## 3 Existing Space Systems  
Of the multitude of Earth observing (EO) spacecraft currently in operation (as of 26 June 2020), only a small number can provide the appropriate GSD, multi-spectral imagery necessary for the archaeological purposes outlined in Sections 1 and 2. These systems are summarised in Table 1, where GSD given are the maximum attainable (normally at nadir). In all cases pansharpened multi-spectral images are available at the stated panchromatic spatial resolution. While the listing of eleven systems (Table 1), with often daily revisit intervals, suggests that suitable data should be readily available, this is not the case. Firstly, the majority of these systems are commercial; as such, not all data are publicly available, and most of the data are certainly not freely available. Moreover, the revisit rates listed indicate the frequency with which a region of the Earth can be observed, but the actual data collection frequency is much lower. This is because the stated revisit time includes the ability of the spacecraft to slew and point their instrument off-nadir. As such, the revisit time represents the time in which any one region of the Earth could be imaged; it does not represent the time in which imagery of the entire Earth can be collected. As the spacecraft can only point at one location at a time, the limiting factor for the complete imaging of a region is the instrument swath width (i.e., the width of the spacecraft imager field of view on the Earth's surface) and, as noted in Section 2, a lower GSD generally corresponds to a smaller field of view. For visual imagery, the images must be taken during daytime and at times of limited cloud cover, further compounding these difficulties. For Scotland, the average cloud cover in May, June and July is 70%, though it may be lower on the east coast of the country [20]. Assuming an equal chance of cloud being present at any time, this indicates that only 30% of daytime satellite passes over the region of interest are likely to be useable. These factors highlight the reality behind the headline figures of revisit rates, and the potential difficulties of securing imagery of given locations at specified periods. Conspicuously absent from Table 1 are the Sentinel-2 spacecraft, part of the EU Copernicus programme, and NASA's Landsat spacecraft, as, in spite of their frequent revisits over Europe (five days for Sentinel-2 A and B combined and eight days for Landsat 7 and 8 combined) and previous use in archaeological prospection and condition monitoring [6,7], they cannot provide the GSD necessary for the purposes outlined in Section 2. In order to assess the suitability of existing satellite systems for archaeological purposes as defined in the scope (Section 2), an approximately 150 km\({}^{2}\) region in Scotland is examined as a sample site (Figure 3). For each of the eleven systems listed in Table 1, the availability of recent data is assessed for the periods from 1 May 2016 to 31 July 2016 and from 1 May 2018 to 31 July 2018. These periods were selected for the favourable cropmark formation conditions that prevailed during dry and hot summers that encouraged crop stress. For all the apparent likelihood of data availability on the basis of the large number of operational systems, for the sample area there are a total of eight images with <30% cloud cover for these two time periods, many of which provide <100% coverage of the area of interest. The low number of images available is striking, with a concomitant low number of time series of multiple overlapping images. The trends of image acquisition seen here reflect the priorities of most satellite data providers/consumers, with high demand, and hence high acquisition, generally focusing on urban areas. Failing a significant change in the global use of satellite data, such trends are likely to continue across all very high-resolution satellite data providers.