Satellite Subsystems ť Electrical ť Communications

	Satellite Subsystems ť Electrical ť Communications

BLUEsat provides two communications channels which can be multiplexed over its pair of receivers and transmitters. The basic networking protocol is the AX.25 protocol, which is an amateur radio adaptation of the ITU-T X.25 protocol. AX.25 allows point-to-point (a single transmitting groundstation to the satellite) and point-to-multipoint (the satellite broadcasts to several listening groundstations) sessions. AX.25 is used in amateur packet radio networks, like long-range wireless networks, and supports the requirements for amateur satellite communication. It has error detection capabilities and is able to recover from some types of errors. The AX.25 protocol represents the Transport layer in the OSI networking model, and are conveyed in HDLC frames (the Link layer). HDLC frames -- streams of bits -- are sent over the radio links (the Physical layer), which use GMSK (Gaussian Minimum Shift Keying) over the 145 MHz (uplink) and 435 MHz (downlink) bands.

On top of the AX.25, the satellite provides an implementation of the PACSAT suite, which comprises the higher levels of the OSI networking model, and provides functionality similar to an internet FTP server. This is the benefit to amateur radio satellite users worldwide; BLUEsat will be able to receive files at one point over the globe, and transmit them to others elsewhere.

Satellite
On BLUEsat, the modems are fairly simple devices, and it is the Flight Computer's (FC's) responsibility to assemble/disassemble PACSAT packets, AX.25 frames and HDLC frames:

Hamtronics TA451 UHF FM Exciter Transmitters
BLUEsat carries a pair of Hamtronics TA451 UHF FM Transmitters, modified to function in LEO. Their function is to perform:

Modulation,
Frequency Multiplexing, and
Amplification

The transmitters are capable of operating from 9600 bps (bits per second) to 56000bps; 9600 bps is BLUEsat's operational bitrate. Each transmitter transmits on a different frequency in the 435 MHz band.

Receivers
BLUEsat also carries a pair of Hamtronics R144 VHF FM Receivers, similarly modified to function in LEO. Their function is to perform:

Amplification,
Selection and Filtering,
Demodulation, and
Frequency Conversion

The receivers operate at 9600 bps only. Each is tuned to a different frequency in the 145 MHz band.

Groundstation
Back on earth, the groundstation requires equipment to communicate with the satellite. This is composed of:

A PC running the relevant software,
A TNC (Terminal Node Controller -- in KISS mode it is not much more than a modem),
A powerful Yaesu FT-847 satellite transciever (transmitter/receiver), and
A pair of cross-yagi directional antennas mounted on the roof of the Electrical Engineering building. These are mounted 6 metres above the roof, on an azimuth-elevation rotator which is controlled by the PC.

When tracking, the tracking software on the PC regularly adjusts the attitude of the antennas to point them at the satellite being tracked, as it moves across the sky. It also adjusts the receive frequency of the transciever to compensate for Doppler shift from the fast-moving satellite.

The data communications setup of the groundstation is outlined in the diagram below:


	Author: Vaenthan Thiru Š The Author, the BLUEsat project or UNSW. All rights reserved. Last Modified: May 18 2006 08:37:41.