THE GPS RECEIVER

Analog and Telecommunication Electronicsprof. Dante Del Corso

- A.Y. 2013/2014 -

Ruggero Balteri – s206828

SUMMARY

1. - Fundamental concepts behind the GPS

Distance measurement – example 1

The Light pulse Transmitter – example 2

2. The structure of GPS

3. Fundamentals of GPS C/A Code Signal Structure

4. GPS Receiver basic architecture

The five steps

The main building blocks of a GPS receiver

5. Practical implementation with an Arduino

Ruggero Balteri - s206828 2

MAIN TASK OF A GPS RECEIVER

Reception of the signal from satellites.

Extract satellite data from the signal.

Measure the distance from the user to each satellite the user receiver tracks.

Calculation of user’s position.

Ruggero Balteri - s206828 3

GPS receivers DO NOT send data.

DISTANCE MEASUREMENT –EXAMPLE 1

Ruggero Balteri - s206828 4

THE LIGHT PULSE TRANSMITTER –EXAMPLE 2

Let us consider:

a car moving in an ideal straight road from A to B.

In A and in the car there is an ideal clock.

Every time a clock counts to 20,000µs (20ms) it rolls over to zero and starts over again.

When the clock in A rolls over (time zero) a light pulse is emitted.

Note that:

Ruggero Balteri - s206828 5

Our final goal is to measure the distance of the car from A.

Distance Time to travel at speed of light

AB 6000km 20msresolution 300m 1µs

Ruggero Balteri - s206828 6

All clocks are synchronized.

FINAL REMARKS

The GPS is not a NOT a pulsed system

By locking the signal sent by satellites you can recreate a copy of the atomic clock in the receive

If you consider another atomic clock in B (in sync with A) you can minimize the error by considering the same bias:

�Δ𝑡𝑡𝐴𝐴 = 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝐴𝐴 − 𝑇𝑇𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 − 𝑇𝑇𝑏𝑏𝑟𝑟𝑠𝑠𝑡𝑡𝐴𝐴Δ𝑡𝑡𝐵𝐵 = 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝐵𝐵 − 𝑇𝑇𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 − 𝑇𝑇𝑏𝑏𝑟𝑟𝑠𝑠𝑡𝑡𝐵𝐵

Ruggero Balteri - s206828 7

⇒ 𝑇𝑇𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 =𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝐴𝐴 + 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟𝐵𝐵 − 𝑇𝑇𝑏𝑏𝑟𝑟𝑠𝑠𝑠𝑠𝐴𝐴 − 𝑇𝑇𝑏𝑏𝑟𝑟𝑠𝑠𝑠𝑠𝐵𝐵 − Δ𝑡𝑡𝐴𝐴 + Δ𝑡𝑡𝐵𝐵

2

Ruggero Balteri - s206828 8

S1

x1

S2

x2

S3

x2

By analogy:

3 satellites are needed to determine the position on a 3D space

BUT

at least 4 satellites are required to properly solve the user position and the user clock error.

THE STRUCTURE OF GPS

To recap: 1. Antenna receives GPS signals2. RF chain3. AD converter4. Acquisition of satellites signals5. Tracking -> Ephemeris data and pseudo-ranges 6. User Position

The Control segment monitor the performance of the GPS satellites

The Space segment 24 satellites at a distance of 20.000km from mean sea level

The User segment Calculate the sent time and received time and multiply by the speed of

light.

Ruggero Balteri - s206828 9

GPS C/A CODE SIGNAL STRUCTURE

Key concepts:1. PRN (pseudo-random noise) generator2. BPSK modulation (Bit Phase shift modulation3. Code division multiple access (CDMA)

Ruggero Balteri - s206828 10

Ruggero Balteri - s206828 11

Ruggero Balteri - s206828 12

GPS RECEIVER BASIC ARCHITECTURE

The basic steps:

RF Down-conversion

Signal Acquisition

Data Demodulation

Data Clock Recovery, SV Clock Replication

“Time Sent-Received information.”

The last step usually requires a microcontroller.

Ruggero Balteri - s206828 13

THE BUILDING BLOCKS OF A GPS RECEIVER

Antenna Uniform gain over

a very wide spatial angle. Frequency selection Reject multi-path effect

Pre-amp

Demodulation 1st BP filter -> image

rejection 1st mixer 2nd BP filter -> out of band signal rejection 2nd mixer -> downconverted

Ruggero Balteri - s206828 14

Ruggero Balteri - s206828 15

Benefits of downconversion:• Easier design of sharp BPF• Correct amplification of the signal without problems related to feedback oscillation• Digital sampling of the signal is easier.

1. Sum frequency components are generated, but these are eliminated by the bandpassfilter

2. Fequency separation of the desired and image signals is twice the IF

3. the filtering becomes difficult if a single downconversion

4. Downconversion is often accomplished in more than one stage, with high first IF (30–100 MHz) to permit image rejection.

5. The frequency of the signal is now low enough to be digitally sampled.

INTERFACING A GPS RECEIVER WITH ARDUINO

Ruggero Balteri - s206828 16

Ruggero Balteri - s206828 17

GPS Shield

Arduino 2009

GPS module

Ruggero Balteri - s206828 18

Final IC Soldering headers GPS Shield Arduino 2009 GPS module

NMEA 0183

ASCII based protocol

There are several NMEA messages, but the most relevant for us is represented by the string $GPRMC

Ruggero Balteri - s206828 19

Example$GPRMC,220516,A,5133.82,N,00042.24,W,173.8,231.8,130694,004.2,W*70N° parameter Actual Value Description

1 220516 Time Stamp2 A validity - A-ok, V-invalid3 5133.82 current Latitude4 N North/South5 42.24 current Longitude6 W East/West7 173.8 Speed in knots8 231.8 True course9 130694 Date Stamp10 4.2 Variation

$GPRMC,235316.000,A,4003.9040,N,10512.5792,W,0.09,144.75,141112,,*19$GPGGA,235317.000,4003.9039,N,10512.5793,W,1,08,1.6,1577.9,M,-20.7,M,,0000*5F$GPGSA,A,3,22,18,21,06,03,09,24,15,,,,,2.5,1.6,1.9*3E

TINYGPS – A SMART ARDUINOLIBRARY

no need to deal with NMEA commands

1. Instance TinyGPS object

2. Feed the method “.read()” with NMEA data

3. If the method “.encode()” returns “true” then the sentence can be parsed

4. Use the methods: “f_get_position()”, “crack_datetime()”, “f_altitude()”… to extract all the needed information

Ruggero Balteri - s206828 20

Ruggero Balteri - s206828 21

Include the libraries:SoftwareSerial.h

TinyGPS.h

Define Constants:RX, TX pins

Baud Rate of GPS and Arduino.

Create:New TinyGPS object

New Uart Connection

Is there any Data on the RX pin?

Feed all received data to the method “.read()”

Does the method “encode()” return “true”?

NO

Call the function “getgps()” and Print on video:Latitude and longitude,

Date and time,Altitude,Speed.

YES

NO

YES

A simple algorithm

OutputLat/Long: 45.04378, 7.67608Date: 5/13/2014 Time: 16:32:43.99Altitude (meters): 248.00Speed(kmph): 0.41

Ruggero Balteri - s206828 22

The end ◊

Title: “Fundamentals of Global Positioning System Receivers - A Software approach”Author: James Bao-Yen Tsui

Title: “Principles of GNSS, Inertial, and Multisensor Integrated Navigation Systems”Author: Paul D. Groves

Title: “GPS & Galileo: Dual RF Front-end Receiver and Design, Fabrication, and Test”Author: Jaizki Mendizabal

Title: “Fundamentals of GPS Receivers: A Hardware Approach”Author: Dan Doberstein

Title: “Global Positioning Systems, Inertial Navigation, And Integration”Authors: Mohinder S. Grewal, Lawrence R. Weill, Angus P. Andrews

Title: “Signal Acquisition And Tracking For A Software Gps Receiver” (Thesis)Author: Sophia Y. Zheng

Datasheet: GLOBALSAT GPS Module EM-506Company: Globalsat Technology CorporationWebsite: www.globalsat.com.tw

Arduino – www.playground.arduino.cc/Tutorials/GPS

NMEA – www.aprs.gids.nl/nmea

TInyGPS – www.arduiniana.org/libraries/tinygps

Sparkfun – www.sparkfun.com