CASSIOPE SPACECRAFT (Swarm-E)
PROCESSED DATA HANDBOOK

GAP Line of Sight (LOS) TEC NetCDF File

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The GAP Line of Sight TEC netCDF file provides the line of sight total electron content between a given e-POP GPS receiver and any locked GPS satellite. The LOS TEC data has been phase-leveled and bias compensated according to Watson, C., et al., (2018) Enhanced Polar Outflow Probe ionospheric radio occultation measurements at high latitudes: Receiver bias estimation and comparison with ground-based observations, Radio Science, 53, 10.1002/2017RS006453.

Filename Format

“GAP_yyyymmdd_HHMMSS_hhmmss _LOS_TEC_RCVRr_V.v.r.nc”

• r: GAP Receiver number (0-3=GAP-A; 4=GAP-O)
• mm: File month
• dd: File day
• yyyy: File year
• HH: File start hour (UT)
• MM: File start minute (UT)
• SS: File start second (UT)
• hh: File end hour (UT)
• mm: File end minute (UT)
• ss: File end second (UT)
• V: Major code version
• v: Minor code version
• r: Reprocessing count

Global Attributes

• Month (integer): Month of file
• Day (integer): Day of file
• Year (integer): Year of file
• RCVR (integer): GAP Receiver number (0-3=GAP-A; 4=GAP-O)
• RES (double): Data time resolution (seconds)
• Start_Time (double): Data start time (hours of UT)
• End_Time (double): Data end time (hours of UT)

Variables

• PRNs (integer): 1D array of GPS PRN numbers
• GPS_DCBs (double): 1D array of GPS differential code biases (DCBs) for each PRN (TEC units)
• RCVR_DCB (double): GAP receiver DCB (TEC units)
• RCVR_DCB_RMS (double): RMS of GAP receiver DCB (TEC units)
• UT (double): 1D array of time tags for each LOS TEC value (hours of UT)
• *LOS_TEC (double): 2D array of LOS TEC with dimensions length(UT) x length(PRNs) (TEC units)
• L1 (double): 2D array of L1 carrier phase observable (wave cycles)
• L2 (double): 2D array of L2 carrier phase observable (wave cycles)
• L1_CN0 (double): 2D array of nominal carrier-to-noise ratio for L1 observations (dBHz)
• L2_CN0 (double): 2D array of Nominal carrier-to-noise ratio for L2 observations (dBHz)
• **DATA_FLAGS (integer): 2D array of data quality flags for LOS_TEC (see description below)
• XGPS (double): 2D array of ECEF x-component locations of GPS satellites (km)
• YGPS (double): 2D array of ECEF y-component locations of GPS satellites (km)
• ZGPS (double): 2D array of ECEF z-component locations of GPS satellites (km)
• XEPOP(double): 1D array of ECEF x-component location of CASSIOPE satellite (km)
• YEPOP(double): 1D array of ECEF x-component location of CASSIOPE satellite (km)
• ZEPOP(double): 1D array of ECEF x-component location of CASSIOPE satellite (km)

*LOS_TEC is phase-leveled and calibrated for satellite and receiver DCBs.

**DATA_FLAGS description:

Data flag values range from 0-255 and can be converted to binary to extract corresponding error:

Value	Description
0	LOS_TEC data is OK
Bit 0 set	Cycle slip detected by receiver (correction applied if Bit 7 set, otherwise data point is set to undefined (NaN))
Bit 1 set	Half cycle ambiguity/loss-of-lock detected by receiver (data point is set to NaN)
Bit 2 set	Low signal strength (data point is set to NaN)
Bit 3 set	High multipath+noise occurrence (data point is set to NaN)
Bit 4 set	Outlier detected (data point is set to NaN)
Bit 5 set	A cycle slip correction was applied at this data point. The cycle slip was detected by receiver software or our processing algorithm.
Bit 6 set	Data point falls within an interval of instrumental-type data gaps (missing data), which are present due to receiver bottle necking. This is a known issue with the GAP receivers. Data is still valid and usable, but missing data is periodically present during intervals when this bit is set. Corresponding data points may either be valid or, when a data gap occurs, set to NaN.
Bit 7 set	Loss of lock has occurred while GPS satellite is in view. Data is unavailable at data point is set to NaN.
-1	There is no LOS_TEC data for this time

Integer value of data flag can be converted to binary.  For example, a data flag value of 97 corresponds to:  1x2⁰+0x2¹+0x2²+0x2³+0x2⁴+1x2⁵+1x2⁶+0x2⁷.  Bit 0, 5, and 6 are set, indicating that a cycle slip was detected by the receiver software, a cycle slip correction was applied, and data point falls within an interval of instrumental data gaps.

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