For the past three years, a project has been underway to develop a precise positioning/navigation system using existing C-band geostationary communication satellites. The motivation behind this effort derived not only from the predicted high accuracy, reliability, and 24-hour coverage that a satellite-based system offers, but also from the fact that there existed (and still exists) considerable uncertainty about the availability of the high accuracy Global Positioning System (GPS) "P-code" service for the civil (offshore) community. This effort has been successful, culminating in a new system called STARFIX, which became operational during the first quarter of 1986. STARFIX was developed by PANAV, a joint venture between Analytical Technology Laboratories, Inc. (ATL) and John E. Chance & Associates. The system has been described in detail elsewhere (e.g. Refs. [1] and [2]), although a brief description for reader benefit is contained i: e Appendix.
The main purpose of this paper is to describe the benefits of such a system for offshore users based on the history of STARFIX utilization over the past few months. Accuracies (present and predicted), reliability, and operational characteristics will be discussed. Special emphasis will be placed on methods of calibration in order to achieve and maintain highest possible accuracy in offshore applications.
STARFIX is a passive system in that the user equipment does not transmit signals, it only receives them. All information necessary to compute the user's position is contained in the received signal, including the phase of the pseudo-random code (PRN) as well as actual satellite positions (determined by the STARFIX tracking network) which are periodically transmitted (Le. modulated) via the same signals. A small computer in the receiver itself performs the necessary fix computations.
There are two major components of the user equipment: the antenna subsystem and the receiver/computing subsystem, same as for any other radiolocation system. However, for STARFIX the antenna subsystem must receive very weak 4 Ghz signals from three satellites located 22,000 miles away, so that it is more complex than the usual "whips" and other simple antennas that offshore users are familiar with.
The antennas are in fact small horns (each about 8 inches long) that are mounted on a rotating platform which keeps them pointed toward the same spots in the sky as the ship moves (shown in Figure 1). A north-seeking device such as a gyrocompass provides the reference information which allows the antenna controller to maintain the correct relative azimuth. The present size of the overall platform is about 28 inches in diameter and about 30 inches high, with a total weight of about 100 pounds. This antenna subsystem is typically mounted on a pedestal such that there is a relatively unobstructed view of the sky in all azimuths. However, it has been found that a mast or other similar structure can be in line of view if it is at least 5 to 7 meters from the antenna itself.
The receiver/computer subsystem can be located up to 500 feet from the antenna.
