function readdat, filepath, datestr, timestr, datacount, channelcount

;open the data file:
dat_fh = 1
openr, dat_fh, filepath, /get_lun                        ;open the data file

;read the GPS NMEA header
nmea_string = string('')
readf, dat_fh, nmea_string
point_lun, dat_fh, 0

;parse date and time from the NMEA string
nmea_chars = strarr(strlen(nmea_string))
for i=0, strlen(nmea_string) - 1 do begin
    nmea_chars(i) = strmid(nmea_string, i, 1)
endfor
delim = where(nmea_chars eq ',')
timestr = strmid(nmea_string, delim(0)+1, 6)
datestr = strmid(nmea_string, delim(8)+1, 6)

;get the size of the file
status = FSTAT(dat_fh)
filesize = status.size

;the next line associates the variable gps_end as a byte array - in other
;words, ALL of the data are interpreted as bytes at this point. This is done
;in order to locate the CR/LF after the NMEA string.
cr        = bytarr(200) ;the maximum length of NMEA string is 80
gps_end   = assoc(dat_fh, cr)

;next line searches the gps_end array for occurrences of a line feed.
last_gps  = where(gps_end(0) eq 10)

;next line gets the index number of the first occurrence of a line feed.
last_gps  = last_gps(0)

;get the channel count and data count
;channelcount = 3
datacount = (filesize - (last_gps + 2)) / (channelcount * 2)

print, filesize, datacount, last_gps, channelcount
;stop

;The next line associates the variable 'data' as a uintarr. In addition, the
;association is made such that the first integer is located at last_gps+4 bytes
;from the beginning of the file. This comes from Paul's information (the 4
;bytes will probably also need to be extracted at some point, but are ignored
;at this point
bdata     = uintarr(datacount * channelcount)
data      = assoc(dat_fh, bdata, last_gps+2)

;the following assigns the first data array to bx, swapping byte ordering in
;the process.
;dbdt      = swap_endian(data(0))
dbdt      = data(0)

;=========================================================
;CHECKING THE GPS BIT:
;gps_bit  = fix((dbdt and (2^15)))/2^15
;indices = indgen(864000) *2
;ccc= gps_bit(indices)   ; these are the gps bit indices - 0's and 1's
;aaa=where(ccc eq 1)
;ddd = (aaa-shift(aaa,1))-10
;eee = where(abs(ddd) gt 0) *10

;kkk=intarr(432000)
;kkk(eee) = 1

;=========================================================

;finally, the 16-bit data are masked to 12 bits, converted to signed integers,
; have the midrange point (2048) subtracted and are scaled.
;dbdt      = (fix((dbdt and (2^12-1))) - 2048)/204.8
dbdt      = (fix((dbdt and (2^12-1))) - 2048)*1.25/2048

;don't forget to close the data file!!!!
free_lun, dat_fh

;at this point, all of the data have been read in, although the various
;channels still need to be extracted. This would be done something like:

x_indices = lindgen(datacount) * channelcount ;to get an array of even numbered indices

;then
data_arr  = fltarr(3, datacount)
data_arr(0, *) = dbdt(x_indices)
data_arr(1, *) = dbdt(x_indices+1)
data_arr(2, *) = dbdt(x_indices+2)

;========================================================================
;plot,data_arr(0,000:4300),yrange=[-1,1]
;oplot,ccc(2000:9000)+2
;oplot,kkk(2000:9000)-2
;======================================================================

;to be done:
;- I have not done anything to determine array sizes, which is something that
;does need to be completed before implemeneting the assoc() function. The
;file information needs to be queried for this, as well as the size of the
;NMEA string.

; Also, I have no function to query the number of channels that are sampled.
;I believe this is contained in the 3 bytes following the NMEA string
return, data_arr
end