Problems and Requirements of Satellite Constellation for Internet

4.1. Satellite Orbit

The orbit of a satellite can be either circular or ecliptic, it can be described by following Keplerian elements [KeplerianElement]:¶

1. Inclination (i)¶
2. Longitude of the ascending node (Omega)¶
3. Eccentricity (e)¶
4. Semimajor axis (a)¶
5. The withdraw BGP routes are redistributed into IGP, IGP starts to flood the LSA to all satellites.¶
6. Argument of periapsis (omega)¶
7. True anomaly (nu)¶

For a circular orbit, two parameters, Inclination and Longitude of the ascending node, will be enough to describe the orbit.¶

4.2. Coverage of LEO and VLEO Satellites and Minimum Number Required

The coverage of a satellite is determined by many physical factors, such as spectrum, transmitter power, the antenna size, the altitude of satellite, the air condition, the sensitivity of receiver, etc. EIRP could be used to measure the real power distribution for coverage. It is not deterministic due to too many variants in a real environment. The alternative method is to use the minimum elevation angle from user terminals or gateways to a satellite. This is easier and more deterministic. [SpaceX-Non-GEO] has suggested originally the minimum elevation angle of 35 degrees and deduced the radius of the coverage area are about 435km and 1230km for VLEO (altitude 335.9km) and LEO (altitude 1150km) respectively. The details about how the coverage is calculated from the satellite elevation angle can be found in [Satellite-coverage].¶

Using this method to estimate the coverage, we can also estimate the minimum number of satellite required to cover the earth surface.¶

It must be noted, SpaceX has recently reduced the required minimum elevation angle from 35 degree to 25 degree. The following analysis still use 35 degree.¶

Assume there is multiple orbit planes with the equal angular interval across the earth surface (The Longitude of the ascending node for sequential orbit plane is increasing with a same angular interval). Each orbit plane will have:¶

1. The same altitude.¶
2. The same inclination of 90 degree.¶
3. The same number of satellites.¶

With such deployment, all orbit planes will meet at north and south pole. The density of satellite are not equal. Satellite is more dense in the space above the polar area than in the space above the equator area. Below estimations are made in the worst covered area, or the area of equator where the satellite density is the minimum.¶

Figure 1 illustrates the coverage area on equator area, and each satellite will cover one hexagon area. The figure is based on plane geometry instead of spherical geometry for simplification, so, the orbit is parallel approximately.¶

Figure 2 shows how to calculate the radius (Rc) of coverage area from the satellite altitude (As) and the elevation angle (b).¶

                       x
            |          |
            |          |
            x      *********
            |     *    |    *
            |    *     |     *
        *********      x      *
       *    |    *     |     *
      *     |     *    |    *
     *      x      *********
      *     |     *    |
       *    |    *     |
        *********      x
            |          |
            |       orbit 2          ^ north
            x                        |
            |                        |
            |                        |
          orbit 1                    +-------> east

Satellite coverage on ground¶

        |<---  2*Rc --->|

                + Satellite
               /|
              / |
             /  |
            / b |
           /-\  +
          /   * |     __Earth surface
         /  *   |    /
        / *_----+----__
        +               +
         *             *
          *           *
           *   2*a   *
            *  ___  *
             *-   -*
              *   *
               * *
                * Earth center

Satellite coverage estimation¶

x

The vertical projection of satellite to Earth¶

Re

The radius of the Earth, Re=6378(km)¶

As

The altitude of a satellite¶

Rc

The radius (arc length) of the coverage, or, the arc length of hexagon center to its 6 vertices. Rc=Re*(a*pi)/180¶

a

The cap angle for the coverage area (the RC arc). a = arccos((Re/(Re+As))*cos(b))-b.¶

b

The least elevation angle that a ground station or a terminal can communicate with a satellite, b = 35 degree.¶

Ns

The minimum number of satellite on one orbit plane, it is equal to the number of the satellite's vertical projection on Earth, so, Ns = 180/(a*cos(30))¶

No

The minimum number of orbit (with same inclination), it is equal to the number of the satellite orbit's vertical projection, so, No = 360/(a*(1+sin(30)))¶

For a example of two type of satellite LEO and VEO, the coverages are calculated as in Table 1:¶

4.3. Real Deployment of LEO and VLEO for Satellite Network

Obviously, the above orbit parameter setup is not optimal since the sky in the polar areas will have the highest density of satellite.¶

In the real deployment, to provide better coverage for the areas with denser population, to get redundance and better signal quality, and to make the satellite distance within the range of inter-satellite communication (2000km [Laser-communication-range]), more than the minimum number of satellites are launched. For example, different orbit planes with different inclination/altitude are used.¶

Normally, all satellites are grouped by orbit planes, each group has a number of orbit planes and each orbit plane has the same orbit parameters, so, each orbit in the same group will have:¶

1. The same altitude¶
2. The same inclination, but the inclination is less than 90 degree. This will result in the empty coverage for polar areas and better coverage in other areas. See the orbit picture for phrase 1 for [StarLink].¶
3. The same number of satellites¶
4. The same moving direction for all satellites¶

The proposed deployment of SpaceX can be seen in [SpaceX-Non-GEO] for StarLink.¶

The China constellation deployment and orbit parameters can be seen in [China-constellation].¶

5.1. Dynamic Ground-station-Satellite Communication

All satellites are moving and will lead to the communication between ground station and satellite can only last a certain period of time. This will greatly impact the technologies for the satellite networking. Below illustrates the approximate speed and the time for a satellite to pass through its covered area.¶

In Table 2, VLEO1 and LEO3 have the lowest and highest altitude respectively, VLEO2 is for the highest altitude for VLEO. We can see that longest communication time of ground-station-satellite is less than 400 seconds, the longest communication time for VLEO ground-station-satellite is less than 140 seconds.¶

Re

The radius of the Earth, Re=6378(km)¶

As

The altitude of a satellite¶

AL

The arc length(in km) of two neighbor satellite on the same orbit plane, AL=2*cos(30)*(Re+As)*(a*pi)/180¶

SD

The space distance(in km) of two neighbor satellite on the same orbir plane, SD=2*(Re+As)*sin(AL/(2*(Re+As))).¶

V

the velocity (in m/s) of satellite, V=sqrt(G*M/(Re+As))¶

G

Gravitational constant, G=6.674*10^(-11)(m^3/(kg*s^2))¶

M

Mass of Earth, M=5.965*10^24 (kg)¶

T

The time (in second) for a satellite to pass through its cover area, or, the time for the station-satellite communication. T= ALs/V¶

5.2. Dynamic Inter-satellite Communication

In order to form a network by satellites, there must be a inter-satellite communication. Traditionally, inter-satellite communication uses the microwave technology, but it has following disadvantages:¶

1. Bandwidth is limited and only up to 600M bps [Microwave-vs-Laser-communication].¶
2. Security is a concern since the microwave beam is relatively wide and it is easy for 3rd party to sniff or attack.¶
3. Big antenna size.¶
4. Power consumption is high.¶
5. High cost per bps.¶

Recently, laser is used for the inter-satellite communication, it has following advantages, and will be the future for inter-satellite communication.¶

1. Higher bandwidth and can be up to 10G bps [Microwave-vs-Laser-communication].¶
2. Better security since the laser beam size is much narrower than microwave, it is harder for sniffing.¶
3. The size of optical lens for laser is much smaller than microwave's antenna size.¶
4. Power saving compared with microwave.¶
5. Lower cost per bps.¶

The range for satellite-to-satellite communications has been estimated to be approximately 2,000 km currently [Laser-communication-range].¶

From Table 2, we can see the Space Distance (SD) for some LEO (altitude over 1100km) are exceeding the celling of the range of laser communication, so, the satellite and orbit density for LEO need to be higher than the estimation values in the Table 1.¶

Assume the laser communication is used for inter-satellite communication, then we can analyze the life time of inter-satellite communication when satellites are moving. The Figure 3 illustrates the movement and relative position of satellites on three orbits. The inclination of orbit planes are 90 degree.¶

                       + North pole
                      /|\
                     | s |
                    s  |  s
                   /   s   \
                   s   |   s
                   |   s1  |
                   s4  |   s6
                   |   s2  |   -------- Equator
                   s5  |   s7
                   |   s3  |
                   s   |   s
                   \   s   /
                    s  |  s
                     | s |
                      \|/
                       +  South pole

Satellite movement¶

There are three scenarios:¶

1. For satellites within the same orbit

The satellites in the same orbit will move to the same direction with the same speed, thus the interval between satellites is relatively steady. Each satellite can communicate with its front and back neighbor satellite as long as satellite's orbit is maintained in its life cycle. For example, in Figure 3, s2 can communication with s1 and s3.¶

2. For satellites between neighbor orbits in the same group

The orbits for the same group will share the same orbit altitude and inclination. So, the satellite speed in different orbit are also same. Similar to the scenario 1, the relative position of satellites in different orbit is relatively steady as long as satellite's orbit is maintained in its life cycle. For this scenario, a satellite can communicate with a satellite ahead in the previous and next orbit, it can also communicate with a satellite behind in the previous and next orbit. For example, the s2 communicates with s4, s5, s6, s7.¶

3. For satellites between different orbits in the different group

The orbits for the different group will have different orbit altitude, inclination and speed. So, the relative position of satellite is not static. The inter-satellite communication can only last for a while when the distance between two satellite is within the limit of inter-satellite communication, that is 2000km for laser [Laser-communication-range].¶

For the scenario 3, the estimation of the life time when two satellite can communicate are as follows.¶

Figure 4 illustrates two satellites move and intersect with an angle¶

               ^ V2
              /
             +-\ A
            /   |
-----------/----+-------> V1
          /
         /
        /

Satellite (speed vector V1 and V2) intersects with Angle A¶

Dl

The laser communication limit, Dl=2000km [Laser-communication-range]¶

Da

Altitude difference (in km) for two orbit plane¶

A

The angle between two orbit's vertical projection on Earth¶

Vl

The speed vector of satellite on orbit 1¶

V2

The speed vector of satellite on orbit 2¶

|V|

the magnitude of the difference of two speed vector V1 and V2, |v|=|V1-V2|=sqrt((V1-V2*cos(A))^2+(V2*sin(A))^2)¶

T

The life time two satellites can communicate, or the time of two satellites' distance is within the range of communication, T = 2*sqrt(Dl^2-Da^2)/|V|¶

Using formulas above, below is the estimation for the life of communication of two satellites when they intersect. Table 3 and Table 4 are for two VLEOs with the difference of 114.1km for altitude. (VLEO1 and VLEO2 on Table 2). Table 5 and Table 6 are for two LEOs with the difference of 175km for altitude (LEO2 and LEO3 on Table 2).¶

7.1. Common Problems and Requirements

For both satellite relay and satellite networking, satellite-ground-station must be used, so, the problems and requirements for the satellite-ground-station communication is common and will apply for both methods.¶

When one satellite is communicating with ground station, the satellite only needs to receive data from uplink of one ground station, process it and then send to the downlink of another ground station. Figure 5 illustrates this case. Normally microwave is used for both links.¶

Additionally, from the coverage analysis in Section 4.2 and real deployment in Section 4.3, we can see one ground station may communicate with multiple satellites. Similarly, one satellite may communicate with multiple ground stations. The characters for satellite-ground-station communication are:¶

1. Satellite-ground-station communication is P2MP.

Since microwave physically is the carrier of broadcast communication, one satellite can send data while multiple ground stations can receive it. Similarly, one ground station can send data and multiple satellites can receive it.¶

2. Satellite-ground-station communication is in open space and not secure.

Since electromagnetic fields for microwave physically are propagating in open space. The satellite-ground-station communication is also in open space. It is not secure naturally.¶

3. Satellite-ground-station communication is not steady.

Since the satellite is moving with high speed, from Section 5.1, the satellite-ground-station communication can only last a certain period of time. The communication peers will keep changing.¶

4. Satellite physical resource is limited.

Due to the weight, complexity and cost constraint, the physical resource on a satellite, such as power supply, memory, link speed, are limited. It cannot be compared with the similar device on ground. The design and technology used should consider these factors, and take the appropriate approach if possible.¶

The requirements of satellite-ground-station communication are:¶

R1. The bi-directional communication capability

Both satellites and ground stations have the bi-directional communication capability¶

R2. The identifier for satellites and ground stations

Satellites and ground stations should have Ethernet and/or IP address configured for the device and each link. More detailed address configuration can be seen in each solution.¶

R3. The capability to decide where the IP packet is forwarded to.

In order to send Internet traffic or IP date to destination correctly, satellites and ground station must have Ethernet hub or switching or IP routing capability. More detailed capability can be seen in each solution.¶

R4. The protocol to establish the satellite-ground-station communication.

For security and management purpose, the satellite-ground-station communication is only allowed after both sides agree through a protocol. The protocol should be able to establish a secured channel for the communication when a new communication peer comes up. Each ground station should be able to establish multiple channels to communicate with multiple satellites. Similarly, each satellite should be able to establish multiple channels to communicate to multiple ground stations.¶

R5. The protocol to discover the state of communication peer.

The discover protocol is needed to detect the state of communication peer such as peer's identity, the state of the peer and other info of the peer. The protocol must be running securely without leaking the discovered info.¶

R6. The internet data packet is forwarded securely.

When satellite or ground station is sending the IP packet to its peer, the packet must be relayed securely without leaking the user data.¶

R7. The internet data packet is processed efficiently on satellite

Due to the resource constraint on a satellite, the packet may need more efficient mechanism to be processed on satellite. The process on satellite should be very minimal and offloaded to ground as much as possible.¶

7.2. Satellite Relay

One of the reason to use satellite constellation for internet access is it can provide shorter latency than using the fiber cable underground. But using ISL for inter-satellite communication is the premise for such benefit in latency. Since the ISL is still not mature and adopted commercially, satellite relay is a only choice currently for satellite constellation used for internet access. In [UCL-Mark-Handley], detailed simulations have demonstrated better latency than fiber network by satellite relay even the ISL is not present.¶

7.3. Satellite Networking

In the draft, satellite Network is defined as a network that satellites are inter-connected by inter-satellite links (ISL). One of the major difference of satellite network with the other type of network on ground (telephone, fiber, etc.) is its topology and links are not stationary, Some new issues have to be considered and solved. Follows are the factors that impact the satellite networking.¶

  /--------\             S1---S2----S3----\               /------\
 /          \            /    IGP domain   \             /        \
+ Local net CE--eBGP--PE_GS1---S4---S5---PE_GS2--eBGP--PE Internet +
 \          /            \                 /             \        /
  \--------/              \---S6---S7---S8/               \------/