To better visualize the problem being described, here is an animation of SpaceX's Starlink system when fully deployed:  When using a **Carrier-sense multiple access** MAC protocol the user verifies the absence of other ongoing data traffic before transmitting on a shared transmission medium. ### Autonomous systems (ASes) An AS is simply a network owned by a commercial (or government) entity which is somehow connected to the broader Internet. ASes are identified by a unique 16bit number. ### BGP The Border gateway protocol, operates among ASes. It is concerned with exchanging reachability information amongst ASes in a scalable way. #### Frequency-division multiple access (FDMA) FDMA involves dividing the frequency bandwidth of the channel you are using into many non-overlapping sub-channels and allocating each sub-channel to a separate user. #### Time-division multiple access (TDMA) In TDMA you allocate users to time slots when they can transmit. #### Code Division Multiple Access (CDMA) CDMA is an example of multiple access, where several users can send information simultaneously over a single communication channel. Today you can get a Satellite Data Hotspot for $250. E.g. [Globalstar Sat-Fi2](https://www.magnumelectronics.com/Globalstar-Satellite-Hotspot-p/SATFI-2-US.htm)  ### Teledesic project Teledesic was an American company founded in the 1990s which made plans to use low-earth orbiting satellites to provide wireless Internet service almost everywhere on Earth. Some plans envisioned a system that could provide uplinks of as much as 100 Mbit/s and downlinks of up to 720 Mbit/s. The system never ended up being fully deployed due to financing problems. The network of 840 satellites would have cost over $9 billion to build.  *Conceptual drawing of a Teledesic satellite* noted Here is an illustration of the footprint of a single geostationary satellite.  ### SATNET Also known as the Atlantic Packet Satellite Network, was an early satellite network that provided connectivity between the US and Europe. The SATNET link was made available in 1973. Due to its different characteristics than the rest of ARPANET, it ended up having a prominent role in pushing the design of the Internet protocol suite towards being able to support very heterogeneous networks.  *ARPANET diagram (SATNET link highlighted)* #### Geostationary orbit height calculation A geostationary orbit is a an orbit where the satellite remains always in the same position from the point of view of a static observer on earth.  The satellite is kept in orbit by the gravitational force of the earth exerted on the satellite ($F_g$). Therefore: $$ F_g = m_{sat} \cdot a_{sat} $$ We can find $F_g$ using Newton's Universal law of gravitation. $$ F_g = G \frac{m_{earth} \cdot m_{sat}}{r^{2}} $$ The acceleration of an object moving around a circle can be expressed using the angular velocity (\omega) of that object (which in this case is the same angular velocity as earth’s angular velocity around its axis): $$ a_{sat} = r \cdot \omega^{2} $$ Earth spins around it’s axis approximately every 24h (86400s). Therefore its angular velocity around its axis is: $$ \omega = \frac{2\pi}{86400}$$ From this we get $$ m_{sat} \cdot r \cdot \omega^{2} = G \frac{m_{earth} \cdot m_{sat}}{r^{2}} $$ $$ \frac{G \cdot m_{earth}}{r^{2}} = r \cdot \omega^{2} $$ $$ \frac{G \cdot m_{earth}}{r^{2}} = r \cdot \omega^{2} $$ $$ r = \sqrt[3]{\frac{G \cdot m_{earth}}{\omega^{2}}} $$ $$ r \approx 4.2232 \cdot 10^7 m $$ Now we just have to subtract the earth’s radius from $r$ to get the altitude of the satellite. $$ 4.2232 \cdot 10^7 - 6.371 \cdot 10^6 \approx 35,861km $$ ### Satellite communication frequency bands The first satellite to orbit the Earth was Sputnik 1, carried two radio beacons on frequencies of 20.005 and 40.01 MHz. Over time satellite communications have been shifting towards higher and higher frequencies. Sputnik 1 (1957 - the first satellite to orbit the earth) carried two radio beacons on frequencies of 20MHz and 40MHz. At these frequencies, amateur radio enthusiasts could easily tune into its transmissions as it passed overhead (you can listen to samples [here](https://www.youtube.com/watch?v=-YSm2qFwRpI) ). The Explorer earth orbiter missions (1958) used VHF (~100MHz). The first Lunar probe (Pioneer III) used UHF (960MHz). Over time the tendency was to increase the frequency, first to S-band (2.3 GHz), then to X-Band (8.4GHz), Ku-Band (10-18GHz) and so on. The two main reasons for this was competition for frequency allocations at lower frequencies (the frequencies you can use to communicate with are tightly regulated) and the need for more spacecraft antenna directivity. ### Signal power degradation with distance in passive and active satellites For a **passive** satellite (effectivel just a reflector, no amplification) the following expression can be used to calculate the received signal power: $$ P_{r} = \frac{P_{t} \cdot G_{t} \cdot G_{r} \cdot \sigma}{(4\pi)^{2} \cdot d_{t}^{2} \cdot d_{r}^{2}}$$ Where: * $P_{t}, P_{r}$: Transmitter and receiver power * $G_{t}, G_{r}$: Gain of transmitter and receiver antennas * $d_{t}, d_{r}$: Distance of ground stations to satellite * $\sigma$: satellite gain/loss factor Assuming $d_{t} \approx d_{r}$ we get: $$ P_{r} = K \cdot \frac{\sigma}{d^{4}} \cdot P_{t} $$ As we can see the received power signal is inversely proportional to the fourth power of the distance. For an **active** satellite we get: $$ P_{r} = \frac{P_{t} \cdot G_{t} \cdot G_{r} \cdot g_{s} \cdot A_{s}}{(4\pi)^{2} \cdot d_{t}^{2} \cdot d_{r}^{2}}$$ Where $g_{s}$ is the transponder amplification coefficient and $A_{s}$ is the effective area of the active satellite antenna. Assuming $g_{s}$ is proportional to $4\pi d_{r}^{2}$ we get: $$ P_{r} = K^{'}\cdot \frac{A_{s}}{d^{2}} \cdot P_{t} $$