Inter-satellite links
On September 3, 2020, SpaceX announced the first tests of inter-satellite link (ISL).
The presence of such channels in the Starlink constellation was announced at the very beginning, but later they were abandoned in the first generation satellites to save time and money.
Inter-satellite channels would make it possible to solve the problem of communication in those regions of the planet where it is impossible to install a gateway on earth with a fiber-optic link connected to it for Internet access. Currently, Starlink cannot provide services in the seas and oceans, except for a short distance from the coastline, thereby cutting itself off from the highly lucrative cruise ship and commercial maritime shipping markets, as well as from most of the long-distance flights in the global civil aviation. ...
Another widely and hotly discussed advantage of ISL is that the speed of propagation of a signal in space is equal to the speed of light, but in optical cable it is less, and theoretically, the delay when using Starlink satellites with ISL will be less than when using transatlantic submarine cables connecting the United States with Europe, Asia and Australia, and this will attract stock brokers trading on the exchanges of these continents.
Before proceeding to the discussion, let us tell a little, in fact, about the technology of laser communication.
Already today, lasers are widely used to transfer huge amounts of data over fiber optic cables. Their use in space has even greater potential; the absence of a physical transmission medium will make it possible to obtain a high speed of information transfer. Another advantage of lasers is that light has a wavelength 10,000 times shorter than the wavelengths of radio waves used in space communications (or a transmission frequency 10,000 times higher). This means that the laser light can travel in a narrower beam, and will require smaller receivers in order to receive a signal sufficient to process the amplitude. In addition to increasing the level of safety of space communications, this will reduce the weight, dimensions of communication equipment,a lot of money is spent on the delivery of which into space.
Figure: View of the onboard set for laser communication LLCD (Lunar Laser Communication Demonstration), which participated in the NASA LADEE (Lunar Atmosphere and Dust Environment Explorer) experiment in 2013: communication between the Earth and a spacecraft in orbit of the Moon.
It should be noted that the bandwidth of the communication channel is determined, among other things, by the diameter of the receiving optics, for example, the ground receiving station for this experiment looked like this:
At the same time, the transmission rate from the lunar orbit reached 622 Mbit / s, but the transmission rate in the opposite direction, despite the large size of the station transmitting from the Earth, was no more than 20 Mbit / s. That is, the size of the receiving optics and the distance between the transmitter and the signal receiver play a key role.
Currently, the main focus is on the use of laser communications to communicate between the Earth and artificial earth satellites. For example, one onboard development kit of Mynaric AG (Germany) for laser communication weighs 7-15 kg. This kit can transmit 10 Gbps over 4500 km. The manufacturer is considering 100 Gbps, but its current products run at 10 Gbps. Note that the receiving terminal on Earth for receiving data at such speeds has more than impressive dimensions.
The figure shows the Mynaric ground laser terminal.
According to Mynaric, targeting, capturing and tracking a spacecraft is the most difficult problem in space laser communications. The fundamental trade-off here is to find a compromise between aiming accuracy and light beam power: the smaller the divergence (scattering) of the light beam, the higher the signal at the receiver, but in this case the requirement for pointing accuracy is higher. The divergence of the light beam of a modern laser can reach 10 Ī¼rad (or 0.00057 degrees). Note that in this case the beam of light at a distance of 1000 km has a diameter of only 10 meters, and the task of "hitting" it with another satellite will be extremely difficult for the guidance system.
It should be remembered that with a connection between a satellite and the Earth, we have on one side an object rigidly fixed in space, with an inter-satellite communication channel, the complexity of organizing a communication session is practically doubled.
If the equipment on the satellite cannot provide such pointing accuracy, then it remains to put up with the wide beam scattering, which, with a fixed transmitter power on board the satellite and the size of the optical receiver, significantly reduces the throughput of such a communication channel.
We also note one more point: if for a single satellite for its communication with the Earth, one set of laser communication is enough, which in a communication session will be oriented to the Earth, then in such a complex and multi-satellite system as Starlink for organizing a service, that is, a continuous communication channel at any time of day, each satellite must have 4 sets of laser communication modules oriented in all four directions. At the same time, it is important that even with four modules, it will be necessary to ensure the deflection of the beam in the module in the range of 90 Ā° (plus / minus 45 Ā° from the axis), which makes the design of such a module extremely complex and may require mechanical rotary devices in the laser communication module. ... If the 45 Ā° deflection angle is not automatically guaranteed,then there are "dead" zones for reception / transmission from a particular satellite, which will lead to the fact that communication will not be organized along the shortest route, and the control of the ISL transmission will require continuous calculation of "dead zones" for each satellite at each moment of time and taking this into account when laying a "route".
A separate issue is the layout of the placement of modules on the satellite. The Starlink satellite is now optimized to fit as tightly as possible inside the fairing of a Falcon 9 rocket and is rectangular in shape with a fairly low height, but it is on this āshortā side that optical modules will need to be placed (one on each side). The question is whether it will be possible to fit them into the current design of the satellite, even taking into account the fact that SpaceX will itself design the modules for laser communication and their optics. Judging by the description of the optical communication equipment, the control of the beam direction is implemented by a lens system, and such an optical part requires rather large dimensions when it comes to transmission with high throughput.
Note also that transmitters for laser communication are new energy consumers on board, and their efficiency does not exceed 25%, that is, the problem arises of utilizing and dumping into space the remaining 75% of the consumed energy, which, although not critical, is nevertheless task requiring an engineering solution.
A separate, much more complex and important problem is traffic management directed to the optical communication channel. Let us recall that the existing "classical" communication satellites in geostationary orbit are repeaters, that is, with mirrors. They receive a signal from the Earth at one frequency and transmit it from a satellite to Earth on another, but without changing the modulation and other parameters of the signal itself.
For understanding, we will show with an elementary example what modulation is and how useful information is transmitted in a radio signal.
A distinction is made between a carrier wave and a baseband signal. If we are talking about the transmission of an analog signal, then another signal is superimposed on the carrier frequency, changing the amplitude of the carrier frequency:
A) the type of the carrier frequency signal,
B) the type of the modulating signal (useful information), C
) the type of the transmitted signal with useful information.
To transmit digital information, the carrier frequency and the modulated signal with useful information look like this:
The main thing here is the absence of signal processing (demodulation) on board the satellite and, accordingly, the equipment for this.
So, when operating in the Ku-band, the signal is transmitted from the gateway to the satellite at frequencies of 14-14.5 GHz, onboard the signal changes the carrier frequency and with constant modulation (useful information) is transmitted down to the subscriber terminal at frequencies 10.7-11 , 2 GHz. However, the inclusion of laser communication channels in the architecture of the Starlink network will require routing on board the satellite and separation of information flows from the subscriber terminal into those that will be transmitted down to the gateway or further through the inter-satellite channel. The simplest way without significantly complicating the design of the satellite itself is the allocation of a special frequency range within the common band, along which the transmitted signal and information, when it enters the satellite, is sent exclusively to the inter-satellite communication channel. That is, a high frequency radio signal carrying datasuperimposed on the light signal before transmission via an optical channel with a wavelength of 1000-1500 nm (RF over fiber technology). It's simpler, but it means:
A) the throughput of inter-satellite channels will be initially limited,
B) the entire frequency resource used for transmitting information transmitted further through inter-satellite communication channels will be excluded for servicing ordinary subscribers during the period when the satellite is flying over an area where there are enough gateways and there is no need in inter-satellite channels,
B) with a high degree of probability, special subscriber terminals operating in dual-frequency mode will be needed.
An alternative to this option is information processing on board the satellite. That is, the radio signal received from the subscriber terminal is demodulated and decoded to the level of IP packets, sent to the router, which already distributes the information to the radio frequency or optical communication channel.
This method allows flexible use of the entire available frequency range and does not require special subscriber terminals, but requires an onboard router capable of processing packets at speeds up to 20 Gbit / s. At the same time, the processor of such a router should not work in a strictly air-conditioned room of a data center with a narrow range of operating temperatures, but in open space, where temperatures, even in the presence of a powerful COTR (cooling and thermoregulation system), will be in a larger temperature range. At the same time, the presence of a powerful SRT will undoubtedly affect the mass and size parameters of the satellite.
Note, however, that all of the above problems are technical in nature and, in principle, can be solved.
The presence of inter-satellite optical channels will lead to the emergence of different services for the consumer. He can access the Internet through a regular gateway at basic rates and with a āstandardā delay in the channel, or he can choose the āfastā connection option, when his information is sent through inter-satellite communication channels and ādescendsā to Earth only at the nearest to the end point gateway. Of course, this "fast" data transmission will be more expensive, and the cost of traffic transmitted in this way will naturally be higher.
Of course, a separate purely commercial task is calculating how much the cost of such "fast" traffic should be higher than usual, and the main thing is whether there will be a sufficient number of customers willing to pay for such a fundamental change in the network architecture and the associated investments in the space segment.
Let me remind you in this regard the words of Jonathan Hofeller, vice president of commercial sales for SpaceX: āWe need to make sure that it is cost-effective before creating this (SL) and implementing it in the Starlink constellation.ā
There is another aspect of having inter-satellite links in the constellation Starlink, which may not have attracted the attention of SpaceX specialists so far The introduction of ISL will allow a Starlink subscriber to access the Internet from the territory of another country or transfer information from one terminal to another, bypassing any ground communication centers.
However, almost all countries, and even more so developed ones, have norms in their legislation that oblige all telecom operators to ensure the ability of special services to access the traffic transmitted in their networks. It is precisely about the guarantee of ensuring access, whether the special services will read the correspondence or not, this is already a matter of the court and other norms of local legislation. But telecom operators must provide this. In the United States, this is regulated by The Communications Assistance for Law Enforcement Act (CALEA), adopted in the era of Bill Clinton, even before the events of 9/11. The norms of this law and the requirements for telecom operators in the United States are not far from the Russian legislation on SORM and the corresponding requirements for Russian telecom operators, the same situation is in most other states.
The requirements for ensuring SORM pose two groups of problems. One is purely US-based - how SpaceX can convince the FBI that it complies with CALEA's requirements. Perhaps it will be a list of Starlink subscribers pre-approved by the FBI who can use the service with ISL, it may be prohibited to send traffic originating outside the United States to a subscriber in the United States, maybe ISL will transmit traffic raised only through gateways in the United States. In general, there are many options and they are the subject of discussion between SpaceX and the FBI, after all, Elon Musk is a good citizen of the United States and a patriot of this country.
But the issue of access of special services to subscriber traffic starts to look completely different if we are talking about another country.
If, prior to the introduction of inter-satellite communication channels, SpaceX could convince any national telecommunications regulator that all traffic for subscribers of this country will go from the gateway on its territory, on which the special services / police will supply the appropriate police interceptor device, then with the presence of ISL they will have to either take the word of a private American company, or sign some kind of cooperation agreement with the FBI, transferring to the FBI part of the authority to intercept the traffic of potential criminals from this country. In any case, we will talk about limiting national sovereignty on their own territory for Starlink subscribers.
Of course, it will most likely be possible to establish data exchange within the United States and its allies in NATO or the Western world, but even in these countries there are internal conflicts, as, for example, in Spain - the issue of separatism of Catalonia, or in Turkey - the confrontation between Erdogan and his opponents where there is no crime or terrorism, but the authorities of the country restrict or may restrict certain sites on the Internet or are interested in the correspondence of individual citizens. That is, in fact, Spain or Turkey should oblige the United States to monitor its political opponents, even if the United States government does not consider them criminals.And
if we recall Saudi Arabia (a US ally), then it is unlikely that it will be ready to open its citizens full access to sites of erotic content or web resources that criticize the current monarch.
In short, the introduction of inter-satellite communication channels in the SpaceX constellation will cause serious problems for its entry into the commercial communication markets of other countries.
Thus, we can say that SpaceX is at a crossroads. If inter-satellite communication channels are introduced, then its service will generate significant interest from the military, as well as cruise and shipping companies based in the United States, but the chances of providing commercial communication services in the markets of other countries will significantly deteriorate.
- Everything about the Starlink Satellite Internet project. Part 1. Birth of the project
- Everything about the Starlink Satellite Internet project. Part 2. Starlink Network
- Everything about the Starlink Satellite Internet project. Part 3. Ground complex
- Everything about the Starlink Satellite Internet project. Part 4. Subscriber terminal
- Ā« StarlinkĀ». 5. Starlink -
- Ā« StarlinkĀ». 6. -
- Ā« StarlinkĀ». 7. Starlink RDOF
- Ā« StarlinkĀ». 8.
- Ā« StarlinkĀ». 9.
- Ā« StarlinkĀ». 10. Starlink
- Ā« StarlinkĀ». 11. Starlink
- Ā« StarlinkĀ». 12. Starlink
- Everything about the Starlink Satellite Internet project. Part 13. Satellite network delay and access to radio spectrum