1

Satellite observation systems and reference systems (ae4-e01)

Applications

E. Schrama

2

ContentsPreprocessing of observations

- example 1: dual frequency ionospheric effect

- example 2: tropospheric range delay effect

- example 3: normal point compression technique

Global Positioning System - Precise point positioning services

- Detection of plate tectonics

- Estimation of wet tropospheric delay

International Earth Rotation Service (IERS)- Earth rotation parameters + LOD

- Interpretation of these Earth rotation variables (AAM)

Satellite altimetry- Technique, Role of POD, Results

Gravity missions- GRACE, GOCE and CHAMP

3

Satellite laser ranging

4

VLBI

5

GPS

6

Preprocessing of observations

• Oftentimes raw observations are NOT suitable for direct

application in parameter estimation algorithms

• Raw observations typically contain non Gaussian errors

like outliers greater than 3 sigma

• Often there are very good reasons to inspect and clean up

the data before you put it into an estimation procedure

• This topic is much depending on the observation

technique, we will just show some well known examples.

7

Preprocessing example 1

The problem is: how do you eliminate the ionospheric delay from dual frequency range data?

22

21

212

22

1

22

212

10

222

211

)( and

)(

)(

ffN

ff

ffr

fNr

fNr

o

o

8

Preprocessing example 2

The air pressure is 1000 mbar, the air temperature is 20 degrees centigrade, the relative humidity is 50%, what is the dry+wet tropospheric delay of a radio signal as a function of the elevation angle for a station at MSL and 50 degrees latitude. The answer is:

• Use the Hopfield model (see Seeber p 45 - 49) to calculate the refractive index

• Use the integral over (n-1) ds to compute the path delay

• For the latter integral various mapping functions exist

9

Dry tropospheric delay example

0 10 20 30 40 50 60 70 80 900

5

10

15

20

25

Elevation angle

Dry

tro

posp

heric

del

ay

Modified Hopfield model This result is entirely depending on the air pressure P, 1% air pressure change (=10 mbar) gives 1% range change. Since air pressure is usually known to within a mbar the dry tropospheric delay error is small. For low elevation angles the delay error increases due to the mapping function uncertainties. Hence elevation cut-off angles are used (typically 10 degrees).

m

a

10

Wet tropospheric delay example

0 10 20 30 40 50 60 70 80 900

0.2

0.4

0.6

0.8

1

1.2

1.4

Elevation angle

Wet

tro

posp

heric

del

ay

Modified Hopfield model The wet tropospheric range depends on the relative humidity which varies more rapidly in time and place compared to air pressure. Variations of the order of 50% are possible. As a result the vertical path delay can vary between 5 and 30 cm. The alternative is the use of a multifrequency radiometer system, see Seeber p 49.

m

a

11

Normal point compression

0 200 400 600 800 1000 1200-0.5

0

0.5

1

1.5

2

2.5

3x 10

4

Method: Use a compression technique (splines, polynomials, etc) that fit the crosses. Evaluation of the model results in the compression points (the circles). This procedure filters out the noise. Horizontal: time, vertical: range

Case: red crosses is SLR data, there are too many of them and there are clear blunders that we don’t accept in the parameter estimation procedure.

12

GPS: precise point positioning

• Concept of differencing– Single differencing– Double differencing– Triple differencing

• Software– Bernse software– GIPSY JPL– Other software

13

Concept of differencing• In the GPS system, many observations are made at the “same” time by

difference receivers.

• All receivers collect pseudo range data, carrier phase data and navigation messages

• The Pseudo range navigation allows you to get a approximate solution for receiver coordinates (approx 3 m)

• More importantly is that the pseudo range navigation solution allows to synchronize all receiver clocks to the (approx 10 nano seconds, nsec).

• The pseudo-range solution requires orbit information

• The dual frequency concept results in ionospheric free ranges and carrier phase estimates

• From this point on we start to work with “differencing techniques”,

14

Broadcast Ephemeris GPS

15

Broadcast ephemeris GPS (2)

16

Single differences

SAT(1) SAT(2)

RCV(a)

r1a r2

a

Single Difference = r1a - r2

a

17

Double differences

SAT(1) SAT(2)

RCV(a)

r1a

r2a

Double Difference = (r1a - r2

a) - (r1b-r2

b)

r2b

r1b RCV(b)

18

Difference data processing

• Single differences (as shown two sheets before this one) are insensitive to receiver clock errors

• Double differences are insensitieve to all receiver and satellite clock errors

• Triple differences (= differences of double differences at consequetive epochs) reveal jumps in carrier phase data.

• Differencing techniques as described above result in observation equations that allow one to solve for coordinate delta’s (improvements)

• Available software to do this: GIPSY (JPL) + Bernese SW

19

GPS to observe deformation around a vulcano on Hawaii

Ref: http://www.unavco.org/research_science/science_highlights/kilauea/kilauea.html

20

Plate Tectonics

Source: Unavco Brochure

21

SE Asia deformations due to 26/12/04 Earthquake

22

GPS: Wet troposphere (cm)

http://www.gst.ucar.edu/gpsrg/realtime.html

23

Ionosphere from GPS (TEC)

http://www.gst.ucar.edu/gpsrg/realtime.html

24

Polar motion

• Lectures on reference systems explained what it is (Your vocabulary contains : precession, nutation, polar motion)

• Typically observed by all space techniques• It is observable because of a differences between

reference systems • Satellite and quasars “live” in an inertial system• We stand with both feet on the ground

25

IERS Earth rotation parameters

26

X-pole solution

27

Y-pole solution

28

IERS: Length of day variations

The atmosphere (left) and the ocean tides (right) correlate with space geodetic observations of the length of day (LOD) source: NASA

29

Satellite Altimetry

By means of a nadir looking radar we measure the reflection of short pulse in the footprint. This footprint is about 4 to 8 kilometer in diameter.

Source: JPL

30

Pulse reflection

time

power

time

power

Sent

Received

31

Radar footprint simulation

32

Significant wave height (JPL)

33

Scalar wind speed (JPL)

34

Ionospheric delay (JPL)

35

Radiometric water vapor (JPL)

36

Technical evolution• SKYLAB 1972 NASA 20 m

• GEOS-3 1975-1978 NASA 3 m

• SEASAT 1978 NASA 2 m

• GEOSAT 1985-1990 US Navy 30 cm

• ERS-1 1991-1996 ESA 4-10 cm

• ERS-2 1995- ESA 4 cm

• T/P 1992- NASA/CNES 2 - 3 cm

• GFO 2000- US Navy

• JASON 2001- NASA/CNES 2 - 3 cm

• ENVISAT 2002- ESA

37

Geosat (1985-1990)

ERS-1 1991-1996ERS-2

1995-

Recent and operational systems

Topex/Poseidon 1992 -

38

Doris tracking network

Source: CNES

39

ERS-1/2 tracking + cal/val

Source: DEOS

40

122

T/P sampling

121

120

119

41

Topex/Poseidon groundtrack

42

Mesoscale Variability

43

Gulf stream (altimeter)

44

Thermal image Gulf stream

45

Permanent currents

46

Schematic overview ocean currents

47

Ship observations (1)

48

To show how difficult it sometimes is at sea (2)

49

More Detail in Gulf Steam

50

Four Seasons from Altimetry

51

El Niño Southern Oscillation

52

53

Speed Kelvin/Rossby waves

54

Kelvin and Rossby wavesEquator: 2.8 m/s 20 N: 8.5 cm/s

55

Pacific decadal oscillation

1977-1999 Since 1999

56

Examples of ocean tides

This shows a 7 meter tidal height difference in Brittany France (Pentrez Plage)

57

M2 tide observed by altimeter

58

Tides in the South China Sea

M2 wave

59

K1 tidal component (23h 56m)

60

Tide constants along the shores

61

Tidal energy dissipation

-3 0 -2 0 -10 0 1 0 2 0 3 0 m W / m2

R R a y, G S F C

62

Gravity from satellite altimetry

63

January 98 August 98

64

Quickscat

You can also observe wind speed AND direction from space with a so-called scatterometer. (A different instrument that looks and works much like a radar altimeter.)

65

Tutorial quickscat

under the radarSide lobes

66

Global windfield patterns

67

Extreme wind conditions (Hurricane DORA)

68

ICE/wind

69

Decade of the Geopotentials

• CHAMP: – Single satellite with accelerometers (why?) and

a space-borne version of GPS (2000->)

• GRACE: – Two CHAMP flying after one another (2002->)

• GOCE: – Four “champs” inside a satellite (2007?)

70

Geoid (=ocean surface without currents)Geoid (=ocean surface without currents)

71

Gravity field of the MoonGravity field of the Moon

72

CHAMP 1

73

CHAMP 2

74

CHAMP launch

75

CHAMP 4

76

CHAMP 5

77

78

79

Principe GRACE

80

GOCE gradiometry mission

81

Concept Gradiometer

Proofmass

Spring

82

Gravity field improvement

0 50 100 150 200 250 30010

-5

10-4

10-3

10-2

10-1

100

degree l

met

ercumulative geoid error