1.

ARCHITECTURE AND TECHNICAL DESCRIPTION

Astrix NS, the newcomer in the Astrix family with more than 6 million hours of cumulated successful flight history in orbit, arises from the needs of the new compact satellite platforms for small, light, low-power, and cost-efficient equipment: 100 x 100 x 100 mm, 1.4 kg and 7 W (see figure 1). It is qualified for a large range of missions, including the demanding GEO 15 years + electric orbit rising.

Figure 1:

Astrix NS and its mechanical design

Astrix NS is a compact solid state 3-axis gyroscope with shared communication and power electronic boards. Its optical architecture corresponds to the architecture of iXblue high performances FOG that has been improved for more than 25 years [4] [5]. It is available at a competitive price thanks to the use of COTS EEE parts that are carefully space-qualified. For Astrix NS, this critical activity of space qualification of the components is undertaken by Airbus Defence & Space. It guarantees a high reliability for all space environments including severe levels of radiations.

Astrix NS reuses many materials and components from the Astrix family that have demonstrated their reliability for space use. It is especially true for the optical part of the gyroscope: the same rad-hard fibers designed and manufactured by iXblue [2], a compact version of the same optical components. The patented core closed feedback loop is the same as in all iXblue products. The coil diameter was reduced to 40mm ensuring high performances while keeping the overall system very compact.

Astrix NS consists of two main parts, as illustrated in Figure 2 with the green-dashed boxes:

Figure 2:

System architecture

Each of them is composed of an optical fiber coil ended by an integrated optical circuit (IOC). These Sagnac interferometers measure the rotation rate around the fiber optic coil axis.

The 3 Sagnac interferometers are arranged on a symmetrical pyramid with orthogonal sensitive axis. This architecture is similar to Astrix 1090 and also to iXblue inertial systems for launchers. This solid heritage guarantees a very good robustness to vibrations and shocks. This pyramidal architecture also ensures a homogeneous behavior of the three sensors to mechanical and thermal variations.

These two parts are mechanically assembled in one shielding case (see Figure 1).

To offer a compact and reliable FOG with the highest cost/performances ratio on the market, iXblue reuses the bricks that made the Astrix series successful and combines them with a COTS approach for the EEE components. This approach was presented at the AAS GN&C conference [3].

Table 1.

Astrix NS main inertial performances

Table 2.

Astrix NS main features

The characteristics of Astrix NS make it a highly versatile gyroscope that has been designed to fit both AOCS propagation during an extended attitude sensor blackout, pointing accuracy and high-speed fine guidance for agile spacecrafts.

2.

ASTRIX NS’ ANGULAR ACCURACY

For Astrix NS, the angular knowledge error is mainly due to the angular random walk (ARW) noise as there is negligible Rate Random Walk (RRW) and Angular White Noise (AWN) as can be seen in Figure 3 with a linear Allan variance from 1mHz to 250 Hz. It allows real-time analysis of reaction wheel and cryocooler induced microvibrations. Signal latency is expected under 2 milliseconds which is compatible with high frequency LOS loops.

Figure 3:

Allan variance of Astrix NS standard and high performance versions

In this paper, we present the pointing measurement results for Astrix NS standard version i.e. with a 5 m °/√h ARW. For the high performance Astrix NS (2.5 m°/√h ARW), the results would be twice better. It will be demonstrated and published in a future paper.

The Astrix NS short time angular accuracy evolves roughly as the square root of time, as can be seen in Figure 4 with the black bold curve. If this experiment is repeated several times, for each duration, we observe a nearly gaussian distribution of angular error as can be seen in Figure 5.

Figure 4:

Angular error recording through time during a single experiment. The three colored light lines show the measurement for the three axes of the gyroscope. The black bold curve shows the 3D error.

Figure 5:

Distribution of measured pointing performances of one axis of an Astrix NS through time. Each “slice” of time distribution has a gaussian shape (skewed by the log axis representation in this figure)

To monitor more easily the evolution of the distribution through time we plot the distribution values at the 10^th, 50^th, 90^th and 99,7^th percentiles as shown on Figure 6.

Figure 6:

Measured pointing performance of Astrix NS at 100 Hz for the three axes. From left to right: x-axis, y-axis and z-axis. Each color represents the maximal error inside a 10% to 99,7% confidence rising through time. For example, there is a 90% probability (red curve) to have an error inferior to 2 μrad after 1 second of pure inertial propagation on one axis. Dash lines are theoretical systems made of only pure 5 m°/√h Angular Random Walk noises (the commercial specification of the standard Astrix NS).

In the representation of Figure 6, theoretically perfect systems with only pure white noise would results in straight parallel lines (dashed lines on Figure 6). A spectral anomaly in the noise spectrum or an electronic defect in the gyroscope would cause the temporal evolution to display bumps and peaks. In Astrix NS case, we don’t observe any significant and annoying bumps which demonstrates that the design is sound.

The really meaningful error is the three-dimensional angular error which is a combination of the three orthogonal physical sensors. The noise of the three fiber gyroscope axes of Astrix NS are uncorrelated which is a favorable case for better three-dimensional performances. For uncorrelated variables, the three gaussian distributions of pointing performances combine according to a third order Chi Square Law, squeezing the error distribution of the real three-dimensional error. On Figure 7, the excellent superposition of the dashed curves (theoretical third order Chi Square Law values) and the plain curves (measured value) demonstrates this point.

Figure 7:

Measured pointing accuracy of the standard performance Astrix NS (3 gyroscopes axis with a 5 m°/√h specification – 500 Hz). Each color represents the maximal error - from 10% to 99,7% - rising through time. The dotted lines are theoretical behavior for a pure 5 m°/√h noise through the third order chi square law formula. The blue circle and perpendicular dashed lines explicit a mean error at 10 milliseconds under 0.3 μrad. A high performance Astrix NS (2.5 m°/√h ARW) would be twice better with an error divided by 2.

Depending on the specific mission and on the architecture of the satellite, the relevant duration and the needed angular knowledge error vary. For instance, significant micro-vibrations in orbit have been measured on laser com satellites [6] and induced micro-vibrations linked to control moment gyroscopes (CMGs) or cryocooler are a concern [7]. As an example, we underline (blue circle on Figure 7) that for a duration of 10 milliseconds, the pointing error will be below 0.3 μrad with a probably of 50%.

We believe that with such high angular precision at 500 Hz in a 100 x 100 x 100 mm cube, Astrix NS can be used inside the telescope itself and monitor some of the telescope structural deformations to improve the satellite performances.

Finally, the space qualification of Astrix NS is planned for Q1 2023 and the first flight models delivery will be made in S2 2023.