## 3 Technical Features and Socio-Legal Characteristics of Africa's Satellite Constellation
### Demonstration of Inaccessible Zones  
It is a principle of law that the law must not mandate the doing of the impossible-_Lex non cogit ad impossibilia_. Sometimes a Court's judgment may be easy to interpret yet implementing it may be impossible ab initio or become impossible as things develop. This can occur as it did in the Cameroon-Nigeria process where the attempt to give effect to the delimitation instruments as prescribed by the World Court meant that surveyors will have to traverse inaccessible territories. A clear advantage of space applications in demarcation exercises is their singular advantage in making it possible for demarcators to deal with inaccessible terrain. The boundary between states tend to be delimited by features like water bodies or treacherous natural features such as dense forests, range of mountain peaks, volcanoes or marshland. As a result verification by traversing the territory for surveys may be impracticable or even impossible. A recurring feature in the lengthy judgment of the ICJ and therefore, in the Cameroon-Nigeria boundary demarcation project is the feature of watersheds as boundary. Note the occurrence of watersheds in the Judgment Paragraphs 79, 86, 103-112, 116-122, 124, 135-140, 147, 161, 162, 164-167, 168, 170, 174, 179, 183. Indeed it was decided that the demarcation exercise must deal with watershed line for approximately 400 km of the boundary. The _Encyclopaedia of International Boundaries_ defines a watershed as \"the elevated area separating headstreams that are tributaries to different river systems or basins. Watershed therefore, is the area of land where all of the water that is under it drains off it and goes to the same place [60]. The Mixed Commission soon found out that in many cases the watershed feature referred to in the applicable treaties and the World Court's judgment are inaccessible or can be accessed only at great cost or danger to personnel. The parties thus, adopted a very modern space science solution in order to find the watershed line. Three options were identified: (i) manual delineation from existing topographic maps with contour lines; (ii) automatic extraction from a Digital Elevation Model (DEM) data using a software application (ESRI, 2002 (a); and (iii) direct identification in the field. The UN Cartographic section successfully proposed the use of DEM data produced by the Shuttle Radar Topography Mission (SRTM) as the source data for the extraction of watershed line as well as for the ortho-rectification (See Cartographic section, department of Peacekeeping Operations, United Nations, Data and methodology Used in watershed line Extraction for Cameroon-Nigeria Preliminary mapping, 17 January 2005. The SRTM uses imaging radar technology to view the Earth's surface. The radar signals can see through clouds, but it does not see through thick vegetation canopies and human-made features. It uses a wave length of 5.6 cm which does not penetrate vegetation competently. The shuttle spacecraft emits radar signals that are reflected on the earth surface. Two antennas receive the returning signals. The radar waves are reflected by the ground features, including bare soil, dense vegetation dense and man-made features. The topography of the terrain causes phase difference between the signals received by the two antennas. This phase difference is translated into topographic (elevation) data -Jet Propulsion Laboratory, Jet Propulsion Laboratory, \"Shuttle Radar Topographic Mission\" California Institute of Technology.) [61]. The Sub Commission on Demarcation selected the option of automatic extraction and specified that the boundary line for watersheds be extracted from \"a fully rectified DEM\" using \"stereo imagery and ground control points\" [62]. In the event the UN Cartographic Section was able to produce only limited coverage of stereo imagery over the boundary area that produced the DEM data required according to the parties specifications [63].