Encoder frame

All the data must be put in a protocol to decode the data on the ground, and we shall refer to this as the encoder frame.

The most naïve encoder frame (which is not used) consists only of the raw digitally sampled 8-bit analog signals being sent down to the ground without any interuptions or insertions. After sending sensor 1 to 8, the frame starts over again with sending sensor 1 and upwards. After a signal dropout, there is no way for the decoders at the ground station to know what the binary data is—when a new sensor is starting or where a new frame is starting. To avoid this we start the frame with a 16-bit sync word which is always fixed to EB90 (in hexadecimal). It is unlikely that a sync word value will occur by accident elsewhere in the encoder frame, and if it does, we would only loose two frames as a worst case scenario after a dropout. When the encoder recognizes the sync word, it knows exactly where in the frame it is and what comes next. The next is a frame counter, a 16-bit long value incrementing by one for each frame so that one can easily see how many frames a dropout has lasted. An overflow of the sync word only happens every 30 seconds or so. After the sync word the eight 8-bit sensor values are sent, and the two 8-bit digital sensors are sent last, before a new encoder frame is started with a sync word and then the incremented frame counter and so on.

The transmitter sends with a data rate of 250 kbps. Knowing that each frame is 14 bytes (two sync bytes, two counter bytes, eight analog signal bytes and two 8-bit digital), we can then easily calculate the number of frames per second. This must then also be the sampling frequency of the sensors.

How the data is received and processed at the ground station is a topic for the campaign period.

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This article is part of a pre-course program used by Andøya Space Education in Fly a Rocket! and similar programs.