Changing GPS co-ordinates into Degrees-Minutes-Seconds

[FromTheCockpit]

Ok, thanks for your time in writing a detailed explanation, BUT little bit of confusion still left. I have the following string :

$GPRMC,123339.000,A,5134.2770,N,00007.8480,E,1.14, 254.38,271009,,*09

Can you please show me as to how to convert "5134.2770" & "00007.8480" into degrees - minutes - seconds in PBP + speed to Km/H.
I have done this online and the answer is
Degrees, Minutes & Seconds
Latitude - Longitude
N51 34 16 - W0 07 50 and 2.111 Km/Hr (Though it should be 0 Km/Hr as Module is stationary)

Also what is checksum? and what it is used for?

Your effort will be much appreciated. Many Thanks

[ScaleRobotics]

You are using one of the newer GPS's with a little higher resolution. With any handheld gps, there will be a little wandering of the output. You will see slight position changes, even though you are standing still. This will result in a display of speed, although it will be very low speed.

The checksum is used to ensure the device reading the gps doesn't receive faulty input. If it is important for your application, the Pic chip can be made to perform a checksum on the sentence, and compare it to the one the gps calculated. If it differs, the data read from the gps is corrupt, and must be read again.

To convert Knotts to km/h, multiply by 1.852. Since PICs don't do decimal, we will have to make our own. A close enough approximation of knotts x 1.852 is (knotts x 2048)/11, but remind yourselft that your result will have two digits past the decimal point. If you want whole numbers for km/h then divide by 100, or do (knotts x 2048)/1100. A handy tool to figure these fractions out is a download at the very bottom of this page: http://www.miscel.dk/MiscEl/miscel.html

To convert, you follow my earlier example, but insert skips to match the format of your NMEA sentence.

For your:

$GPRMC,123339.000,A,5134.2770,N,00007.8480,E,1.14, 254.38,271009,,*09

it would be something like:

SerIn2 GPSin,baudGPS,Timeout,Nogps,[wait("$GPRMC"),skip 14,DEC2 lat_deg,DEC2 lat_min,skip 1,DEC3 lat_min_dec ....]

you should be able to enter the rest for longitude now....

The degrees and whole minutes are already in the format you want. We just need to convert decimal minutes into seconds. To do this, we use the same conversion I showed you in the previous example (except you will see that I forgot to add another digit for the seconds result). Here you have .2770 minutes. We only care about the first three digits. In fact, we might care about even fewer digits, but since we used three of the digits in the last example, lets just use three again.

The following will multiply your 277 by 6. This will give you a result of 1662, which really is 16.62, but again, the PIC can't do decimal, so we will use the whole number, and just format it for output.

lat_seconds = lat_min_dec * 6
DEBUG DEC lat_deg," deg ",DEC lat_min," min ",DEC lat_seconds dig 3,DEC lat_seconds dig 2,".",DEC lat_seconds dig 1, DEC lat_seconds dig 0," sec",13,10

That would print out:
"51 deg 34 min 16.62 sec", which matches pretty well with the 51 34 16 that you converted on line.

[FromTheCockpit]

The checksum is used to ensure the device reading the gps doesn't receive faulty input. If it is important for your application, the Pic chip can be made to perform a checksum on the sentence, and compare it to the one the gps calculated. If it differs, the data read from the gps is corrupt, and must be read again.

Thanks a lot. I think I need to sit down with the manual for sometime to learn about different conversions and decimal substitutes. Seems little complicated for now.

But I still wanted to ask you that how do I checksum the data in PIC? I mean my PIC gets everything in a string which is comming from the GPS in the first place. Why it is a number, what does this number do?

Can you please put up the code you have done as it will be far far easier to understand somthing working than perform a reverse engineering technique on it. From a beginners point of view what you explained above about checksum has gone past way above my head.

Thanks

[ScaleRobotics]

...Seems little complicated for now.

But I still wanted to ask you that how do I checksum the data in PIC? I mean my PIC gets everything in a string which is comming from the GPS in the first place. Why it is a number, what does this number do?

Can you please put up the code you have done as it will be far far easier to understand somthing working than perform a reverse engineering technique on it. From a beginners point of view what you explained above about checksum has gone past way above my head.

Well, I didn't post my code because I do something a little different. I convert from deg min.min to deg.deg so I can perform some math on the waypoints. My code is very ugly and sometimes confuses me.

I think it is very helpfull for people to be able to write code, and "make it their own". That is the only way they will be able to progress, and really make the code do things they want. So I try to give enough data for people to start playing around with the parts of code. The link to MeLabs sites GPS code is a fully functional code that will not have to be reverse engineered. Only needs to be edited slightly to read different digits by changing the skip 34, and a few other small things.

I suggest you ignore pretty much everything I have written below this line for now, and get your hands fully around the above code by playing around with the serin2 command, and seeing what you can get to print out to your computer.

This is NOT the code that I suggest playing around with, but if you must see it, here is a snippet. Darrell and others would probably say they needed a lot of toilet paper and a few Amodiums to go with it. If after seing it, you don't get sick to your stomach, I will try to explain the garbage. I would not say it is advanced. It is just too messy to explain properly. There are a couple errors I need to fix, with a few parenthesis in the wrong place, etc.

This is assuming you use an array called gpsdata.....

        IF gpsdata[char_A] = "A" Then              'was 9, Needs to be 13 for EB-85A                                       
            lathome_hi = ((gpsdata[char_A+2] - 48) *10)+((gpsdata[char_A+3] - 48))
            lathome_low = ((((gpsdata[char_A+4] -48)*1000)+((gpsdata[char_A+5]-48)*100)+((gpsdata[char_A+7]-48)*10)+(gpsdata[char_A+8]-48))*5)/3
                lathome_min = ((((gpsdata[char_A+9]-48)*10) + (gpsdata[char_A+10]-48))*5)/3    'lathome_min
                lathome_min = lathome_min + 33*((((gpsdata[char_A+4] -48)*1000)+((gpsdata[char_A+5]-48)*100)+((gpsdata[char_A+7]-48)*10)+(gpsdata[char_A+8]-48))*5)//3
                tempbyte = lathome_min /100
                lathome_low = lathome_low + tempbyte
                lathome_min = lathome_min //100
            if gpsdata[char_A+12]=78 then north =1         '="N"
            lonhome_hi = ((gpsdata[char_A+14]-48)*100)+((gpsdata[char_A+15]-48)*10)+(gpsdata[char_A+16]-48)
            lonhome_low = ((((gpsdata[char_A+17]-48)*1000)+((gpsdata[char_A+18]-48)*100)+((gpsdata[char_A+20]-48)*10)+(gpsdata[char_A+21]-48))*5)/3
                lonhome_min = ((((gpsdata[char_A+22]-48)*10) + (gpsdata[char_A+23]-48))* 5)/3
                lonhome_min = lonhome_min + 33*((((gpsdata[char_A+17]-48)*1000)+((gpsdata[char_A+18]-48)*100)+((gpsdata[char_A+20]-48)*10)+(gpsdata[char_A+21]-48))*5)//3
                tempbyte = lonhome_min/100
                lonhome_low = lonhome_low + tempbyte
                lonhome_min = lonhome_min//100
            if gpsdata[char_A+25] = 87 then west = 1 'check to see if we are W or E char 87 = "W"

Ok, now you got that, have your wife or girlfriend grab you another roll of toilet paper, and we will move on to checksums.....

I also, purposely did not go into check sums. You really only need to worry about checksums if your life, or your equipment is at risk because of the way you are using your GPS data. The reason it can be important, is that the NMEA sentences can be rather long. The one you are working with is in the neighborhood of 70 to 80 characters, though I have not counted. There are plenty of chances that by transmission through wires, or wireless, that one bit here or there could be missed, giving you a completely different location. Your equipment might not realize there was an error, unless it compared the checksum from the gps to its own calculated checksum on the data received.

Basically, the checksum is a sum of all the data in the NMEA sentence. It does something similar to adding up all the ascii codes.

I have some example code from an RCAP autopilot project, but this too is not really important if you are not making an autopilot, a hazard avoidance product, etc. So it can pretty much be left out of the equation for most applications.

DoCS:
    i = 0                             ' Initialize the position counter
                
    While gpsdata[i] != "*"           ' Count everthing until the *
        cs = cs ^gpsdata[i]           ' XOR for checksum
        i = i + 1                     ' Increment position counter
    Wend

    ' A quick and easy atoh function
    ' Make it a seperate label if we add more atoh conversions in the future
    LookDown gpsdata[i+1],["0123456789ABCDEF"],tempbyte
    gpscs = tempbyte * 16        
    LookDown gpsdata[i+2],["0123456789ABCDEF"],tempbyte                
    gpscs = gpscs + tempbyte
        
    ' Flag bad data "V" if checksum doesn't add up. 
    ' Or if the sentence is less then 20 chars long
    ' Which means there is no active waypoint to go to.
    ' Should be "A" otherwise.
    IF (cs != gpscs) OR (i < 20 )Then 
          gpsvalid = 0
    Else 
          gpsvalid = 1
    EndIF  

Return

[FromTheCockpit]

You are right. Toilet paper is over now. I am going to play with the code for next day or two and will put my findings and problems here. Thanks for the advise.

[FromTheCockpit]

Hi, Ok, I have the following code now:

STRING: $GPRMC,123339.000,A,5134.2770,N,00007.8480,E,1.14, 254.38,271009,,*09

ab:
DEBUGIN 2000,ab,[WAIT("$GPRMC"),skip 14,DEC2 ladeg,DEC2 lamin,skip 1,DEC4 lasec1,skip 3,DEC3 lodeg,DEC2 lomin,skip 1,DEC4 losec1]
losec=lasec1 * 6 : lasec=lasec1 * 6
pause 1000
DEBUG "Lat ",DEC2 ladeg," Deg ", DEC2 lamin," Min ",DEC2 lasec," Sec ",10,"Lon ",DEC3 lodeg, " Deg ", DEC2 lomin," Min ",DEC2 losec," Sec ",13,10,10
goto ab

And the WRONG result on my screen is the following:
Lat 51 Deg 34 Min 36 Sec
Lon 000 Deg 07 Min 36 Sec

Can you advise me please of what am I doing wrong here?

[FromTheCockpit]

I got the co-ordinates working now. Just wondering how to go about getting the speed into variable as it keeps changing, 1.14 knots now, may be 100.20 knots later!