GPS decoding problems

[btaylor]

Here is a routine that will extract all fields of the RMB sentence. You can modify it for other strings.

Cheers
Brian

GetData:
' Add code here for GPRMC, etc

GetRMBData: ' capture 84 character frame in real time
serin2 rx232, 188, 2000, nogps,[WAIT("$GPRMB")] ' find header
for i = 0 to 83 'grab data stream
serin2 rx232, 188, [a[i]]
next i
' now we have an 84 character array starting with the first comma after
' $GPRMB. $GPRMB is of variable length depending on field values.
' Processing is now done while the next GPS frames pass by. Need a bigger
' RAM to handle RMB & RMC on every GPS frame.

UnpackRMB: ' unpack frame & show on SEROUT
CommaCnt = 0
for i = 0 to 83
if a[i] = "," then
CommaCnt = CommaCnt + 1 ' found a new field
serout bootout, 2, [$0D, $0A, #CommaCnt , ","]
if CommaCnt = 1 then getvalidity
if CommaCnt = 2 then getxte
if CommaCnt = 3 then getsteer
if CommaCnt = 4 then getorigin
if CommaCnt = 5 then getdestination
' if CommaCnt = 6 then getdestnlatt
' if CommaCnt = 7 then gethemisphere
' if CommaCnt = 8 then getdestlong
' if CommaCnt = 9 then geteorw
' if CommaCnt = 10 then getrangetogo
if CommaCnt = 11 then getbearing
' if CommaCnt = 12 then getclosingvelocity
if CommaCnt = 13 then getarrival
if CommaCnt = 14 then getdata 'all done so get next RMC
GetFieldData:
for j = i+1 to (i+15) 'get the next character
if a[j] = "," then
goto getnextfield
endif
serout bootout, 2, [a[j]]
next j
endif
GetNextField:
next i

goto getdata

GetValidity:
' This is needed since both Track and Bearing collapses to 0.0 when no
' GPS data is being received. This could be falsely interpreted as
' traveling due North with the destination dead ahead.
' A FailSafe check must be done here to prevent this. Currently omitted.
if a[i+1] = "V" then
serout bootout, 2, ["V = bad frame"]
goto GetNextField
endif
if a[i+1] = "A" then
serout bootout, 2, [ "A = good frame"]
goto GetNextField
endif
serout bootout, 2, [ "NO sync"]
goto GetNextField

GetXTE:
xte = 0
for j = (i+1) to (i+8) ' get (up to) the next 8 characters
if a[j]="," then xtedone ' end of field or blank field found
if a[j]<>"." then ' ignore any decimal point
e = a[j] - 48
'ASCII representation - subtract 48 to get numeric value
xte = 10* xte + e
endif
next j
XTEDone:
' at this point we have XTE * 10
serout bootout, 2, ["XTE = ", #xte/10, ".", #xte//10]
goto GetNextField

GetSteer:
if a[i+1] = "L" then
serout bootout, 2, ["Turn LEFT"]
goto GetNextField
endif
if a[i+1] = "R" then
serout bootout, 2, [ "Turn RIGHT"]
goto GetNextField
endif
serout bootout, 2, [ "NO turn info"]
goto GetNextField

GetOrigin:
'This is an alphanumeric field - needs unpacking
serout bootout, 2, ["Origin = "]
for j = (i+1) to (i+15) ' get (up to) the next 15 characters
if a[j]="," then origindone ' end of field or blank field found
serout bootout, 2, [ a[j]]
next j
OriginDone:
goto GetNextField

GetDestination:
'This is an alphanumeric field - needs unpacking
serout bootout, 2, ["Destination = "]
for j = (i+1) to (i+15) ' get (up to) the next 15 characters
if a[j]="," then destinationdone ' end of field or blank field found
serout bootout, 2, [ a[j]]
next j
DestinationDone:
goto GetNextField

GetBearing:
bearing = 0
for j = (i+1) to (i+8) ' get (up to) the next 8 characters
if a[j]="," then bearingdone ' end of field or blank field found
if a[j]<>"." then ' ignore any decimal point
e = a[j] - 48
'ASCII representation - subtract 48 to get numeric value
bearing = 10* bearing + e
endif
next j
BearingDone:
' at this point we have BEARING * 10
serout bootout, 2, ["Bearing = ", #bearing/10, ".", #bearing//10]
lcdout $FE, $01, "Track ", #track/10, ".", #track//10
lcdout $FE, $C0, "Bearing ",#bearing/10, ".", #bearing//10
goto GetNextField

GetArrival:
if a[i+1] = "A" then
serout bootout, 2, ["Arrived"]
goto GetNextField
endif
if a[i+1] = "V" then
serout bootout, 2, [ "not there yet"]
goto GetNextField
endif
serout bootout, 2, [ "NO arrival info"]
goto GetNextField

NoGPS:
lcdout $FE, $01, "No GPS data " , $FE, $C0, "Check GPS "
pause 1000
goto startup

[sradayda]

anybody can provide full description for this project ???

[Heckler]

Here is how I decode the $gprmc sentence...

Partial code below

GPRMC VAR BYTE[64]

hh      VAR BYTE  'hours
mm      VAR BYTE  'minutes
ss      VAR BYTE  'seconds
sss     var word  'milliseconds
fix     VAR WORD  'GPS fix
latdeg  VAR BYTE  'degrees latitude
latmin  VAR BYTE  'minutes latitude  
latminn var word  'fractional minutes latitude
NS      VAR BYTE  'north or south
londeg  VAR BYTE  'degrees longitude
lonmin  VAR BYTE  'minutes longitude
lonminn var word  'fractional minutes longitude
EW      VAR BYTE  'east or west
Knots   VAR byte  'speed in knots (units)
Knotss  var byte  'speed in fractional knots
course  var word  'heading
dy      VAR BYTE  'day
mt      VAR BYTE  'month
yr      VAR BYTE  'year


'============= Main Program ==========================================
Main:
high gpwr
pause 3000

GPSin:
    serin2 gps,188,3000,tmout,[WAIT("$GPRMC,"),STR GPRMC\63\"$"] 'Data IN from GPS
    
    arrayread gprmc,[DEC2 hh,DEC2 mm,dec2 ss,_
      wait(","),fix,wait(","),DEC2 latdeg,DEC2 latmin,wait("."),dec4 latminn,_
      wait(","),NS,wait(","),DEC3 londeg,DEC2 lonmin,wait("."),dec4 lonminn,_
      wait(","),EW,wait(","),dec knots,dec Knotss,dec course,_
      wait(","),DEC2 dy,DEC2 mt,DEC2 yr]      'parse the GPS array into parts

if fix = "V" then gpsin  'jump to "waiting for fix" display

low gpwr
high xpwr
    pause 1000
    
    SEROUT2 xbtx,188,[$0D,$0A,"$GPRMC,",STR GPRMC\63]            'Data OUT to XBEE
    pause 500

Hope this helps

[sradayda]

Thanx so much .. I really appreciate it
Could you please provide me with GPS hardware connections with the pic ?

[mackrackit]

What are you using for the GPS? A handheld unit or a module?

[sradayda]

What are you using for the GPS? A handheld unit or a module?

I am using a module.

[mackrackit]

The module data sheet should give the pin out. RX and TX.