1.

INTRODUCTION

In order to accelerate the communication speed between satellites and ground stations, free-space optical (FSO) communication will be a key technique. Therefore, the demand for FSO communication has greatly increased in the last few years. However, FSO communication between space and ground is affected by the atmosphere, for example, cloudy skies and atmospheric turbulence. One solution is site diversity; however, a large number of fixed stations are required to implement this approach successfully. In addition, in case of emergency, FSO communication at an arbitrary location would be required, since fixed ground stations cannot respond to this situation. Thus, we proceed to develop a transportable optical ground station (TOGS), which can be moved to a better place or to a location requiring a communication site with space and can deploy the functionality of an optical station. Development of TOGS was made by the Institute of Communications and Navigation of the German Aerospace Center (DLR)^1,2 and by NICT with a compact 2×2×4 m³ ground station transportable with a truck and capable of 40 Gbit/s lasercom with an aircraft³. Currently, NICT has started the development of another TOGS. In this paper, we introduce NICT’s plan to develop this TOGS.

2.

CONCEPT OF TOGS

The basic concept of a TOGS is that it works as an optical ground station in any place. Main requirement is to be transportable by a vehicle that can travel on public roads in order to move from a standby place to a site where communication is required. The deployed location of the TOGS is assumed to be any place, and the setup of the TOGS should be completed as soon as possible at that location. To achieve this requirement, the telescope is preferably integrated with the transportation vehicle, and in order to handle the changing weather conditions, it is necessary to store the telescope and other instruments immediately in case of rain. To be able to establish stable communication links with aircrafts or satellites, we have to suppress the telescope vibrations. Based on this concept, we organize the requirements for TOGS.

Figure 1.

Concept of TOGS.

3.

REQUIREMENTS FOR THE TOGS

The basic requirement is that the vehicle as well as the enclosure of TOGS can travel on a public road and can be operated by a regular truck driver’s license in Japan. This is also a restriction in order to design the system. It is necessary to provide a vibration-absorbing function on the vehicle while moving so that the communication instruments are not affected. This function must work while moving on off-roads because the deployment location might be far from public roads. In order to deploy the ground station as soon as possible, we have to promptly complete the setup of the telescope for pointing to the aircraft or satellite. Therefore, the telescope must be integrated within the vehicle, and the vehicle must have a function to open the roof of the vehicle in order to receive the optical beam from the aircraft or satellite. The field of view of the telescope needs to have an arbitrary angle of elevation beyond the horizon with respect to all azimuthal directions. We require that the telescope installed in the vehicle has the largest aperture possible.

For the stability of optical communication, the vehicle must have a mechanism to fix vibrations. It is necessary to provide a mechanism to obtain the location information of the ground station deployment, and to calculate the relationship between the direction of the telescope and the position on the celestial sphere for setup of the TOGS. We require an optical system that efficiently transmits the photons collected by the telescope to the receiver, and an interface with an external power supply because an internal power supply would generate vibrations. It is also necessary to provide an interface to an external communication network. These requirements are summarized in the tables 1 and 2. We prepared the first specification of transportable optical ground station according to these requirements.

Table 1.

Required Specification of the vehicle.

Table 2.

Required specifications of the telescope.

4.

REQUIRED SPECIFICATION OF TOGS

In order to determine the specifications according to the requirements, we divided the elements of this development into three components: the vehicle, the telescope, and the fine-pointing optics. The vehicle and its additional parts provide the base of a system that transports a communication device without instruments failure and can be quickly deployed at any destination. The telescope must be able to efficiently collect the photons propagating in space and to provide a pointing accuracy sufficient to acquire a light source into the field of view of the telescope. Since the pointing accuracy of the telescope is not enough to stabilize the communication, we must install the fine-pointing optics that enable to stabilize the signal intensity which is inputted to the receiver.

4.1.

Vehicle

The specifications of the vehicle are as shown in Table 1. An 8-ton truck is a realistic assumption for the maximum size allowable to travel on public roads with a regular license in Japan. Air suspension during the transportation is required, such as a vibration-absorbing mechanism. An outrigger mechanism is also required to fix the vehicle to the ground of a deployed place as an additional equipment for the vehicle. The pedestal on which the telescope is installed needs to be equipped with a vibration-isolation mechanism in the floor of the vehicle. The fine-pointing optics also needs to be covered by this vibration-isolation system, because this optics must be fixed to the telescope. A retractable roof above the telescope is required as well. In our design, while only three directions will be available for communications, we can arrange the vehicle towards any arbitrary orientation. Therefore, we will be able to adjust the direction of the vehicle in accordance to the orbit of the spacecraft. In addition, space for installing the receiver and its control instruments is also required, as well as space for the maintenance. Currently, the design of the vehicle is assumed to be like the one shown in figure 2.

Figure 2.

Vehicle and mounted components.

4.3.

Fine-pointing optics

The fine-pointing optics should stabilize the intensity of light supplied from the telescope to the receiver. This optical system is shown in figure 3. The fine-pointing optics consist of a sensor for measuring positional deviation and a mirror for correcting the deviation. The measurement of positional deviation is performed by a sensor dividing the wavefront into four parts. The mirror that controls the tip-tilt is operated to correct the positional shift. As mentioned above, the optical bench on which the fine-pointing optics is constructed is fixed to the vehicle. Therefore, these optics should be implemented considering to prevent any misalignment of the optics by vibration while moving.

Figure 3.

Fine-pointing optics of TOGS.

5.

DISCUSSION

In this development, we have to investigate how to develop a system that can be set up quickly, and we have to evaluate the vibration resistance of the whole system. The required time for system setup must be as short as possible to reduce the risk of weather or atmospheric conditions changes meanwhile. The problem to be solved is how to quickly fix the station on the ground, the horizontal leveling, and the pointing analysis of the telescope. In particular, the pointing analysis using stellar coordinates cannot be set up during the day time. We need to investigate other methods to quickly point the telescope. Furthermore, the fine-pointing optics is an important component to achieve the required accuracy. We will also consider installing adaptive optics to compensate the influence of the atmospheric turbulence on the optical system.