Contributed by Stan Armington.
Originally posted here.

This information is based on experimentation, and is probably not scientifically correct. I'd certainly appreciate any input or advice that would help improve this document. I wrote it in response to several requests from the NSBasic group for advice on how to link a GPS to a palm. You can direct advice, corrections or suggestions to stan© mallatreks.com. For some excellent links to GPS information, go to joe.mehaffey.com. I also have some information and waypoints at www.treks.com.np/gps

The Palm OS platform is excellent for GPS data collection because it allows you to collect the data you want in your own way. All commercial GPS units are designed to accommodate the needs of a specific application - either navigating to a waypoint, collecting continuous data (tracks), collecting discrete points, plotting on a map, etc. I want to handle the data in my own way - which none of the commercial units allow - and therefore wrote my own system.

For an outstanding example of what is possible with GPS, look at the system written by Robin Lovelock at http://www.gpss.co.uk/. This system was what gave me the confidence to go off and start manipulating my own GPS data.

The only units I have found that attach to the Palm are the ones by Rand McNally (RM) - one model (made by Navman in New Zealand) for the Palm 3 and another (made by Magellan) for the Palm V. I've also seen a note about a Korean unit that can be attached to a Handspring. I have not tried the Magellan Palm V unit, but the Palm 3 version takes a long time to lock on. Therefore, for testing and actual use I'm using a Garmin Etrex ($US$119) and a cable ($38) to connect to the Palm cable (though the hotsync cradle can also be used). You need a null modem with a male-male gender changer to collect the two cables. This makes a complicated and cumbersome system that almost defeats the purpose of using a Palm. I'd appreciate any information on other units to try. Also look at www.celia.mehaffey.com/dale/pilotgps.htm for suggestions.

Most units (including the RM) will output NMEA sentences. These are strings that start with a $ and end with a carriage return. The most straightforward discussion of these sentences is in werple.net.au/~gnb/gps/nmea.html The sentence of most interest is the $GPGGA (Global Positioning System Fix Data) which has latitude, longitude, elevation, number of satellites and an estimate of precision.

Remember that even with Selective Availability turned off, GPS is still prone to errors. This is almost always at least 10 metres horizontally and up to 50 metres vertically. This can be reduced by differential correction using a base station. I have not investigated how to convert the data I collect into something that can be differentially corrected. For precise altitude measurements I still use a US $3000 Trimble Geo Explorer and correct the data using the Trimble software.

You can also improve precision by averaging the recorded positions. That's what I'm doing and it makes things a bit slower and more complicated. I would like to collect data in the background (so that one could enter information about a position at the same time that the GPS is averaging the readings) but this may not be possible with the Palm OS.

Note that NMEA data comes in as degrees plus decimal minutes. This means you must convert into decimal degrees if you are going to do any calculations.

NS Basic issues:

1. Data Capture

The serial input via NSBasic is slow. I tried collecting characters one by one and lost a bunch (I wonder if ths could be solved by allowing a timeout of less than one second in the serialreceive function?). I finally wrote the following code, which works. Fortunately the NMEA sentences just keep rolling in and it's no big deal if you miss one. I don't know how one would handle a situation where it was critical to capture all the serial input. Ideally one would have a routine that somehow worked in the background to capture and average the data. The following routine is in a loop that ends when the user pushes a stop button. NMEA data comes in at a baud rate of 4800 (8-1-none).

err=serialreceive(Buffer,150,1)    
   if err=0 then   
   i=instr(1,Buffer,~$~,0)     'sentence start?
   j=instr(I+1,Buffer,&h0d,0)    'sentence end?
   if I>0 and j>0 then       'yes - then grab it
     line=mid(Buffer,I,j-I)       
     term.text=line                      'display the NMEA sentence
     if anydata=false then
       GPS_Status.text=~Receiving~    'put up a flag saying we're finally receiving data
       anydata=true       
     end if  
     if left(Line,6)=~$GPGGA~ then
       showlocation(line)                       'convert and save
     end if 
    end if             
   end if

2. Auto Off

George says he is going to handle this in a future version, but for now it's not possible to turn off the palm auto shutoff feature. If you don't do this, the unit shuts down and stops capturing data (though it does save what it has). The easiest way I found to do this is to use the Autooff program. This turns the Autooff OFF. When you are done with the GPS, you MUST go back to the Palm preferences and set the auto-off time back to 1,2 or 3 minutes. Autooff is by Joe Grand, The Grand Design, http://www.mindspring.com/~jgrand

3. Checksum

The NMEA 0183 Standard allows for an optional checksum. The checksum is preceded by the asterisk (*) character placed after the last data field. It is a two-character field equal to the hex value obtained by XOR'ing all the character bytes of the sentence.

This is impossible to calculate (at least in any straightforward manner) in NSBasic. I've had to do a lot of error checking and validation of data as a workaround. George Henne says that bit manipulation is also in the works.

4. UTM conversion

It would be nice to display UTM co-ordinates so that it would be easy to relate toe display to map co-ordinates. Unfortunately this is a long complex calculation that takes about FIVE SECONDS on the palm. Here's the NSBasic code:

NS Basic routine to convert Latitude/Longitude to UTM

"sub LLtoUTM(Lat as float, Long as float)        
' results are global variables
'formula adapted from C code on url http://www.gpsy.com/gpsinfo/geotoutm/gantz/UTMConversions.cpp          
'East Longitudes are positive, West longitudes are negative. 
'North latitudes are positive, South latitudes are negative
'Lat and Long are in decimal degrees
'If input is in dddmm.mm then need to convert
Dim LongOrigin as float
Dim  eccPrimeSquared as float
Dim  N as float
Dim  T as float
Dim  C as float
Dim  M as float
Dim  A as float
Dim  LatRad as float
Dim  LongRad as float
Dim  LongOriginRad as float
Dim  Longtemp as float
Dim  ZoneNumber as Integer
Dim k0 as float
Dim eradius as float
Dim eccSquared as float
Dim ECCcube as float

k0 = 0.9996
  eradius = 6378137  'WGS84 EquatorialRadius
  eccSquared = 0.00669438 'WGS84 eccentricitySquared  
  ECCcube=pow(eccSquared,3)      
  LongTemp = (Long+180)-int((Long+180)/360)*360-180 '-180.00 .. 179.9
  LatRad = DegtoRadians(Lat)
  LongRad = DegtoRadians(LongTemp)
  ZoneNumber = int((LongTemp + 180)/6)+1
  if (Lat >= 56.0) and (Lat < 64.0) and (LongTemp >= 3.0) and (LongTemp < 12.0 ) then
    ZoneNumber = 32   
  end if  
  LongOrigin = (ZoneNumber - 1)*6 - 180 + 3  '+3 puts origin in middle of zone
  LongOriginRad = DegtoRadians(LongOrigin)
  UTMZone=ZoneNumber
  eccPrimeSquared = (eccSquared)/(1-eccSquared)
  N = eradius/sqrt(1-eccSquared*Pow(sin(LatRad),2))
  T = pow(tan(LatRad),2)
  C = eccPrimeSquared*pow(cos(LatRad),2)
  A = cos(LatRad)*(LongRad-LongOriginRad)
  M = (1 - eccSquared/4 - 3*pow(eccSquared,2)/64 - 5*EccCube/256)*LatRad 
  M=M - (3*eccSquared/8 + 3*pow(eccSquared,2)/32 + 45*EccCube/1024)*sin(2*LatRad) 
  M=M + (15*pow(eccSquared,2)/256 + 45*EccCube/1024)*sin(4*LatRad)
  M=M - (35*pow(eccSquared,3)/3072)*sin(6*LatRad)
  M=eradius*M	
  UTMEasting = A+(1-T+C)*pow(A,3)/6
  UTMEasting=UTMEasting + (5-18*T+pow(T,2)+72*C-58*eccPrimeSquared)*pow(A,5)/120
  UTMEasting=k0*N*UTMEasting+ 500000.0
  UTMNorthing = M+N*tan(LatRad)*(pow(A,2)/2+(5-T+9*C+4*pow(C,2))*pow(A,4)/24)
  UTMNorthing = UTMNorthing + (61-58*T+pow(T,2)+600*C-330*eccPrimeSquared)*pow(A,6)/720
  UTMNorthing = k0*UTMNorthing 
  if Lat < 0 THEN
    UTMNorthing = UTMNorthing+10000000.0 '10000000 meter offset for southern hemisphere'
  end if
end sub"

Revised 8 Sep 2000