Navigating Low Altitude Satellites Using the Current Four NAVSTAR/GPS Satellites

A greater degree of survivability is a significant requirement of future space systems. Toward this end, an important aspect is an autonomous navigation capability. For many space systems, an obvious choice is the use of the Navstar/Global Positioning System (GPS). With the operational 18-satellite GPS constellation (plus three active spares), low altitude user satellites will have continuous tracking data available for navigation. On the other hand, with the current constellation of four satellites, there is less available data and, consequently, the accuracy of navigation varies a great deal throughout the user satellite orbit. This paper discusses the navigational performance that can be achieved with this constellation. Last summer, the Ladsat 4 satellite was placed into a near circular, sun synchronous, orbit at an altitude of 705 km, and at an inclination angle of 98.3 deg. This satellite contains a Navstar GPS receiver (called GPSPAC), and is the first user satellite to employ GPS. Using this satellite as an example, an error analysis was performed of the navigation accuracy that can be obtained for a low altitude satellite. During the course of a Landsat 4 orbit, it will be able to observe from none to four Navstar/GPS satellites. When four satellites are available, the three-dimensional accuracy can be expected to be about 11 m. Since Landstat 4 is sun synchronous, i.e., its orbit plane rotates in longitude with the sun, there will be times during the year when this satellite and the GPS satellites simultaneously come together over the continental United States. This is where Landsat 4 is primarily used, and is the ideal geometric relationship between Landsat 4, the GPS satellites, and the earth. This favorable condition occurred last November. This paper discusses the use of Navstar/GPS to navigate Landsat 4 under this ideal geometric situation, as well as at other times during the year. The paper presents the results of a theoretical accuracy analysis. The actual on-orbit experience with Landsat 4 is not covered here.