Global Navigation Satellite System

based Positioning

Presented By: Mehjabin Sultana (437589)

Sami Romo (69293A)

Nuno Silva (335872)

A little bit of history

● First navigation satellite belonged to Russia (former Soviet Union): they launched the first artificial Earth satellite “Sputnik 1”, in 1957. The initial approach towards the position location was based on measuring the Doppler shift of the satellite.

● US was also a pioneer in the development of navigation satellites: “Transit” system was introduced in the 1960s, mostly for targeting submarine-launched long-range missiles. “Transit” was effective but required expensive receiver systems

● US moved to the Global Positioning System (GPS) or Navstar system in the 1980s.

History (cont..)

● Russia also developed a GPS-like system named GLONASS (abbreviation for the Russian “Globalnaya Navigatsionnay Sputnikovaya Sistema”) in the 1980s.

● Nowadays, both Japan and the European Space Agency (ESA) are working on GPS augmentation systems, such as MTSAT (Multifunction Transport Satellite Space based Augmentation System), EGNOS (European Geostationary Navigation Overlay System)

● Also new stand-alone satellite navigation systems are developed, such as Galileo (the future European satellite system) and Compass (in China).

● Most common abbreviation for a generic satellite positioning system is

GNSS (Global Navigation Satellite System).

GNSS System ComparisonGPS (US) GALILEO (Europe) GLONASS (Russia) COMPASS (China)

First launch 1978 2011 1982 2007

Full Operational Capability

(FOC)

1995 2018 2011 2020

Number of satellites 32 30 31 35

Orbital planes 6 3 3 3

Access Scheme CDMA CDMA FDMA/CDMA CDMA

Current Status 32 operational 4 IOV satellites,

22 operational

satellites budgeted

24 operational,

1 in preparation,

2 on maintenance,

3 reserved and

1 on test

14 operational satellites,

full coverage on Asia

pacific region

Why do we need satellite-based positioning?Satellite-based positioning provides us some services:

Location = determining a basic position (e.g., emergency calls)

Navigation = getting from one location to another (e.g., car navigation)

Tracking = monitoring the movement of people and things (e.g., fleet

management, workforce management, lost child/pet tracking)

Mapping = creating maps of the world

Timing = bringing precise timing to the world

GNSS system architecture

GNSS systems are quite complex, involving many different components.

- All GNSS systems are based on the same architecture (3-segment

architecture):

● Space segment: satellites

● Ground segment: monitoring, controlling and uploading stations

● User segment: user community/GNSS receivers

- The number of satellites and monitor stations differ according to the

GNSS system (GPS, Glonass, Galileo, ...)

The three segments of GNSS

Tasks of different segmentsThe space segment is formed by the satellites, also abbreviated by SV (Satellite Vehicle). The functions of a

satellite are:

● It receives and stores data from the ground control segment.

● It maintains a very precise time. In order to achieve such a goal, each satellite usually carries several

atomic clocks of two different technologies (e.g., cesium and rubidium), depending on the generation of

the satellite.

● It transmits data to users through the use of several frequencies

● It controls both its altitude and position

● It may enable a wireless link between satellites

Tasks of ground segment

● The main functions of the ground segment are to:

● Monitor the satellites; activate spare satellites (if available) to maintain system availability; check the SV

health

● Estimate the on-board clock state and define the corresponding parameters to be broadcast (with

reference to the constellation’s master time)

● Define the orbits of each satellite in order to predict the ephemeris data, together with the almanac;

● Ephemeris = accurate orbit and clock corrections for the satellites. Each satellite broadcast only its

ephemeris data. In GPS, ephemeris is broadcast every 30 s.

● Almanac= coarse orbital parameters/information of the satellites (valid for up to several months)

Tasks of different segments (cont)

Tasks of user segment

● The main functions of a GPS receiver are:

● Receive the data from the satellites belonging to one or several constellations (e.g. GPS; Galileo) on one

or several frequencies. If several constellation => multi-system receivers. If several frequencies => multi-

frequency receivers (dual-frequency GPS-GLONASS receivers are rather common nowadays)

● Acquire the signal from each satellite on sky (acquisition = identification of satellite code and coarse

estimation of time delays and Doppler shifts)

● Track the signal received from the satellites on sky (tracking = fine estimation of time delay and Doppler

shifts)

How GNSS works? : Time Difference

- The GNSS receiver compares the time a signal

was transmitted by a satellite with the time

it was received.

- The time difference tells

the GNSS receiver how far

away the satellite is.

How GNSS works? : Travel Distance

Velocity x Time = Distance

Radio waves travel at the speed of light, roughly 299 792 458 m/s (i.e.,

around 3*108 m/s)

If it took, for example, 0.067 seconds to receive a signal transmitted by a

satellite floating directly overhead, use this formula to find your distance

from the satellite.

Travel Distance: 299792458 m/s x 0.067 s = 20086094.69 m

How GNSS works? : Triangulation in 2D (I)

Geometric Principle:

You can find one location if

you know its distance from

other, already-known locations.

Location can be anywhere on

the periphery of the circle.

How GNSS works? : Triangulation in 2D (II)

Location can be any of

the two intersecting

points (red dots)

How GNSS works? : Triangulation in 2D (III)

Location is exactly

at the intersecting

point of the three

circles (red dot)

How GNSS works?: 3D Trilateration

1 Satellite

2 Satellites

3 Satellites

How GNSS works? Position Determination

● GNSS systems use the concept of Time-Of-Arrival (TOA) of signals +

triangulation/trilateration to determine user position.

● Minimum 3 satellites

needed in order to

determine the user

coordinates xu, yu, zu

(horizontal, vertical &

height). The 4th satellite

is needed to determine

the clock error.

4 unknowns

Applications - Military

● Joint Direct Attack Munition (JDAM) smart bomb, Tomahawk cruise missile...

o No need for ground support

o Guidance system can be used in all weather conditions

o Reverts to inertial navigation when GPS signal is lost

● Combat Survivor Evader Locator (CSEL)

o All weather availability

o Ease of use (fast and accurate)

Applications - Agriculture

● Tractor guidance

o Tractor drives itself minimizing over-lap and under-lap

o Shortens the amount of time used per field

o Ability to work in low visibility conditions increases productivity

● Yield mapping

o GPS with grain flow and grain moisture sensors

o Processed yield maps can be used to investigate factors affecting the yield

Applications - Marine

● Automatic Identification System (AIS) is used for vessel traffic control around busy seaways

o http://www.landsort-ais.se

Applications - Surveying and Mapping

● Survey vessels combine GPS positions with sonar depth soundings to make the nautical charts that alert mariners to changing water depths and underwater hazards

Applications - Aviation

● Enhanced Ground Proximity Warning System (EGPWS) reduces the risk of controlled flight into terrain, a major cause of many aircraft accidents

Applications - Rail

● Positive Train Control (PTC) systems prevent collisions, derailments, work zone incursions, and passage through switches in the wrong position

Applications - Recreation

● Geocaching

● “Checking-in” in social media

● Sports tracker: saving and sharing your jog route with friends

● Andropas journey planner: see the bus or train location real time, will you make it to the bus or not?

Applications - Space

● Launch vehicle tracking

o Replacing or augmenting tracking radars with higher precision, lower-cost GPS units for range safety and autonomous flight termination

● Timing solutions

o Replacing expensive spacecraft atomic clocks with low-cost, precise time GPS receivers

● Constellation control

o Providing single point-of-contact to control for the orbit maintenance of large numbers of space vehicles such as telecommunication satellites

Applications - Timing

● Wireless telephone and data networks use GPS time to keep all of their base stations in sync

● Major investment banks use GPS to synchronize their network computers located around the world

● Integration of GPS time into seismic monitoring networks enables researchers to quickly locate the epicenters of earthquakes and other seismic events

● The U.S. Federal Aviation Administration (FAA) uses GPS to synchronize reporting of hazardous weather from its 45 Terminal Doppler Weather Radars located throughout the United States

● By analyzing the precise timing of an electrical anomaly as it propagates through a grid, engineers can trace back the exact location of a power line break

References

● An introduction to GPS, mms.nps.gov/gis/gps/How_GPS_Works.ppt

● http://www.colorado.edu/geography/gcraft/notes/gps/gps_ftoc.html

● European GNSS Supervisory Authority GSA - www.gsa.europa.eu

● European Space Agency ESA - http://www.esa.int/esaNA/galileo.html

● ION Institute of Navigation - http://www.ion.org/

● www.GPS.gov

Thanks!!