Star Tracker's Quaternion seems to be off

Hi everyone,

I am developing an attitude mode to align a vector in ECI (N) frame with the b2 axis, similar to what the Sunsafe mode does. To move from the inertial frame to the body frame I need to retrieve the inertial quaternion of the body, which should be provided by the Star Tracker in the generic_adcs/fsw/src/generic_adcs_ingest.c script as:

void Generic_ADCS_ingest_generic_st(CFE_MSG_Message_t * Msg, Generic_ADCS_DI_St_Tlm_Payload_t *St)
{
    GENERIC_STAR_TRACKER_Device_tlm_t *st = (GENERIC_STAR_TRACKER_Device_tlm_t *)Msg;

    St->valid = st->Generic_star_tracker.IsValid;
    double q[4] = {st->Generic_star_tracker.Q0, st->Generic_star_tracker.Q1, st->Generic_star_tracker.Q2, st->Generic_star_tracker.Q3};
    QxQ(q, St->qbs, St->q);
}

In particular, the QxQ method should move from the Sensor frame to the Body frame, by using the quaternion specified in generic_adcs/fsw/cfg/Inp_DI.txt as :

!!!!!!!!!!!!!!! Star Tracker !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
-0.5510871 -0.4059919 -0.5108982 0.5200544 ! Quaternion from sensor to body frame

As far as I understand, then the resulting quaternion stored in the DI_St_Tlm_Payload data structure should be the body quaternion that I am looking for. However, when I confront this quaternion with the one directly retrieved from 42 in the 42Truth Sim, they are totally different. This leads to a wrong conversion between my ECI and body frame, while the conversion becomes correct when, instead of retrieving the ST quaternion, I retrieve directly the body quaternion from 42.

Is there something that I am missing, or is perhaps the quaternion specified in Inp_DI the wrong one?

Also, to get the body quaternion from 42, I had to modify the 42 code itself to return the ST quaternion as S->B[O].qn instead of the one computed by the method in the (42sensors.c script). For some reason, modifying the data_provider.cpp code of the ST in this way:

qn_key.append("SC[").append(std::to_string(_sc)).append("].B[0].qn"); 
std::string qn_values = _dp.get_value_for_key(qn_key);

_generic_star_tracker_data_is_valid = (valid_value == "1");
std::vector<double> data;
parse_double_vector(qn_values, data);

leads to the collapse of the ST sim.

Thanks in advance for the support, I really appreciate it!

nasa / nos3

Star Tracker's Quaternion seems to be off #351