COMS Groundsegment Definition

This page contains information used to determine the requirements on the ground segment, data handling and storage and control systems as well as radio operator licenses required.

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.

Virtual Ground Station


SATELLITE CONTROL:


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


COMMUNICATIONS SUBSYSTEM

REQUIREMENTS ON GROUND SEGMENT

The key requirements on…:
1. Tracking to determine the position of the satellite in orbit
2. Telemetry operations to acquire and record satellite data and status
3. Commanding operations to interrogate and control various functions of the satellite
4. Controlling operations to determine orbital parameters
5. Data processing operations to present all the engineering and scientific data formats required for successful progress of the mission

The main hardware components involved in satisfying these requirements are an antenna, a receive-transmit system, data recorders, computers and their peripheral and control consoles.

ANTENNA

The main hardware component of a ground station is the antenna, whose support functions will include tracking, telemetry and command.

The antenna diameter, D, required for any particular mission is primarily a function of maximum satellite range, d (km), carrier frequency fc (Hz), data rate b (bits/s) and satellite transmitter power tp (watts). A useful approximation for D can be found from:

Thus for a polar orbiting satellite, for example, transmitting 1Mbit/s data at S-band with a 1 watt on-board transmitter, D is calculated to be of the order of 9m.

TRACKING AND DATA RELAY SYSTEM

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.

The 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.

Decoding software examples are available by one of the “big guns” in Europe who have the equipment and interest to monitor satellites continuously. Mike Rupprecht from Germany. http://www.dk3wn.info/sat/afu/sat_hitsat.shtml (In German)

GROUND STATION CONFIGURATION

Although the antenna is the main item of hardware at the groundstation, the other items (a receive-transmit system, data recorders, computers and their peripheral and control consoles) are an equally integral part of its facilities.

To support spacecraft and their payloads, the ground system must command and control them, monitor their health, track them to determine orbital position, and determine space-craft attitude from sensor information, although some of these functions are currently carried out on-board the spacecraft. The ground system controls the spacecraft and its instruments or payloads by transmitting command data to the spacecraft and uses spacecraft housekeeping telemetry and mission data to carry out these functions. It acquires mission data from a spacecraft and its instruments and transfers it to the data users. The ground system also supplies any telemetry and tracking information that they may need.

DATA HANDLING AND STORAGE

SOFTWARE
Pre-pass software:
- orbit determination and prediction

  • to track the satellite correctly during a station pass
  • to reconstruct the orbit and hence the position of the satellite at any time during the mission and
  • to predict ahead many weeks for the purposes of mission planning

- observation planning and scheduling
//* necessary to know where the satellite will be in the future

  • antenna control software needs to know where to find it during passes
  • data processing programmes need to know where it was when the data was accumulated//

- command list generation

- simulation

REAL-TIME SOFTWARE
- Tracking
- Command Uplink and verification
- Data reception
- Status checking

POST-PASS SOFTWARE
- Health assessment

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.

CW Telemetry
E.g. from HITSAT, Mike Rupprecht DK3WN

HIT1 Bureau call sign: JR8YJT

HIT2 Absolute time information:
A Month [hex-> 1-Jan, 2-Feb, … 9-Sep, A-Oct, B-Nov etc.]
BB Day [01..31]
CC Hour [00..23]
DD Minute [00..59]
EE Second [00..59]
FF RSSI [hex-> value(dec) * 5/256 in volts]

HIT3 Each temperature information:
GG +X face temp [hex-> value(dec)*(-2.4021422) + 214.94 in °C]
HH -X face temp [hex-> value(dec)*(-2.4021422) + 214.94 in °C]
II +Y face temp [hex-> value(dec)*(-2.4021422) + 214.94 in °C]
JJ -Y face temp [hex-> value(dec)*(-2.4021422) + 214.94 in °C]
KK +Z face temp [hex-> value(dec)*(-2.4021422) + 214.94 in °C]
LL -Z face temp [hex-> value(dec)*(-2.4021422) + 214.94 in °C]
MM Transceiver [hex-> value(dec)*(-2.4021422) + 214.94 in °C]
NN Battery temp [hex-> value(dec)*(-2.4021422) + 214.94 in °C]

HIT4 Power supply information:
OO prim. battery [hex-> value(dec)/256 * 10 in volts]
PP sec. battery [hex-> value(dec)/256 * 10 in volts]
QQ 5V [hex-> value(dec)/256 * 6.06383 in volts]
RR 5.5V [hex-> value(dec)/256 * 6.06383 in volts]

HIT5 Amateur radio service: TNX[Callsign1],TNX[CallSign2],

- Data processing

- Orbit determination

- Data analysis

This software resides in the space-craft’s own on-board computer

CONTROL SYSTEMS

RADIO OPERATOR LICENSES REQUIREMENT

International regulations for communications systems are set in place to control possible interference between different systems to ensure compatibility between the various national systems that may be connected end to end.

Main satellite comms bands.

Radio operating licenses can be obtained from John Heath and Andy Thomas via Amsat. We will need to go through the frequency application process with them – time is tighter than was once thought.


REFERENCES:

Fortescue & Stark, “SpaceCraft Systems Engineering”

Mike Rupprecht (DK3WN) - HITSAT –
http://www.dk3wn.info/sat/afu/sat_hitsat.shtml


This section can be downloaded here

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