NL2005664C2

NL2005664C2 - An attitude determination system suitable for a spacecraft.

Info

Publication number: NL2005664C2
Application number: NL2005664A
Authority: NL
Inventors: Eberhard Karl Albert Gill; Georgi Nadeltchev Gaydadjiev; Rouzbeh Amini
Original assignee: Univ Delft Tech
Priority date: 2010-11-10
Filing date: 2010-11-10
Publication date: 2012-05-14

Description

An attitude determination system suitable for a spacecraft

The invention relates to an attitude determination system.

The prior art teaches an attitude determination system suitable for a spacecraft, which is connected or connectable to 5 a power source and which comprises multiple sensors for measuring attitude-signals of the spacecraft, and a computation unit for calculating the attitude from the measurement signals of the multiple sensors, and a transmission system for entertaining communication between the multiple sensors and the computation 10 unit.

Such an attitude determination system is for instance known from US 5,109,346. In this known system star sensors and gyros are used and the information therefrom is accumulated in a Kalman filter to provide continuous estimates of position and 15 orbit for use when the sun or moon is not visible. The best estimate of position is selected and is used to control the spacecraft without having to rely on ground-based eguipment or other spacecraft for the determination of position and orbit.

Data communication between the sensors and the computa-20 tion system according to US 5,109,346 is with fixed wiring, yet it is also feasible and even preferable that such a system is provided with a wireless communication system of the type that is disclosed in US2007/0049204. In both cases however the attitude determination system may have limited power capabilities 25 which necessitates that keeping energy consumption low will be a prime design consideration. What this prior art does not teach are the features that differentiate the invention from the prior art, which will be explained furtheron in this description following the introduction of the invention's objectives.

30 It is a first objective of the invention to provide an attitude determination system with improved energy consumption, i.e. an energy consumption that is lowered in comparison with prior art solutions, which is one of the first challenges the invention seeks to overcome.

35 Further objectives of the invention are to provide the possibility of increased modularity and autonomous operation of the attitude determination system of the invention.

2

It is remarked that application of onboard wireless communication between spacecraft subsystems is particularly desirable since it improves the flexibility of the spacecraft design, integration and testing. Statistics show that 6 to 10 per-5 cent of the mass of a spacecraft is due to wires and electrical interfaces. Furthermore, wireless communication can address other problems of wired communication as well, such as: failures of wires and connectors, high cost of late design changes, undesired ground loops. A need therefore exists in particular for 10 integrated autonomous wireless onboard modules which require a low-energy demand.

In order to promote the objectives of the invention, it is proposed that the attitude determination system has the features of one or more of the appended claims.

15 In a first aspect of the invention the attitude deter mination system comprises sensors with a variable sampling rate and an energy manager module for saving on energy consumption by determining the value of the variable sampling rate of the sensors depending on an estimated measurement error in the calcu-20 lated attitude. The invention is also embodied in a method to determine the attitude of a spacecraft, making use of such a system in which the value of the variable sampling rate of the sensors is varied depending on an estimated measurement error in the attitude of the spacecraft. This is a very effective way to 25 save on energy consumption of the system, considering that the sensors are then only employed when they are needed to improve the accuracy of the attitude determination.

The objective of the invention is best attained when the variable sampling rates of the sensors are lowered when the 30 estimated measurement error in the calculated attitude is reducing, and that the variable sampling rate of the sensors is increased when the estimated measurement error in the calculated attitude is increasing.

For arranging a stable operation of the attitude deter-35 mination system of the invention it is preferable that the variable sampling rates of the sensors are updated when the estimated measurement error surpasses a pre-determined threshold.

As mentioned above with reference to the attitude determination system known from US 5,109,346, it is known that the 40 computation unit can be provided with a Kalman filter. In con- 3 nection with the invention it is preferable that the Kalman filter is arranged to estimate the measurement error in the calculated attitude based on a difference between predicted measured signals of the multiple sensors and actually measured signals of 5 the multiple sensors. Therefore this Kalman-filter can perform a function not only in the attitude determination process, but also in the energy-management of the system by the energy manager module .

The invention will hereinafter be further elucidated 10 with reference to the drawing of an exemplary embodiment of the attitude determination system of the invention. This embodiment is not restrictive with respect to the appended claims, and can be varied in many respects without departing from the scope of protection of said claims.

15 In the drawing: -Figure 1 is a functional flow diagram of the system; and -Figure 2 is a block diagram showing the data flow between different components of the system of the invention.

20 Wherever in the figures the same reference numerals are applied, these numerals refer to the same parts.

Making reference first to Figure 1, a block diagram is shown pertaining to a functional flow diagram of the system, which is generally denoted with reference 1.

25 While the system 1 is running, the attitude measure ments are performed by the applied attitude sensors, which may be a gyro 5, a magnetometer 6, and a sun sensor 7 (see in particular figure 2). The frequency of the measurements by said sensors is variable. Block 5, 6, 7 in figure 1 represents the 30 sensor measurements which are communicated to block 2 where filtering occurs. Preferably the applied filter is an Extended Kalman Filter (EKF) algorithm which may perform two functions. The Kalman filter calculates the attitude information from the sensor 5, 6, 7 measurements and from spacecraft kinematics ac-35 cording to a dynamic model 2a. Furthermore, the filter 2 produces an estimate of the sensor measurements in each sample.

This estimation is used to calculate in module 2b an estimate of the measurement errors. These errors are calculated as the difference between the estimated sensor measurements and the actual 40 measurements from the sensors.

4

The determined attitude information is transferred to a wireless transmitter 4 to be communicated to a spacecraft onboard computer. The calculated measurement errors are communicated to an energy manager 3. The energy manager 3 compares the 5 errors with a predesigned threshold 3a. If the errors are small enough, it is possible to use the estimated sensor measurements for the attitude determination. In that case the energy manager 3 reduces in block 3b the sampling rate of the sensors 5, 6, 7. Thus, the energy manager 3 schedules the so-called wake-up time 10 of the sensors 5, 6, 7 to tune the energy consumption of the system 1.

Figure 2 shows a data flow diagram for the proposed system 1 according to the invention. As it is shown in this flow diagram, data is communicated between wireless transmitter 4 15 that connects to the onboard computer, and a filter 2 of the attitude determination system. The filter 2 is providing data to the energy manager 3. At the same time the filter 2 is receiving measurement signals from sensors 5, 6, 7, notably a three axis gyroscope 3, a three axis magnetometer 6, and a two axis sun-20 sensor 7. The energy manager 3 continuously communicates an updated measurement-rate to the sensors 5, 6, 7 while it simultaneously receives information about the available energy and the status of the power source 8, which may be an energy harvester in the form of a solar cell in combination with a battery.

Claims

A position-determining system (1), connected or connectable to a voltage source (8) and comprising a plurality of sensors (5, 6, 7. for measuring position signals, a calculating unit for calculating the position from the measuring signals of the multiple sensors ( 5, 6, 7), and a transmission system (4) for maintaining communication between the multiple sensors (5, 6, 7. and the computer unit, characterized in that the sensors (5, 6, 7.) have a variable sampling frequency and the system comprises an energy manager module (3) for saving energy consumption by determining the value of the variable sampling frequency of the sensors (5, 6, 7) in dependence on an estimated measurement error in the calculated position .

2. A position-determining system (1) according to claim 1, characterized in that the variable sampling frequency of the sensors (5, 6, 7) is reduced when the estimated measurement error decreases in the calculated position, and in that the variable sampling frequency of the sensors (5, 6, 7) is increased when the estimated measurement error increases in the calculated position.

Position determination system (1) according to claim 1 or 2, 20, characterized in that the variable sampling frequency of the sensors (5, 6, 7) is updated when the estimated measurement error exceeds a predetermined threshold.

4. A position-determining system (1) according to any one of the preceding claims, wherein the computing unit is provided with a Kalman filter (2), characterized in that the Kalman fliter (2) is adapted to estimate the measurement error in the calculated position based on a difference between predicted measurement signals from the multiple sensors (5, 6, 7) and actually measured signals from the multiple sensors (5, 6, 7).

Method for determining position with a system (1) connected or connectable to a voltage source (8) and comprising multiple sensors (5, 6, 7) with a variable sampling frequency for measuring the position signals, characterized in that the value of the variable sampling frequency of the senors (5, 6, 7) is varied depending on a measured error in the measured position for saving energy consumption of the system (1).