while the Sun, Moon, and stars would traverse the skies behind it. Such orbits are useful for telecommunications satellites. A perfectly stable geostationary orbit is an ideal that can only be approximated. In practice the satellite drifts out of this orbit because of perturbations such as the solar wind, radiation pressure, variations in the Earth's gravitational field, and the gravitational effect of the Moon and Sun, and thrusters are used to maintain the orbit in a process known as station-keeping. Eventually, without the use of thrusters, the orbit will become inclined, oscillating between 0° and 15° every 55 years. At the end of the satellite's lifetime, when fuel approaches depletion, satellite operators may decide to omit these expensive manoeuvres to correct inclination and only control eccentricity. This prolongs the life-time of the satellite as it consumes less fuel over time, but the satellite can then only be used by ground antennas capable of following the N-S movement. Geostationary satellites will also tend to drift around one of two stable longitudes of 75° and 255° without station keeping. Elliptical and inclined geosynchronous orbits Many objects in geosynchronous orbits have eccentric and/or inclined orbits. Eccentricity makes the orbit elliptical and appear to oscillate E-W in the sky from the viewpoint of a ground station, while inclination tilts the orbit compared to the equator and makes it appear to oscillate N-S from a groundstation. These effects combine to form an analemma (figure-8). Satellites in elliptical/eccentric orbits must be tracked by steerable ground stations. Tundra orbit The Tundra orbit is an eccentric geosynchronous orbit, which allows the satellite to spend most of its time dwelling over one high latitude location. It sits at an inclination of 63.4°, which is a frozen orbit, which reduces the need for stationkeeping. At least two satellites are needed to provide continuous coverage over an area. It was used by the Sirius XM Satellite Radio to improve signal strength in the northern US and Canada. Quasi-zenith orbit The Quasi-Zenith Satellite System (QZSS) is a four-satellite system that operates in a geosynchronous orbit at an inclination of 42° and a 0.075 eccentricity. Each satellite dwells over Japan, allowing signals to reach receivers in urban canyons then passes quickly over Australia. Launch Geosynchronous satellites are launched to the east into a prograde orbit that matches the rotation rate of the equator. The smallest inclination that a satellite can be launched into