GNSS Remote Sensing

• GNSS Atmospheric Sounding (ground based)

• GNSS Radio Occultation (satellite based)

• GNSS Reflectometry (uses variety of receivers)

GNSS (Global Satellite Navigation System) is an umbrella term for GPS, GLONASS, Beidou, Galileo, QZSS.

GNSS uses (artificial) satellites to determine positions on earth.

Measurements between the receiver and satellite allow for calculating the receiver's position.

GNSS requires at least four satellites to be visible for accurate positioning (line of position!)

GNSS consists of a space segment (satellites), a control segment (monitor stations, master control stations, data uploading stations), and a user segment (receivers and processing software).

Observables are:

• Doppler observables, used for velocity determination

• Code pseudo-ranges, derived from PRN code

• Carrier-phase measurements

• Signal-to-noise-ratio (SNR), indicated signal quality

Orbit velocity? Altitude? How many satellites?

GNSS-RS uses GNSS satellite signals to observe the earth. (…)

Satellite data isn’t constricted to land mass and can collect data even for remote areas that aren’t easily accessible.

• refraction/bending: signal bends as it passes from a vacuum (space) to another medium (atmosphere) due to density. The bending occurs due to a change in the speed of the signal (denser medium = slower). For the ionosphere, refraction occurs due to changes in electron density.

• dispersion: different frequencies of the signal travel different ways. This is caused by the ionosphere which is a dispersive medium.

• degradation: signal becomes weaker over time

• (de-)focussing: caused by ionosphere. When travelling through irregularities, for example due to iones, the intensity of the signal changes due to focussing or defocussing (like for glasses).

• multipath: caused mostly by ground environment. Signal comes from several ways due to reflection -> what is the right signal?

During disturbed times, the ranging delay can increase to more than 100m.

Distrubances can be caused by space weather events such as

• solar flares (burtst of radiation from the sun)

• coronal mass ejections CME (clouds of solar plasma)

• geomagnetic storms (often following a CME)

GNSS Atmopsheric sounding is the technique of observing the earth with GNSS signals that travel from GNSS satellites to a ground station. For the IGS (International GNSS service), there are currently 525 stations worldwide. With the GNSS signal errors, there can be info derived about water vapor content, ionospheric delay and ionospheric disturbances. When locating the station with GNSS signals, additionally info about its location & displacement over time (due to continental drift, land slides, uplift etc.) can be derived.

Observations are:

• timing

• positioning (location of the station + displacement over time)

• tracking (time derivative of positioning)

• L1/L2 signal analysis (signal errors, collected info:

GNSS Radio Occultation is the technique which uses LEO satellites which receives GNSS signals and aalyzes them in order to observe the atmosphere. The observations are therefore also restricted to the atmosphere.

It’s the only technique with altitude resolution and can collect information about the neutral atompshere (temperature, water vapor content, pressure) and ionosphere (electron density, disturbances).

Observables are:

• profiles of temperature, pressure,water vapor content (neutral atmosphere)

• electron density profiles, SNR profiles (ionosphere)

The LEO satellite receives the signal of GNSS satellites while they rise/set (on the other side of earth). Because the signal travels through the atmosphere, it is disturbed depending on the altitude. Due to the signal being sent many times, the time differences between these signals can be observed which gives information about where exactly this signal is disturbed.

So with this we know the altitude - but how do we know the latitude/longitude?

In GNSS-RO, we assume that the atmosphere and ionosphere is spherically symmetric around the earth so that the atmospheric conditions only depend on altitude.

GNSS reflectometry works by receivers analyzing GNSS signals that have been reflected by the earth’s surface. These receivers can be satellites, ground stations, or even ships and aircrafts.

The reflected GNSS signal is weaker and also can change its polarization. With this, info about

• altimetry (water level)

• soil moisture, snow depth, vegeation info (surface)

can be derived - mainly surface parameters.

• Geosphere:

  • continental drift, land uplift, vulcano monitoring, landslides (Ground stations)

• Biosphere:

  • bee flight tracking, lynx monitoring (Ground stations)

  • vegeation observation (Reflectometry)

• Hydrosphere:

  • water level monitoring (Reflectometry/Ground stations)

  • tsunami early warning (ground stations)

  • soil moisture (Reflectometry)

• Atmosphere:

  • weather forecast (Radio Occulation)

  • temperature in troposphere/stratosphere (Radio Occultation)

  • Gravity wave detection

  • footprints of quakes, volcanos, tsunamis (ground stations)

• Cryosphere:

  • glacier motion (Ground stations)

  • snow depth (Reflectometry)