Satellite Semantic Addressing for Satellite Constellation

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. Argument of periapsis (omega)¶
6. True anomaly (nu)¶

The circular orbit is widely used by proposals of satellite constellation from different companies and countries.¶

For a circular orbit, we will have:¶

• Eccentricity e = 0¶
• Semimajor axis a = Altitude of satellite¶
• Argument of periapsis omega = 90 degree¶

So, three parameters, Altitude, Inclination and Longitude of the ascending node, will be enough to describe the orbit. The satellite will move in a constant speed and True anomaly (nu) can be easily calculated after the epoch time is defined.¶

4.2. Satellite Constellation Compositions

One satellite constellation may be composed of many satellites (LEO and VLEO), but normally all satellites are grouped in a certain order that is never changed during the life of satellite constellation. Each satellite constellation's orbits parameters described in Section 4.1 must be approved by regulator and cannot be changed either. Follows are characters of one satellite constellation:¶

1. One Satellite Constellation is composed of couple of shell groups of satellites.¶
2. The same shell group of satellites will have the same altitude and inclination angle.¶
3. The total No orbit planes in the same shell group of satellites will be evenly distributed by the same interval of Longitude of the ascending node (Omega). The interval equals to (360 degree/No). As a result, all orbit planes in the same shell group will effectively form a shell to cover earth (there will be a coverage hole for the shell on the sky in both polar areas if the inclination angle is less than 90 degree).¶
4. Each orbit plane in the same shell group will have the same number of satellites, all satellites in the same orbit plane will be evenly distributed angularly in the orbit plane. Assuming there are Ns satellites in each orbit plane, then the angular interval of satellites equals to (360 degree/Ns).¶
5. All satellites in the same shell group are moving in the same circular direction. As a result, at any location on earth, we can see there will have two group of satellites moving on the opposite direction. One group moves from south to north, and another group moves from north to south. Section 5.5 has more details.¶

4.3. Communication between Satellites by ISL

When ISL is used for the communication between satellites, each satellite will have a fixed number of links to connect to its neighbor. Due to the cost of ISL and the constraints of power supply on satellite, the number of ISL is normally limited to connect to its closest neighbors. In 3D space, each satellite may have six types of adjacent satellites, each type represents one direction. The number of adjacent neighbors in one direction is dependent on the number of deployment of ISL device on satellites, for example, the laser transmitter and receiver for ISLL. Figure 1 illustrates satellite S0 and its adjacent neighbors.¶

       /           /           /
      /           /           /
     /           /           /
    S7          S8          S9
   /           /           /
  /           /           /
 /           /           /
       /           S1          /
      S5          /           S3
     /           /           /
    /           S0          /
   /           /           /
  S6          /           S4
 /           S2          /
       /           /           /
      /           /           /
     /           /           /
    S10         S11         S12
   /           /           /         ^ Moving direction
  /           /           /         /
 /           /           /         /
orbit      orbit       orbit

All adjacent satellites of S0 in Figure 1 are listed below:¶

1. The front adjacent satellite S1 that is on the same orbit plane as S0.¶
2. The back adjacent satellite S2 that is on the same orbit plane as S0¶
3. The right adjacent satellites S3 and S4 that are on the right orbit plane of S0¶
4. The left adjacent satellites S5 and S6 that are on the left orbit plane of S0¶
5. The above adjacent satellites S7 to S9 that are on the above orbit plane of S0¶
6. The below adjacent satellite S10 to S12 that are on the below orbit of plane S0¶

The relative position of adjacent satellites will directly determine the quality of ISL and communication. From the analysis in [I-D.lhan-problems-requirements-satellite-net], The speed of satellite is only related to the altitude of the satellite (on circular orbit), all satellites with a same altitude will move with the same speed. So, in above adjacent satellites, some adjacent satellite's relative positions are steady and the ISL can be alive without interruption caused by movement. Some adjacent satellites relative positions are changing quickly, the ISL may be down since the distance may become out of reach for the laser of ISL, or the quick changed positions of two satellite make the tracking of laser too hard. Below are details:¶

• The relative position of satellites in the same orbit plane will be the steadiest.¶
• The relative position of satellites in the direct neighbor orbit planes in the same shell group and moving in the same direction will be steady at equator area, but will be changing when two orbits meet on the polar area. Whether the link status will be flipping depends on the tracking technology and the range of laser pointing angle of ISL. See Figure 2.¶
• The relative position of satellites in the neighbor orbit planes in the same shell group but moving in the different direction will not be steady at all times. More details are explained in Figure 8¶
• The relative position of satellites in the neighbor orbit planes in the different shell group will be dependent on the difference of altitude and inclination. This has been analyzed in [I-D.lhan-problems-requirements-satellite-net].¶

             \      /
              P3   P4
               \  /
                \/
                /\
               /  \
              P1   P2
             /      \

* Two satellites S1 and S2 are at position P1 and P2 at time T1
* S1's right facing ISL connected to S2's left facing ISL
* S1 and S2 move to the position P4 and P3 at time T2
* S1's left facing ISL connected to S2's right facing ISL
* So, if the range of laser pointing angle is 360 degree and
  tracking technology supports, the ISL will not be flipping
  after passing polar area; Otherwise, the link will be flipping

5.1. Indexes of Satellite

As described in Section 4.2, one satellite has three important orbit related information as described below.¶

1. Index for the shell group of satellites in a satellite constellation¶
2. Index for the orbit plane in a shell group of satellites¶
3. Index for the satellite in an orbit plane¶

It should be noted that for all type of indexes, it is up to the owner to assign the index number. There is no rule for which one should be assigned with which number. The only important rule is that all index number should be in sequential to reflect its relative order and position with others. Below is an example of assignment rules:¶

1. The 1st satellite launched in an orbit plane can be assigned for the 1st satellite index (0), the incremental direction of the satellite index in the same orbit plane is the incremental direction of "Argument of periapsis (omega)"¶
2. The 1st orbit plane established can be assigned for the 1st orbit plane index (0), the incremental direction of the orbit plane index is the incremental direction of "Longitude of the ascending node (Omega)".¶
3. The shell group of satellites with the lowest altitude can be assigned for the 1st shell group index (0), the incremental direction of shell group index is the incremental direction of altitude.¶

It should also be noted that for all type of indexes assignment, there are no strict requirement for the physical positions of satellite. Due to the launching time difference, the shifing of the satellite orbit after some time, the orbit parameters of satellites always have some difference and do not follow the theoritical values. For example:¶

1. The altitude of all satellites in the same shell group might not be exactly same.¶
2. The inclination angle of all satellites in the same shell group might not be exactly same.¶
3. The Longitude of the ascending node (Omega) of all satellites in the same orbit plane might not be exactly same.¶
4. The interval of the Longitude of the ascending node (Omega) of all orbit plane in the same shell group might not be equal¶
5. The angular interval of all satellites in the same orbit plane might not be equal.¶

Figure 3 and Figure 4 illustrate three types of indexes for satellite¶

       /           /           /   \
      /           /           /    |
     /           /           /     |
    S           S           S      > shell group3
   /           /           /       |
  /           /           /        |
 /           /           /         /
       /           S           /   \
      S           /           S    |
     /           /           /     |
    /           S           /       > shell group2
   /           /           /       |
  S           /           S        |
 /           S           /         /
       /           /           /   \
      /           /           /    |
     /           /           /     |
    S           S           S       > shell group1
   /           /           /       |
  /           /           /        |
 /           /           /         /
orbit     orbit      orbit           ----> Earth self-rotation
plane1    plane2     plane3

Shell Group and Orbit Plane Indexes for Satellites¶

         , - ~ S1 ~ - ,
     S2 '              ' S8
   ,                       ,
  ,                         ,
 ,                           ,      Indexed
 S3                          S7 <-- satellite
 ,                           ,      in one orbit plane
  ,                         ,
   ,                       ,      ^  move direction
     S4                 , S6     /
       ' - , _ S5_ ,  '         /

Three types of Index for satellites¶

5.2. The Range of Satellite Indexes

The ranges of different satellite indexes will determine the range the dedicated field for semantic address. The maximum indexes depend on the number of shell group, orbit plane and satellite per orbit plane. The number of orbit plane and satellite per orbit plane have relationship with the coverage of a satellite constellation. There are minimum numbers required to cover earth. [I-D.lhan-problems-requirements-satellite-net] has given the detailed math to estimate the minimal number required to cover the earth. There are two key parameters that determine the minimal number of satellite required. One is the elevation angle, another is the altitude. StarLink has proposed two elevation angles, 25 and 35 degrees [SpaceX-Non-GEO]. The lowest LEO altitude can be 160km according to [Lowest-LEO-ESA]. The Table 1 and Table 2 illustrate the estimation for different altitude (As), the coverage radius (Rc), the minimal required number of orbit planes (No) and satellite per orbit plane (Ns). The elevation angle is 25 degree and 35 degrees respectively.¶

The real deployment may be different as above analysis. Normally, more satellites and orbit planes are used to provide better coverage. So far, there are only two proposals available, one is StarLink, another is from China Constellation. For proposals of [StarLink], there are 7 shell groups, the number of orbit plane and satellites per orbit plane in all shell groups are 72 and 58; For proposals of [China-constellation], there are 7 shell groups, the number of orbit plane and satellites per orbit plane in all shell groups are 60 and 60;¶

It should be noted that some technical parameters, such as the inclination and altitude of orbit planes, in above proposals may be changed during the long-time deployment period, but the total numbers for indexes normally do not change.¶

From the above analysis, to be conservative, it is safe to conclude that the range of all three satellite indexes are less than 256, or 8-bit number.¶

5.3. Other Info for satellite addressing

In addition to three satellite indexes described in Section 5.1, other information is also important and can also be embedded into satellite address:¶

1. The company or country code, or the owner code. In the future, there may have multiple satellite constellations on the sky from different organizations, and the inter-constellation communication may become as normal that is similar to the network on the ground. This code will be useful to distinguish different satellite constellation and make the inter-constellation communication possible. One satellite constellation will have one code assigned by international regulator (IANA or ITU). Considering the following facts:¶

   • The space of LEO satellite orbits is limited. New LEO satellite orbits need ITU's approve.¶
   • The spectrum for LEO satellite communication is limited. New spectrum needs ITU's approve.¶
   • The costs of satellite constellations in launching, maintenance and operation are considerably high.¶

   We can predict the total number of satellite constellation is very limited. So, the size of code is limited. In the draft, we propose to use one octet for Owner code.¶

2. The Interface Index. This index is to identify the ISL or ISLL for a satellite. As described in Section 4.3, the total number of ISL is limited. So, the size of interface index is also limited.¶

5.4. Encoding of Satellite Semantic Address

The encoding for satellite semantic address is dependent on what routing and switching (L2 or L3 solution) technologies are used for satellite networking, and finally dependent on the decision of IETF community.¶

Follows are some initial proposals:¶

1. 32-bit satellite semantic address (Figure 5) can be used for Router ID if IGP, i.e, OSPF, is used for the routing within the satellite network. Note, this does not hint the current OSPF can be used for satellite network without any changes. Separate drafts should be written to describe the details about the modified OSPF for satellite network routing.¶
2. When satellite network is using L3 or IPv6 solution, the satellite semantic address is encoded as the interface identifier (i.e., the rightmost 64 bits) of the IPv6 address for IPv6. Figure 6 shows the format of IPv6 Satellite Address.¶
3. When satellite network is using L2 solution, the satellite semantic address can be embedded into the field of "Network Interface Controller (NIC) Specific" in MAC address [IEEE-MAC-Address]. But due to shorter space for NIC, the "Index for the shell group" and "Index for Interface" will only have 4-bit. This is illustrated in Figure 7. This encoded MAC address can also be used for L3 solution where the interface MAC may be also needed to be configured for each ISL.¶
4. Recently, some works suggested to use Length Variable IP address for routing and switching [Length-Variable-IP] or use flexible IP address [I-D.jia-flex-ip-address-structure] or shorter IP address [I-D.li-native-short-addresses] to solve some specific problems that regular IPv6 is not very suitable. Satellite network also belongs to such specific network. Due to the resource and cost constraints and requirement for radiation hardened electronic components, the ISL speed, on-board processor and memory are limited in performance, power consumption and capacity compared with network devices on ground. So, using IPv6 directly in satellite network is not an optimal solution because IPv6 header size is too long for such small network. From above analysis, 32-bit to 64-bit length of IP address is enough for satellite networking. Using 128-bit IPv6 will consume more resource especially the ISL bandwidth, processing power and memory, etc. If shorter than 128-bit IP address is accepted as IETF work, the satellite semantic address can be categorized as a similar use case. Figure 5 illustrates a 32-bit Semantic Satellite Address format. The final coding for the shorter IP address can be decided by the community. How to use the 32-bit Semantic Satellite address can be addressed later on in different document.¶

  0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Owner Code  |  Shell_Index  |  Orbit_Index  |   Sat_Index   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Owner Code: Identifier for the owner of the constellation
Shell_Index: Index for the shell group of satellite in a satellite
             constellation
Orbit_Index: Index for the orbit plane in a shell group of satellite
Sat_Index: Index for the satellite in an orbit plane

  0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~                     Subnet Prefix (64 bits)                   ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Owner Code  |  Shell_Index  |  Orbit_Index  |   Sat_Index   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Intf_Index  |                    Reserved                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Owner Code: Identifier for the owner of the constellation
Shell_Index: Index for the shell group of satellite in a satellite
             constellation
Orbit_Index: Index for the orbit plane in a shell group of satellite
Sat_Index: Index for the satellite in an orbit plane
Intf_Index: Index for interface on a satellite
Reserved: 24-bits reserved

          3 Octets             3 Octets
    /---------^--------\ /--------^--------\
    +-------------------+-------------------+
    |        OUI        |     Sat Address   |
    +-------------------+-------------------+
                                 |
                                 |
 +-------------------------------+
 |
 |
 v

  0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Shell |  Orbit_Index  |   Sat_Index   |Intf_Id|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

OUI: Organizationally Unique Identifier assigned by IEEE
Shell: 4-bit Index for the shell group of satellite in a satellite
       constellation
Orbit_Index: Index for the orbit plane in the group of satellite
Sat_Index: Index for the satellite in the orbit plane
Intf_Id: 4-bit Index for interface on a satellite

5.5. Link Identification by Satellite Semantic Address

Using above satellite semantic addressing scheme, to identify steady and un-steady links is as simple as below:¶

Assuming:¶

1. The total number of satellites per orbit plane is M¶
2. The total number of orbit planes per shell group is N.¶

3. Two satellites have:¶

   • Satellite Indexes as: Sat1_Index, Sat2_Index¶
   • Orbit plane Indexes as: Orbit1_Index, Orbit2_Index¶
   • Shell group Indexes as: Shell1_Index, Shell2_Index¶

Steady links:¶

1. The links between adjacent satellites on the same orbit plane, or, the satellite indexes satisfy:¶

   • Sat2_Index = Sat1_Index + 1, when Sat1_Index < M-1; Sat2_Index = 0, when Sat1_Index = M-1; and¶
   • Orbit1_Index = Orbit2_Index, Shell1_Index = Shell2_Index.¶

2. The links between satellites on adjacent orbit planes on the same altitude. and two satellites are moving to the same direction, or, the satellite indexes satisfy:¶

   • Orbit2_Index = Orbit1_Index + 1, when Orbit1_Index < N-1; Orbit2_Index = 0, when Orbit1_Index = N-1; and¶
   • Shell1_Index = Shell2_Index.¶
   • Sat1_Index and Sat2_Index may be equal or have difference, depend on how the link is established.¶

Un-Steady links:¶

1. The links between satellite and ground stations.¶
2. The links between satellites on adjacent orbit planes on the same altitude. Two satellites are moving to the different direction. Or, the satellite indexes do not satisfy conditions described in above #2 for Steady links.¶

3. The links between satellites on adjacent orbit planes on different altitude. Or, the satellite indexes satisfy:¶

   • Shell1_Index != Shell2_Index.¶

Figure 8 illustrates the links for adjacent orbit planes (#2 for Steady Link and Un-steady Link above). From the figure, it can be noticed that some links may have shorter distance than steady link, but they are unsteady. For example, the links between S1 and S4; S4 and S2; S2 and S5, etc.¶

            i+N/2         i+1+N/2       i+2+N/2
          / \           / \           / \
         /   \         /   \         /   \
        S1............S2............S3    \
       /       S4 ..........S5............S6
      /         \   /         \   /         \
     /           \ /           \ /           \
     i-1           i             i+1

* The total number of orbit planes are N
* The number (i-1, i, i+1,...) represents the Orbit index
* The bottom numbers (i-1, i, i+1) are for orbit planes on
  which satellites (S1, S2, S3) are moving from bottom to up.
* The top numbers (i+N/2, i+1+N/2, i+2+N/2) are for orbit
  planes on which satellites (S4, S5, S6) are moving from up
  to bottom.
* Dot lines are the steady links