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.

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 or for areas that are politically unstable. Satellite data can get us a good overview on large scale structures from space and, additionally,

globally consistent data sets which can be broadly diversified.

The 2 observations equations for code and carrier phase which are very similar, but not identical. The upper one is the code obs. eq., the lower equation carrier observation eq.:

The only differences are that the phase ambiguity is included in the carrier phase obervation equation and that the ionospheric delay is negative for the carrier observations but positive for the code observations.

So, for code measurements the ionosphere delays the signal and for carrier phase measurements the signal is faster by the same amount. This si due to

• 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; bending effect increases with density). For the ionosphere, refraction occurs due to changes in electron density. For the atmosphere, bending occurs due to changes in air & water vapor densities, although they are small compared to electron density effects. Bending allows that the signal can be received even when the transmitter is slightly behind the earth.

• dispersion: different frequencies of the signal travel different ways in a dispersive medium (example: rainbow!) -> caused by the ionosphere which is a dispersive medium (refractivity depends on the frequency in the ionosphere!). Dispersion causes spatial separation in different wavelengths.

• degradation: signal becomes weaker over time due to atmospheric absorption; appears due to strong ionospheric disturbances but also due to water vapor content and air density

• (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, e.g. at surfaces of buildings -> what is the right signal?

Effect of rain: Rain and clouds usually don’t affect GNSS signals due to the diameter of raindrops (is too low). However, when it’s raining a lot, it could have an effect (like ~ 10cm/h).

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.

• 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)

  • flooding prediction

  • 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)

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.

The Global Positioning System is a satellitebased navigation system owned by the United States Space Force. It is one of the global navigation satellite systems (GNSS) that provide geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.

NOT equal to GNSS, because GNSS is an umbrella term for many systems including GPS.

With the launch of the Soviet Sputnik satelllite in 1957, it was discovered that the signal the satellite sent are Doppler shifted which meant that the signals could be used to determine its position. This discoverey led to the idea that maybe, by turning this around, with the satellites position known the position of the receiver could be tracked.

In the 1960s, the Transit system (also known as NAVSAT or NNSS) of the US was launched, which was the first operational satellite navigation system. These satellites were quite small, about 1m in diameter, relatively light weight, and had a low polar orbit.

The GPS project began in 1973. After in 1983, a plane was shot down due to it being in a prohibited zone because its system had a navigation error, US President Reagan announced that when GPS is completed it would be avaiable for civilian use. Since 1995 GPS is fully operational and with that the first GNSS.

Today, we have Multi-GNSS - 4 GNSS systems (GPS, GLONASS, Beidou and Galileo) and 2 regional satellite systems (QZSS and IRNSS).

Additionally:

GPS: project started in 1973, fully operational since 1995

Galileo: limited services since 2016, fully op. since 2020; first sat. launched in 2005 -> independence from U.S. civil service

BeiDou: geostationary sat. since 2000, extended to pacific region in 2012, fully op. since 2020

GLONASS: construction from 1982-1996, almost down in 2001 but was recompleted: recently 27 satellites! New “K1” satellites with only one frequency

GNSS satellites transmit continuous electromagnetic waves; the phase of the carrier wave which has a high frequency is tracked by the receiver. The receiver then measures the phase difference between the transmitted and the received signal - this difference allows for the calculation of the distance between the satellite and the receiver.

With Carrier-phase measurements, there is an ambiguity in the number of complete wavelengths between satellite & receiver which needs to be resolved to determine the true distance accurately.

This can be achieved by combining pseudoranges and phase observations but needs a (more expensive) receiver which is able to do so! (Most smartphones, for example, can’t)

Once that is resolved, carrier-phase measurements can achieve millimeter-level precision.

Augmentation systems are designed to improve the performance of existing GNSS signals, for example to improve accuracy, reliability and avaiability.

They do this by providing additional information like correction data (e.g. for clocks) and atmospheric delay corrections to users by broadcastng differential signals.

There are satellite-based augmentation systems (SBAS) and ground-based augmentation systems (GBAS).

Examples are:

• WAAS (North America)

• EGNOS (Europe)

• GAGAN (India)

• QZSS-SBAS (Japan)

A GEO (geosynchronos equatorial orbit) is just a dot because they rotate with earth in the same time (alt. ~ 36000 km); they all have an inclination of 0° and are therefore always on the equator.

A IGSO (inclined geosynchronous orbit) is geosynchronous (same altitude!) but with an inclination, which produces 8-shaped groundplots which stay on the latitude and are in between the latitude of their incliantion. With no eccentricity, the loops cross at the equator. They are often eccentrical as well, making one of the “ribbons” of the 8 larger than the other (where it’s larger: farther away) and makes it so that they are not crossing directly at the equator.

A MEO (medium earth orbit) is an orbit with an altitude of around 2000 - 30000 km. Most GNSS satellites are in this orbit. They need around 3 - 12 hours to orbit earth, meaning that they produce wide waves on a groundplot. A satellite with a 12h-orbit produces 24/12 = 2 waves on the groundplot. For a 3h orbit it would be 24/3 = 8. Most satellites fly eastwards, therefore the groundplot shifts westwards. The reach as high in latitude as their inclination allows.

A LEO (low earth orbit) is an orbit with an altitude of up to 2000km. Most GNSS RS satellites are in this orbit, as are most scientific satellites. They need around 90 minutes to orbit earth. Therefore, the groundplot has 24/1.5 = 16 waves. They reach as high in latitude as their inclination allows - a lot of them have polar orbits. Most satellites fly eastwards, therefore the groundplot shifts westwards.

GPS has 6 orbital planes, so while all satellites have the same inlcination, they are shifted in longitude to ensure maximum coverage. For GPS, this is a 60° shift in longitude. Even though the inclination is only at around 55°, the polar areas can still receive enough signals due to the high altitude of the GPS satellites.

Regional satellite system provide independent positioning, navigfation and timing but not globally. Instead they provide these services in a limited area.

Examples are:

• QZSS (Japan); Quasi-Zenith Satellite System, has 4 satellites from which one is geostationary and the other one is inclined geostationary

• IRNSS/NaviC (India); Indian Regional Navigation Satellite System, op. since 2018, has 7 satellites

Skyplot: Imagine you’re looking into the sky and project that half-sphere (up until the horizon!) onto a circular map. The satellites can be seen as dots. They show the elevation of satellites (how high they are above the horizon) and the azimuth (the direction where the satellite is).

For groundplots, one can see how the satellites move around earth (“what they see”).

The neutral atmosphere and the ionosphere blocks certain wavelengths for electromagnetic waves on the earth (Acoustic waves obviously can’t be used as they need a medium). There are some windows which wavelengths are let through: Visible light, infrared windows, some microwave windows and a big radio window. Long-wavelength radio waves are blocked by the ionosphere, so the wavelength can’t be too long (this is how old radios work: they send long-wavelength radio waves which get reflected by the ionosphere to be received somewhere else in the world!)

GNSS systems use different bands to broadcast their signals. For GPS, these are L1, L2 and L5. With the exception of L2, these bands are also similar fot the other GNSS systems, but they are typically a bit wider so they can be received at a ider range of frequency.

They range from ~ 1100 - 1600 MHz, having a wavelength of ~ 0.27 - 0.18m.

A GNSS signal consists of several different things (GLONASS not included as they work a bit differently):

• carrier wave: radio frequency sinusoidal signal

• PRN code: sequences of randomly distributed 0s and 1s (binary, here: follows Gold Code), is repeated over and over, usually 1 ms long -> used to identify satellite as it’s unique for every satellite

• navigation data: binary coded message providing info about satellite’s position and velocity as well as clock bias, satellite health etc. -> necesary info for user to perform positioning service

Positioning is done with help of the carrier wave (carrier-phase measurements) and/or with the PRN code (Code-phase measurements).

PRN code can be the P- or the C/A-code. C/A code is for civilian use (L1 band) and P-Code (more precise) can be encrypted, preventing unauthorized access. P-Code works a bit different than C/A code:

The P-Code is a very long code which would take a week to repeat. Each satellite is assigned a portion of it, repeating it every 7 days. It’s broadcasted on L1 and L2 frequencies.

• Doppler observables, used for velocity determination

• Code pseudo-ranges, derived from PRN code (Pseudorange and not range due to e.g. clock errors (mainly from receiver)!)

• Carrier-phase measurements

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

However, the primary observable is of course the time delay.

Code-phased measurements use a known, repeating sequence of signals (pseudorandom noise PRN, series of binary numbers 0 or 1) which is transmitted by the satellite at a very high speed. For GPS, this is often the C/A (Coarse/Acquisition) code. The receiver generated an identical PRN code and shifts it until it matches the one it receives from the satellite.

The time delay between when the receiver's code matches the satellite's incoming code indicates the signal’s time of arrival (ToA).

The receiver calculates how long it took for the signal to travel from the satellite to the receiver by determining the shift required to synchronize the codes. This shift is always half of the width of tau (basically half of the 1 or 0s that make up the code). To know which satellites code has been received, the receiver needs to “know” all the codes emitted by the GNSS satellites. This time is called Time of Flight.

Code-phase measurements from C/A Code have an accuracy of 5 -10 meters without augmentation.

They are used for the initial estimation of the receivers position.

Spoofing & Jamming can lead to being unable to do positioning. Other than that, ionospheric delays can cause a location error up to 100m.

Dual- or even triple-frequency receivers can compensate ionospheric delay: they remove up to 99.9% of ionospheric effects. Single-frequency receivers work best when the ionosphere is undistrurbed and can correct up to 75% of the delay when the ionosphere is calm.

• tunnels: solution could be a series of antennas aboveground. A software could simulate satellite’s signals and GNSS receivers could then use the “GNSS” signals transmitted by the antennas

• underwater: GNSS signals can’t penetrate water (this is why GNSS Reflectometry works in the first place); acoustic waves could be used instead

Refractivity depends on different parts of the atmosphere:

• dry: depends on atmospheric pressure & temperature

• wet: depends on water vaopr partial pressure and temperature

• Ionosphere: depends on electron density in the ionosphere and the carrier frequency

The main challenge for calculating refractivity models is the spatial & temporal change of the refractivity

Less than a second. If we assume the signal travels with the speed of light at 300000 km/s and the satellite is ~ 20000 km away, the signal needs 300000/20000 = 0.066 s, so around 70 ms.

The C/A code as well as the p code are ranging codes used by GPS.

C/A-Code, also Clear/Aquisition code, is code that can be used for measurements by the general public. It is usually less accurate (~ 5-10 m) and is broadcasted on a single frequency (L1). It’s also around a ms long.

The P-Code, also Precision Code, is code that is much more precise (~ 30-40cm) and broadcasted onto two frequencies (L1 and L2). It’s different for each satellite and much (!) longer.

Refractivity is what happens when a signal is bent in a medium. For the atmosphere, the refractivity can be calculated with the Smith Weintraub Formula.

The lower the elevation of the satellite is, the more atmosphere the signal has to cross which results in larger delays. For example in the polar regions, there are no GNSS satellites which are in zenith. The delay is exponential:

5° -> 25m

10° -> 13m

20° -> 7m

90° (zenith) -> 2.3m