioning (2/2)

 In other words, the residual ionospheric error on one _{In other words, the residual ionospheric error on one}

individual receiver-to-satellite path depends on :

– the vertical TEC ;_{the vertical TEC ;}

– the vertical TEC difference (gradient) between the 2 stations (IP);_{the vertical TEC difference (gradient) between the 2 stations (IP);}

– the GPS constellation geometry._{the GPS constellation geometry.}

 _{The error on the position depends on how the individual}_{The error on the position depends on how the individual}

ionospheric residual errors will “combine” in the data

processing algorithm (least squares) which uses all

satellites in view satellites in view 12 2 1 1 2 1 2 2 40.3 1 1 1 ( ( ) ( ))

cos( ) cos( ) cos( )

IP IP IP

I VTEC VTEC VTEC

f Z Z Z

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SW and DGPS (1/2)

 DGPS (= Differential GPS) allows to measure positions in real _{DGPS (= Differential GPS) allows to measure positions in real}

time with an accuracy of a few meters

 _{Based on (ranging) code measurements}_{Based on (ranging) code measurements}

 Uses the corrections broadcast by a reference station_{Uses the corrections broadcast by a reference station}

 The accuracy depends mainly on the distance between the _{The accuracy depends mainly on the distance between the}

observer and the reference station

 _{Distances up to 1000 km (w.r.t. the reference station) can be}_{Distances up to 1000 km (w.r.t. the reference station) can be}

considered

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SW and DGPS (2/2)

 _{Provided the distances (up to 1000 km) and the accuracy (a few}_{Provided the distances (up to 1000 km) and the accuracy (a few}

meters), only (strong) large scale gradients in TEC will have an

influence

 At mid-latitudes, large scale gradients having a potential _{At mid-latitudes, large scale gradients having a potential}

influence on DGPS error budget are mainly observed at solar

maximum or during geomagnetic storms

 _{BUT NOT ONLY these gradients influence the final position}_{BUT NOT ONLY these gradients influence the final position}



 TEC (gradient) maps can only give a rough idea of the error on the final TEC (gradient) maps can only give a rough idea of the error on the final

user position

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- 2 5 - 2 0 - 1 5 - 1 0 - 5 0 5 1 0 1 5 2 0 2 5 3 0 L o n g i t u d e 3 5 4 0 4 5 5 0 5 5 6 0 6 5 7 0 L a tit u d e

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0 5 1 0 1 5 2 0 2 5 T i m e 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 TE C p re di ct ed b y K lo bu ch ar (T E C U ) S l a n t T E C b y s a t e l l i t e p r e d i c t e d b y t h e K l o b u c h a r m o d e l P O T S D A M d a y : 0 0 1 y e a r : 2 0 0 3 T E C b y s a t e l l i t e - 0 . 2 - 0 . 1 0 0 . 1 0 . 2 0 . 3 R es id ua l E rro r b y sa te lli te (m et er s) R I N E X S I M U L A T E D D A T A ( i o n o K l o b u c h a r ) R e s i d u a l e r r o r B Y S A T E L L I T E P O T S D A M L a t i t u d e d a y : 0 0 1 y e a r : 2 0 0 3 D G P S r e f e r e n c e : B R U S S E L w i t h d g p s c o r r e c t i o n 0 5 1 0 1 5 2 0 2 5 T i m e - 1 . 2 - 0 . 8 - 0 . 4 0 0 . 4 0 . 8 R e si d u a l E rr o r (m e te rs ) R I N E X S I M U L A T E D D A T A ( i o n o K l o b u c h a r ) R e s i d u a l e r r o r P O T S D A M L a t i t u d e d a y : 0 0 1 y e a r : 2 0 0 3 D G P S r e f e r e n c e : B R U S S E L w i t h d g p s c o r r e c t i o n s

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SW and Real Time Kinematic (1/2)



RTK (Real Time Kinematic) allows to measure a

_{RTK (Real Time Kinematic) allows to measure a}

mobile user position in real time with an accuracy of a

few centimetres



Based on carrier beat phase measurements (which are

_{Based on carrier beat phase measurements (which are}

ambiguous)



Mobile receiver uses the measurements (and the

_{Mobile receiver uses the measurements (and the}

corrections) broadcast by a reference station



The accuracy depends on the distance between the

_{The accuracy depends on the distance between the}

observer and the reference station

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SW and Real Time Kinematic (2/2)

 Distances up to 10-20 km can be considered (mainly depending _{Distances up to 10-20 km can be considered (mainly depending}

on ionospheric conditions)

 _{Ambiguities are solved during a (static) initialization procedure}_{Ambiguities are solved during a (static) initialization procedure}

(search technique is based on the stochastic model)

 Usually : assumption that there is no residual (ionospheric) error _{Usually : assumption that there is no residual (ionospheric) error}

in the differenced observations

 _{Provided the distances (up to 20 km) and the accuracy (a few}_{Provided the distances (up to 20 km) and the accuracy (a few}

centimetres), small scale gradients in TEC will have an

influence on the error budget

 At mid-latitude, small scale gradients in TEC are mainly due to _{At mid-latitude, small scale gradients in TEC are mainly due to}

TID’s and “ionospheric noise”

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Small-scale structures

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Small scale structure climatology

 Frequency of occurrence depends on solar activity._{Frequency of occurrence depends on solar activity.}

 Are strongly related to _{Are strongly related to}Space Weather_{Space Weather} and, in particular, to _{and, in particular, to}

geomagnetic storms (Kp index).

 _{Severe geomagnetic (ionospheric) storms are the origin of}_{Severe geomagnetic (ionospheric) storms are the origin of}

strong « noise-like » TEC variations which severely degrade

real-time solutions (ambiguity resolution).

 BUT : there are also strong TID’s during so-called quiet _{BUT : there are also strong TID’s during so-called quiet}

ionosphere activity periods which can give strong residual

errors even on short distances (4 km baselines)

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0 . 0 0 4 . 0 0 8 . 0 0 1 2 . 0 0 1 6 . 0 0 2 0 . 0 0 2 4 . 0 0 U T T i m e ( h o u r s ) - 4 . 0 0 - 2 . 0 0 0 . 0 0 2 . 0 0 T E C v a ria tio n w ith ti m e (T E C U /m in )

Geomagnetic storms and TEC variability

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1 1 . 2 1 1 . 6 1 2 1 2 . 4 1 2 . 8 1 3 . 2 1 3 . 6 T e m p s 2 4 9 1 1 2 2 4 9 1 1 2 . 4 2 4 9 1 1 2 . 8 2 4 9 1 1 3 . 2 2 4 9 1 1 3 . 6 2 4 9 1 1 4 D is ta n ce ( cy cl e s) D o u b l e D i f f é r e n c e ( A m b i g u i t e s ) s a t 1 7 - 2 1 S A N S S A U T S B R U S - G I L L J O U R : 3 5 9 A N N E E : 2 0 0 4 d d _ L 4 1 1 . 2 1 1 . 6 1 2 1 2 . 4 1 2 . 8 1 3 . 2 1 3 . 6 - 5 6 8 7 3 4 . 8 - 5 6 8 7 3 4 . 4 - 5 6 8 7 3 4 - 5 6 8 7 3 3 . 6 - 5 6 8 7 3 3 . 2 d d _ L 1 - ( f / c ) * d i s t 2 4 6 8 T e m p s - 5 3 5 6 6 . 8 - 5 3 5 6 6 . 4 - 5 3 5 6 6 - 5 3 5 6 5 . 6 - 5 3 5 6 5 . 2 D is ta n ce ( cy cl e s) D o u b l e D i f f é r e n c e ( A m b i g u i t e s ) s a t 2 9 - 2 6 S A N S S A U T S B R U S - G I L L J O U R : 3 5 9 A N N E E : 2 0 0 4 d d _ L 4 2 4 6 8 1 0 6 4 1 5 7 . 2 1 0 6 4 1 5 7 . 6 1 0 6 4 1 5 8 1 0 6 4 1 5 8 . 4 1 0 6 4 1 5 8 . 8 d d _ L 1 - ( f / c ) * d i s t

TID’s and ambiguity resolution (1/2)

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TID’s and ambiguity resolution (2/2)



Rather difficult to predict how such TID’s will affect

_{Rather difficult to predict how such TID’s will affect}

ambiguity resolution process



As 1 cycle is about 20 cm, uncorrectly solved

_{As 1 cycle is about 20 cm, uncorrectly solved}

ambiguities can result in errors of several decimeters

(even during «quiet » ionospheric activity)



(Strong) TID’s are observed even at solar minimum

_{(Strong) TID’s are observed even at solar minimum}

and they have an annual peak during winter time

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Conclusions



Ionospheric phenomena which have to be taken into

_{Ionospheric phenomena which have to be taken into}

account in the mitigation techniques depend very much

on the application (large scale/small scale TEC)



TEC (and all his « drivers » - from SW activity) is the

_{TEC (and all his « drivers » - from SW activity) is the}

main parameter but satellite geometry is also very

important



_

??? TEC maps ?????



No simple relationship between ionospheric activity

_{No simple relationship between ionospheric activity}

and the positioning error (geometry, ambiguity search,

least square process)



A given ionospheric activity will NOT ALWAYS result in A given ionospheric activity will NOT ALWAYS result in the same positioning error