| Internet-Draft | Problems, Requirements for Satellite Net | July 2023 |
| Han, et al. | Expires 6 January 2024 | [Page] |
This document presents the detailed analysis about the problems and requirements of satellite constellation used for Internet. It starts from the satellite orbit basics, coverage calculation, then it estimates the time constraints for the communications between satellite and ground-station, also between satellites. How to use satellite constellation for Internet is discussed in detail including the satellite relay and satellite networking. The problems and requirements of using traditional network technology for satellite network integrating with Internet are finally outlined.¶
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Satellite constellation for Internet is emerging. Even there is no constellation network established completely yet at the time of the publishing of the draft (June 2021), some basic internet service has been provided and has demonstrated competitive quality to traditional broadband service.¶
This memo will analyze the challenges for satellite network used in Internet by traditional routing and switching technologies. It is based on the analysis of the dynamic characters of both ground-station-to-satellite and inter-satellite communications and its impact to satellite constellation networking.¶
The memo also provides visions for the future solution, such as in routing and forwarding.¶
The memo focuses on the topics about how the satellite network can work with Internet. It does not focus on physical layer technologies (wireless, spectrum, laser, mobility, etc.) for satellite communication.¶
The traditional satellite communication system is composed of few GSO and ground stations. For this system, each GSO can cover 42% Earth's surface [GEO-Coverage], so as few as three GSO can provide the global coverage theoretically. With so huge coverage, GSO only needs to amplify signals received from uplink of one ground station and relay to the downlink of another ground station. There is no inter-satellite communications needed. Also, since the GSO is stationary to the ground station, there is no mobility issue involved.¶
Recently, more and more LEO and VLEO satellites have been launched, they attract attentions due to their advantages over GSO and MEO in terms of higher bandwidth, lower cost in satellite, launching, ground station, etc. Some organizations [ITU-6G][Surrey-6G][Nttdocomo-6G] have proposed the non-terrestrial network using LEO, VLEO as important parts for 6G to extend the coverage of Internet. 3GPP has been working on the NTN integration with 5G and beyond. SpaceX has started to build the satellite constellation called StarLink that will deploy over 10 thousand LEO and VLEO satellites finally [StarLink]. China also started to request the spectrum from ITU to establish a constellation that has 12992 satellites [China-constellation]. European Space Agency (ESA) has proposed "Fiber in the sky" initiative to connect satellites with fiber network on Earth [ESA-HydRON].¶
When satellites on MEO, LEO and VLEO are deployed, the communication problem becomes more complicated than for GSO satellites. This is because the altitude of MEO/LEO/VLEO satellites are much lower. As a result, the coverage of each satellite is much smaller than for GSO, and the satellite is moving very fast on the ground reference and not relatively stationary to the ground. This will lead to:¶
In Section 4, we will discuss couple of topics of satellite network integration with Internet, such as using satellite network for broadband access and wireless access, the current 3GPP works for satellite network in 5G and beyond.¶
Finally, the problems and requirements for satellite network integration with Internet will be discussed and analyzed in Section 5.¶
As the 1st satellite constellation company in history, the SpaceX/StarLink will be inevitably mentioned in the draft. But it must be noted that all information about SpaceX/StarLink in the draft are from the public. Authors of the draft have no relationship or relevant inside knowledge of SpaceX/Starlink.¶
Since there is no complete satellite network established yet, all following analysis is based on the predictions from the traditional GEO communication. The analysis also learnt how other type of network has been used in Internet, such as Broadband access network, Mobile access network, Enterprise network and Service Provider network.¶
As a criteria to be part of Internet, any device connected to any satellite should be able to communicate with any public IP4 or IPv6 address in Internet. There could be three types of methods to deliver IP packet from source to destination by satellite:¶
Using the above methods for IP packet delivery via satellite network, we will have two typical use cases for satellite network. One is for the general broadband access (see Section 4.1), another is for the integration with 3GPP wireless network including 4G and 5G (see Section 4.2 and Section 4.3).¶
For this use case, the end user terminal or local network is connected to a ground station, and another ground station is connected to Internet. Two ground stations will have IP connectivity via a satellite network. The satellite network could be by satellite relays or by inter-satellite network.¶
Follows are typical deployment scenarios that a Satellite network is used for broadband access of Internet.¶
S1----\ /-----------\
/ \ / \
T---GW--GS1--S2--GS2-------PE Internet +
\ / \ /
\---S3/ \-----------/
S1----\ S4----\ /-----------\
/ \ / \ / \
T---GW--GS1--S2--GS2---S5--GS3---PE Internet +
\ / \ / \ /
\---S3/ \---S6/ \-----------/
S1-----S2-----S3--\ /----------\
/ \ / \
T---GW--GS1--S4----S5---S6---GS2-------PE Internet +
\ / \ /
\---S7----S8----S9/ \----------/
/-----------\ S1----\ /-------\ / \ / \ / \ + Local network CE------GS1--S4--GS2-------PE Internet + \ / \ / \ / \-----------/ \---S7/ \-------/
/-----------\ S1----\ S4----\ /-------\ / \ / \ / \ / \ + Local network CE----GS1--S2--GS2--S5--GS3---PE Internet + \ / \ / \ / \ / \-----------/ \---S3/ \---S6/ \-------/
/-----------\ S1-----S2-----S3---\ /------\ / \ / \ / \ + Local network CE------GS1--S4----S5---S6---GS2-------PE Internet+ \ / \ / \ / \-----------/ \---S7----S8----S9/ \------/
In above Figure 1 to Figure 6, the meaning of symbols are as follows:¶
For this use case, the wireless access network (4G, 5G) defined in 3GPP is used with satellite network. By such integration, a user terminal or local network can access Internet via 3GPP wireless network and satellite network. The End user terminal or local network access Internet through satellite network and Mobile Access Network. There are two cases: 1) From mobile access network to satellite network or 2) From satellite network to mobile access network, Satellite network includes inter satellite network and relay network. See Figure 7 for mobile access network to satellite network, and Figure 8 for satellite network to mobile access network.¶
+--------------+ +-------------+ +---------+ +--------+ | T or | |Mobile Access| |Satellite| |Internet| | Local network+----+ Network +----+ Network +----+ | +--------------+ +-------------+ +---------+ +--------+
+--------------+ +---------+ +-------------+ +--------+ | T or | |Satellite| |Mobile Access| |Internet| | Local network+---+ Network +---+ Network +---+ | +--------------+ +---------+ +-------------+ +--------+
3GPP SA Working Groups (WG) feature a couple of satellite-related projects (or SIDs).¶
For Release 18, 3GPP has finished the project 'Study on 5G System with Satellite Backhaul' [TR-23.700-27] and 'Study on 5GC enhancement for satellite access Phase 2' [TR-23.700-28].¶
For Release 19, 3GPP will study more topics for satellite network used for 5G system, such as Regenerative payload generic architecture study, Store and Forward Satellite operation, etc.¶
One key aspect is to investigate the potential architecture requirements and enhancements to deploy UPFs on satellites (LEO/MEO/GEO) with gNBs on the ground. Specifically, it targets at enhancing the local-switching capability for UE-to-UE data communication when UEs are served by UPFs on-board satellite(s). Similarly, the SA1 WG proposed a new satellite-based SID in which the service end points (could also be called UEs in a broader sense) may continuously move in a fast way. The UEs can be ships, boats, and cars, etc., which are located in remote regions that need the connection to LEO's for achieving communication.¶
In all the SIDs, satellite based backhaul is important for mission critical scenarios in remote areas. Here, we want to clarify that while 3GPP documents TS 23.501 [TS-23.501] and 23.502 [TS-23.502] specify that a ground base station, i.e., gNB, may have multiple types of satellite backhauls (BH), e.g., GEO BH, LEO BH and LEO-BH with ISL, this use case focuses specifically on the LEO-BH with ISL. ISL stands for inter-satellite link.¶
Clearly, when a satellite backhaul involves multi-hop ISL path connected via different satellites, the capabilities provided by the satellite path would be changed and adjusted dynamically. For example, in the LEO case, the peering relationship between two neighboring satellites changes roughly every 5 minutes thanks to the orbital movement (see Table 2). This will definitely impair the networking performance and stability, and, in worst case, may cause the loss of connectivity. Even if some overlay tunneling mechanisms could be used to address the multi-hop ISL issue, the extra delay and potentially less bandwidth as introduced naturally by the ever-changing backhaul path would still impact the traffic engineering over the links.¶
The following diagram Figure 9 demonstrate the dynamic characteristics of satellite backhaul between two UEs. In the figure, UEs are connected, via gNBs, to UPFs on-board satellites. Both UPFs are connected via multi-hop ISLs to the 5G core (5GC) on the ground. There are two different multi-hop ISL paths: o A UE has to rely on a multi-hop ISL path to connect to 5GC on the ground. o When two UEs intend to communicate via the local data switching on satellite(s), some new ISL-based peering has to be established which would bring in the multi-hop ISL scenario. For example, the ISL between the Sat#1 and Sat#2 helps form a multi-hop path (marked N19 in the diagram) between the two UEs. Note that if the UPF-based local data switching involves only one UPF, then it is designated as intra-UPF local switching and relatively simpler. This is compared to the case of inter-UPF local switching as shown in the diagram.¶
Sat#: Satellite GS: Ground Station
UPF: User Plane Function (5G) gNB: Next Generation NodeB
Sat#1
+----------+ +--------+
| UPF#1 | (ISL) | Sat#.. |
UE#1--gNB#1--|(on-board)|- - - -| |-----+
+----------+ +--------+ |
: | --------
:(N19) v / \
:(ISL) GS ---+ 5G Core +
: ^ \ /
+----------+ +--------+ | --------
| UPF#2 | (ISL) | Sat#.. | |
UE#2--gNB#2--|(on-board)|- - - -| |-----+
+----------+ +--------+
Sat#2
In this diagram, both UEs are served by different satellite backhauls. If the local data switching via LEO UPFs on-board could be established (via the N19 ISL forwarding), then the system efficiency and QoE improvement would be achieved. Here, since UEs are served by different satellites, a multi-hop ISL scenario must be supported. But, this scenario posts challenges due to the dynamic satellite network topology and distinguished transmission capabilities from different satellites.¶
For example, if the UE-to-UE session has to maintain a service over longer time (> 5 minutes) such that the Sat#1 and Sat#2 move apart, then a new ISL path with potentially a new N19-ISL might be established. In worst case, if newly-involved satellites in the path happen to be polar-orbit ones and they do not support cross-seam ISLs, the communication latency may change dramatically when cross-seam transits or leaves. In another example, if both UEs belong to the same entity and need to form a 5G-VN group, then the 5G LAN-type service with PSA UPF-based local-switching must be applied among them.¶
Regardless, more efficient satellite communication mechanisms must be adopted, e.g., running efficient satellite-based routing protocols, establishing tunnels between LEO UPFs on-board, etc., for better local-data switching.¶
Further, 5GS may collaborate with satellite networks to improve QoS. One 5GC NF (i.e., SMF) can initiate UP path monitoring, and accordingly receive UP path monitoring results indicating observed delay. After that, the SMF takes corresponding actions like further verifying network statistics, updating sessions, etc. The coordination with the satellite networks would improve the process, which suggests satellites networks respond better to the (monitor-based) polling from 5GS.¶
One more thing we want to point out is that, while the propagation delay of satellite backhaul paths may change dramatically with the movement of satellite, this kind of change normally be periodic and can be well predicated based on the operation information of satellite constellation. Thus, making use of these information would also help for better services.¶
As described in Section 4, satellites in a satellite constellation can either relay internet traffic or multiple satellites can form a network to deliver internet traffic. More detailed analysis are in following sub sections. There might have multiple solutions for each method described in Section 4, following contexts only discuss the most plausible solution from networking perspectives.¶
Section 5.1 will list the common problems and requirements for both satellite relay and satellite networking.¶
Section 5.2 and Section 5.3 will describe key problems, requirement and potential solution from the networking perspective for these two cases respectively.¶
For both satellite relay and satellite networking, satellite-ground-station communication 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 1 illustrates this case. Normally microwave is used for both links.¶
Additionally, from the coverage analysis in Appendix A.2 and real deployment in Appendix A.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:¶
The requirements of satellite-ground-station communication are:¶
One of the reasons to use satellite constellation for internet access is it can provide shorter latency than using the fiber 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.¶
One satellite relay is the simplest method for satellite constellation to provide Internet service. By this method, IP traffic will be relayed by one satellite to reach the DGS and go to Internet.¶
The solution option and associated requirements are:¶
S1. The satellite only does L1 relay or the physical signal process.¶
For this solution, a satellite only receives physical signal, amplify it and broadcast to ground stations. It has no further process for packet, such as L2 packet compositing and processing, etc. All packet level work is done only at ground station. The requirements for the solution are:¶
In addition to the above requirements, following problem should be solved:¶
S2. The satellite does the L2 relay or L2 packet process.¶
For this solution, IP packet is passing through individual satellite as an L2 capable device. Unlike in the solution S1, satellite knows which ground station it should send based on packet's destination MAC address after L2 processing. The advantage of this solution over S1 is it can use narrower beam to communicate with DGS and get higher bandwidth and better security. The requirements for the solution are:¶
In addition to the above requirements, the problem P1-1 for S1 should also apply.¶
For this method, packet from SGS will be relayed through multiple intermediate satellites and ground station until reaching a DGS.¶
This is more complicated than one satellite relay described in Section 5.2.1.¶
One general solution is to configure both satellites and ground-stations as IP routing nodes, proper routing protocols are running in this network. The routing protocol will dynamically determine forwarding path. The obvious challenge for this solution is that all links between satellite and ground station are not static, according to the analysis in Appendix B.1, the lifetime of each link may last only couple of minutes. This will result in very quick and constant topology changes in both link state and IP adjacency, it will cause the distributed routing algorithms may never converge. So this solution is not feasible.¶
Another plausible solution is to specify path statically. The path is composed of a serials of intermediate ground stations plus SGS and DGS. This idea will make ground stations static and leave the satellites dynamic. It will reduce the fluctuation of network path, thus provide more steady service. One variant for the solution is whether the intermediate ground stations are connected to Internet. Separated discussion is as below:¶
S1. Manual configuring routing path and table¶
For this solution, the intermediate ground stations and DGS are specified and configured manually during the stage of network planning and provisioning. Following requirements apply:¶
In addition to the above requirements, the problem P1-1 in Section 5.2.1 should also apply.¶
S2. Automatic decision by routing protocol.¶
This solution is only feasible after the IP continuity problem (P1-1 in Section 5.2.1) is solved. Following requirements apply:¶
In addition to the above requirements, the problem P1-1 in Section 5.2.1 should also apply.¶
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.¶
The 1st question to answer is should the satellite network be configured as L2 or L3 network? As analyzed in Appendix A.2 and Appendix A.3, since there are couple of hundred or over ten thousand satellites in a network, L2 network is not a good choice, instead, L3 or IP network is more appropriate for such scale of network.¶
If we assume the orbit is circular and ignore other trivial factors, the satellite speed is approximately determined by the orbit altitude as described in the Appendix B.1. The satellite orbit can determine if the dynamic position of two satellites is within the range of the inter-satellite communication. That is 2000km for laser communication [Laser-communication-range] by Inter Satellite Laser Link (ISLL).¶
When two satellites' orbit planes belong to the same group, or two orbit planes share the same altitude and inclination, and when the satellites move in the same direction, the relative positions of two satellites are relatively stationary, and the inter-satellite communication is steady. But when the satellites move in the opposite direction, the relative positions of two satellites are not stationary, the communication lifetime is couple of minutes. The Appendix B.2.2 has analyzed the scenario.¶
When two satellites' orbit planes belong to the different group, or two orbit planes have different altitude, the relative position of two satellite are unstable, and the inter-satellite communication is not steady. As described in Appendix B.2, The life of communication for two satellites depends on the following parameters of two satellites:¶
From the examples shown in Table 4 to Table 7, we can see that the lifetime of inter-satellite communication for the different group of orbit planes are from couple of hundred seconds to about 18 hours. This fact will impact the routing technologies used for satellite network and will be discussed in Section 5.3.3.¶
When the satellite network is integrated with Internet by traditional routing technologies, following provisioning and configuration (see Figure 10) will apply:¶
The work on PE_GS1 are:¶
The work on PE_GS2 are:¶
/--------\ S1---S2----S3----\ /------\ / \ / IGP domain \ / \ + Local net CE--eBGP--PE_GS1---S4---S5---PE_GS2--eBGP--PE Internet + \ / \ / \ / \--------/ \---S6---S7---S8/ \------/
Local access Internet through inter-satellite-networking¶
On PE-GS1, due to the fact that IGP link between PE_GS1 and satellite is not steady; this will lead to following routing activity:¶
Similarly on PE_GS2, due to the fact that IGP link between PE_GS2 and satellite is not steady; this will lead to following routing activity:¶
For the analysis of detailed events above, the estimated time interval between event 1 and 5 for PE_GS1 and PE_GS2 can use the analysis in Appendix B.1. For example, it is about 398s for LEO and 103s for VLEO. Within this time interval, the satellite network including all satellites and two ground stations must finish the works from 1 to 4 for PE_GS1 and PE_GS2. The normal internet IPv6 and IPv4 BGP routes size are about 850k v4 routes + 100K v6 routes [BGP-Table-Size]. There are couple critical problems associated with the events:¶
As a summary, the traditional routing technology is problematic for large scale inter-satellite networking for Internet. Enhancements on traditional technologies, or new technologies are expected to solve the specific issues associated with satellite networking.¶
This memo includes no request to IANA.¶
Security considerations for communication between satellite and ground station, or between satellites are described in corresponding sections. There is no extra security issue introduced by this memo.¶
This section will introduce some basics for satellite such as orbit parameters, coverage estimation, minimum number of satellite and orbit plane required, real deployments.¶
The orbit of a satellite can be either circular or ecliptic, it can be described by following Keplerian elements [KeplerianElement]:¶
For a circular orbit, two parameters, Inclination and Longitude of the ascending node, will be enough to describe the orbit.¶
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 is 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 satellites required to cover the earth surface.¶
It must be noted, SpaceX has recently reduced the required minimum elevation angle from 35 degrees to 25 degrees. The following analysis still use 35 degrees.¶
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:¶
With such deployment, all orbit planes will meet at north and south pole. The density of satellite is 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 11 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 12 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
|<--- 2*Rc --->|
+ Satellite
/|
/ |
/ |
/ b |
/-\ +
/ * | __Earth surface
/ * | /
/ *_----+----__
+ +
* *
* *
* 2*a *
* ___ *
*- -*
* *
* *
* Earth center
For a example of two type of satellite LEO and VEO, the coverages are calculated as in Table 1:¶
| Parameters | VLEO1 | VLEO2 | LEO1 | LEO2 |
|---|---|---|---|---|
| As(km) | 335.9 | 450 | 1100 | 1150 |
| a(degree) | 3.907 | 5.078 | 10.681 | 11.051 |
| Rc(km) | 435 | 565 | 1189 | 1230 |
| Ns | 54 | 41 | 20 | 19 |
| No | 62 | 48 | 23 | 22 |
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:¶
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].¶
Unlike the communication on ground, the communication for satellite constellation is much more complicated. There are two mobility aspects, one is between ground-station and satellite, another is between satellites.¶
In the traditional mobility communication system, only terminal is moving, the mobile core network including base station, front haul and back haul are static, thus an anchor point, i.e., PGW in 4G or UPF in 5G, can be selected for the control of mobility session. Unfortunately, when satellite constellation joins the static network system of Internet on ground, there is no such anchor point can be selected since the whole satellite constellation network is moving.¶
Another special aspect that can impact the communication is that the fast moving speed of satellite will cause frequent changes of communication peers and link states, this will make big challenges to the network side for the packet routing and delivery, session control and management, etc.¶
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.¶
The "longest communication time" is for the scenario that the satellite will fly over the receiver ground station exactly above the head, or the ground station will be on the diameter line of satellite coverage circular area, see Figure 11.¶
| Parameters | VLEO1 | VLEO2 | LEO1 | LEO2 | LEO3 |
|---|---|---|---|---|---|
| As(km) | 335.9 | 450 | 1100 | 1150 | 1325 |
| a(degree) | 3.907 | 5.078 | 10.681 | 11.051 | 12.293 |
| AL(km) | 793 | 1048 | 2415 | 2515 | 2863 |
| SD(km) | 792.5 | 1047.2 | 2404 | 2503.2 | 2846.1 |
| V(km/s) | 7.7 | 7.636 | 7.296 | 7.272 | 7.189 |
| T(s) | 103 | 137 | 331 | 346 | 398 |
In order to form a network by satellites, there must be an inter-satellite communication. Traditionally, inter-satellite communication uses the microwave technology, but it has following disadvantages:¶
Recently, laser is used for the inter-satellite communication, it has following advantages, and will be the future for inter-satellite communication.¶
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 lifetime of inter-satellite communication when satellites are moving. The Figure 13 illustrates the movement and relative position of satellites on three orbits. The inclination of orbit planes is 90 degrees.¶
+ North pole
/|\
| s |
s | s
/ s \
s | s
| s1 |
s4 | s6
| s2 | -------- Equator
s5 | s7
| s3 |
s | s
\ s /
s | s
| s |
\|/
+ South pole
There are four scenarios:¶
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.
\ /
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
For satellites on different orbit planes with same altitude, the estimation of the lifetime when two satellite can communicate are as follows.¶
Figure 16 illustrates a general case that two satellites move and intersect with an angle A.¶
^ V2
/
/
+-
/ \ A
-------------+----+----> V1
/
/
More specifically, for orbit planes with the inclination angle i, Figure 17 illustrates two satellites move in the opposite direction and intersect with an angle 2*i.¶
^ move from south to north
\ /
\ /
\ /\
\/ | A = 2*i
/\ |
/ \/
/ \
/ V move from north to south
Follows are the math to calculate the lifetime of communication. Table 3 are the results using the math for two satellites with different altitudes and different inclination angles.¶
| i (degree) | 80 | 80 | 65 | 65 | 50 | 50 |
|---|---|---|---|---|---|---|
| Alt (km) | 500 | 800 | 500 | 800 | 500 | 800 |
| |V| (km/s) | 14.98 | 14.67 | 13.79 | 13.5 | 11.66 | 11.41 |
| T(s) | 267 | 273 | 290 | 296 | 343 | 350 |
For satellites on different orbit planes with different altitude, the estimation of the lifetime when two satellite can communicate are as follows.¶
Figure 18 illustrates two satellites (with the altitude difference Da) move and intersect with an angle A.¶
^ V2
/
/
-------+ /
Da /| +-
/ |/ \ A
----------/--+----+----> V1
/ /
/
/
/
Follows are the math to calculate the lifetime of communication¶
Using formulas above, below is the estimation for the life of communication of two satellites when they intersect. Table 4 and Table 5 are for two VLEOs with the difference of 114.1km for altitude. (VLEO1 and VLEO2 on Table 2). Table 6 and Table 7 are for two LEOs with the difference of 175km for altitude (LEO2 and LEO3 on Table 2).¶
| Parameters | VLEO1 | VLEO2 |
|---|---|---|
| As(km) | 335.9 | 450 |
| V (km/s) | 7.7 | 7.636 |
| A (degree) | 0 | 10 | 45 | 90 | 135 | 180 |
|---|---|---|---|---|---|---|
| |V| (km/s) | 0.065 | 1.338 | 5.869 | 10.844 | 14.169 | 15.336 |
| T(s) | 61810 | 2984 | 680 | 368 | 282 | 260 |
| Parameters | LEO1 | LEO2 |
|---|---|---|
| As(km) | 1150 | 1325 |
| V (km/s) | 7.272 | 7.189 |
| A (degree) | 0 | 10 | 45 | 90 | 135 | 180 |
|---|---|---|---|---|---|---|
| |V| (km/s) | 0.083 | 1.263 | 5.535 | 10.226 | 13.360 | 14.461 |
| T(s) | 47961 | 3155 | 720 | 390 | 298 | 276 |