The objective of the study is to analyze the communication architecture for a mission in the equatorial region of the Moon, based on the lava tube exploration concept. Usually, the analysis of a communication architecture of a complex scenario like a lunar mission starts with different possibilities. For example, one of the main design drivers to choose the need or not of relay satellites is the power onboard the exploration system and which antenna can be used. After deciding the most suitable general architecture, two processes are optimized in parallel: (1) coverage analysis based on the antenna used, (2) decision on which frequency and bandwidth use for a given amount of telemetry and payload data. Our algorithm is an original contribution that can harmonize and decide upon these last two steps. To this end, we combine the coverage analysis with the other communication parameters to find the fittest architecture in one quick run. Moreover, the proposed approach is scalable and adaptable depending on the specific mission. It can deal with any number of ground stations and with any number of satellites, exploring the best communication configuration depending on the mission requirements. Although it produces coherent results with expectations and previous missions, the data are still rough estimations to be refined in more detailed and specific analysis afterwards. However, the output already helps the designers shape their mission architecture and start planning the operations for future exploration systems. The resulting set-up should achieve a global coverage scenario where at least one satellite is in Line-of-Sight (LOS) per Earth day with the site of interest during the overall mission evaluation period. This will create redundancy in the communications relays: if a satellite fails or enters into safe mode, another satellite can replace it, operating on its behalf. Moreover, at least one communication window per day is required in order to maximize the access toward Earth of the lava tubes’ exploration systems, which in turn, will maximize the possible scientific return of the mission.
Orbital and ground segment configuration
To comply with the aforementioned principles, the space segment of the satellite constellation is based on the Lunar global coverage network from [18]. In particular, the constellation is constituted by six satellites around the Moon with a semi-major axis of 6500 km. Three of them have a Right Ascension of the Ascending Node (RAAN) \(\Omega =0^{\circ }\), an inclination \(i=40^\circ\) and different mean anomalies \(\nu\). The remaining three satellites have similar characteristics but have a Right Ascension of the Ascending Node (RAAN) equal to 90\(^\circ\). The orbital parameters and satellites’ characteristics are summarized in Table 2.
Table 2 Satellites’ parameters: semimajor axis a, eccentricity e, inclination i, true anomaly \(\nu _0\), RAAN \(\Omega\) and argument of perigee \(\omega\) The ground segment, on the other hand, is comprised of Deep Space Network (DSN) ground stations [19], in particular the ones in California (Goldstone), Australia (Canberra) and Spain (Madrid). The geographic coordinates, with respect to the Earth’s or Moon’s Latitude and Longitude, are presented in Table 3.
Table 3 GSs (Earth) and Marius Hills (Moon) geographic coordinates To evaluate how the communication changes during the various orbits of the Moon around the Earth, the communications opportunities are analyzed for a year long. During the year, the overall elements of the mission have been modeled using the mission analysis tool System Tool Kit (STK), from AGI company [20]. Nonetheless, any tool able to compute accesses between a satellite and a point of interest may be used to create the inputs for the explained simulation framework.
Channel and data handling
Following the guidelines in [21] and looking at previous reference missions [22,23,24], the payload data communication band was set to X-Band. The telemetry data employs S-Band [23]. The X-Band has relatively few constraints in terms of maximum bandwidth, and it is one of the most used bands for space missions around or on the Moon and Mars. Moreover, it is less susceptible to Earth weather variation like the K or Ku band, that are even less restricted in bandwidth. The data rate was set to 150 Mbit/s, following the ITU suggestions [21]. Nevertheless, the overall analysis is parametric and different values of data rate can be used to plan the communications between Earth and Moon using the proposed approach.
Data volumes have been estimated considering the huge number of imaging material from the scientific payload, which should be sent to Earth. Following the approach in [25], a preliminary data volume of 160 Gb per single transfer was defined for the proposed mission. This value is just a parameter in the simulation: changing the daily data volume is possible to appreciate how the communication schedule can change, and this modification is easily implementable in the proposed model. Indeed, the data volume sensitivity evaluation can be quite interesting during the preliminary phase of the design of a mission, in the domain of conceptual design. This volume of data will be then compressed at a ratio 15:1, following the suggestions of [26].
The possible envisioned data handling approaches are: (1) simple BPs communication, (2) S&Fs, (3) SC&Fs architecture. The BP communication relays on the creation of a physical bridge between the lunar site and the DSN network. In this communication mode, the satellite should be in LOS of both the scientific target and the Earth. In the S&F architecture, the data should be all received within a communication window and should be all sent in another communication window. In the SC&F architecture, the fraction of data not sent toward Earth from the satellites are stored, and the next useful communication window is used to forward them to Earth. The SC&F is following the new philosophy of the Delay Tolerant Networks (DTNs) to facilitate and automate communication planning in space-related networks.
Evaluation framework
After defining the physical layer architecture, the satellite constellation and the position of the site of interest, the study continues analyzing the connectivity among Earth, the satellites and the lunar site of interest. Subsequently, all accesses between Marius Hills, each satellite, and each ground station, have been incorporated into a single algorithm for post-processing. Figure 2 presents a simplified flowchart of the proposed evaluation algorithm, while Algorithm 1 shows a minimum logical working example.
The procedure requires in input: the accesses among each satellite and GSs, the chosen data volume D (set at 160 Gb for this analysis), and the selection of the coupling elements, i.e. the whole set of possibilities and combination of satellites and GSs,. Once these parameters are set, the algorithm combines all accesses in a single file sorted by the starting time.
In the first stage, possibilities to upload data from Marius Hills to the first available satellite are considered. Once the upload is completed, that given satellite is excluded from subsequent uploads, but the others can still receive data. Each satellite that has received the data can perform the download phase, by looking for the first subsequent GSs available. The process repeats with all the possible GSs, if available. Downlink contacts that end first, are prioritized in the algorithm; namely, there is a reward process that favors satellites that are usually faster than others to perform the whole upload and download process.
More specifically, both the upload and the download phases can be completed either in a single access window or several. Depending on the chosen D, the same access window may be sufficiently large or not to allow a complete transmission, given \(t_{0, i}\) and \(t_{f, i}\), with \(i = 1, n\), where n is the total number of available accesses over the whole year. The generic transmission requires a total time \(T = f \left( D, t_{del}, SF \right)\), where the delay time \(t_{del}\) considers 32 bits used for the inter-packet time, both at the beginning and at the end of the transmission. The single packet size is here considered equal to \(SPS=6400\) bits, and a Safety Factor \(SF=20\) s is added to the computed minimum transmission time to account for a safe link establishment between transmitter and receiver.
Therefore, T is a value that remains constant once the initial parameters are selected (i.e., all transmissions, both in upload and download, require the same total time). Therefore, a complete transmission can take place if \(t_{0, i} + T <t_{f, i}\). In this case, the assumed communication architecture is S&F. .
If the transmission cannot be completed in a single time window, the e SC&F architecture is still possible and the algorithm calculates the remaining data volume \(D_{rem}\) to then proceed to search for the following opportunities to complete the download phase. It should be noted that the further evaluated contacts assume the same combination of satellite-Ground Station (both upload and download) to verify if it is more convenient to proceed with a e SC&F or to select a new satellite later in time; this evaluation terminates once \(D_{rem} = 0\). Total occurrences of the e S&F and e SC&F architecture utilization are accumulated for the whole evaluation year for each satellite/Ground Station pair. It is assumed that the first satellite to complete the upload can proceed with the immediate download, and the first Ground Station to receive the entire download is the chosen one among those in range.
The access times are always analyzed by increasing the initial time. However, before proceeding with future access windows, the algorithm first checks if the satellite that completes an upload phase can perform a download in the remaining upload window. This is the only exception that allows a single bi-directional exploration and that does not violate any physical law. This particular operation requires further explanation and, to this end, we present Algorithm 2.
Let \(t_{f,up}=t_{0,i}+T\) be the moment at which the upload phase ends for a specific satellite in the i-th access. The algorithm first checks if there is enough time left (\(t_{f,up}+T<t_{f,i}\)) in the same window, which spans between \(t_{f,up}\) and \(t_{f,i}\). If this applies, it searches through previous accesses started before (\(t_{0,dw}<t_{f,up}\)) or after (\(t_{0,dw}>t_{f,up}\)) the upload completion but still not concluded. Once identified the moment \(t_{0,dw}\) at which the download can start, if \(t_{f,dw}=t_{0,dw}+T<t_{f,i}\), then the same satellite can perform the download phase immediately after the upload without seeking subsequent accesses. This may happen in the best case at \(t_{0,dw}=t_{f,up}\), i.e. as soon as the upload is completed if there is a previous active downlink access window; otherwise in a later moment.
Regardless of this particular bi-directional search, the specific satellite with the uploaded data start to search in all future accesses the occasions to perform the download exploiting a greedy search algorithm. To this end, the satellite considers the type of e S&F or e SC&F architecture it uses. Once this happens, the satellite is re-inserted in the list of eligible candidates for the upload, and the cycle begins again.