The analysis for this chapter assumes traditional RF link equations, parameters, and efficiencies for a deep space mission. As outlined in the light analysis chapter, several efficiencies and losses are assumed for this deep space communications link. These can be revisited below in Table 8.1. Most of these parameters have been referenced from typical deep space link parameters. Other parameters, such as the receiver efficiency and data rate, are values defined by the Deep Space Network. The 70 m dishes are just over 70% efficient at X-Band and even more efficient at S-Band. The lowest data
rate for downlink is 10 bps[66]. The transmit efficiency is calculated in the Deployable Subsystem chapter.
Table 8.1: Communications Parameters for Deep Space Link
Parameter Value
Signal-To-Noise Ratio (SNR or Eb/No) 10 dB
Signal Link Margin (SLM) 3 dB
Data Rate (R) 4 bps
Transmitter Efficiency (ηT) 0.4
Receiver Efficiency (ηR) 0.6
System Noise Temperature (Ts) 75 K
Transmit & Receive Antenna Pointing Loss (each) (LP L) -3 dB
Polarization Loss (LP) -2 dB
Line Loss (Ll) -2 dB
Implementation Loss (Limp) -3 dB
Atmospheric Loss (Latm) -0.3 dB
Total Losses before Space/Noise Losses -10.3 dB
In addition, with the UHF and Stanford SRI case requiring enormous amounts of power, the Deep Space Network is the only ground station option. The 70 m dishes operate at two RF frequencies, S-Band and X-Band. The DSN also operates at Ka-Band using its 34m beam waveguide antenna.
The link analysis uses typical RF communications link equations. These culminate in a variety of equations. The main equation is the signal-to-noise ratio equation shown in Equation 8.1, where Pt is the RF power in Watts, Gt is the transmitter gain
in Watts, Gr is the receiver gain in Watts, k is the Boltzmann constant in J/K, Ts is
the system noise temperature in Kelvin, λ is the RF wavelength in meters, d is the distance between the two antennas in meters, and LT P L and LRP L is the transmit
antenna pointing loss and receive antenna pointing loss, respectively, in Watts. Eb No = PtGtGrLpLlLatmLimpLRP LLT P L kTsR(4πdλ )2 (8.1)
The gain for a parabolic dish is calculated using Equation 8.2, where ηt is the
transmitter efficiency and Dt is the diameter of the dish in meters.
Gt=
ηtπ2D2t
λ2 (8.2)
The receiver gain can be calculated with a very similar equation shown in Equa- tion 8.3, although using the receiving dish parameters.
Gr =
ηrπ2Dr2
λ2 (8.3)
The equations can be rearranged to solve for required RF power for the various ground station options along the Voyager 1 trajectory. Using the values listed in Table 8.1 and assuming the spacecraft’s determined dish size of 7.75 m in diameter, the required RF power to maintain a signal to noise ratio of 10 dB with an added signal link margin of 3 dB can be calculated as a function of the spacecraft’s distance for various ground station solutions. This is shown in Figure 8.1.
Figure 8.1: RF Power Required for a 7.75 m Diameter Dish at 10 bps at Various Distances From the Sun
As seen in the curve, X-Band and Ka-Band appear to be the most attractive options once reaching large distances of up to 100 AU, requiring the least RF power. However, it should be noted that S-Band can close the link with 1 W of RF power when near Jupiter. Due to its lower frequency, S-Band will result in a higher beamwidth for the antenna, requiring less maneuvers to track Earth. This may be optimal for the spacecraft’s ADCS system when maneuvers are required more frequently.
While Ka-Band appears to be a prime option, high frequencies are highly sensitive to surface irregularities. Because the spacecraft will be using an inflatable dish, there are bound to be surface irregularities. This issue is highlighted in Alessandra Babuscia’s paper, in which she details that using Ka-Band with the inflatable antenna will be almost impossible. However, she states that X-Band is indeed feasible given the surface irregularities. Therefore, the spacecraft will need to operate on X-Band
in order to minimize the required RF power with the inflatable dish. Therefore, operating on X-Band, the spacecraft will need to generate 23 W of RF power. The power chapter of this paper determined that the spacecraft will need to transmit once a week to keep the average power consumption low. For this analysis, the data volume transmitted each week will be 1 KB or 8000 bits. The spacecraft will be using the minimum data rate to keep the electrical power requirement low, and using 10 bps, the transmission will need to occur for 800s to downlink 1 KB. While this appears low, the instruments will be recording data infrequently. While the true required data volume for transmission must be analyzed further, 1 KB will provide a good reference point for determining requirements down the line.
The transmitter will need to be highly specialized, requiring low standby power consumptions while able to generate over 23 W of RF power. While the exact spec- ifications of the transmitter should be investigated in further detail, the transmitter will be assumed as a solid state power amplifier (SSPA), a compact and low cost spacecraft amplifier. Typical SSPAs range from 0.5 to 1.5 kg and can be up to 30% efficient. The SSPA is assumed to occupy a 1U volume. The feed for the antenna will be presumed as the patch antenna used in Babuscia’s inflatable antenna design. This patch antenna is assumed to be 10 cm x 10 cm in area.