GPS­-­When­people­talk­about­"a­GPS,"­they­usually­mean­a­GPS receiver.­The­Global Positioning System­(GPS)­is­actually­a­constellation­of­27­Earth-orbiting­satellites­(24­in­operation­and­three­extras­in­case­one­fails).­

--­The­U.S.­military­developed­and­implemented­this­satellite­network­as­a­military­navigation­system,­but­soon­opened­it­up­to­everybody­else.

--­Each­of­these­3,000-­to­4,000-pound­solar-powered­satellites­circles­the­globe­at­about­12,000­miles­(19,300­km),­making­two­complete­rotations­every­day.­The­orbits­are­arranged­so­that­at­any­time,­anywhere­on­Earth,­there­are­at­least­four­satellites­"visible"­in­the­sky.

--­A­GPS­receiver's­job­is­to­locate­four­or­more­of­these­satellites,­figure­out­the­distance­to­each,­and­use­this­information­to­deduce­its­own­location.

--­This­operation­is­based­on­a­simple­mathematical­principle­called­trilateration.

--­Trilateration­in­three-dimensional­space­can­be­a­little­tricky,­so­we'll­start­with­an­explanation­of­simple­two-dimensional­trilateration.­

­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­GPS Working

2-D Trilateration

Imagine­you­are­somewhere­in­the­United­States­and­you­are­TOTALLY­lost­--­for­whatever­reason,­you­have­absolutely­no­clue­where­you­are.­You­find­a­friendly­local­and­ask,­"Where­am­I?"­He­says,­"You­are­625­miles­from­Boise,­Idaho."This­is­a­nice,­hard­fact,­but­it­is­not­particularly­useful­by­itself.­You­could­be­anywhere­on­a­circle­around­Boise­that­has­a­radius­of­625­miles,­like­this:You­ask­somebody­else­where­you­are,­and­she­says,­"You­are­690­miles­from­Minneapolis,­Minnesota."­Now­you're­getting­somewhere.­If­you­combine­this­information­with­the­Boise­information,­you­have­two­circles­that­intersect.­You­now­know­that­you­must­be­at­one­of­these­two­intersection­points,­if­you­are­625­miles­from­Boise­and­690­miles­from­Minneapolis.

If­a­third­person­tells­you­that­you­are­615­miles­from­Tucson,­Arizona,­you­can­eliminate­one­of­the­possibilities,­because­the­third­circle­will­only­intersect­with­one­of­these­points.­You­now­know­exactly­where­you­are­--­Denver,­Colorado.

3-D Trilateration

Fundamentally,­three-dimensional­trilateration­isn't­much­different­from­two-dimensional­trilateration,­but­it's­a­little­trickier­to­visualize.­Imagine­the­radii­from­the­previous­examples­going­off­in­all­directions.­So­instead­of­a­series­of­circles,­you­get­a­series­of­spheres.

In­order­to­make­this­simple­calculation,­then,­the­GPS­receiver­has­to­know­two­things:

•The­location­of­at­least­three­satellites­above­you•The­distance­between­you­and­each­of­those­satellites

GPS­Solutions

TC6000GN

• GPS module NMEA output• GPS­all-in-one­module• GPS­tracking­sensitivity­-162dBm• Very­small­package• One­single­power­supply• Low­power­consumption• Integrated­LNA­and­TCXO

TC6000GN

• TC6000GN-P1­is­a­high­performance­fully­autonomous­GPS­module­that­outputs­NMEA­on­a­serial­port­and­minimizes­the­load­on­the­host­processor. Protocol communication­is­NMEA­(provides­time,­position,­speed,­satellite­status,­and­course)

• Adding­an­antenna,­a­32,768Hz­clock­and­a­single­1.8V­power­supply­will­form­a­complete­antenna–to-­NMEA­high­performance­GPS­solution.

• TC6000GN-P1­is­a­fully­autonomous­GPS­module­that­outputs­NMEA­on­a­serial­port­with­minimal­requirement­from­a­host­processor.­

• Depending­on­the­application,­different­configurations­and­additional­components­must­be­added.

Reference­circuit

• The­circuit­shows­an­implementation­with­a­level­shifter­that­allows­to­connect­TC6000GN-P1­to­3.3V­system.­The­3.3V­to­1.8V­regulator­(LDO­or­switch­mode)­is­not­shown.­

• ­A­LED­driver­is­shown­to­indicate­the­current­fix­mode­by­a­LED­• ­The­PUSH_TO_FIX­pin­is­available­on­the­input­of­the­level­shifter­to­allow­

an­external­controller­to­put­the­TC6000GN-P1­to­deep­sleep.­• For­the­antenna,­two­different­options­are­shown.­A­passive­antenna­with­

matching­and­a­connector­for­an­active­antenna­alternatively.­The­active­antenna­configuration­includes­the­decoupled­antenna­supply­voltage.­The­coil­blocks­the­RF­path­to­the­supply,­the­capacitor­blocks­the­DC­path­towards­the­GPS_RF­input­

• The­GPS­reset­is­shown­with­three­options:­reset­through­R/C­circuit,­through­microcontroller­or­through­a­special­reset­chip,­that­generates­the­reset­pulse.­

• ­The­32,768Hz­clock­can­be­provided­by­a­dedicated­oscillator­circuit­or­by­feeding­an­external­clock­­

--­­UART­module­in­the­MSP430­to­enable­communications­with­a­computer.

--­The­Universal­Asynchronous­Receiver/Transmitter­(UART)­module­is­part­of­both­the­Universal­Serial­Communication­Interface­(USCI)­Module­and­the­universal­synchronous/asynchronous­receive/transmit(USART)­Module.

--­Depending­on­which­derivative­of­the­MSP430­you­are­using,­you­will­use­either­USCI­or­USART­and­enable­one­of­the­modes­of­operation­such­as­UART,­SPI,­I2C,­etc.

UART

Hardware Connection

--­UART­is­simple­to­connect­and­it­uses­2­lines:­TX­(Transmit)­and­RX­(Receive).­No­clock­is­required­because­it’s­asynchronous.

--­When­interfaced­to­another­module­that­uses­UART,­you­will­need­to­cross­connect­the­lines.­The­TX­of­one­device­will­connect­to­the­RX­of­another­device,­while­the­RX­will­be­connected­to­the­TX­of­the­second­device.

--­RS232­is­a­common­connection­between­PC­and­devices,­and­it­can­be­easily­converted­to­RS232­using­a­level­converter.

--­Another­popular­solution­is­to­connect­the­UART­of­the­MSP430­to­a­USB­to­UART­converter­such­as­the­FT232RL­and­FT2232­from­FTDICHIP.­These­Ics­creates­a­virtual­COM­port­on­the­host­machine­that­allows­you­to­seamlessly­transfer­bytes­between­the­host­machine­and­the­MSP430.

UART connectivity on the MSP430F5435