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Using GPS in Embedded Applications Pascal Stang Stanford University - EE281 November 13, 2002 PowerPoint Presentation
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Using GPS in Embedded Applications Pascal Stang Stanford University - EE281 November 13, 2002 I NTRODUCTION Brief history of GPS Transit System NavStar (what we now call GPS) Started development in 1973 First four satellites launched in 1978

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Using GPS in Embedded Applications

Pascal Stang

Stanford University - EE281

November 13, 2002

i ntroduction
INTRODUCTION
  • Brief history of GPS
    • Transit System
    • NavStar (what we now call GPS)
      • Started development in 1973
      • First four satellites launched in 1978
      • Full Operational Capacity (FOC) reached on July 17, 1995
      • System cost of $12 billion
      • GPS provides both civilian and military positioning globally
      • GPS comprised of three “segments”
        • Space Segment (the satellites)
        • Ground Segment (the ground control network)
        • User Segment (GPS receivers and their users)

*Selective Availability (S/A) deactivated May 2000

Stanford University - EE281 Presentation

gps s pace s egment
GPS SPACE SEGMENT
  • GPS Constellation
    • 24 satellites (Space Vehicles or SVs)
    • 20,200km altitude (12 hour orbit period)
    • 6 orbital planes (55° inclination)
    • 4 satellites in each plane
  • GPS Satellite Details
    • Manufactured by Rockwell International, later by Lockheed M&S
    • ~1900 lbs (in orbit)
    • 2.2m body, 7m with solar panels
    • 7-10 year expected lifetime

Stanford University - EE281 Presentation

gps b lock iir s atellite
GPS BLOCK IIR SATELLITE

Stanford University - EE281 Presentation

t he gps s ignal
THE GPS SIGNAL
  • C/A code and P(Y) code
  • All SVs transmit at 1575.42MHz
  • Each SV modulates using a unique 1023-bit pseudorandom (PRN) code sent at 1.023Mcps (chips per second)
  • PRN allows spread-spectrum CDMA management of GPS transmit frequency
  • Receiver’s distance to the SV can be determined by measuring the PRN time skew between the transmitted and received signals
  • GPS system data (ephemeris, clock, and atmospheric parameters) are transmitted by further modulating the PRN code at 50bps

1023 chips (1ms)

PRN

code

1 chip

PRN

PRN

PRN

PRN

PRN

20 codes (20ms)

Data

Bits*

0

0

1

0

1

0

50bps (20ms/bit)

*PRN code inverts to signify bit transition (0/1)

Stanford University - EE281 Presentation

h ow t o g et a p osition
HOW TO GET A POSITION
  • Need signal from at least four SVs for 3D position
  • One SV provides a time reference
  • Distance to three remaining SVs is determined by observing the GPS signal travel time from SV to the receiver
  • With three known points, and distances to each, we can determine the GPS receiver’s position (trilateration)

Stanford University - EE281 Presentation

e mbedded gps r eceivers

Trimble GPS Patch Antenna

EMBEDDED GPS RECEIVERS
  • Typical GPS Receivers
    • Trimble SK8/ACE GPS receiver ($60)
    • Garmin GPS35 ($160)
    • DeLorme Earthmate ($85)
    • Have seen some as cheap as $50
  • Interface
    • Single or dual serial port
    • Protocols: NMEA-0183, TSIP, TAIP, Garmin, Rockwell Binary, others…
  • Power
    • Typical requirements: 5V @ 200mA
  • Where to buy
    • Electronics outlets (Fry’s, GoodGuys, etc)
    • Electronic Surplus (Halted, All electronics, etc)
    • Internet (where else!?!)

Stanford University - EE281 Presentation

gps r eceiver rf f ront e nd
GPS RECEIVER RFFRONT END

Stanford University - EE281 Presentation

r eceiver c orrelator p rocessor
RECEIVERCORRELATOR & PROCESSOR

Stanford University - EE281 Presentation

nmea 0183
NMEA-0183
  • National Marine Electronics Association 0183 (NMEA-0183)
    • (Inter)National standard for navigation data exchange among marine electronics (GPS, LORAN, wind/water speed sensors, autopilot, etc)
    • Adopted by GPS community as defacto standard for simple output-only Position-Velocity-Time reporting
    • Available on nearly every commercial GPS with a serial port
    • Uses standard serial port (RS-232C) at 4800,8,N,1 default
    • Output-only ASCII-only comma-delimited string-based protocol
    • NMEA strings:
      • $GPGGA – GPS fix data message (lat, lon, time, #SVs, etc)
      • $GPGGL – Geographic position (lat, lon, time)
      • $GPGSA – GPS DOP and active satellites (SVs, P,H,VDOP)
      • $GPGSV – GPS satellites in view (SV elevation/azimuth, SNR, etc)
      • $GPVTG – GPS velocity and heading
      • $GPZDA – Time & Date message
    • NMEA strings are followed by a precisely defined number of fields which carry the data. Data recovery can be as easy as using sprintf(…).

Stanford University - EE281 Presentation

t rimble tsip taip
TRIMBLE TSIP/TAIP
  • Trimble Standard Interface Protocol (TSIP)
    • Binary Packet Communications Protocol over RS-232C (9600,8,O,1 default)
    • Available on nearly all Trimble GPS products
    • Best for complete embedded control of GPS receiver
    • Allows reading and control of:
      • All processed GPS data (position, velocity, time)
      • All raw GPS data (pseudoranges, carrier phase, PDOP, TDOP, signal quality, SVs used, GPS system messages)
      • GPS receiver mode & parameters (serial port protocols, DGPS mode, SV selection mode, and more)
      • GPS hardware control (oscillator offset, mixer/integrator control, test modes, fast-acquisition modes, and more)
  • Trimble ASCII Interface Protocol (TAIP)
    • Provides basic subset of TSIP commands in ASCII-only format
    • Great for low-overhead use of GPS receiver in projects with limited processor speed or RAM
    • Easy to learn

Stanford University - EE281 Presentation

gps r esourses
GPS RESOURSES
  • Trimble Embedded Receivers
    • Spec Sheet: http://www.trimble.com/products/catalog/oem/lassen2.htm
    • Full manual: ftp://ftp.trimble.com/pub/sct/embedded/pubs/lassensk2man.pdf
      • Includes excellent NMEA, TSIP, and TAIP reference
  • Garmin Embedded Receivers
    • GPS35 Full manual: http://www.garmin.com/manuals/spec35.pdf
  • Stanford GPS courses
    • AA272C – GPS Theory and Operation
    • AA272D – Integrated sensor navigation (GPS, INS, etc)
  • GPS links
    • General Info: http://www.gpsy.com/gpsinfo/

Stanford University - EE281 Presentation