To Receive EntrySat

The beacon :

Entrysat plans to remain in orbit for at least six months before launching its scientific mission to study the re-entry environment into the atmosphere. During its operation in orbital mode, Entrysat will emit a beacon every minute. This 50-byte message contains basic information such as the satellite’s operating mode, supply voltages and currents, and platform temperatures (see below). As it approaches the ground station (Toulouse, France), additional information about the cubesat’s sensors will be sent to the ground.

The radio link :

The link to the ground uses a BPSK modulation at 9600 baud on the frequency 436.95 MHz in the amateur radio bands. Moreover, Entrysat has an amateur radio call sign ON2FR which is essential for transmitting on this band. There are several ways to receive and demodulate its signal. This is left to each individual’s discretion to experiment with receiving digital data. However, we propose a solution through an SDR receiver with its software. (Gqrx of OZ9AEC for inatance) and a demodulation software like SoundModem of UZ7HO (version G3RUH-BPSK-9K6). The SDR receiver must be in USB mode, with a bandwidth of at least 24kHz and a frequency offset of -12kHz to compensate for the USB offset (or +12kHz if you choose LSB mode)

 

The decoding of the :

The beacon uses an AX25 frame to deliver a PUS packet according to the ECSS standard. Here is an example of a received frame:

8c 6c 96 a8 82 40 e0 : F6KTA-0 (ASCII<<1)
9e 9c 60 64 8c a4 61: ON02FR-0 (ASCII<<1)
03 f0 : Control Bit / Protocol Identifier (fixed)
—————–Information Field ——-
00 00 00 00 : Time not used
————— TM PACKET————–
08 01 : packet ID
c7 29 : packet sequence control
00 12 : packet length
10 : Spare/TM source packet number/ Space (fixed)
03 19 : service / service sub_type (3,25)
23 fe bd cd 17 : time (0x23256^3 + 0xfe256^2 + 0xbd256 + 0xcd + 0x17256^-1) = 2019/02/19 @ 13:14:53
time since 2000/1/1 @ 00:00:00
————–SOURCE DATA——————
06 : SID (6)
00 : Mode Safe
f1 : EPS_VBATT ( 3+0.05×241 = 15.05V ) Battery Voltage
6b : EPS_BATT_VCURRENT ( -1+0.0078740×107 = -157.48mA ) Battery consumption
00 : EPS_3.3V Current (0.025×0 = 0A) 3.3V current sink
00 : EPS_5V Current (0.025×0 = 0A) 5V current sink
9e : TRX_TEMP (-15+0.25×158 = 24.5°C) Radio Transceiver temperature
a0 : EPS_TEMP ( 25 °C) Power Management module temperature
98 : BATT_TEMP ( 23°C ) Battery temperature
1f c6 : Packet CRC
b0 : Frame status, AX.25 transfer frame information field (fixed)
09 be fe 23 : Time, AX.25 transfer frame information field, Coded in Little Endian (last packet sent)

In this example, the cubesat is in survival mode. Therefore, it does not consume current on its 3.3V and 5V power lines for the payloads (EPS 3.3V and 5V), Current=0A.

A decoding software has been written by AMSAT-FR and is available for download. It interfaces with the demodulation software through its KISS server.

Link to the EntrySat Decoder (thank you Xtof)