Roy E. Anderson

Satellite-aided mobile communications: Experiments, applications, and prospects

NASAs application technology satellites (ATS) were used in a series of communications and position-fixing experiments with automotive vehicles, ships, and aircraft. Applications of the communications were demonstrated and evaluated for public services including law enforcement, search and rescue, medical emergency, and for commercial uses in the land and maritime transportation industries. The technical success of the experiments and the demonstrated potential value of the communications prompted a study that concluded an operational satellite-aided system would be a valuable augmentation of planned trunking or cellular-type terrestrial mobile radio telephone systems.

Land mobile communications and position fixing using satellites

In a recent series of experiments, the General Electric Corporate Research and Development Center demonstrated effective satellite-aided land mobile communications, analog and digital data relay, and automatic real-time vehicle position fixing. In one sequence of tests a station wagon was equipped with a specially designed antenna, a slightly modified commercial VHF transceiver, and a digital tone-code ranging responder that operated within the communications bandwidth. The General Electric Radio-Optical Observatory near Schenectady, NY, was the major earth terminal. A commercial VHF base station with a satellite antenna deployed first in an office building in Washington, DC, and later in Tucson, AZ, represented a headquarters or a sector office ground station. Communications were relayed by NASAs ATS-3 geosynchronous satellite. Both ATS-1 and ATS-3 were used for position fixing the vehicle. Voice, slow scan television, audio test tones, prerecorded intrusion sensor data, and telephone patches were relayed by the satellite to and from the vehicle under a variety of conditions in greater Washington, DC, and in the southwestern United States. The experiment demonstrated continent-wide communication of a quality comparable to fringe area reception of present local VHF mobile communications but with a notable lack of multipath flutter. Due to the high angle of signalling path from the vehicle to the satellite, solid structures such as buildings, mountains, bridges, or tunnels degraded communications only occasionally. An absolutely clear line-of-sight signalling path was not required. Trees directly in the signal path seldom interrupted communications. Noisy radio environments such as power lines and vehicle ignitions degraded signals received in the vehicle but did not affect vehicle transmissions. Vehicle positions to within one quarter mile were achieved in real time and within several hundred feet after post experiment recalibration and analysis. In another sequence of tests, similar equipments plus biomedical sensors and a medical telemetry unit were installed in an ambulance. NASAs ATS-3 satellite relayed two-way voice communications between a hospital and the ambulance and electrocardiograms from the ambulance to the hospital. Signals were received with excellent quality from various points within the United States, all well beyond the range of conventional line-of-sight communications. The future of operational systems depends not only on technology but on the need to define user requirements, international frequency allocations, and a commitment to support the initial hardware investment. One key technology that would have to be developed is a multibeam spaceborne antenna with low sidelobes. It has yet to be shown that a large space structure will be low enough in cost to attract the large number of subscribers needed to make it an attractive business venture. It appears likely, but not certain, that the cost of a satellite system to serve large, thinly populated areas may be less than a network of terrestrial repeaters that serve the same area. The likelihood is sufficient to justify further studies.

Satellite-aided land mobile radio experiments, applications and problems☆

Abstract Radio communications between vehicles or individuals and base stations are essential to many public safety, law enforcement and commercial users in urban areas. Present day communications are limited to nearly line-of-sight distances from the vehicle or hand portable units to a base station or repeater. It is probable that many potential users in rural, remote and offshore areas would benefit if the communications were available everywhere on land and territorial waters. Geosynchronous satellites can serve as repeaters for mobile and personal radios, and they appear to be a cost effective means of providing the service. Voice bandwidth communication and automatic position monitoring of an automobile were tested using the VHF transponders of NASAs ATS-3 and ATS-1 satellites. Voice communications were reliable and position fixes were accurate to one fourth mile, with 0.3 mile precision of individual fixes. Demonstrations for potential users have included emergency medical voice and telemetry communications between ambulances and hospitals. While technical feasibility of satellite-aided mobile and personal communications has been demonstrated, and its potential value is appreciated by a small segment of the user community, many problems remain before an operational system could be implemented. Solutions to the problems require an experimental satellite with a multibeam antenna. The satellite or its associated ground terminals should incorporate computer controlled network switching to test demand assignment of channels. Prolonged experience by many users of the experimental satellite in their routine operations would aggregate user needs and define the capital investment that would be justified to implement a commercial operating system.

Satellite augmentation of cellular type mobile radio telephone systems

Abstract NASAs ATS-6 satellite relayed voice bandwidth communications between five trucks and the trucking company dispatchers as the trucks traveled throughout the north-eastern quarter of the contiguous United States. The experiment, conducted over a seven month period, demonstrated that propagation characteristics are much different for the satellite-mobile links than for terrestrial-mobile links. A properly designed satellite system can provide high quality, reliable voice and data communications except where the vehicle-satellite path is shadowed by a structure or terrain feature. Mobile equipment in the experiment was adapted from commercial mobile radios. The vehicle antennas were 75 cm tall, 2 cm diam. Another experiment proved the feasibility of vehicle position surveillance using active two-way tone-code ranging through ATS-6 to provide one line of position and passive one-way ranging by measuring the time-of-arrival of a signal from an independent satellite. A position fix was printed out at an earth station 1 sec after it sent the interrogation signal to the distant vehicle, a towboat on the Mississippi River. The line of position from ATS-6 was accurate to 0.1 nautical mile using a voice bandwidth ranging signal. The line of position from the NOAA GOES satellite was accurate to 2 miles, using 100 Hz signal bandwidth. If the signal from the independent satellite had the same bandwidth and signal-to-noise ratio as ATS-6, the fixes would have been accurate to about 0.1 nautical mile. A concept study concluded that satellites might be a cost effective augmentation of terrestrial cellular type mobile radio telephone systems. The satellites would serve thinly populated areas where terrestrial systems are not cost effective. In the United States, the satellites would serve about 90% of the land area where 20% of the population resides. A multibeam satellite with many channels in each beam would be compatible with the urban terrestrial systems and together they would provide a nearly ubiquitous mobile radio telephone service.

EXPERIMENTAL INVESTIGATION OF AERONAUTICAL AND MARITIME COMMUNICATIONS AND SURVEILLANCE USING SATELLITES

The experimental program described included an evaluation of every significant factor affecting the accuracy of position fixing by range measurements from geostationary satellites. The tone-code ranging technique was found to provide a precision of roughly 0.1 n mi at mid-lattitudes with one range measurement at L-band and one at VHF. The factors which introduce bias errors were studied analytically and experimentally, and means of rectifying the bias errors were developed. An operational system at L-band may be expected to provide position fix accuracy of 0.1 n mi over large regions of the earth. The ranging technique developed within this program is digital, and uses a single signalling waveform and a single modem for communications and ranging.