I'm watching this because I too have a IIIc.
The full text from the manual is :-
APPENDIX 05: Skymap IIIC Serial Data Output Sentences.
Skymap IIIC can output standard aviation RS232 data at 9600 baud or NMEA 0183 format RMC,
GGA and RMB sentences at 4800 baud. The data output type is Setup on Screen 35 Data
input/output Setup and Test Screen.
NMEA 0183 Data Format.
4800 baud, 8 data bits, 1 stop bit, no parity. Each sentence is followed by a checksum (*hh), carriage
return (CR) and a line feed (LF). All three sentences sent every two seconds.
NMEA – RMC Sentence.
$GPRMC,<1>,<2>,<3>,<4>,<5>,<6>,<7>,<8>,<9>,
<10>,<11>*hh<CR><LF>
<1> UTC time of position fix, hhmmss format.
<2> Status, A= Valid position, V=warning
<3> Latitude,ddmm.mmm format.
<4> Latitude hemisphere, N or S.
<5> Longitude, dddmm.mmm format.
<6> Longitude hemisphere, E or W.
<7> Speed over ground (000.0-999.9 knots).
<8> Course over ground (000.0-359.9 true).
<9> Date of fix ddmmyy format.
<10> Magnetic variation 000.0-180.0o.
<11> Variation direction E or W.
Sample sentence.
$GPRMC,132404,A,5111.465,N,00102.845,W,090.0,304.1,020492,004.3,W*7A
NMEA – GGA Sentence.
$GPGGA,<1>,<2>,<3>,<4>,<5>,<6>,<7>,<8>,<9>,M,<10>,M,,*hh<CR><LF>
<1> UTC time of position fix, hhmmss.
<2> Latitude, ddmm.mmm format.
<3> Latitude hemisphere, N or S.
<4> Longitude dddmm.mmm format.
<5> Longitude hemisphere, E or W.
<6> GPS quality. 0 = no fix, 1 = fix good, 2 = fix good, differential mode.
<7> Number of sats in use, 00 to 08.
<8> DOP, 01.0 to 99.0.
<9> Antenna height above MSL (metres).
<10> Geoidal offset in metres.
Sample sentence.
$GPGGA,091636,5119.607,N,00122.001,E,1,04,1.8,18.3,M,-47.0,M,,*5D
NMEA – RMB Sentence.
$GPRMB,<1>,<2>,<3>,<4>,<5>,<6>,<7>,<8>,<9>,<10>,<11>,<12>,<13>*hhCRLF
1. Position valid (A = valid, V = invalid).
2. XX.XX Cross track error in nautical miles.
3. X Direction to steer (L = left, R = right).
4. XXXX Four character start waypoint identifier. (first four characters of name).
5. XXXX Four character destination waypoint identifier.
6. DDMM.MM Destination Lat.
7. X Destination hemisphere (N or S).
8. DDDMM.MM Destination Long.
9. X Destination hemisphere (E or W).
10. XXX.X Range to destination in nautical miles.
11. XXX.X Bearing to destination TRUE.
12. XXX.X closing velocity in knots.
13. X Arrival flag. (A = arrival, V = not arrival).
Sample sentence.
$GPRMB,A,00.00,L,1,2,5142.20,N,00217.10,W,048.8,304.0,090.0,V*18
Notes on the RMB sentence.
Note the direction in field 3 is direction to steer, i.e. off track to the right, steer left.
If not in NAV or DIRECT TO mode, the flag in field 1 will show invalid (V).
If in DIRECT TO mode there will be no start point identifier so field 4 will be blank (spaces). If in DIRECT
TO mode where the DIRECT TO point is just a point in space with no name, field 5 will be blank (spaces)
also.
The arrival flag in field 13 is set to A when the TP imminent message shows on screen and stay set until
the next leg is selected even if the TP passed message comes up on screen.
At the end of the flight plan or DIRECT TO when the unit drops out of NAV mode, the flag in field 1 will
change to a V.
The closing velocity in field 12 is the vectored velocity towards the waypoint. There is no provision for
showing negative numbers so if answer is negative, all zeros will be sent.
For what its worth, the RS-232 data:-
RS-232C AR-NAV Data Format
9600 baud 8 data bits, 1 stop bit, no parity. Type 1 sentence only sent every two seconds. Each line of
data is followed by a carriage return (CR) and a line feed (LF) character except the STX and ETX lines.
Header Data Description.
STX Hex 02.
z aaaaaCRLF GPS altitude in feet.
A s dd mmhhCRLF Latitude where s = N or S, dd = degrees
mm = minutes, hh = hundredths of minutes.
B s ddd mmhhCRLF Longitude designated as above.
C dddCRLF Magnetic track in whole degrees.
D sssCRLF Groundspeed in knots.
E dddddCRLF Distance to waypoint in 1/10ths of nm.
G snnnnCRLF Cross track error where s = L or R (of track)
and nnnn = error in 100ths of an nm.
I ddddCRLF Desired magnetic track in 10ths of a degree.
i.e. BRG to waypoint if you were on track.
K cccccCRLF Destination waypoint name. Dashes sent after
name if shorter than 5 characters.
L ddddCRLF Actual magnetic bearing to WPT (in 1/10ths
of a degree).
Q sdddCRLF Variation. s= E or W. ddd = 1/10ths degrees.
S ----eCRLF e = NAV flag. - = NAV valid, N = NAV not valid.
T ---------CRLF Indicates end of sentence type 1.
ETX Hex 03.
Notes on the AR-NAV sentence.
If the unit is not in NAV mode or does not have a fix, dashes (---) will be sent in fields E, G, I, K, L. Part e
of field S will be set to N.
If the unit does not have a fix, dashes will be sent in the latitude, longitude, track and ground speed fields.
If the variation value is greater than 99.9°, a fourth character will be sent in the Q field (i.e. sddddCRLF).
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I hope that all means something to someone! 
