Global email to hand-held terminals

by Chris Turner, G7TZO.

Cycom stands for cyclic communication. Cycom-1 might be a possible future amateur LEO microsat which, despite it's small size and cost, still manages to communicate with hand-held ground terminals possessing omni-directional antennas. The satellite would use a very high gain directional antenna for transmit and receive for user communication. The hi-gain antenna is arranged to rotate continuously like the lens of a lighthouse such that it illuminates only those points on the earth surface which would see the satellite low on the horizon. The view from the ground terminal is that the satellite has a very intense signal when at a low angle above the horizon but this signal intensity is cyclic and "flashes" on for about 0.1 seconds every second. When the satellite gets higher in the sky the signal disappears.

The hi-gain directional antenna can only point in one direction at a time and so to achieve communication between arbitrary points it is necessary to store and delay received messages for a known period of time before retransmitting them by which time the antenna will have rotated to be pointing at the target. A graphic analogy is provided by imagining jelly (or other material) being thrown at a fan; the jelly arrives, clings to the fan blade for a short while and then is flung off in another direction.

Refer to the diagram of Figure 1. The dotted line show the track of the satellite (nearest earth point) as it continues its orbit. The top circle show the path of the rotating beam where it hits the earth's surface at times close to t0. The lower circle is the path of the rotating beam at times close to t1. The beam has a clockwise rotational speed of w radians per second.

Earth terminal A can communicate with earth terminals B or D in half duplex with a latency of less than 1 second by setting the delay time on messages. In order for A to send a message to C the delay time will be:-

d = (2*PI*n+(a+b))/w where n is a positive integer chosen such that d is close to (t1-t0).

C will be able to reply to A during the same pass if A sent its message when the satellite was rising for A, the message was delivered when the satellite was rising for C and the satellite had not yet set for A. Otherwise the reply will delay for a whole orbit.

This mode is the autonomous hand-held to hand-held mode. Obviously for email gateways to ground networks one would use a single high cost ground station with tracking antenna to provide a continuous datalink for the whole pass.

The Cycom-1 technique has unique advantages. The directional beam gives:-

The high gain of the antenna gives:-

The low angle operations give:-

Disadvantages of the Cycom-1 technique are :-

The author can be reached on the internet at http://www.cycom.co.uk/ There is a published patent relating to this subject.