## 2 Inter-Satellite Communication
### Small Satellite ISL Missions for IoT  
The advantages of small satellite constellations with ISL capabilities have been thoroughly discussed and several missions have been proposed; for instance, in [7]. Only a handful of missions, however, have been successfully realized in the past, and were mostly for technological demonstration. Table 2 provides an overview of some notable small satellite ISL missions which have contributed to technological advancement in this field. The majority of past missions used narrow-band UHF links with P2P communication between only two nodes. The robustness and simplicity of UHF technology made it the primary choice for TM/TC in small satellites, from their early development up to present. Despite their limited transmission rate of some tens of kbps, the advantages of low power consumption, very simple antenna design, and cost-effective ground station set-up has made it preferable for universities and research institutions. The very first mission to demonstrate ISL performance was the SNAP-1 mission from the UK/China in 2002, even though it only demonstrated one-way UHF communication between a nanosatellite and a larger mother-satellite [8]. In CanX-4&5, two satellites were designed to perform formation flight with sub-meter tracking error accuracy. The ISL in this case was, again, a two-way crosslink using S band with an effective data rate of about 10 kbps at distances of up to 5 km [9], in order to exchange position, velocity, and attitude information between the satellites. Admittedly, past missions, such as SNAP-1 or CanX-4&5, did feature an crosslink; however, it was only on a very basic level. These appear insufficient for the envisioned autonomous nanosatellite swarms, with communication distances of at least several tens of km, data rates in the vicinity of 100 kbps, and the capability of multi-hop routing for reasonably effective data exchange. Recent concepts and missions have implemented more sophisticated ISL techniquesm such as the increase of bandwidth in the S band, extension of communication range, and multi-hop routing protocols. EDSN [10], by NASA, although suffering failure during launch, implemented a multi-hop routing protocol with eight satellites and two Cubesats. The DIAMOND and S-NET missions were among the first nanosatellite missions ever to successfully demonstrate ISL using SDR-based configurable S band radio. Albeit Iridium Next spacecrafts do not fall into the small satellite category; however, they are listed as the Iridium constellation was the first LEO mission to use ISL commercially. \begin{table}
\begin{tabular}{l l l l l l l l l l l l} \hline \hline \multirow{2}{*}{**Mission**} & **Org.** & \multirow{2}{*}{**(**Nat)**} & \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Mass**} & \multirow{2}{*}{**ISL**} & \multirow{2}{*}{**ISL**} & \multirow{2}{*}{**Data**} & **Rate** & **Mission** & **Launch** & **Ref** \\  & & **No.** & & **[kg]** & & & **Payload** & **Band** & **[kbps]** & **[km]** & **Purpose** & **Status** & **Ref** \\ \hline SNAP-1 & Surrey (UK) & 2 & 6.5/49 & - & UHF & 9.6 & 2 & tech demo & 2000, success & \\ CanX-4&5 & SFFTL (CAN) & 2 & 5 & - & S & 10 & 5 & PFF demo & 2014, succ & [11] \\ EDSN & NASA (USA) & 8 & 2 & MHz2420 & UHF & 9.6 & 20 & multi-point & 2015, LV fail & [12] \\ Iridium & Lockheed & & & & & & & & & mobile & 2017, started & \\ Next & (USA) & 66 & 860 & - & Ka & 12,500 & 4500 & communication & 2017, started & \\ Diamond & SAS (UK) & 3 & 6 & - & S & 2000 & - & ISL demo & 2017, success & [13] \\ S-NET & TUB (GER) & 4 & 9 & SLINK & S & 100 & 400 & ISL demo & 2018, success & [2] \\ CPOD & Tyvak (USA) & 2 & 3 & - & S & 250 & 25 & rendezvous, proximity & 2020, expected & \\ PROBA-3 & ESA (EU) & 2 & 320/180 & Gamalink S &? & tbd & formation, autonomy & 2019, expected & [14] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Selected list of notable past and future small satellite ISL missions and parameters.