Accuracy of dual-Doppler - PowerPoint Presentation

Slide1

Accuracy of dual-Doppler lidar retrievals of near-shore winds

Nikola

Vasiljević

and Michael Courtney

WindEurope Resource Assessment Workshop 2017

Edinburgh, UK

Slide2

What a lidar measures?

2

Slide3

What a lidar measures?

Beam direction

Line-of-sight

3

Slide4

What a lidar measures?

Range gate

Range gate

center

4

Slide5

What a lidar measures?

V

r

Radial wind speed or line-of-sight (LOS) wind speed

V

wind

5

Slide6

What about two

lidars

?

V

wind

V

r1

V

r2

Two independent LOS measurements => direct retrieval of

u

and

v

6

Slide7

What about two

lidars

?

V

wind

V

r1

V

r2

From

u

and

v

horizontal wind speed

V

h

and wind direction

𝚯

can be derived:

7

Slide8

Single

lidar

measurement accuracy

How accurately can we determine radial wind speed?

How accurately can we determine where we are probing the atmosphere?

(1

)

LOS speed retrieval

accuracy

(2

) Pointing accuracy

(3

) Ranging accuracy

8

We aim at assessing (1),(2) and (3

) individually!

Slide9

How to assess LOS retrieval accuracy

Lidar LOS measurements vs. Mast measurements

V

ane

Cup

Mast

LOS calibration at 5 km range (see [1])

9

Slide10

How to assess pointing accuracy

Map hard targets

Acquire reference positions of

lidar

and targets

Compare mapped positions with reference positions

More details in [2,3]

10

Slide11

How to assess pointing accuracy

Map hard targets

Acquire reference positions of

lidar

and targets

Compare mapped positions with reference positions

More details

in [2,3]

11

Slide12

How to assess pointing accuracy

Elevation error

Azimuth

Map hard targets

Acquire reference positions of lidar and targets

Compare mapped positions with reference positions

12

Slide13

How to assess pointing accuracy

Azimuth error

Azimuth

Map hard targets

Acquire reference positions of lidar and targets

Compare mapped positions with reference positions

13

Slide14

How to assess ranging accuracy

Hit a hard target with laser light and find distance at which CNR is maximum

Compare this distance with calculated one

CNR max at 957 m, whereas the calculated distance was 947 m

14

Slide15

Accuracy of single-Doppler retrievals

V

r

Assuming:

- Uniform wind field

- Wind gradient with height follows power law:

R

15

Slide16

Accuracy of single-Doppler

retrievals

V

r

16

Slide17

Accuracy of dual-Doppler retrievals

V

wind

V

r1

V

r2

Accuracy of retrieved

V

h

and

𝚯

depends on

U

Vr1

and

U

Vr2

:

17

Slide18

Partial

derivatives for

U

Vh

V

r1

V

r2

18

Slide19

Partial

derivatives for U

𝚯

V

r1

V

r2

19

Slide20

RUNE dual-Doppler setup

Three scan lines (50, 100 and 150 m

asl

)

45 measurement points distributed along each line

20

More

details on RUNE

see

[4]

Slide21

RUNE dual-Doppler setup

21

Slide22

Intersecting and elevation angles at 100 m

asl

22

Slide23

Accuracy simulation for RUNE setup

Uniform horizontal wind speed

V

h

= 10 m/s

Vertical wind speed

w

= 0 m/s

Wind speed gradient with height assumed to follow power law

Shear

exponent:

-

𝛼 =

0.1 (open water)

Wind direction:

-

𝚯

= 270˚

-

𝚯 =

180˚

Trajectory 100 m line

Accuracy:

- U

LOS

= 0.1

m/s

- U

𝚹

=U𝞿= 0.05˚- UR = 1 m23

Slide24

H=100 m, 𝛼

=

0.1

,

V

h

=10 m/s

Wind

Wind

24

Not

to trust

Slide25

Accuracy map

25

Slide26

Accuracy map

26

Slide27

Accuracy map for 𝚯 = 270˚ 27

Wind

Infinity

Slide28

Accuracy map for 𝚯 = 180˚ 28

Wind

Infinity

Slide29

ConclusionsWe derived a very simple and effective accuracy model- we look at biases since they give errors in wind resource assessment

Accuracy of horizontal wind speed and wind direction depends on:

- LOS accuracy- Pointing accuracy- Range accuracy

Beware of intersecting angle between two laser beams !Main accuracy drivers are:- Pointing accuracy in elevation in relation to measurement range- Intersecting angle between two laser beamsRule of thumb, keep keep your intersecting angle bigger than 30˚29

Slide30

Future workWe are preparing several publications:- Review of static errors in pointing accuracy

- Method to assess static errors

- Accuracy assessment of dual-Doppler retrievalsFor more detailed presentation on this topic come to

PhD Summer School: Remote Sensing for Wind Energy (see [5])RUNE blind comparison (contact Alfredo Diaz: aldi@dtu.dk)RUNE data will be made publically available through the MARINET II project via the e-WindLidar platform

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Slide31

ReferencesLea, G., & Courtney, M. (2016). Validation of long-range scanning lidars deployed around the

Høvsøre

Test Station. DTU Wind Energy. (DTU Wind Energy E; No. 0114).Vasiljevic

, N. (2014). A time-space synchronization of coherent Doppler scanning lidars for 3D measurements of wind fields. DTU Wind Energy. (DTU Wind Energy PhD; No. 0027(EN)). Vasiljevic, N., Lea, G., Courtney, M., Cariou, J.-P., Mann,J. & Mikkelsen, T. (2016). Long-Range WindScanner System. Remote Sensing, 8(11). DOI: 10.3390/rs8110896

Floors, R. R., Peña, A., Lea, G., Vasiljevic, N., Simon, E., & Courtney, M. (2016). The RUNE Experiment—A Database of Remote-Sensing Observations of Near-Shore Winds. Remote Sensing, 8(11). DOI: 10.3390/rs8110884

PhD Summer School: http://www.vindenergi.dtu.dk/english/Education/PhD-Summer-School

31

Slide32

Thank you!32

Nikola Vasiljević

niva@dtu.dk

https://dk.linkedin.com/in/nvasiljevichttp://www.nikolavasiljevic.com

https://www.youtube.com/user/cadenza83/videos

^ youtube videos Jakob Mann mentioned