- Define link budget
- Define housekeeping/mission data that is to be transmitted to ground station.
- Assign word sizes, sample frequencies, etc. to data.
- Calculate total orbit time
- STK may be used for this step
- Calculate amount of data gathered in one orbit
- Calculate time in contact with ground station
- Calculate constants
- Antenna gain
- Path loss
- Antenna orientation
- Power
- Control Electronics
- Attenuation
- Define uplink and downlink
- At Strathclyde, it is likely that the STAC station shall be used as both uplink and downlink.
- Any satellite designs must be fitted to the STAC station.
- Integration of communication with other subsystems.
This describes the base station that will be used, any others that can be used, the relay to these stations either by other satellites or by direct communication. It should also include the specific system layout of the satellite based on the generic model, the model is adapted by removing any components that are not necessary in the design and by showing the path that data will take, possibly including an estimate of internal error and path loss to the antenna.
Introduction
A link budget is a summary of all the losses and gains that are experienced by a signal during its transmission during a medium.
In the case of satellite design, the link budget is essential in the design of the communications system.
This is split into two parts: the downlink (from satellite to ground station) and uplink (from ground station to satellite). The budget must contain all the constants of the transmission (see link budget document) as well as the pass over time and power requirements. The link budget is very important and must use all the previous values calculated (although don’t worry about going back over things and fixing them!). The budget now becomes the core document for the communications, all its properties are interdependent so changing one will result in all others being updated at the same time, setting it out in a spreadsheet is a good way to do this (an example is provided in document).
StrathSat Communications
The book Space Mission Analysis and Design (SMAD) has been used as the foundation for the link design process and the data involved within this report. The method used was an iterative design process that followed the outline in the previously mentioned text. In this way complexity was built into the budget as each segment was completed.
The mission brief given for Strathclyde Satellite (StrathSat) is to design and build a 1U cubesat that will be launched into Middle Earth Orbit (MEO). From this orbit it must be capable of performing orbit and attitude information gathering so that the controller knows where the cubesat is. It must be able to deploy its primary payload, a deorbiting balloon that will increase the eccentricity of the cubesat until drag effects cause it to fully de-orbit. After the balloon has been deployed the orbit and attitude data should take readings to confirm the effect the balloon is having on the orbit. The secondary payload is a Geiger–Müller tube that will take accurate readings of both the inner and outer Van-Allen belts, whilst the orbit and attitude data plots an accurate position for each reading. The hope is to use the Strathclyde Satellite Tracking and Control Station (STAC) to monitor and control the cubesat.
From this brief the important criteria for the link budget is:
- The orbit must have an inclination great enough to bring it over Glasgow
- The signal will need to have a power great enough to transmit from MEO and through radiation belts
- The bandwidth must be great enough to transmit all orbit data on each pass over
- As system is placed on a base of 10cm the antenna must be physically small
- With only a 1U system the power requirements must be minimised
- The deorbiting balloon forces the cubesat to face in a particular orientation and so reduces the pointing time at the ground station, this dictates the data transfer rate.
- The de-orbit balloon will constrict the pointing of the craft so unidirectional antenna or large area coverage will be required.
- As a student proposal costs must be minimal.
- Cubesat will need to conform to the systems that STAC already has installed.
STAC Station Overview
The majority of the information for STAC is contained within a separate document. From this the key points for this design are:
- It has VHF, UHF and S-Band communications.
- 50 W, for the transmitter at the station.
- 15 dBi gain at STAC.
- The s-band disc being commissioned will only be 0.6 m in diameter, this is not suitable for 1M bps so a lower data rate than this is required.
- STAC has BPSK and AFSK, if QPSK is the modulation method chosen then alternative arrangements will need to be made to communicate with the satellite.
GENSO Overview
Global Educational Network for Satellite Operations (GENSO) is an organization that is a collection of Universities that allows access to each member’s ground station and antenna. By being a member of this it theoretically increases the time the satellite is available for from a number of minutes to 24 hours. Other benefits would include an error checking, as there as several stations possibly communicating with the satellite then the information sent can be referenced against what others received. However GENSO is being advertised as a solution for Low Earth Orbit (LEO) communication, as this cubesat is going to MEO it may not be suitable for communicating with. Members of GENSO do have S-band transmitters so these may be the solution to the issue raised in the STAC section with regards to QPSK modulation.
Telemetry, tracking and command (TT&C)
Telemetry refers to any data that is transmitted over distance, tracking can be achieved by a combination of IR Earth sensors and Sun sensors. Command refers to the control that the ground station is capable of exerting on the spacecraft, so this data rate will be to and from the CPU and any receptors on the subsystems. Each system should be outlined at this stage to include roughly to what extent they will be used in the particular satellite, so will we use tracking or rely on NORAD tp provide coordinates etc.
Data collection
The health of each subsystem must be known throughout the mission, in order to know this some property of the system has to be measured and checked to ensure that it represents a healthy and working system. An example is on a solar cell, there must a temperature measure and a voltage measure, from these two measurements it can be seen if the cell is operating correctly and within safe conditions. Each data has a word length that will decide the accuracy of the measurement and a data storage value in bytes. These measurements will be taken at intervals throughout a period and stored with the time taken. From the system architecture the number of sensors on each subsystem can be decided and then a value can be given to the amount of data that will be stored per orbit. This is known as the house keeping data.
Data Relay
The data that has been stored throughout the orbit needs to be transmitted to the ground station, the only time to do this is when the satellite and the ground station are in line of sight with each other, this value is effected by; location of ground station in relation to satellite, the eccentricity of the satellite and the inclination of the satellite. Calculate the worst case time to transmit and design the bit rate around this so that you can get all house keeping data from one orbit in one pass over, also include any payload data that is required to be transmitted.
Calculations
The value for the data rate is based on the amount of data needing to be transmitted and the time available to transmit the data. The cubesat will be in MEO but the exact orbit is not known until we have a launch selected so in the attached link budget the altitude starts at 10,000 km and increase in increments of 500 km until 20,000 km. Also as this is a MEO mission the transmitter and receiver selected would need to be radiation hardened, (Space Micro inc). Approximate data values are provided in table 1 to show where the data rate has been calculated from.
Component | No. of components | Interval (mins) | Bits per sample | Total per period |
---|---|---|---|---|
Sun Sensor | 4 | 10 | 10 | 10 |
Earth IR | 1 | 10 | 10 | 1391 |
Solar Cell Temperature | 1 | 5 | 10 | 696 |
Solar Cell Voltage | 1 | 10 | 10 | 348 |
Battery Voltage | 1 | 10 | 10 | 348 |
Battery Temperature | 1 | 10 | 10 | 348 |
Transceiver Temperature | 1 | 10 | 10 | 348 |
Science Payload | 1 | 5-10 | 10 | Max=696 |
Table 1 - Component Data Rates
The orbit has been taken to be at 10,000km altitude for the calculations used in the table, from this and equation 1, the period of the cubesat can be shown to be 20859.5 seconds.
Due to the effect of the balloon on the cubesat the pointing time at the earth is greatly reduced, it is estimated that only around 5% of the orbit time will be available for the transceiver to point at the ground station. From this the time available for downlink is 1042 seconds. To transmit the 4523 bytes of data in that time will require a minimum data transfer rate of 5 bps, to allow for loss of data or decreased transmission time a minimum data transfer rate of 10 bps should be achieved. This target can be met easily with the S-band transmitter and allows us to put in place a back up omni-directional antenna that will operate in emergency situations to provide minimal contact with the cubesat.
The orbital period of the satellite must first be calculated using
Equation 1 T=2π√(a^3/GM)
Where T is the orbital period (s), a is the semi-major axis of a circular or elliptical orbit (m), G is the gravitational constant (6.67384 X 10-11 m3 kg-1 s-2) and M is the mass of the Earth (5.9742 X 1024 kg).
Due to the effect of the balloon on the cubesat the pointing time at the earth is greatly reduced and it is estimated that only around 5% of the orbit time will be available for the transceiver to point at the ground station. From this the time available for downlink is 1042 seconds. To transmit the 4523 bytes of data in that time will require a minimum data transfer rate of 5 bps, to allow for loss of data or decreased transmission time a minimum data transfer rate of 10 bps should be achieved. This target can be met easily with the S-band transmitter and allows us to put in place a back up omni-directional antenna that will operate in emergency situations to provide minimal contact with the cubesat.
The link budget equation may be expressed thusly:
Equation 2 Eb/No =(P*Ll*Gt*Ls*La*Gr)/(k*Ts*R)
Where the symbols have the following meanings:
Variable Definition Units
Eb Energy per bit J
No Power spectral density ratio None
P Transmitter power W
Ll Transmitter to antenna line loss dB
Gt Transmit antenna gain dB
Ls Space loss dB
La Path loss dB
Gr Receive antenna gain dB
k Boltzmann constant (1.38 x 10-23) m2 kg s-2 K-1
Ts System noise temperature K
R Data rate b s-1
Table 2 – Nomenclature for Link Budget Equation
There are a multitude of calculators and spreadsheets available online that have already been configured for calculating the link budget.
Integration
The physical dimensions of the system required to provide the link budget must now be calculated and priced. If the result is unrealistic then the link budget should be changed to fit the physical constraints of the budget and the dimensions of the satellite.
S-Band
The S band is defined by an IEEE standard for radio waves with frequencies that range from 2 to 4 GHz, crossing the conventional boundary between UHF and SHF at 3.0 GHz. It is part of the microwave band of the electromagnetic spectrum. The S band is used by weather radar, surface ship radar, and some communications satellites, especially those used by NASA to communicate with the Space Shuttle and the International Space Station. The 10-cm radar short-band ranges roughly from 1.55 to 5.2 GHz.
[Reference: http://en.wikipedia.org/wiki/S_band]
PSK –Phase-shift Keying
Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, each assigned a unique pattern of binary digits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal — such a system is termed coherent (and referred to as CPSK).
[Reference: http://en.wikipedia.org/wiki/Phase-shift_keying]
BPSK – Binary Phase-shift Keying
BPSK (also sometimes called PRK, Phase Reversal Keying, or 2PSK) is the simplest form of phase shift keying (PSK). It uses two phases which are separated by 180° and so can also be termed 2-PSK. This modulation is the most robust of all the PSKs since it takes the highest level of noise or distortion to make the demodulator reach an incorrect decision. It is, however, only able to modulate at 1 bit/symbol (as seen in the figure) and so is unsuitable for high data-rate applications when bandwidth is limited.
QPSK – Quadrature Phase-shift Keying
Sometimes this is known as quaternary PSK, quadriphase PSK, 4-PSK, or 4-QAM. (Although the root concepts of QPSK and 4-QAM are different, the resulting modulated radio waves are exactly the same.) With four phases, QPSK can encode two bits per symbol— sometimes misperceived as twice the BER (Bit Error Rate) of BPSK. The mathematical analysis shows that QPSK can be used either to double the data rate compared with a BPSK system while maintaining the same bandwidth of the signal, or to maintain the data-rate of BPSK but halving the bandwidth needed. In this latter case, the BER of QPSK is exactly the same as the BER of BPSK - and deciding differently is a common confusion when considering or describing QPSK.
AFSK – Audio Frequency-shift Keying
Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented by changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone. Normally, the transmitted audio alternates between two tones: one, the "mark", represents a binary one; the other, the "space", represents a binary zero.
[Reference: http://en.wikipedia.org/wiki/AFSK#Audio_FSK]
Path Loss
Path loss (or path attenuation) is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system. Path loss may be due to many effects, such as free-space loss, refraction, diffraction, reflection, aperture-medium coupling loss, and absorption.
[Reference: http://en.wikipedia.org/wiki/Path_loss]