Transmitting Data - g7kse/CanSat GitHub Wiki

Transmitting the data from the CanSat is not as complex as it might seem. Much of the work has already been done with the Adafruit libraries. In the example code we'll be sending the following data to the receiver on the ground

Data	Unit	Note
Temperature	Deg C	Primary mission data - taken from the BMP280
Pressure	hPa	Primary mission data - taken from the BMP280
Mission Time	s	Time from 'Power On' in seconds
Altitude	m	Inferred from the pressure difference at the time of launch

Hardware

The Pico needs to interface with 2 devices. The BMP280 (I2C) temperature and pressure sensor (that also can infer altitude from 2 points - we'll go through this below) and the RFM9x LoRa transceiver (SPI) that will send the data. Because we are using both I2C and SPI we need to make use of a few more pins so the Pin allocations are as below. Note the modules I used are both 5v tolerant so you can use the 5V VBus line off the Pico and any of the ground pins. Data pins are:

Board	Pin
BMP280 - SDA	GPIO Pin 14
BMP280 - SCL	GPIO Pin 15
RFM9x - SCK	GPIO Pin 2
RFM9x - MISO	GPIO Pin 3
RFM9x - MOSI	GPIO Pin 4
RFM9x - CS	GPIO Pin 6
RFM9x - RST	GPIO Pin 7

Software

The transmit code example is available in the repo and an explanation of the makeup of the code is within the comments.The software reads the BMP280 sensor every second then passes the data to the payload message. The altitude calculation is containing within the library and is based on a relative difference between 2 pressures. So you can either take the sea level pressure on the day of launch (this is sometimes called 'QNH' in aeronautical Q codes) and it will return the height above sea level or an alternative would be to take an accurate measure of pressure at the time of launch and update that line of code to get a height above ground at the launch site.

The data will be sent as a string in the format CanSat example,mission time,temperature,pressure,altitude