## 5 Discussion  
From an examination of appropriate currently operational multi-spectral imaging spacecraft, it is clear that the ideal requirements of the archaeological community, illustrated here with a case study of Scotland, would be challenging to meet using existing systems. What is readily available as a by-product of existing acquisitions may not be suitable, and even with expensive bespoke tasking there is no guarantee of acquiring the data required. Although the global revisit time of many of these systems appears sufficient (1-4 days), this only expresses the ability to view all areas of the globe, and does not correspond to the ability to image all areas. Tasking patterns, slewing and imaging constraints  
Figure 7: Maps of the south region of interest as described in Table 6 and indicated on Figure 2. The maps are oriented with north to the top and cover an area of approximately 63 km (N-S) \(\times\) 90 km (E-W). The colours overlaid on the maps indicate the (**a**) average and (**b**) maximum revisit times of each location in the region after 30 days. Coverage is calculated at approximately 5 km intervals and interpolated to provide the coverage estimates for the full region of interest. (Map data: Google, TerraMetrics, www.google.com/maps [retrieved 16 March 2020]). mean that the useable imagery available of any area is collected at a much lower frequency than might be expected. For archaeological applications, the useable imagery collected is even lower because commercial imperatives mean that a high proportion of historical imagery is centred on urban areas. Considering eleven satellite systems (as described in Table 1) only eight appropriate archive images were identified of a 150 km\({}^{2}\) region of archaeological interest in Scotland over a six-month period during the summers of 2016 and 2018. This highlights the significant compromises inherent in archaeological applications of commercial satellite systems for site detection and monitoring, and the potential benefits of a bespoke system. When a constellation of 12 spacecraft were tasked with imaging this region in July 2020, 150 images were collected, though due to cloud cover only 17% of these images (a total of 26) were usable. This underlines the requirement for a system tasked consistently to improve the chances of securing the necessary imagery for archaeological observations. It also highlights the major impact of cloud cover, which is not further considered here, but is clearly a significant consideration in sensor choice and an imperative to developing uses of radar systems, for example. It is of note that the method of coverage analysis and orbit design presented in this paper is independent of sensor type, and the swath widths for very-high-resolution synthetic aperture radar (SAR) spacecraft are similar to those of the multi-spectral optical systems presented. For example, the recently launched ICEYE constellation can collect SAR imagery at \(<\)1 m GSD with a corresponding swath of 5 km [42] and Capella SAR offers \(<\)1 m GSD with a swath of approximately 10 km [43]. The spacecraft in both constellations have a mass of approximately 100 kg each, similar to the optical systems considered here. As such, a spaceborne SAR system designed to meet the outlined requirements would likely have similar parameters to the presented optical constellation. TerraSAR-X [44] and COSMO-SkyMed [45] are larger spacecraft, but both offer similar \(<\)1 m GSD SAR data and have been successfully used in archaeological prospection and cultural heritage monitoring, confirming the viability of this approach [7; 45]. In designing a bespoke satellite for archaeology in Scotland, an inclined 60 deg orbit offers an increased revisit rate over a more common sun-synchronous orbit. The main drawback to this orbit is that even though there is a higher revisit rate, there may be issues with coverage due to the chance of passing over during eclipse, when an optical payload is ineffective.