I.

INTRODUCTION

This paper describes the Engineering & Qualification Model (EQM) thermo-mechanical design of the Coarse Lateral Sensor (CLS) Optical Box (OB) developed by Thales Alenia Space España and LIDAX.

The Coarse Lateral Sensor is a low cost, low mass optical metrology sensor able to provide medium accuracy angular measurements and able to be easily accommodated on all the identified formation flying missions, acting as the final positioning system or as a bridge between the Radio Frequency (RF) and the fine optical metrology systems. The operational measurement distance of the CLS goes from 25m to 250m and the working angular range is ± 5°, with an angular accuracy better than ± 5 arc sec.

The Coarse Lateral Sensor is composed of three main components:

CLS is able to operate in a large temperature range, between -20°C and +50°C, thanks to its active thermal control system based on heaters and thermistors, which avoids misalignments and ensures an appropriate operation within the accuracy requirements.

CLS functional concept is shown in Figure 2.

Figure 1:

CLS instrument (left), CLS Optical Box (center), Optical Assembly (right)

Figure 2:

CLS Concept

The doublet of the Optical Assembly sends the light from a LED to the Corner Cube (CC) placed in the satellite to be controlled; then the CC reflects the light beam which is recovered by the lenses doublet and directed to the CMOS sensor by means of a beamspliter. Additional optical elements such as filters and diaphragms are included to guarantee the optical performances.

This paper outlines the thermo-mechanical design of the CLS OB EQM, summarizing design drivers, mechanical and thermal concept, and main performances.

The design of the CLS Optical Box is focused on three main objectives:

Figure 4 shows main parts of the OB assembly.

Figure 3:

CLS Optical Box general view

Figure 4:

CLS Product Tree

Figure 5:

CLS OB

Figure 6:

CLS OB Optical Assembly, Baseplate, and Housing

Figure 7:

Heaters & Thermal Sensors on Optical Assembly (LEFT), Optical Assembly supported by Bipods

Figure 8:

CLS Optical Box external finishing

II.

DESIGN DRIVERS & MAIN REQUIREMENTS

The main objective of this opto-mechanical design is to guarantee the acquisition of the receiver satellite’s position with a lateral displacement accuracy of ± 5arcsec.

Taking into account the operative temperature range of the CLS, from -20° to 50°C, an active thermal control is necessary to avoid misalignments or measurement errors during the CLS operation.

On the other hand, from a structural point of view it is necessary to provide the optics with a support able to guarantee an eigenfrequency above requirement, and support launch loads as well as defined quasi-static loads.

Thermal insulation and structural stiffness are conflicting requirements so a commitment solution must be reached between both to fulfil specifications.

Finally, the necessary optics size compared to the total envelope requirement results in a very small space available for integration tasks; hence the design must also be oriented towards ease of assembly.

In conclusion, the following issues have been identified as design drivers:

Main requirements are shown in Table 1.

Table 1:

CLS OB Main Requirements

III.

MECHANICAL DESIGN

CLS Optical Box mechanical design is focused on solving the points shown previously and fulfilling requirements shown in Table 1; it is composed of the elements shown in Figure 4.

The Optical Box contains both the light source and the optics in a unique assembly which is connected to the Electronics Box (TAS-E responsibility) by means of a harness.

Both the optics and the focal plane are placed in the same integral bracket, the Optical Support. This assembly is located in a baseplate + housing where the LED and the IR filter are also integrated.

The light source, a LED with a peak wavelength of 455nm, is mounted at the necessary distance from the optics by means of a baffle, which is supported by the housing. In the same way, the IR filter and diaphragm are located at the Optical Box exit.

IV.

THERMAL CONCEPT

The thermal design is based on maintaining a constant temperature inside the CLS to provide a stable optical configuration, and hence a reliable measurement of the reflector satellite in any mission configuration.

This concept aims to guarantee a good thermal stability along time once the CLS is in the operative mode. After the launch, the CLS thermal environment could range from -20°C to +50°C; the Sensor should then be calibrated at the hottest temperature of the range and the thermal control will be in charge of maintaining the same temperature conditions for the optics independently of the thermal environment.

Therefore an active thermal control system composed of four thermofoil heaters and six thermal sensors is used to always keep optical elements at the nominal working temperature (≈50°C), independently of environmental conditions. It was decided to always work in the hottest area of the range since it is more cost effective to warm up the optics-assembly than to cool it down, and the power budget is limited to 8W. When the CLS environment is at the coldest temperature (-20°C), the sensor is heated in order to always keep the optics support at the maximum temperature (+50°C), which is defined as the nominal operation temperature.

At the same time, thermal design is focused on maintaining a high thermal stability to minimize gradients and avoid rotations and displacements of the optical elements which could cause deterioration of the sensor features.

In order to fulfil these requirements the following directives have been established:

V.

PERFORMANCES

Comparison between specification and performances is shown in Table 2.

Table 2

CLS OB performances

VI.

CONCLUSIONS

The following conclusions can be established: