Space missions face many constraints based on financial and technological limits, from the overall dimensions of their tools to the quality of their transported instruments. Communication, especially for missions far from the Earth, takes on considerable importance regarding transmission power and distance, which can compromise data rates.
Laser technologies are used to conduct high-precision measurements of space between an observer on Earth and an optical receiver placed on a satellite. Over the years, these free-space techniques have benefited from some improvements in both optical technology and data modelling.
The laser communication system allows you to connect with Earth’s satellite, reaching download speeds of 622 Mbit/s to communicate at a distance of over 384 thousand km, faster than any radio-based system. Advanced laser systems and communication/cryptography techniques were employed to cope with the turbulence of space and atmosphere, and those systems are some of the most promising visions for the future of aerospace communication.
The Laser
In most free-space systems, the light source of the transmitter is intensity-modulated to encode digital signals. In this context, various techniques have been developed to study the spatial dynamics of the Earth-satellite/moon system. These are particularly crucial for gravitational physics and new communication techniques using high-performance optics and lasers. Lasers are distinguished from other light sources by their spatial coherence; they can reach very high irradiance or very low divergence with the aim of concentrating their power at a great distance. Their wavelength in a vacuum characterises lasers. Although temporal coherence implies monochromaticity, some lasers emit a broad wavelengths spectrum simultaneously (see figures 1 and 2).
Figure 1. Laser beam sent from the earth to a satellite for free-space communications.
Infrared laser communications offer significant advantages regarding the volume and weight of the equipment, energy consumption, and bandwidth compared to traditional radio communications.
Figure 2. A laser used in spaces communications.
Design Challenges
Today’s advanced military electronics systems for various applications, such as communications, multi-computing, and signal processing, are subject to some challenges related to a significant increase in dissipated power compared to previous generations. Thermal management has become a crucial issue in these new systems. Electronics are not just for sensors or processing; the electric power is emerging in various electric aircrafts, electric warships, and hybrid-electric vehicles, in which batteries, bipolar isolated gate transistors (IGBTs), and transformers need cooling.
Designers are addressing a range of challenges regarding power and thermal issues. They need to meet specific standards; in military systems, different standards govern power quality: one for air systems, one for ground systems, and another one for a wide EMI interference standard that covers all types of naval chassis and planes substantially. Engineers are faced with the design of countless features in systems that must meet weight and energy consumption (SWaP) requirements.
Free-Space Communication
The theory behind free-space optics is the one concerning the transmissions in optical fibres, with the difference that the light signal is sent through the ether from source to destination and not confined in a light guide (i.e., in an optical fibre). At the source level, the light signal is modulated with the data to be transmitted. At the destination, the beam is intercepted by a photodetector, with extraction or demodulation of the data, which is followed by amplification. It is then sent to the hardware to use the information in the form of electrical signals.
Atmospheric laser communication equipment can transmit a variety of data, voice, and image information without electromagnetic interference. Compared to fibre optic communication, atmospheric laser communication offers the advantages of flexible and fast network construction, safe operation, and easy upgrades (see figure 3).
Figure 3: A free-space communication network.
Communication links using laser beams are inherently resistant to interception by exploiting quantum cryptography. Long optical wavelengths allow high antenna gains to establish connections over extremely long distances and allow the use of small transmitters and receivers.
Quantum cryptography makes use of the properties of modern physics, such as the quantum no-cloning theorem and the Heisenberg uncertainty principle. Quantum cryptography is designed to be safe for three main reasons. One, the quantum no-cloning theorem states that an unknown quantum state cannot be cloned. Two, in a quantum system, which may be in one of two states, any attempt to measure it will disturb the system itself. A quantum message that is intercepted and read by malicious users will become useless to the recipient. Three, the effects produced by the measurement of a quantum property are irreversible, which means that an interceptor cannot modify a quantum message to its original state (see figure 4).
Figure 4. Quantum key distribution.
The choice of a receiver and transmitter for free-space communication is driven by the desire to achieve extremely high transmission speeds. This allows the designer to keep smaller optical devices and an ultra-low power for the laser. Adaptive optics can flatten the received wavefronts and enable light coupling in monomodal sensing systems. Such single-mode receivers include consistent or optically pre-amplified architectures.
Conclusion
From the ever-increasing demand for low-Earth orbit satellites for typical applications and low orbital levels, such as CubeSats, SmallSats, MicroSats, and other small satellites, the need arises to realise less expensive space electronic systems than the “inherited space systems” that we’ve used up to now. The use of laser technologies allows the use of optical communication techniques with advanced cryptographic techniques.
Laser communication has some advantages; in addition to the high bandwidth, it requires less mass, power, and size compared to RF systems on spacecraft.
Another advantage is the flexibility in collecting data on planetary missions, including the ability to monitor dynamic phenomena.
Laser communications for planetary missions do not provide the data transmission rates of the gigabit order, but they are far better than the current radio frequency communications offered by the technology. Missions on the moon could enjoy data transmission speeds of around 500 megabits per second. Those on Mars could work up to 100 megabits per second. Even spacecrafts further in the solar system could still return data over 1 to 2 megabits per second.
