Coms Ground Segment

National Space Centre was visited by Adam Bark, Peter Hague, and Phillip Peterson on 09/09/2009, to ascertain what ground segment equipment is available to the COMS team.

Overview

The ground segment equipment we will have access to is located on a trolley, currently in storage in the gallery of the National Space. It consists primarily of a transceiver and a PC.

Connectors to the aerial are available in the gallery at present, but with some modifications the ground segment could be connected elsewhere in the building.

Transceiver

There is a Yaesu FT-847 Transceiver available for our use on the site. We have verified that this unit can handle the band (435MHz) and modulation (FM) currently used by the COMS team.

PC

The transceiver connects to a PC via two 3.5mm audio plugs which attached to the line in and line out ports on the soundcard of the PC. An interface box isolates transciever audio from the computer sound card. There is also a 9-pin D-sub connector which plugs into the serial port of the PC.

Two audio lines are required, because a PC sound card can only either send or receive on a particular audio line. The plug which connects to 'line out' carries the signal transmitted to the satellite from the ground station. The plug which connects to 'line in' carries the signal received by the ground station from the satellite.

The PC runs Windows XP, and will come installed with Ham Radio Deluxe and DM780 software. All necessary control functions (direction of the antenna, whether or not the transceiver is listening or sending) can be controlled through this software.

Ham Radio Deluxe comes with DM780 included by default, and is available as a free proprietary download here: http://www.ham-radio-deluxe.com/

Initial acquisition

After a successful launch, we will receive a list of TLEs from NORAD ( http://cubesat.wikidot.com/adcs-norad-data-format )representing the orbits of all the cubesats released - but will not know which one corresponds to our satellite. There is no means to detect the initial signal from PLUME except to point the antenna at each of the cubesats in turn until we receive the signal that uniquely identifies ours.

Scheduling

The hardware at the NSC is not exclusively for our use, and we must plan our ground segment strategy understanding that other users will need the equipment most of the time.

Communication capabilities

The amateur satellite VO52 was given to us as an example of a satellite this equipment has been used to communicate with in the past. The specifications of this satellites transmitter, along with its orbital elements, allows us to calculate an upper bound for the weakest signal the ground segment can process.

According to this site:

http://www.amsat.org/amsat-new/satellites/sat_summary/hamsat.php

VO52 has a transceiver with an output of 1 watt or 30dBm, and according to this site:

http://www.amsat.org/amsat-new/satellites/satInfo.php?satID=2

It has an apogee of 646km and a perigee of 607km. It is also in a highly inclined orbit, so we can assume it could appear anywhere in the sky visible from the NSC.

Radius of Earth, R=6378km

Further distance between NSC and VO52 is when VO52 is at apogee and is exactly on the horizon. The centre of the Earth, the NSC, and VO52 would then form a right angled triangle with the hypotenuse being the distance from the centre of the Earth to VO52. Therefore;

(1)
\begin{equation} R^2+Xmax^2=(R+646)^2 \end{equation}
(2)
\begin{equation} Xmax=2942km \end{equation}

Where Xmax is the distance between the NSC and VO52. The smallest possible distance between the NSC and VO52 is when VO52 is at zenith and also at its perigee. This distance is simple the altitude of the satellites perigee so:

(3)
\begin{equation} Xmin=607km \end{equation}

This information can be used to calculate the received power at the ground station using a link budget equation.

(4)
\begin{equation} Prx=Ptx-Lfs \end{equation}

where Prx is the received power, Ptx is the transmitted power, and Lfs is the path loss, and is according to http://en.wikipedia.org/wiki/Free_space_loss given by

(5)
\begin{equation} Lfs=20 log D + 20 log f +32.44 \end{equation}

Where D is the distance in kilometres between the transmitter and receiver, and f is the frequency in MHz.

For the maximum distance,

(6)
\begin{equation} Prx=30dBm-20 log 2941 - 20 log 435 - 32.44 = -124.6dBm \end{equation}

and for the minimum distance,

(7)
\begin{equation} Prx=30dBm-20 log 607 - 20 log 435 - 32.44 = -110.9dBm \end{equation}

Note that these calculations are lacking several loss and gain terms, and are thus rough estimates. They should be refined before being used as the basis for any design decisions. A good place to start is this Wikipedia page:

http://en.wikipedia.org/wiki/Link_budget

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