1 Satellite observation systems and reference systems (ae4-e01) Applications E. Schrama

2 Contents Preprocessing 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?

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 ) 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 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 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 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) r1ar1a r2ar2a Single Difference = r 1 a - r 2 a

17 Double differences SAT(1)SAT(2) RCV(a) r1ar1a r2ar2a Double Difference = (r 1 a - r 2 a ) - (r 1 b -r 2 b ) r2br2b r1br1b 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:

20 Plate Tectonics Source: Unavco Brochure

21 SE Asia deformations due to 26/12/04 Earthquake

22 GPS: Wet troposphere (cm)

23 Ionosphere from GPS (TEC)

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 SKYLAB1972NASA 20 m GEOS NASA 3 m SEASAT1978NASA 2 m GEOSAT US Navy 30 cm ERS ESA 4-10 cm ERS ESA 4 cm T/P1992-NASA/CNES cm GFO US Navy JASON2001-NASA/CNES cm ENVISAT2002-ESA

37 Geosat ( ) ERS ERS Recent and operational systems Topex/Poseidon

38 Doris tracking network Source: CNES

39 ERS-1/2 tracking + cal/val Source: DEOS

T/P sampling

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 waves Equator: 2.8 m/s20 N: 8.5 cm/s

55 Pacific decadal oscillation 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

62 Gravity from satellite altimetry

63 January 98August 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 radar Side 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)

71 Gravity 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