The XMM orbital target visibility period of 145 ks can in practice not be used for continuous observations, because of incomplete ground station coverage, as displayed in Fig. 79. There is a telemetry gap of ca. 1 hour (constant to within a few minutes during the first 1000 orbits) close to apogee, during which no data can be transmitted. Since XMM data transmission is via telemetry during direct contact, it does not carry onboard data storage devices and no observations can be recorded during these gaps. Thus, each orbit is split into two visibility periods. Fig. 80 shows how this gap shifts over time. When it is located at ``24 hours'', it is exactly during apogee and thus splits the orbital visibility period exactly into halves (about days 550-750, i.e., orbits 275-375). In all other cases, one visibility period is longer than the other. In the beginning, e.g., the offset of the gap with respect to apogee is ca. 4 hours (about 14.4 ks), making one visibility period approximately 56 ks long, the other about 85 ks.