Telemetry : define received data

[wanysteus]

• Define precisely the sensor data we will receive
  • Sensor type/name
  • Data type
  • Raw data ? Processed data ?
  • If raw data, how do we process it ?

[wanysteus]

Background

From the datasheets (see README.md in #13) :

• (did not read the IMU datasheet yet but usually with Invense there is calibration data on read-only memory)
• The static pressure sensor has calibration data for temperature and pressure on read-only memory on-chip
• The magnetometer calibration data is automatically processed on-chip -> no need do deal with it
• The dynamic pressure sensor does not need calibration
• (did not read the GPS receiver datasheet yet)

We can work with only the raw data for some sensors but for others, we need to get the calibration data on the Ground Station somehow

• Should we use special frames to send out the calibration data? Yes
• Should we get the calibration data during board manufacturing and hardcode it ? No

Flow

When powered up, the OBC activates the telemetry transmitter and does a complete health check. This health check is (hopefully) received by the Ground Station. During this health check, the header and some calibration data is sent. Some status messages are also sent. For example "health check : OK" or "No response from IMU1".

Once this is complete, sensor readings are sent continuously.

The telemetry transmitter can be turned off and turned on again by the ground station. When it is turned on again, a full health check is performed.

Types of frames received

Frames are separated by a newline (\n) character

Data frames

Just data from the sensors. This will be received very often -> Stored in a .csv file

Frame structure:

#data1&data2&...&dataN

data.csv:

# Header
TimeReception, Complete, TimeMeasurement, RawData1Sensor1, ..., RawDataNSensorM
# Data
ValueFrame1, True, ValueFrame1, ValueFrame1, ..., ValueFrame1
ValueFrame2, False, ValueFrame2, ValueFrame12 ..., MissingValue
...

Context frames

Those contain the description of the data frame fields and calibration data from the sensors -> Calibration data stored in a .yaml file and header stored on the first line of the above .csv

As those frames are needed to process the received data frames on the ground station, they should be sent on regular intervals (every few seconds). The ground station will wait for them to process the data

NB: The header content must be unambiguous and knowing it must be enough to know what data it is and how to convert it

Frame structure:

@header1&header2&...&headerN (header) %cal1:value1&cal2:value2&...&calM:valueM (calibration)

calibration.yaml:

BMP280_dig_T1: 27504
BMP280_dig_T2: 26435
...

NB: it is easy to parse a .yaml file as a dict in python with PyYAML

Status frames

Those can contain error messages, events (like parachute deployment) -> Stored in a logging file

Frame structure:

$message

status.log:

2019-10-03T14:17:25 Error : something bad happened
2019-10-03T14:17:36 Info : parachute deployed
...

[wanysteus]

Defined on the drive