A Dropkick log is a CSV text file. It is composed of NMEA 183 standard GPS records interspersed with the extension records described below.
A new log file is automatically created and opened when the device detects that a climbout to jump altitude has started. Logging continues in that same file until the jumper reaches the ground.
Some NMEA records include a UTC timestamp computed by the GNSS receiver. A native clock in an Arduino application is the millis() system call. It is a unsigned 32-bit value reflecting the number of milliseconds elapsed since boot time. Both time values will appear in a log file. These two timelines are designed to be correlated using a $PTH record, described below.
A single instance of this record appears at the start of a log. It documents the software version used to create the log.
| Description | |
|---|---|
| $PVER | Record identifier |
| id string | ID of this app version |
| version number | an integer version number of the application |
$PVER,"Dropkick, version 0.52",52
PIMU record logs sensor information from the MPU6050 IMU IC. The MPU6050 is mounted on the Dropkick PCB such that the postive X-Axis projects out the SD card slot, the positive Y-Axis projects out the face opposite the USB port, and the positive Z-Axis projects out the "top" of the enclosure. Since the device might be carried by a skydiver in almost any orientation, it would be up to analysis software to discern the body orientation inferred by actual readings.
| Description | |
|---|---|
| $PIMU | Record identifier |
| millis() timestamp | Time of sample in milliseconds |
| X-accel | expressed in meters per second squared |
| Y-accel | expressed in meters per second squared |
| Z-accel | expressed in meters per second squared |
| X-rate | raw X rotation rate; expressed in radians per second |
| Y-rate | raw Y rotation rate; expressed in radians per second |
| Z-rate | raw Z rotation rate; expressed in radians per second |
$PIMU,13925127,7.01,5.31,0.49,-0.00,0.01,0.11
This record logs pressure information captured from the DPS310 sensor IC and a resistor ladder used to monitor the VBATT battery line voltage level.
| Description | |
|---|---|
| $PENV | Record identifier |
| millis() timestamp | Time of sample in milliseconds |
| static air pressure | expressed in hPa |
| estimated altitude | based on static air pressure reading for a standard day; expressed in feet |
| VBATT voltage | battery voltage level (3.5V - 3.8V typ.); sampled once every 30 seconds |
$PENV,13925040,984.62,791.18,3.79
A PTH record is used to correlate Arduino millis() time with the GNSS computed time of day clock information that appears in NMEA records. A PTH record will appear immediately following each NMEA GGA or GGL record. The timestamp present in the record reflects the millis() time at the arrival of the first character of the NMEA record.
$PTH,13925296
This records the estimated surface altitude of the landing area.
| Description | |
|---|---|
| $PSFC | Record identifier |
| estimated surface altitude | Expressed in feet, MSL. Note that the device rarely rests on the ground surface while operating -- no effort is made to take into account the resting height of the device for these samples. |
$PSFC,880
Sensor records are written to the file at these rates:
| Record Type | Reporting Rate |
|---|---|
| GNSS position report | 2 Hz |
| PIMU | 40 Hz |
| PENV | 4 Hz |
| PTH | follows each GGA and VTG record |
Valid for version 52 of the Dropkick firmware.