The total time delay between the fibre clocks, as a function of height, for a fixed fibre length and fibre refractive index. The dashed lines mark lowest and highest altitudes at which measurements could be performed (see feasibility analysis). In this region the time delay is dominated by the Shapiro delay. The choice of fibre length and refractive index is discussed in Section 3. The altitudes and orbital parameters are discussed in Section 5.