You can monitor the LabSat 4 NMEA output alongside the RF during replay. The NMEA output is available via the USB port on the rear panel of LabSat 4. Connect to a PC to view the output in a terminal program.
The NMEA output from LabSat 4 will resemble the screenshot below. Highlighted within the red square is an individual packet of the NMEA string.
GGA NMEA Format Definition
$GPGGA_hhmmss.ss_llll.ll_a_yyyyy.yy_a_x_xx_x.x_x.x_M_x.x_M_x.x_xxxx*hh
1 = UTC of Position
2 = Latitude
3 = N or S
4 = Longitude
5 = E or W
6 = GPS quality indicator (0=invalid; 1=GPS fix; 2=Diff. GPS fix)
7 = Number of satellites in use [not those in view]
8 = Horizontal dilution of position
9 = Antenna altitude above/below mean sea level (geoid)
10 = Meters (Antenna height unit)
11 = Geoidal separation (Diff. between WGS-84 earth ellipsoid and mean sea level. -=geoid is below WGS-84 ellipsoid)
12 = Meters (Units of geoidal separation)
13 = Age in seconds since last update from diff. reference station
14 = Diff. reference station ID#
15 = Checksum
GSA NMEA Format Definition
$GPGSA_A_3______16_18__22_24___3.6_2.1_2.2*3C
$GPGSA_A_3_19_28_14_18_27_22_31_39_____1.7_1.0_1.3*35
1 = Mode:
M=Manual, forced to operate in 2D or 3D
A=Automatic, 3D/2D
2 = Mode:
1=Fix not available
2=2D
3=3D
3-14 = IDs of SVs used in position fix (null for unused fields)
15 = PDOP
16 = HDOP
17 = VDOP
GSV NMEA Format Definition
$GPGSV_3_1_11_03_03_111_00_04_15_270_00_06_01_010_00_13_06_292_00*74
$GPGSV_3_2_11_14_25_170_00_16_57_208_39_18_67_296_40_19_40_246_00*74
$GPGSV_3_3_11_22_42_067_42_24_14_311_43_27_05_244_00____*4D
$GPGSV_1_1_13_02_02_213__03_-3_000__11_00_121__14_13_172_05*67
1 = Total number of messages of this type in this cycle
2 = Message number
3 = Total number of SVs in view
4 = SV PRN number
5 = Elevation in degrees, 90 maximum
6 = Azimuth, degrees from true north, 000 to 359
7 = SNR, 00-99 dB (null when not tracking)
8-11 = Information about second SV, same as field 4-7
12-15 = Information about third SV, same as field 4-7
16-19 = Information about fourth SV, same as field 4-7
GLL NMEA Format Definition
$GPGLL_3751.65_S_14507.36_E*77
$GPGLL_4916.45_N_12311.12_W_225444_A
4916.46_N Latitude 49 deg. 16.45 min. North
12311.12_W Longitude 123 deg. 11.12 min. West
225444 Fix taken at 22:54:44 UTC
A Data valid
$GPGLL_5133.81_N_00042.25_W*75
1 2 3 4 5
1 = 5133.81 Current latitude
2 = N North/South
3 = 00042.25 Current longitude
4 = W East/West
5 = *75 checksum
RMC NMEA Format Definition
$GPRMC_081836_A_3751.65_S_14507.36_E_000.0_360.0_130998_011.3_E*62
$GPRMC_225446_A_4916.45_N_12311.12_W_000.5_054.7_191194_020.3_E*68
225446 Time of fix 22:54:46 UTC
A Navigation receiver warning A = OK, V = warning
4916.45_N Latitude 49 deg. 16.45 min North
12311.12_W Longitude 123 deg. 11.12 min West
000.5 Speed over ground, Knots
054.7 Course Made Good, True
191194 Date of fix 19 November 1994
020.3_E Magnetic variation 20.3 deg East
*68 Mandatory checksum
$GPRMC_220516_A_5133.82_N_00042.24_W_173.8_231.8_130694_004.2_W*70
1 2 3 4 5 6 7 8 9 10 11 12
1 = 220516 Time Stamp
2 = A validity - A-ok, V-invalid
3 = 5133.82 current Latitude
4 = N North/South
5 = 00042.24 current Longitude
6 = W East/West
7 = 173.8 Speed in knots
8 = 231.8 True course
9 = 130694 Date Stamp
10 = 004.2 Variation
11 = W East/West
12 = *70 checksum
$GPRMC_hhmmss.ss_A_llll.ll_a_yyyyy.yy_a_x.x_x.x_ddmmyy_x.x_a*hh
1 = UTC of position fix
2 = Data status (V=navigation receiver warning)
3 = Latitude of fix
4 = N or S
5 = Longitude of fix
6 = E or W
7 = Speed over ground in knots
8 = Track made good in degrees True
9 = UT date
10 = Magnetic variation degrees (Easterly var. subtracts from true course)
11 = E or W
12 = Checksum
VTG NMEA Format Definition
$GPVTG_360.0_T_348.7_M_000.0_N_000.0_K*43
$GPVTG_054.7_T_034.4_M_005.5_N_010.2_K
054.7_T True track made good
034.4_M Magnetic track made good
005.5_N Ground speed, knots
010.2_K Ground speed, Kilometers per hour
$GPVTG_t_T___s.ss_N_s.ss_K*hh
1 = Track made good
2 = Fixed text 'T' indicates that track made good is relative to true north
3 = not used
4 = not used
5 = Speed over ground in knots
6 = Fixed text 'N' indicates that speed over ground in in knots
7 = Speed over ground in kilometers/hour
8 = Fixed text 'K' indicates that speed over ground is in kilometers/hour
9 = Checksum