COMS Science Requirements Document

27/06/2007 - To be updated more once have catalogue from M.K.Consultants.

INTRODUCTION TO COMS SUBSYSTEM:

The communications subsystem (COMS) will perform two functions. It will firstly provide the telemetry downlink and telecommand uplink functionality. Besides that, it will provide a linear transponder service which can be used by amateur radio operators to communicate with each other.

ORBITAL CONSIDERATIONS:

In terms of communications, the orbit is a huge factor to consider. It is absolutely necessary to be able to communicate with your satellite to be able to obtain any data from it. It is required that the orbit is chosen from a launch site such that it passes over the chosen ground station.

The point and orientation of launch heavily affects the orientation of the satellite's orbit. A {satellite is launched from a point a on the surface of the earth. The launch latitude and longitude are given by L and λ respectively. The satellite is launched with an azimuth angle of B.

We can calculate i as:

(1)
$$cos(i) = sin(B)cos(L)$$

We can see from this result that a direct orbit must have a launch azimuth between 0° and 180°. A retrograde orbit must have a launch angle between 180° and 360°.

It can be seen that satellites cannot be put into an equatorial orbit if the launch site is not on the equator. A launch site cannot obtain an orbital inclination smaller then the launch latitude. For that reason, launch sites closer to the equator have a larger range of possible orbits. A launch site directly on the equator can put a satellite into any orbit.

One needs to achieve an inclination of at least about 40-45 degrees as the satellite will never be seen above the horizon.

The orbit of our Cubesat should be mainly selected so the satellite will fly over the ground station located in Leicester (52°38'N 1°08'W).

Possible launch sites:
Russia, Baikonur Cosmodrome - Tyuratam
Latitude 45°36'N Longitude 63°24'E

GROUND STATION MANAGEMENT:

The ground station is preliminary set to be held at the National Space Centre in Leicester (52°38'N 1°08'W), where equipment is available for amateur packet radio. Licensing is easy to obtain from the amateur radio operators and team leaders Andy Thomas and Jon Heath will help us with this.

Data packets containing images and housekeeping information are then downlinked at 1200 Baud from the satellite using the AX.25 protocol; AX.25 designed for use by amateur radio operators, thus being able to function with the equipment. The system will periodically emit a beacon signal with the satellite signature and basic system information data for which the AX.25 protocol is used.

Cubesat will have a downlink in the amateur satellite segment of the UHF amateur radio frequency band. Telemetry decoding software should be made available to participating amateur radio operators which allows them to decode and display real time data. Furthermore, the software should allow a data upload to the central Cubesat ground station at the National Space Centre, Leicester via the Internet using TCPIP (Transmission Control Protocol / Internet Protocol) for data processing.

The University of Leicester Cubesat team should invite all interested radio amateurs to receive, decode and forward telemetry data to the Cubesat ground station. As participation in the project, the amateur radio operator involved in the TCPIP could get a sticker as a form of gratitude.

A few examples of audio from DK3WN, HitSat:
http://www.dk3wn.info/sat/afu/sat_hitsat.shtml

The bit rate, above needs to be considered carefully to be able to send data at a fast enough rate. We need to work with PAY and find what kind of resolution and memory size the pictures are going to be to get an idea how long it will take to send the data. It is necessary to choose the Baud rate such that we can capture the data as it passes over the ground station. It may be necessary to consider compression algorithms for this data. Typically it should be assumed that a satellite will take 5-10 minutes to pass over the visible sky.

SATELLITE CONTROL:

Andy came across this example of commanding small satellites on the amateur radio frequencies so he thought he'd share it.

It is possible to use packet radio to encrypt the data for control using such a sequence.

HARDWARE:

This may be made available by Simon G8PAN, a radio amateur and a professional rf designer. Andy Thomas mentioned our project in confidence to him and he can get us a free "beacon" for 435 MHz. We need to look at the catalogue and enter discussions seriously! M.K.Consultants make very small transmitters etc, unfortunately their full catalogue is not available from there website. The COMS team is looking into this ASAP as the system requirements are vital. Unfortunately the originally proposed 2.4 GHz (MHX-2400) Development Platform was found not to be suitable as the Doppler shift is too high and the frequency band is not legal in this country. We seriously need to check import/export restrictions, licenses when considering the project. A possibility is we could make the kit ourselves. For any hardware, vibration & vacuum testing is required.

Generally, in terms of hardware COMS require:
A modem - frequency is modulated to 8-bit binary numbers from the microcontroller into something like morse code. Morse code is encoded into AX.25 protocol by terminal node controller (TNC). TNC ‘packetizes’ data and ‘keys’ the transmitter (adds satellite ‘call sign’ to data)

A transceiver - to transmit and receive data. The transmitter sends signal to ground station via an antenna. Radio operating at UHF at ground station receives data signal and encodes stream to form required for data analysis.

A Ground Station (provided by NSC): Ground Station transmits commands to the satellite (e.g. on/off commands for instruments). Commands received by receiver and processed by microcontroller.

Need to consult catalogue for M.K.Consultants to get further information about certain specifications.

COSTING

COMS need to consult the catalogue for M.K.Consultants which is not yet available