Radio Communication Link
For Doppler tracking/ranging to be carried out a radio link is needed with the spacecraft. A signal must be sent from the ground to the craft. This must be picked up by the craft and sent back to the ground. The radio waves sent from the spacecraft need to have enough power to be detectable at the ground station.
To make sure that the radio waves are received at both the craft and the ground a link budget is used to take into account all of the factors which diminish the power of the radio waves. This project has designed a link budget for both the ground to primary (uplink) and primary to ground (downlink) part of the mission. The detailed background and work is contained below.
To summarise: with current systems and technology a reasonable link can be established at the end of the mission life, 100 A.U. Not only can the carrier wave be detected but also science data modulated onto it can be received with appropriate levels of error.
A spacecraft receives commands and transmits data over a radio frequency (RF) link. This link is also used for Doppler tracking and ranging. Information is modulated onto a RF carrier wave which is then transmitted to the spacecraft via a ground station and vice versa. It is the frequency shift of the carrier wave which is used for Doppler tracking. For Doppler ranging special ranging codes are modulated onto the carrier wave, which when received provide information about the range.
There are several essential components to a communication link. They are:-
The fundamental component of a link is the RF carrier wave. The carrier wave is generated onboard the spacecraft from an oscillator and is transmitted towards the ground station through an antenna. The antenna at the ground station receives the carrier wave and detects it with a receiver circuit; a link has been made.
A major concern when designing a space communication link is whether the carrier wave can be detected at the ground station. This is because the carrier wave must travel huge distances (several tens of A.U.) before reaching Earth and in travelling that distance it loses power and picks up noise.
To ensure that the carrier wave can be detected there are various design features to be considered in a communication link:-
In the case of a spacecraft to Earth link the transmitted power is constrained to that available from the onboard power source. In theory a ground based transmitter could have a limitless supply of power but onboard a spacecraft the only power available is that provided by the solar panels or a nuclear generator.
The gain of an antenna is directly related to its size. An ideal isotropic radiator would have a gain of 1. Parabolic reflectors, which are the usual choice for satellites, focus radio energy towards a target increasing the gain.
On a spacecraft
space and mass are at a premium, especially inside the launch vehicle. The new ESA Vega launch vehicle has a payload fairing diameter of
2.6m. Any craft launched on a Vega
launcher would either have to have an antenna size of less than 2.6m or a
space deployable antenna. The gain of
the ground station antenna is not constrained in the same way as a spacecraft
antenna. Instead a ground based radio
antenna’s size is constrained by current engineering methods. The largest dishes used for deep space
communication are the NASA Deep Space Network 70m dishes in Goldstone,
The value of the antenna gain multiplied with the transmitted power is known as the effective isotropic radiated power (EIRP). EIRP is a measure of the performance of the transmitter system.
The path loss factor is a physical aspect of radio wave propagation in free space. It is related to the wavelength of the carrier and the distance over which the carrier propagates. As the carrier wave propagates through free space the wave front expands loosing power.
Path loss given in equation 2 attenuates the power of the wave transmitted. The carrier wave is also attenuated as it passes through the atmosphere and the radiation belts of the Earth. Along with physical passage through the atmosphere weather attenuates a radio signal especially clouds and rain. As weather is unpredictable it is not included in this basic design.
System noise affects the quality of the received signal by adding noise power to the received signal. The signal to noise ratio (SNR) is used to measure the quality of the received signal. System noise is a composite of all of the sources of RF noise in the link. There are two types of noise source in a link: external and internal. External noise comes from the Sun, the sky, the atmosphere and any other natural feature which emits in the radio region. Internal noise comes from the operating temperature of the receiver and transmitter, noise from transmission down cables and any electronics in the system. The list is extensive as nearly everything between the transmitter and receiver will generate radio noise.
All of the sources of noise can be equated to individual temperatures which when added together give a system noise temperature which is used to compute the noise power of the link. Much investigation has been carried out on the characteristics of external noise sources, and they can be summarized in a composite link noise for different carrier wave frequencies. For internal sources of noise equipment manufacturers provide specifications of either noise values or operating temperatures.
The figure of merit for a receiver system is the ratio or antenna gain (Gr) to system temperature (T): (G/T)
Once all of the attenuating factors have been taken into account the strength of the received signal must be above the sensitivity level of the receiving circuit. If the carrier wave has lost too much power then it will not be picked up and the link cannot be established. Also the SNR needs to be above a set level in order for the link to be effective.
The variable which is not constrained by other design factors is the carrier wave wavelength. Therefore the communication link designer theoretically has a free hand in the choice of wavelength for the carrier wave.
The communication link is evaluated by received power level and the margin between the required SNR and the actual SNR. The evaluation is done in a link budget, where all of the factors in the link are taken into account. An example can be found in Space Vehicle Design page 547. The link analysis has to be done for the spacecraft to ground station link (downlink) and the ground station to spacecraft link (uplink).
Although the link designer has a theoretical free choice in frequency used for the link there are practical constraints to do with use of the electromagnetic spectrum. The Pioneer Mission used S-band (2-4 GHz). Recently there has been a change to using X-band (8-12 GHz) for space communication with the Deep Space Networks being upgraded to support the new carrier frequency. There are also advantages to using the higher frequency. A higher frequency carrier wave has a higher gain value for a specific antenna.
The greatest advantage of using X-band over S-band is in relation to radio wave propagation through the solar corona and ionised gases. It is something not mentioned in this section but an X-band radio waved experiences less noise and attenuation due to propagation through the solar corona than an S-band radio wave. For deep space missions where links are made for extended periods of time through the corona this is an advantage. Also the higher frequency improves the performance of Doppler measurements by reducing Doppler jitter and increasing Doppler velocity resolution. This is shown in the section on Doppler Measurement Error.
Any future deep space missions will therefore use X-band as the primary means of communications. Looking to the future, projects are attempting to use Ka band (18-40 GHz) for better performance on the downlink (spacecraft to ground station).
Taking into account all of the information presented above a preliminary link budget for a mission to test the Pioneer Anomaly can be designed. It is presented below in table 1.
Table 1, showing the values used for the uplink and downlink link budgets for the current mission design.
The baseline for this preliminary link budget is the ESA ground station at New Norcia, and the Small Deep Space Transponder (SDST) developed by Motorola for NASA. The design is using a fixed parabolic antenna of diameter 2.27m. It has been designed to fit inside the standard payload fairing of the ESA Vega launch vehicle. Table 3 shows how the gain scales with antenna diameter. The antenna efficiency factor has been taken from a NASA progress report; ‘Comparative Deep Space Link Performance’. This report also provided a basic mass for a 2.27m antenna of 10 kg based on current antennas currently being flown.
This link budget has been designed to test whether or not the carrier can be detected at 100 A.U.; end of mission. The required SNR for both uplink and downlink has been set according to the data rates on the respective links. Table 1 also contains the link budget for the uplink. For both the uplink and downlink the link margin is acceptable the area of concern is the received power level. The SDST has a receiver threshold of -186 dBW. This is compared to the uplink received power of -177 dBW.
The downlink is
received with a power of -139 dBW.
Currently there is no information on the sensitivity of the ESA New
Norcia ground station but in 2002 the 70m NASA antenna in
Table 2, shows a brief correlation between antenna diameter and gain.
The two link budgets presented here do not contain some specific noise factors such as polarization loss. Also it neglects losses due to errors in the antenna and feed positioning. Instead they are meant to be a guide to verify that communication and Doppler tracking/ranging can be carried out with a craft at 100 A.U.