The SAREX antenna is mounted in a window of the orbiter shortly after the Shuttle achieves orbit. Frequently, the orbiter must fly in a particular attitude, or orientation with respect to Earth, that "hides" certain windows from view from the ground. Part of the scheduling process for determining the pass times takes this into account and the antenna will be moved from window to window as needed to obtain the best view of the ground. However, it is not always possible to do this, and when this is the case, the antenna may only be visible to the ground during part of the pass. If you are looking at the shuttle as it moves across the sky, you will see different sides of it as it moves past you. This can put the body of the orbiter between you and the antenna, blocking the signals completely. For this reason, you need to be prepared to either lose contact shortly after it peaks in the sky or not be able to gain contact until after TCA. This can occur for either direct or telebridge contacts.
The first four reasons apply mainly to direct contacts. See the section below onGround Station Design for guidelines to help avoid these pitfalls. The last two are the primary reasons why a telebridge contact would fail, but also apply to direct contacts. SAREX is considered a "secondary payload", and while included in the timeline, this status means that it can be dropped if the crew encounters difficulties with the primary payloads, or needs to focus extra attention on other, higher-priority duties.