GB2118002A

GB2118002A - Mobile communications by satellite

Info

Publication number: GB2118002A
Application number: GB08301536A
Authority: GB
Inventors: Arcas Ambrosio Lorente
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-01-20
Filing date: 1983-01-20
Publication date: 1983-10-19
Also published as: ES508924A0; ES8308180A1; GB8301536D0

Abstract

A satellite communications terminal, wherein transmission is effected through the search and rescue channels of the Marecs satellites and reception is through the normal satellite to vessel communications channels of the same satellites.

Description

SPECIFICATION Mobile communications by satellite Page 1 of the drawings shows a general diagram of satellite communications, where one may see how, via the satellite and earth station, the mobiles (ships and planes in the drawing) can communicate with all points of the earth they wish to, via the existing fixed network of communications, schematized by no. 1 of the figure.

The forward channel is the name given to the union, via satellite, between the earth station and the mobile no. 2 and no. 3 its ascending direction is constituted by the earth-to-satellite path no. 2 and the descending direction the satellite-tomobile path, no. 3. Similarly, the return channel is the union, via satellite, between the mobile and the earth station no. 4 and no. 5, its ascending direction being the mobile-to-satellite path no. 4 and the descending the satellite-to-earth station path no. 5.

Terminai Units Page no. 2 of the drawings represents a simplified block diagram of a terminal unit, where the antenna subsystem no. 1, and the transponder no. 2 have been shown schematically.

In transmission, the low frequency signal from the peripheral units no. 3 passes to the modems, no. 4, where it is suitably modulated, and from these it goes to the channel selector, no. 5, where the channel on which it is desired to transmit is selected, and from the selector to the frequency converter no. 6, where, by the suitable frequency injection by the local oscillator, no. 7, the final frequency is obtained corresponding to the channel selected, which is then suitably amplified in the power amplifier no. 8, from where it is sent to the antenna no. 9, to be transmitted.

In reception, the signal picked up by the antenna no. 10, is filtered in the band pass filter no. 1 amplified in the preamplifier no. 12, and subjected to the necessary frequency reductions by the intermediate frequency unit no. 13, suitably fed by the local oscillator no. 7, later passing through the channel selector no. 5, in which the frequency of the channel being received has previously been selected, from the selector it passes to the modems, no. 4, to be demodulated leaveing these, now at low frequency to the suitable peripheral units, no. 3, to be used.

Present Day Terminals and Procedure The satellite communication terminal units are designed in accordance with the characteristics of the link provided by the satellite and the performance it is wished to obtain from the system.

The Marecs satellites have been conceived by the European Space Agency to act as links between terminals on ships and the earth sections. These terminals must be very powerful, have a low noise level and their aerials must be high gain, this last characteristic making necessary the use of complicated guidance systems for them, in order not to lose the alignment with the satellite when the vessel moves with the swell.

These characteristics lead to the high cost and large size of the terminal units used for the Marecs satellites, which means that they can only be fitted to vessels of large tonnage, of the order of 10,000 tons.

Below are the specifications of the standard A terminals used at present: - Equivalent isotropic radiated power EIRP = 37 dBW - Antenna gain/noise temperature G/T = -4 dB/OK - Information speed = 1200 bits/sec.

-Approximate cost of unit = 6,000,000 Pesetas.

The communications channels used by the Marecs satellite terminals, have provision for operating the following frequency bands: - forward channel, descending path (reception) 1 537.8-1542.5 MHz return channel, ascending path (transmission) 1683.6-1644.3 MHz Improvement to Procedure and Terminals By modifying the communications procedure used at present by the Marecs satellite terminals, as described in this memorandum, terminals may be designed for the said satellites at less cost and size, i.e. suitable for use by vessels of small tonnage and even aircraft.

The procedure consists in transmitting towards the satellite, using one of the search and rescue channels of the same, instead of using the channels used for transmission by the existing terminals. The reception is on normal channels.

The search and rescue channel is incorporated into the Marecs satellites, to receive the position signals emitted by the emergency buoys of vessels in danger, using the 1644.3-1 644.5 MHz frequency band.

As the procedure proposed necessitates transmission on a frequency different from that of the transmission of normal terminals, the terminals suggested have as their primary distinctive characteristic the fact that their local oscillators and aerials must be specifically designed to transmit on the 1644.3-1 644.5 MHz frequency band.

On using these transmission frequencies and due to the reception characteristics of the satellite, on the search and rescue channel, which are different from the reception characteristics on the normal communications channels, the transmission of information is possible at slow speed but also at very low power and small antenna gains.

As the ascending return channel is the most critical of the link via Marecs satellites, as it is possible to regulate the emission power from the satellite to the mobiles, the reception antenna gain of the terminal unit may also be small, so it can be stated that with the procedure proposed, the corresponding terminal units will be of low power and small antenna gain.

These characteristics make it possible for the terminal units to have a much smaller size than present terminals, at a lower cost, which will make it possible to fit them in vessels of less than 300 tons and even in aircraft. That is to say that the potential market of these terminals is far greater than that of the present ones and consequently, the communications service provided by the Marecs satellites can also be far greater than at present, originating with this a new source of profit, both in financial terms and also social benefits for the new service provided.

Below are stated the mean specifications the terminal units using the proposed procedure may have: - Equivalent isotropic radiated power EIRP = 16 dBW - Antenna gain/noise temperature G/T = -24 dB/OK - Information speed = 300 bits/sec - Estimated cost of unit: = 2,000,000 pesetas.

The cost stated reflects the smaller exigencies of they characteristics of the unit, e.g. it can operate with a 30 Watt power amplifier and a small antenna of 2 dB gain.

The new procedure, then, leads to new terminal units, which entail great advantages in comparison with existing ones. The fundamental characteristic of the new terminal units is transmission to the Marecs satellites in the (1 644.3-1644.5) MHz frequency band, which is obtained with a suitable design of them, which in turn makes possible the obtainment of characteristics suitable for the provision of communications, via the Marecs satellites, to a high number of mobiles which, without these units, could not use the satellites, as specified above.

Having studied the earth station-mobile, link budgets, page no. 1 of the drawings, the conclusion is reached that communications of sufficient quality may be obtained in the (75 1200) bits/sec. speed range using terminal units defined by the following parameters: G -28 < - < -22 dBOK T 12 < ElRP < 18 dBW This range of values may be obtained with the following specific characteristics of the essential parts of the terminal units: Effective power of amplifier (1 5-60) W Antenna gain (0--4) db Equivalent noise temperature (400 -- 600)OK It can be seen, then, that the proposed communications terminal units, based upon a totally original procedure, have characteristics totally different from the terminal units at present used by the Marecs satellites.

Claims

1. A satellite communications terminal unit is claimed, characterised because the transmission is effected through the search and rescue channels of the Marecs satellites, and reception using the normal satellite-to-vessel communications channels, of the same satellites.

2. A satellite communications terminal unit, according to the preceding claim, characterised because its transmitting frequency is in the (1644,3-1644.5) MHz band.

3. A satellite communications terminal unit, according to the preceding claims, characterised because its gain to noise temperature ratio is in the range of values -28 < G/T < -22 dB/OK

4. A satellite communications terminal unit, according to the preceding claims, characterised because its equivalent isotropic radiated power is in the range of values 12 < ElRP < 18 dBW.

5. A satellite communications terminal unit.