2m DP yagi, on rotator, at ITR, Mawson Lakes.  The Tx/Rx switch (and LNA) is in a box just behind the driven elements.  A separate rotator is used for the 70cm band. 

SUSat status


Since launch we have not be able to contact SUSat, despite many attempts from ITR in Adelaide.  This page provides a summary of the recent attempts to communicate with our QB50 cubesat.  


Outline of Commissioning Process


SUSat has two communications payloads (CPs), each with their own monopole antenna.  The UHF payload is licensed to transmit anywhere on 436.775 MHz and the VHF on 145.835 MHz can only be used over Australia.  Both payloads should have power and are initially configured for receive-only operation.  After receiving a valid uplink packet, either payload should start beacon transmissions in FSK at 1200 bit/s for a three minute period.  (The UHF beacon can be enabled for continuous operation, once per 10 sec frame or multiple frames,  by uplink control packets.)  Except for some initial commissioning, the CPs only need 3V and 5V and are not dependent on the OBC. 


The SUSat CPs, and their respective antenna modules (AMs), are largely independent.  It seems unlikely that both UHF and VHF systems have failed.  The AMs each use a hot resistor to burn nylon ties which hold the coiled 6mm steel tape before deployment.  Both AM burning circuits require a 12V supply to be turned on by the OBC. It was difficult to avoid this single point of failure in the cubesat design and it seems the most likely reason we haven't been able to communication with SUSat.  A problem in the 12V supply could be due to software, battery or power control issues.  


Outline of Testing Process. 


A large number of the CPs tests for cubesat internal comms protocols and electrical interfaces were conducted in the lab over short distances.  Apart from some early balloon tests, we also carried out a number of 'distance' tests of the final prototype CPs and actual ground station,  over about 30 km (e.g. Mawson Lakes to/from Mt Lofty), with appropriate attenuation of signal levels to simulate realistic link distances (e.g. adding a 30 dB attenuator between CP and AM etc) .   (These tests led to some GS fault rectification!)  


During normal communications the cubesat and GS must maintain careful frame sync over the 10 sec TDM frame period, described on previous pages.  This synchronisation isn't required for initial communication, as the GS repeatedly sends uplink telecommands over several seconds and then listens for a response from the cubesat.  After hearing a valid downlink packet, during normal communications the GS then (continuously) adjusts its frame timing to match the  downlink transmissions.  This GS tracking is not required for a simple test of whether the cubesat is able to receive a telecommand and send a downlink status packet in response. 


The AMs were tested many times in the lab.  We did have one or two failures in antenna deployment during these trials, which resulted in improved procedures for stowing the antennas, burn durations etc.  


Recovery Options 


At present (mid July, 2017) we are still hoping it might be possible for the Dutch 25m facility (Dwingeloo) to transmit a UHF uplink command to SUSat.   The huge gain in uplink EIRP might** allow packet reception on SUSat without AM deployment.    One complication in this approach is that the Dutch facility uses a conventional transceiver in packet radio mode, whereas we have used SDR methods e.g. using discrete-time processing for FSK  modulation, followed by replay of stored IQ data files.  

 

** If the AMs have not been deployed,  it is difficult to estimate the current antenna "gain".   From a quick 70cm test in the lab, over a distance of about 10m to a "tinsat model" we used during early stages of the project,  the loss appears to be in the vicinity of 40 to 50 dB.