Reducing Data Latency with Inter-Satellite Links in LEO Constellations: Trade-off Analysis and Impact on Concept of Operations

Abstract

Information on its own has a value but equally the point in time when that information is received is also an important consideration. Data latency, the time needed to perform the measurement with the satellite’s payload and deliver the actionable information to the customer, is a key design decision for ensuring the customer’s needs are met whilst ensuring the mission’s costs are viable. The conventional approach in satellite missions has been to wait until the next booked ground station is overflown, transmitting the data to the ground station and subsequently transferring the data to the data processing center where the derived products are generated. Each of these steps take time and, in their sum, contribute to the total duration of the data latency. When designing or assessing concepts for monitoring activities globally on the Earth with LEO satellite constellations, the constellation itself offers new ways of reducing the data latency. In addition to the existing data transmission service from LEO via GEO to the ground, the data latency can also be reduced either by employing inter-satellite links or edge computing. In this paper we compare the approaches of using i) a ground station network, ii) optical inter-satellite links and iii) edge computing to affect the data latency and compare their use on the annual operational expenditure for the mission. Our analysis shows that incorporating inter-satellite links reduces the data latency by a factor of 2 when compared to constellations solely relying on a ground station network. Whilst onboard processing offers advantages, such as reducing the amount of data that has to be downlinked, it does not reduce the data latency. The interference of the optical link’s performance due to sunlight is not considered to have an operational constraint or significant impact on the data latency. Inter-satellite links importantly allow the locations of ground stations to be placed where desired. To achieve an equivalent data latency without inter-satellite links and solely a ground station network requires a factor of 5 more OPEX, i.e. that saving can be used to finance the CAPEX of the inter-satellite links or compute the break-even point. Alternatively, a constellation using solely a ground station network achieves a 90th percentile data latency that is slower by a factor of nearly 6 using an equivalent OPEX that constellations with inter-satellite links require.