gpstime.sh

# !bin/sh
#
# getloc - read GPS location and echo status, latitude and longitude
#          separated by a space. Status 0 is success.

# set the GPS port and speed
gpsport=/dev/ttyS1
gpsspeed=9600
# define the file for storing the current position (maybe useful for other things ;-)
posdata=/tmp/position.gps
locdata=/tmp/location.gps

closePort() {
  exec 5<&-
}

openPort() {
  stty -F $gpsport $gpsspeed
  exec 5<$gpsport
}

# Pass the GPS value and direction (N/S/E/W) to get the
# decimal latitude/longitude.
gpsDecimal() {
    gpsVal=$1
    gpsDir="$2"
    # Integer part of the lat/long
    gpsInt=`echo "scale=0;$gpsVal/100" | bc`
    # Minutes part of the lat/long
    gpsMin=`echo "scale=3;$gpsVal-100*$gpsInt" | bc`
    # Convert minutes to a full decimal value
    gpsDec=`echo "scale=5;$gpsInt+$gpsMin/60" | bc`
    # South and West are negative
    if [ $gpsDir = "W" -o $gpsDir = "S" ]
    then
      gpsDec="-$gpsDec"
    fi
    echo $gpsDec
}

# Return statuses
STATUS_OK=0
STATUS_NOFIX_SATDATA=1
STATUS_TIMEOUT=2
STATUS_NOTFOUND=3
STATUS_NOFIX_LOCDATA=4

# Status and counter values
foundReliability='false'
foundLocation='false'
linesRead=0

openPort
while [ $linesRead -le 100 ]
do
  # Read the next line from the GPS port, with a timeout of 10 seconds.
  read -t 10 RESPONSE <&5
  if [ $? -eq 1 ]
  then
    # Read timed out so bail with error
    closePort
    echo "$STATUS_TIMEOUT 0 0"
    exit 1
  fi

  # Fallthrough: line was read. Count it because we have a threshhold
  # for the number of sentences to process before giving up.
  linesRead=`expr $linesRead + 1`

  # Get the sentence type.
  sentenceType=`echo $RESPONSE | cut -d',' -f1`

  #echo $sentenceType

  if [ $sentenceType = '$GPGSA' -a $foundReliability = 'false' ]
  then
    #echo "$RESPONSE"

    # Found the "fix information" sentence; see if the reliability
    # is at least 2.
    fixValue=`echo $RESPONSE | cut -d',' -f3`
    #echo "$fixValue"
    if [ $fixValue -ne 2 -a $fixValue -ne 3 ]
    then
      # Insufficient fix quality so bail with error
      closePort
      echo "$STATUS_NOFIX_SATDATA 0 0"
      exit 1
    fi
    # Fallthrough: reliability is sufficient
    foundReliability='true'
    #echo "foundReliability=$foundReliability"
  fi # GPGSA sentence

  # $GPRMC,141220.00,A,xxxx.06666,N,xxxxx.26833,W,0.064,,100916,,,D*65
  if [ $sentenceType = '$GPRMC' -a $foundLocation = 'false' ]
  then
    # Found the "recommended minimum data" (GPRMC) sentence;
    # determine if it's "active", which means "valid".
    #
    statusValue=`echo $RESPONSE | cut -d',' -f3`
    if [ $statusValue = 'V' ]
    then
      # Void status; can't use the reading so bail
      closePort
      echo "$STATUS_NOFIX_LOCDATA 0 0"
      exit 1
    fi

    #echo "$RESPONSE"
    #echo "$statusValue"

    # extract the components of the date/time out of the GPRMC sentence
    #   1      2       3     4      5      6      7    8  9  10 11 12 13
    # $GPRMC,141220.00,A,xxxx.06666,N,xxxxx.26833,W,0.064,,100916,,,D*65
    gpsdate=`echo $RESPONSE | cut -d',' -f10`
    gpstime=`echo $RESPONSE | cut -d',' -f2`
    gpsdate_DD=$(expr substr "$gpsdate" 1 2)
    gpsdate_MM=$(expr substr "$gpsdate" 3 2)
    gpsdate_YY=$(expr substr "$gpsdate" 5 2)
    gpstime_hh=$(expr substr "$gpstime" 1 2)
    gpstime_mm=$(expr substr "$gpstime" 3 2)
    gpstime_ss=$(expr substr "$gpstime" 5 2)
    # execute the set date command with the data from GPRMC sentence
    # date -u -s $gpsdate_MM$gpsdate_DD$gpstime_hh$gpstime_mm"20"$gpsdate_YY"."$gpstime_ss
    #echo "date -u -s 20$gpsdate_YY$gpsdate_MM$gpsdate_DD$gpstime_hh$gpstime_mm"."$gpstime_ss"
    date -u -s 20$gpsdate_YY$gpsdate_MM$gpsdate_DD$gpstime_hh$gpstime_mm"."$gpstime_ss >/dev/null
    #date

    # Fallthrough: active status, so we can use the reading.
    foundLocation='true'
    #echo "foundLocation=$foundLocation"
    latitudeValue=`echo $RESPONSE | cut -d',' -f4`
    latitudeNS=`echo $RESPONSE | cut -d',' -f5`
    latitudeDec=$(gpsDecimal $latitudeValue "$latitudeNS")
    longitudeValue=`echo $RESPONSE | cut -d',' -f6`
    longitudeEW=`echo $RESPONSE | cut -d',' -f7`
    longitudeDec=$(gpsDecimal $longitudeValue "$longitudeEW")
  fi # $GPRMC sentence

  if [ $foundReliability = 'true' -a $foundLocation = 'true' ]
  then
      linesRead=`expr $linesRead + 200`
  fi

done # read-line loop

closePort

# If we get to here and location and reliability were OK, we
# have a fix.
if [ $foundReliability = 'true' -a $foundLocation = 'true' ]
then
  echo "$STATUS_OK $latitudeDec $longitudeDec"
  echo "$latitudeDec, $longitudeDec" > $locdata
  exit 0
fi

# Fallthrough to here means too many lines were read without
# finding location information. Return failure.
echo "$STATUS_NOTFOUND 0 0"
exit 1