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MotivationApplicationsSatellite-based positioning systemsBasic principleError sourcesError mitigationGNSS receiver operationSummary

- Bibliography:Nel Samama, ’Global Positioning- Technologies and Performance’, Wiley & Sons

book.K. Borre, D. Akos, N. Bertelsen & al. ” A software-defined GPS and Galileo

receiver- Single Frequency approach”, Birkhäuser Boston Ed., 2007.

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With satellite-based positioning we can acquire: global coverage (with some limitations, for example indoors) good accuracyIntegrity, especially if several Global Navigation SatelliteSystems (GNSS) are used together.

Inexpensive satellite navigation receivers:Magellan (980588-01) handheld GPS receiver at about 30 EUR, a Garmin NUVI215 GPS receiver with UK and Ireland maps at about 100 EURAccording to the available maps inside the GPS receiver, price can reach few hundred EUR (e.g., TomTom GO750 with navigation routes currently at about 300 EUR)

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transportation (air, land, maritime), surveying and mapping, agriculture, telecommunications, natural resources exploration, commercial, etc.

The current global market of applications and services of positioning systems is estimated to about few billion US dollars (according to the estimation source) and it is expected to grow significantly in the future. According to http://www.gpspls.com/, the revenue for GPS chipsets is expected to grow at an average annual growth rate of 11 %”

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School Bus Tracking: enable school officials to continuously know the location of all buses, route them more efficiently, monitor speeding and cut costs.Snow Plow Tracking: keep track of the location of plows and record the areas which have been serviced. GPS-Equipped Bus Routes: the city of Edinburgh, Scotland has a "Bus-tracker" system that tracks the location of city buses. When an emergency or crime occurs the bus location is immediately available. In addition the bus transmits its location and speed and this information is used to display estimated time of arrivals at street-side bus stops. GPS-Equipped Taxi Cabs: when a call for a cab comes in, the dispatcher can use a GPS tracking system to locate the cab nearest to the pick-up.

Surveying: the technique and science of accurately determining the terrestrial or three-dimensional space position of points and the distances and angles between them.

CostUsabilityAccuracy

Mapping: Cartography

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Tractor Guidance: Farmers can plow their fields with a recording GPS system. Then, the tractor can then be programmed to follow the same route for cultivating, fertilizing, pest control and harvesting. Tracking Livestock: The location of valuable animals on a large farm can be monitored by GPS transmitters attached to the animals collar

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They can be summarized into 5 broad categories:

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)Mapping = creating maps of the worldTiming = bringing precise timing to the world

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SEARCH & RESCUE

TRIP PLANNING & SAFETY

MOBILE ROBOT PERSONAL NAVIGATION FLEET TRACKING

GAMES/INFOTAINMENT

PERSONAL ASSISTANCE

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Autonomous Lawn Mower

GPS digital compass

(99.99$ in Amazon)

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Navstar Global Positioning (GPS)Military controlledDevelopment started in early 70s Fully operationalUnder modernization process

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Source: http://gpsmagazine.com/2008/05/ (data from 2007)

EU GPS market share US GPS market share

GLObal NAvigation Satellite System (GLONASS)Russian Space ForcesBack in full operation soon

GalileoJoint effort: European Space Agency & EU CommissionUp and running by 2019?Interoperable with GPSBetter (?)

BeiDou (COMPASS)China's ongoing self-developmentPlanned to be operational in 2020

Lets see where satellites are now…

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Frequency bandsL-bands for GPSE-bands for Galileo

GPS SignalsCoarse/Acquisition (C/A) (fc=1.023 MHz)P(Y) (fc=10.23 MHz)

Galileo SignalsOpen Service (OS), Commercial Service (CS), SoL(Safety of Life), Public Regulated Service (PRS)

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FDMA (current Russian satellite navigation system) or CDMAtechnology (all the other satellite navigation systems).FDMA= Frequency Division Multiple AccessCDMA = Code Division Multiple Access

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freq

Sat1 Sat2 Sat3 Sat4 Sat5 Sat6

time

code

freq

code

time

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Triangulation techniqueIf at least 3 satellites are availablethen, their positions can be found (ephemeris data)and the distance between the receiver and each satellite can be computed (how?).Knowledge of the satellite positionsand the their distances from the user is all what is needed to find the position of the user (xu,yu,zu).

Is the (xu,yu,zu) position we see in the screen of our GPS receiver?

The C/A code has chip rate: 1.023MHz

We need to find what is the propagation time

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A.Yes

B.No

C.Depends

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1

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4

1 = 23222 km (0.0774 s)2 = 20200 km (0.0673 s)3 = 24556 km (0.0819 s)4 = 21635 km (0.0721 s)

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Atomic clock: ± 1 second in 1 million years Price: $$$

Accuracy,Cost

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Ionosphere50-1000 km above the earthAffects both pseudo-range and carrier phase measurementsThe transmitted signal hits the free electrons and its speed is reduced.The ionospheric delay is frequency-dependent

Total Electron Content (TEC) depends on latitude of the receiver, season, time of day, solar activity, etc.

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TroposphereCaused by the signal refraction in the electrically non-ionized atmospheric layerTropospheric delay is a function of the satellite elevation angle and the altitude of the receiverDepends on atmospheric pressure, temperature, and water vapor pressureUnlike ionospheric effects, tropospheric ones do not depend on the signal’s frequency

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Signal multipath: This occurs when the GNSS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors (similar with cellular systems).

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Non Line-Of-Sight (NLOS): obstruction of the LOS wave due to thick obstacles (e.g., wave propagation indoors or in urban canyons with tall buildings).

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Receiver clock errors: a receiver's built-in clock is not as accurate as the atomic clocks on-board the satellites. Therefore, it may have very slight timing errors.

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Orbital errors: also known as ephemeris errors, these are inaccuracies of the satellite's reported location.Number of satellites visible: the more satellites a GPS receiver can see, the better the accuracy (we need at least 4).

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Geometrical distribution of satellites: the geometry of satellites influences the position accuracy (see plot below)Other sources: intentional jamming of the satellite signals, atmospheric effects that may degrade the satellite signal quality, etc.

(a) Poor and (b) goodconstellation geometries

smaller intersection area =>

better precision

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Source Potential error sizeSatellite clock model clock modeling error: 2 m (rms)Satellite ephemeris prediction

Component of the ephemeris prediction error along the line of sight: 2 m (rms)

Ionospheric delay Delay in zenith direction 2-10 m, depending upon user latitude, time of the day and solar activity

Tropospheric delay Delay in zenith direction at sea level 2.3-2.5 m; lower at higher altitudes

Multipath In a “clean” environment:Code: 0.5-1 mCarrier: 0.5-1 cm

Receiver noise Code: 0.25-0.5 m (rms)Carrier phase: 1-2 mm (rms)

•From the book: Understanding GPS: Principles and Applicationsby Elliott D. Kaplan and Christopher Hegarty

Advanced baseband processing (e.g., multipath reduction schemes and NLOS detection and mitigation schemes): more will be discussed later in this courseProcessing at navigation layer, when combining several measurementsUsage of troposperic & ionospheric models

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Differential GPS (DGPS)To cancel out common errors (e.g., clock, atmospheric, ephemeris) The position of GPS receiver is needed (reference station)The reference station estimates the errors and transmits them to the other GPS receiver

Relative positioningThe reference station broadcasts its time-tagged measurements of pseudoranges The other GPS receiver estimates its position relative to the reference receiver

Dual frequency receivers

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Basic operations shown in the following block diagram

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Navigation unit(computation of

position, velocity and time: PVT solution)

Hervanta, Tietotalo

Sat 1Sat 2

Sat 3Sat 4

Front-end and ADC

Baseband processing

(acquisition, tracking, data

extraction

Coordinatesconversion ->map

position

01011...

(x,y,z; t)

fD, ...

Sat =Satellite

AntennaBandpass filterSignal amplificationDown-conversionBandpass filterAnalogue-to-digital converter

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Captures the satellite signals (e.g. in L bands for GPS and E bands for Galileo)Antenna types

Quad helixPatch antenna (most popular)HelicesMicrostrip (patch antenna is a special case of this)Planar or choke rings (multipath resistance)

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Remove the interference in RF and IF bandsSelect the desired RF or IF frequenciesTypical bandwidths in IF

4 MHz for mass-market receivers20-24 MHz for professional applications

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Two rx architectures (most common)Direct conversion

Conversion directly to baseband, no IF stagesNot commercially available (under research investigation)

Super-heterodyneDown-conversion from RF to IF (usually very low)

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AcquisitionIdentify all the visible to the user satellitesProduce coarse estimates for the frequency and the code phaseDifferent search methods:

SerialParallelHybrid

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TrackingFine estimate of the code and carrier“Keep estimating precisely”

Navigation bit extractionBit synchronization: determine the start/stop of each bitFrame synchronization: determine the start/stop of the navigation data framesData decoding: extract the necessary parameters from the transmitted bits

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Satellite-based positioning offers global coverage with high accuracyDuring the last 3 decades there has been a tremendous development of satellite-based products and applicationsOne of the main challenges is the provision of position information to indoor usersThere is still plenty of space for improvement

Our group’s web-page:http://www.cs.tut.fi/tlt/pos/

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Examples of GPS/GNSS applicationsHow is the position calculated based on pseudoranges?Which are the most common sources of errors in satellite-based positioning ?Basic block diagram of a GNSS receiver (with basic explanations about the functionality of each block)

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