The communications system will be responsible for the exchange of all diagnostic, telemetry, and system control information between the orbiting satellite systems and the ground-based control stations. Each ground site will maintain its own independent communications station capable of both uplink and downlink connections with a single satellite. Redundancy networks built into both the ground stations and satellite stations will allow for information exchange to occur between any satellite/ground-station pair.
Operating Frequency
The basic two-way communications link is maintained at 418 MHz, a frequency residing at the lower end of the UHF spectrum in a band generally set aside for government satellite use (including meteorological satellites and military satellites). Additionally, the relatively low-bandwidth requirements of system information and data flow facilitated the selection of an operating frequency in this range.
Earth Station Communications System
Solid performance of the earth-station antenna is paramount to the goal of minimizing the transmission power required for the orbiting satellite. Additionally, a robust ground-station antenna also helps to maintain overall system integrity during poor environmental conditions (rain fading, ambient electronic noise, targeted interference, etc). Dynamic power transmission may also be used to overcome poor operating conditions.
Each ground-based station incorporates a 40-meter dish antenna optimized for both uplink and downlink operation in the selected band. Detailed parameters and specifications of the Earth station antennas are included in Table 1.
Table 1. Earth Station Antenna Parameters
Design of this antenna was idealized for high-gain, narrow beamwidth applications and performed using standard dish antenna gain calculations. Equation 1 includes the standard gain calculation for dish antennas.
Equation 1. Dish Antenna Gain Calculation
As stated in Table 1, the given parameters result in a gain of approximately 45dB for the Earth station antenna.
Link Budget Calculations
To estimate the necessary satellite antenna parameters, a link budget calculation was performed for the 418 MHz operating frequency. The linear free-space path loss (FSPL) equation is shown in Eq. 2. The link budget equation is included in Eq. 3.
Equation 2. FSPL @ 418 MHz
Equation 3. Linear Link Budget Equation
FSPL at 418MHz is found to be approximately 176dB. For the given antenna parameters and a satellite in geostationary orbit, a transmission power of 10 Watts will result in a received signal power of 0.75 nW at the Earth ground station.
Note that pending available satellite resources, the transmission power may be increased to provide a stronger signal to the Earth station. Due to dish size and power availability, the earth station is capable of transmitting a much stronger signal (up to 100kW).
Satellite Communication System
Consideration of satellite antenna designs was guided by a goal of unobtrusive, low-power solutions. After review of the link budget equation and calculation of the earth station dish antenna performance, the satellite antenna was designed using a similar methodology. Parameters are included in Table 2.
Table 2. Space Station Antenna Parameters
Design of this antenna was optimized for high-gain, low-power applications and performed using the standard dish antenna gain calculations mentioned previously. As seen in Table 2, the given parameters result in a gain of approximately 30dB for the space station antenna.
Channel Capacity
The Shannon capacity defines the upper bound on the amount of information that can be reliably transmitted over a given channel. To define the maximum available bitrate for the communications link, the Shannon-Hartley theorem defined in Eq. 4 is invoked. Here, BW indicates the system bandwidth and SNR is the signal-to-noise ratio. Equation 5 displays the calculation of noise power, a necessary component to determine the SNR of the system.
Equation 4. Channel capacity calculation
Equation 5. Noise power calculation
The maximum experienced noise power is calculated as -161dB for a 20kHz signal bandwidth.
Modulation and Coding Schemes
Given the power and noise requirements of the system, QPSK is chosen as the method of modulation. QPSK is a common modulation scheme in satellite transmissions since it can achieve a higher bitrate per given bandwidth than BPSK schemes.
Additionally, the communications system will employ a CDMA coding scheme, which employs a special spread spectrum technology emphasizing secure data transmission between the satellite and the ground station. Since CDMA effectively rejects narrow-band interference, this coding scheme will be an effective countermeasure against intentional jamming, enhancing the security and reliability of the communications link.
References
http://transition.fcc.gov/Bureaus/OPP/working_papers/oppwp38chart.pdf
Balanis, C. Antenna Theory: Analysis and Design. 3rd Edition.
