Satellite geodesy (ge-2112)

Observation systems

E. Schrama

Contents• Techniques (how they work)

– Global positioning system

– Satellite laser ranging

– Very Long Baseline Interferometry

– Doppler techniques

– Satellite to Satellite Tracking and gradiometry

– Satellite altimetry

– SAR and Interferometric SAR

• Physical Limitations– Some general remarks

– Tropospheric signal delay

– Ionospheric delay

Global positioning system• Reference: GPS book G. Husti, chapters 3 and 4.• GPS consists of a control segment, a space segment

and a user segment• The control segment consists of a number of tracking

stations, there are 5 in total with one command center • The space segment consists of 21 satellites and three

sparse, there are different blocks.• The user segment: an “infinite” number of users that

receive the GPS radio signals

GPS signal (1)• The GPS signals are on two frequencies, L1 or

1575.42 Mhz, L2 of 1227.60 MHz.• CA-code or clear access code is transmitted on L1,

everyone can access CA code. Bandwidth: 2 MHz• P-code (or newer codes, it is secret military code)

go on L1 and L2, the bandwidth is 20 Mhz

• Data modulation (50 bits/sec) on L1 and L2 • Spread spectrum techniques are used, one a simple

radioreceiver will not hear a GPS signal

GPS signal (2)

• Inside the receiver a pseudo random noise code (PRN-code) for a particular satellite is replicated

• The correlation of the PRN code with the incoming satellite signal resuls in a phase measurement

• Each satellite emits a unique and known code

• C/A code: 1000 ns chiplength, length 1023 bits or 1 msec.

• P: 100 ns chiplength, length: 2,36 million bits or 267 days

• Accuracy C/A code is about 3 m, P code is about 0.3 m (= old situation, there are now newer receivers with better correlators and other P codes)

GPS signal (3)• The following information is contained in the GPS signal

– Pseudo range information comes from C/A or P code (these are ranges biased by a receiver and satellite clock error)

– Carrier phase information (range rate time integrated information) can be extracted, accuracy carrier phase is a few mm.

– Navigation messages contain: Clock parameters, Ephemeris, Messages, UTC/ionospheric parameters and Almanac (long term orbit prediction) information

• Receivers:– Numerous receivers are sold nowadays, depending on their design they

can extract some or all of the above listed information

– Receivers have different modes, either they measure ranges, either they look for transmitters in the PRN/Doppler domain

GPS receivers

Satellite Laser Ranging• Optical: range measurements to satellites and moon• Method: send out short laser pulse, count the time

until you receive a return• Dry tropospheric signal delay does apply• Wet tropospheric signal delay does NOT apply• There is NO Ionospheric signal delay• You need people (operators) near the instrument• Depending on local weather conditions (clouds)• World wide there are only a few tracking stations

Very Long Baseline Interferometry (VLBI)

• Receive natural signals from quasars at two or more radiotelescopes.

• Time tag the received signals with separate clocks• Cross correlate different signals• Dry and Wet tropospheric delay and ionospheric

signal delay are all relevant to this technique• You need operators near the telescopes• Somewhat dependent on weather conditions• Globally seen only a few stations

Very Long Baseline Interferometry

Correlation of noisy signalsSignal from star 1 Signal from star 2

Autocorrelation 1 Crosscorrelation 1 and 2

VLBI signal correlation

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Doppler techniques

• Observe the Doppler effect of a transmitter and a receiver in motion

• This is a relative velocity measurement • In satellite geodesy the Doppler effect is only

observed with radio techniques• As a result: dry and wet tropospheric and

ionospheric signal delay• There are automatic stations and there is a well

maintained global network (the DORIS system)

Doris tracking network

Source: CNES

How the Doppler effect works

Waves eminating from a stationary transmitter

Waves eminating from a moving transmitter

Doppler equations

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Doris in action• The Doris configuration consists of about 40 Doppler

beacons and a receiver on a low Earth orbiting satellite

• The beacons transmit at two frequencies, the dual frequency configuration allows to compensate for ionospheric signal delays.

• The satellites equipped with a Doris receiver listen to the beacons and observe the Doppler shift

• The beacons transmit additional housekeeping (clock+meteo) information

• The DORIS network is very useful for Precise Orbit Determination, real time applications even exist.

Satellite to Satellite tracking

• Method:– range or range rate between two or more satellites

– various configurations

• Goal: – easier communication to LEO’s

– more accurate orbit determination

– research on the Earth’s gravity field

– autonomous navigation systems (DIODE)

– docking and rendez vous (several spaceflight appl.)

Satellite to satellite tracking 2

• Examples– Tracking and data relay satellite system:

TDRSS– GPS on a low Earth Orbiter

• T/P demonstration experiment

• CHAMP mission

– GRACE mission

Tracking and Data Relay Satellite System

There are two TDRS satellites in a geostationary orbit. The system allows to communicate with LEOs below your horizon, you can also track a LEO in this way.

Satellite gradiometry

• Accelerometer measurement in a satellite

• Observe differential accelerations, ie. observe the gradient of the gravity vector

• The goal is gravity field research

• Satellite to Satellite Tracking for POD

• The European Space Agency (ESA) plans a gradiometer mission in 2006

GRACE GOCE

Concept Gradiometer

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Realization of SGG

• The instrument needs very sensitive and accurate accelerometers that need to be calibrated in flight

• Proposed techniques: electrostatic and cryogenic• Rotational accelerations and gravity gradients

need to be separated from one another• Trajectory of the spacecraft must be known to

within a reasonable accuracy• Self gravitation and structural stability of the

spacecraft are design drivers of the system

Satellite altimetry

• Radar altimeter in high inclination orbit (800 to 1350 km)

• Precise orbit determination to determine coordinates and clock at satellite

• Goal: – observe heigth profiles at the sea level

– make a geoid model

– make a dynamic topography model,

– chart the tides, etc etc

• An operational technique since 1985

Satellite altimetry (2)

Source: JPL

SAR and INSAR

• From space you can make photos of the Earth. This can be done in the visible light, the near infrared or near ultraviolet, or even in the radio domain

• Several satellites, and in particular ERS-1 and 2 and the Space shutlle have made SAR (synthetic aperture radar) images of the Earth.

• SAR images can see through clouds and allow a resolution of approx 3 by 3 meter.

How SAR works

Flight Direction

Illuminated patches

DD D: Doppler line

R: range line

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SAR image San Fernando Valley

What is INSAR• SAR images made on different times can be put on top of

one another, a SAR image is nothing more than a matrix with complex numbers that basically describe amplitude and phase of scatterer in the image

• SAR images that are relatively close to one another (few hunderd meter at satellite altitude) can be used to “subtract” matching numbers, it results in interferograms that often show fringes.

• The fringes (contours) and reveal changes in distances (modulo 2 pi), either due to the topography of the scatterer or due to change in the terrain between the exposures.

• Applications: monitoring earthquake deformations, etc

Interferogram Earthquake Izmit Turkey 17-8-1999

Physical limitations• Measurement technique related

• radiotechniques: antennas, multipath, phase center

• optical: phase center, pulse width definition

• accelerometers: thermal noise, drift etc

• altimetry: definition of the actual sea level

• Signal delay related• Difference between group and phase delay

• tropospheric delay

• ionospheric delay

• In general it is required to invoke external information (models and measurements or both) in order to apply corrections for signal delay.

What is signal delay

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t: range delay term

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s: observed distance

d: signal delay effect

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Tropospheric signal delay

• Troposphere: “everything below 100 km”

• Dry tropospheric correction– n is a function of properties of atmospheric gas d can be determined if air pressure is known

• Wet tropospheric correction– n is a function of water vapor content d to be determined by relative humidity (in-

situ, meteo model data or radiometer)

Radiometers and wet delay• A radiometer is nothing more than a radio receiver that

observes the amount of EM radiation of a particular object,

• Any object hotter than 0 K emits EM radiation, a radiometer therefor observes brightness temperatures (BT)

• At some frequencies (like 22 GHz) the opaqueness of the atmosphere is determined by water vapor

• By measuring the BT’s at frequencies around 22 GHz you can map the integrated water vapor content in a path.

• This technique is successfully applied on spaceborn radar systems and VLBI.

What is the ionosphere• Ionisation of atmospheric gasses

from circa 70 km height, ions and free electrons are formed.

• Level of ionisation is determined by solar radiation and charged particles entering the Earth’s magnetic field. (Day/Night effect, and Solar wind are the main drivers)

• There are several layers in the ionosphere, short wave radio signals up to 30 MHz can reflect against these layers (AM and SW fading effects)

Image: Copyright the Regents of the University of Michigan

Ionospheric delay• The concentration of free electrons determines the refractive

index n

• Ionospheric range delay is dispersive and thus depends on the frequency of the signal. The delay is inversely proportional to the square of the frequency (ie. high frequencies have less ionospheric delay)

• Remedy: measure ranges at more than one frequency,

• Linear combinations of ranges result in an ionospheric free observation of the distance.

• Group and phase speeds have an opposite sign as far as the

ionospheric signal delay is concerned