5.6 Methods of Large-Scale Navigation
363
reference direction vector of the pulsar in the spacecraft’s orbit coordinate system
can be easily obtained in the case of the spacecraft’s orbit parameters known. When
two pulsars are observed simultaneously on the spacecraft, and two direction vectors
are obtained, the attitude parameters of the spacecraft can be estimated by the method
of attitude determination using two vectors.
Using the X-ray detectors installed on the spacecraft to measure the direction
vectors of pulsars, the spacecraft’s attitudes in BCRS or in the orbit coordinate system
can be determined with high precision, getting to the several arc-seconds, which is
comparable to the level of current optical star sensors. Moreover, the attitude determination using the X-ray pulsars can avoid the complex star pattern recognition
algorithms in the traditional celestial navigation. However, it takes time to detect the
X-ray photons and extract the pulse profiles, so that there is a time lag for obtaining
the attitude parameters. Furthermore, the measurement noise of the X-ray detector
will also affect the attitude determination accuracy. The inertial attitude measurements using the gyroscopes have the advantages of complementary with the attitude
measurements using the X-ray detectors. The inertial attitude has rapidly dynamic
response; the random measurement error is small of the high-precision gyroscopes;
the gyroscope drifts can be calibrated by the X-ray detector measurements. In practical application, the measurement data of the X-ray detectors and inertial gyroscopes is often combined to process. Thereby, within sampling interval, the inertial
attitudes are measured by the gyroscopes to use for the spacecraft’s navigation, and
at sampling point of the X-ray detector unit, the measured direction vectors of the
pulsars is used to correct the inertial attitudes for suppressing the accumulated error
caused by the gyroscope drifts, so as to obtain highly accurate, continuous and stable
attitude parameters of the spacecraft.
Supposed that the spacecraft’s attitude measurement system is composed of the
X-ray detector unit and the inertial attitude unit, in which the latter consists of threeaxis strapdown rate integrating gyroscopes in the spacecraft-body coordinate system,
the data processing flow of the combined attitude measurement of the spacecraft is
shown in Fig. 5.14, where t g is the sampling interval of the inertial attitude unit;
t d is the sampling interval of the X-detector unit; ˆ ε is the estimation of correction
vector of the gyroscope drift; ˆ
ε is the estimation of vector of the gyroscope drift; ˆ
θ
is the estimation of correction vector of the spacecraft’s attitude; ˆ
q is the estimation
of the quaternion vector; ˆ
ω is the estimation of the rotational angular velocity of the
spacecraft; A ob is the outputted attitude matrix of the spacecraft. In order to improve
the efficiency of data processing, a special data processor is configured in the X-ray
detector unit, which is used for the X-ray photon measurement data acquiring, pulse
profile folding, and pulsar’s image extracting and positioning, so as to obtain the
measuring direction vectors of the pulsars in the spacecraft-body coordinate system.
The measurement data from the inertial attitude unit and the X-ray detector unit
are sent to the onboard computer, respectively. The attitude determination process
includes two parts: one is based on the data of inertial attitude unit, and the attitude
estimation values of the spacecraft are computed by the attitude quaternion integral;
another is to use the measurement data from the X-ray detectors to correct the attitude
estimation error of the spacecraft and calibrate the gyroscope drifts. Finally, the
363
reference direction vector of the pulsar in the spacecraft’s orbit coordinate system
can be easily obtained in the case of the spacecraft’s orbit parameters known. When
two pulsars are observed simultaneously on the spacecraft, and two direction vectors
are obtained, the attitude parameters of the spacecraft can be estimated by the method
of attitude determination using two vectors.
Using the X-ray detectors installed on the spacecraft to measure the direction
vectors of pulsars, the spacecraft’s attitudes in BCRS or in the orbit coordinate system
can be determined with high precision, getting to the several arc-seconds, which is
comparable to the level of current optical star sensors. Moreover, the attitude determination using the X-ray pulsars can avoid the complex star pattern recognition
algorithms in the traditional celestial navigation. However, it takes time to detect the
X-ray photons and extract the pulse profiles, so that there is a time lag for obtaining
the attitude parameters. Furthermore, the measurement noise of the X-ray detector
will also affect the attitude determination accuracy. The inertial attitude measurements using the gyroscopes have the advantages of complementary with the attitude
measurements using the X-ray detectors. The inertial attitude has rapidly dynamic
response; the random measurement error is small of the high-precision gyroscopes;
the gyroscope drifts can be calibrated by the X-ray detector measurements. In practical application, the measurement data of the X-ray detectors and inertial gyroscopes is often combined to process. Thereby, within sampling interval, the inertial
attitudes are measured by the gyroscopes to use for the spacecraft’s navigation, and
at sampling point of the X-ray detector unit, the measured direction vectors of the
pulsars is used to correct the inertial attitudes for suppressing the accumulated error
caused by the gyroscope drifts, so as to obtain highly accurate, continuous and stable
attitude parameters of the spacecraft.
Supposed that the spacecraft’s attitude measurement system is composed of the
X-ray detector unit and the inertial attitude unit, in which the latter consists of threeaxis strapdown rate integrating gyroscopes in the spacecraft-body coordinate system,
the data processing flow of the combined attitude measurement of the spacecraft is
shown in Fig. 5.14, where t g is the sampling interval of the inertial attitude unit;
t d is the sampling interval of the X-detector unit; ˆ ε is the estimation of correction
vector of the gyroscope drift; ˆ
ε is the estimation of vector of the gyroscope drift; ˆ
θ
is the estimation of correction vector of the spacecraft’s attitude; ˆ
q is the estimation
of the quaternion vector; ˆ
ω is the estimation of the rotational angular velocity of the
spacecraft; A ob is the outputted attitude matrix of the spacecraft. In order to improve
the efficiency of data processing, a special data processor is configured in the X-ray
detector unit, which is used for the X-ray photon measurement data acquiring, pulse
profile folding, and pulsar’s image extracting and positioning, so as to obtain the
measuring direction vectors of the pulsars in the spacecraft-body coordinate system.
The measurement data from the inertial attitude unit and the X-ray detector unit
are sent to the onboard computer, respectively. The attitude determination process
includes two parts: one is based on the data of inertial attitude unit, and the attitude
estimation values of the spacecraft are computed by the attitude quaternion integral;
another is to use the measurement data from the X-ray detectors to correct the attitude
estimation error of the spacecraft and calibrate the gyroscope drifts. Finally, the