**3.1. Internet access continuity in space tourism**

Space tourism subscribers are conveyed in a space vehicle that hosts communication subsystems which enables internet access. The space vehicle begins its journey from a terrestrial location with access to terrestrial wireless networks. The space tourists have access to the internet via the gateway of the terrestrial wireless network. In the context of the LTE-A utilizing the eNB, the P-GW and S-GW are the gateway entities. A handover is required to ensure the continuity of internet access as the space vehicle travels from the terrestrial location to outer space. Three handover levels are required in the proposed solution, these are:


the APSH becomes necessary as the space vehicle's altitude increases as it approaches low earth orbit. Existing approach consider that satellites should handover to stratospheric platforms in reaching the subscribers. The case here is different because the satellite network is in the last mile.

**4. Sub-orbital Intersatellite Handover (SISH):** The execution of the SISH is required to ensure that the in-orbit space vehicle connects to the satellite enabling it to have the highest throughput and lowest latency. The SISH redefines the role of satellites in accessing cloud based information via the internet. This is because inter-satellite links have often been used with the aim of achieving global coverage using satellite networks; and not in the context of providing seamless high QoS internet connections to subscribers as a last mile technology.

The space tourist subscriber requires internet access for obtaining content from the internet or storing content for storage and later access. This should be realized without significant space segment acquisition costs. The contexts implied in the APSH and SISH require novel mechanisms and accompanying network architecture. This is because the APSH and SISH phases are peculiar to the space tourist subscriber. The relations between the TWNH, TWAH, APSH and SISH are shown in **Figure 2**.

**Figure 2.** Relations between the TWNH, TWAH, APSH and SISH.

The scenario in **Figure 2** shows a space vehicle sojourning from a terrestrial location to outer space. The space vehicle passes through the terrestrial plane, aerial plane and the space plane. The space vehicle(s) is connected to the terrestrial wireless network base stations and access the cloud based content via the internet through the gateways. In the aerial plane, the space vehicle is connected to the high altitude platform.

The high altitude platform receives contents from select ground based stations. These ground stations are those being used for radio astronomy. However, they are not engaged in receiving radio astronomy signals during epoch of use by high altitude platform. The use of such ground stations is feasible considering the emergence of multi-mode ground stations that can be used for radio astronomy and packet processing [45]. The multi-mode ground station is connected to the computing infrastructure of the astronomy organization. The computing infrastructure is linked to the cloud computing platform hosting the content to be accessed by the space tourist subscriber. Idle multi-mode ground stations relay cloud based content to the space vehicle in the space plane. The cloud platform sends the cloud content to be sent to select ground stations that communicate with stratospheric platforms in the aerial plane. The select ground stations are also used to enable communications between satellites and the space vehicle in the space plane.

Aerial platforms communicate with each other using inter-platform links that utilize free space optics to ensure low latency. This is done when the space vehicle moves from the coverage of a high altitude platform to the coverage of another high altitude platform. In moving from the aerial plane to the space plane i.e. executing the APSH, the space vehicle does not have line of sight and communicates with the satellite via the ground station. In the space plane, the space vehicle is able to move between satellites. This is enabled by satellite communications with selected ground stations.

The proposed handover mechanism requires that the space vehicle conveying space tourists pass overhead through radio astronomy observatories. This provides the added benefit of enhancing astro-tourism and enables space tourists to have an aerial view of astronomical observatories. The re-use of existing astronomy infrastructure [46] reduces the cost associated with launching an anchor satellite to maintain high QoS internet connectivity for the concerned space vehicle. The use of selected ground station infrastructure improves the revenue potential for astronomy organizations; and increases the utilization of the high performance infrastructure and ground stations. The space vehicle connects to a geostationary communications satellite [47–51]. The ground segment of the geostationary satellite is an idle multi-mode ground station.

The handover algorithm that enables the provision of seamless internet connectivity for the space vehicle comprises entities that function in the space vehicle, ground stations, high altitude platforms and satellites. The proposed distributed handover algorithm (DHA) functions are for the aerial and space modes. The DHA executes the TWAH and the SISH in the aerial mode and space mode, respectively.

The space vehicle host mechanisms that enable it to execute the TWAH, and the handover between aerial platforms. However, these mechanisms are not designed since they have received considerable research attention. Let *γ,* Ϸ and ƿ be the set of terrestrial wireless network base station entities, stratospheric platforms and satellites, respectively.

$$\boldsymbol{\gamma} = \{\boldsymbol{\gamma}\_1, \boldsymbol{\gamma}\_2, \dots, \boldsymbol{\gamma}\_d\} \tag{19}$$

$$\Phi = \{\Phi\_1, \Phi\_2, \dots, \Phi\_h\} \tag{20}$$

$$\mathbf{p} = \{\mathbf{p}\_1, \mathbf{p}\_2, \dots, \mathbf{p}\_n\} \tag{21}$$

In addition, let *P ϰ; tj* � �*, <sup>ϰ</sup>* <sup>E</sup> *<sup>γ</sup>b;* <sup>Ϸ</sup>*l;* <sup>ƿ</sup>*<sup>t</sup>* � �*, <sup>γ</sup><sup>b</sup>* <sup>E</sup> *<sup>γ</sup>;* <sup>Ϸ</sup>*l*<sup>E</sup> <sup>Ϸ</sup>*;* <sup>ƿ</sup>*t*<sup>E</sup> <sup>ƿ</sup> denote the strength of the signal form entity *ϰ* at epoch *tj*. Given that *Pth*ð Þ *γ* is the threshold signal strength for terrestrial wireless network base station entity; the space vehicle measures the value of *P ϰ* ¼ *γb; tj* � � and *P ϰ* ¼ Ϸ*l; tj* � � and retains connectivity to the terrestrial wireless network if:

$$\frac{1}{dw}\sum\_{b=1}^{d}\sum\_{j=1}^{w}P(\varkappa = \gamma\_b, t\_j) > P\_{th}(\gamma) \tag{22}$$

$$\frac{1}{hw}\sum\_{b=1}^{d}\sum\_{j=1}^{w}P\left(\varkappa = \boldsymbol{\nu}\_{b}, t\_{j}\right) > \frac{1}{hw}\sum\_{l=1}^{h}\sum\_{j=1}^{w}P\left(\varkappa = \boldsymbol{\Phi}\_{l}, t\_{j}\right) \tag{23}$$

If (22) does not hold true, then (23) is also invalid. The APSH should be executed if:

$$\frac{1}{nhw} \sum\_{l=1}^{d} \sum\_{j=1}^{w} P(\varkappa = \Phi\_l, t\_j) > P\_{th}(\Phi) \tag{24}$$

$$\frac{1}{hw} \sum\_{l=1}^{h} \sum\_{j=1}^{w} P\{\varkappa = \Phi\_l, t\_{\dagger}\} > \frac{1}{hw} \sum\_{b=1}^{d} \sum\_{j=1}^{w} P\{\varkappa = \boldsymbol{\gamma}\_b, t\_{\dagger}\} \tag{25}$$

The space vehicle is in the terrestrial plane if (22), (23) hold true and is in the aerial plane when (24), (25) holds true. The space vehicle moves from the aerial to the space plane if:

$$
\ddot{Y}\_1 < \ddot{Y}\_2 < \ddot{Y}\_3 \tag{26}
$$

$$\Psi\_1^\* = \frac{1}{h \times (j + j')} \sum\_{l=1}^h \sum\_{j=1}^{j+j'} P(\varkappa = \Phi\_l, t\_j) \tag{27}$$

$$\Psi\_2^\* = \frac{1}{h \times (\alpha')} \sum\_{l=1}^h \sum\_{j=j+j'+1}^{j+j'+\alpha'} P(\varkappa = \Phi\_l, t\_j) \tag{28}$$

$$\bar{Y}\_3 = \frac{1}{h \times (w - (j + j' + \alpha' + 1))} \sum\_{l=1}^h \sum\_{j=j+j'+\alpha'+1}^w P\{\varkappa = \mathbf{b}\_l, t\_j\} \tag{29}$$

The transition in (26)–(29) involves a movement of the space vehicle from the aerial plane to the space plane. This handover is executed in the APSH. A set of relations describing the handover and the associated transition involving movement from the terrestrial plane to the aerial plane has not been presented. This kind of handover has been sufficiently addressed in the literature focused on aerial–terrestrial communications [45, 52, 53]. However, the context being addressed here is that of ensuring connectivity with a manned aerial vehicle (MAV) i.e. the space vehicle intended for space tourism.

The handover and transition implied in the SISH becomes activated when *P ϰ* ¼ Ϸ*l; tj < P ϰ* ¼ Ϸ*<sup>l</sup>*þ<sup>1</sup>*; tj ;* <sup>Ϸ</sup>*<sup>l</sup>*þ<sup>1</sup> <sup>E</sup> <sup>Ϸ</sup> and the space vehicle selects satellite <sup>Ϸ</sup>*<sup>l</sup>*þ1. The flowchart in **Figure 3** describes the relations executed in a handover procedure. The MAV searches for other networks of aerial platforms if the satellite signal is detected given that (26)–(29) holds true. In **Figure 3**, it is assumed that the space vehicle is able to connect to the concerned entities; i.e., high-altitude platforms or satellites depending on the decision context. The space vehicle connects to the entity with the highest transmit power.
