The mission will consist of three communication links illustrated in Figure 1. 1) Earth-to-Satellite (ES) link, 2) Satellite-to-VIRGIL (SV) link, and 3) Inter-VIRGIL (IV) links. For the communication between the Earth and the satellite, NASA Deep Space Network (DSN) is employed [1]. The DSN is a world-wide network of located in California, Spain and Australia. The antennas are placed 120-degree apart to allow constant observation of space crafts. The link operates at X-band, and the uplink (from Earth to the satellite) operates at 7.3 GHz containing the coded instructions and the downlink (from the satellite to Earth) operates at 8.4 GHz containing the seismic data. At the ground station, the 34-m high gain antenna is used to receive the signal from the satellite. To overcome low signal-to-noise ratio (SNR), the modulation scheme employed in our mission is BPSK (Binary phase shift keying). For the error correction, the 1/2 turbo code is employed for error correction to achieve high channel capacity. The schematics of the decoder is shown in Figure 5 [2].

Leveraging NASA's knowledge of the Venusian atmosphere from the Mariner 2 and Mariner 5 missions in Figure 6, choosing a wavelength of 4.1cm (roughly 7.3GHz) will give us an optimum frequency [3].

Figures 7 and 8 detail the Downlink and Uplink, respectivly. Figure 7 shows that receiving a signal from the orbiter results in 102dB of gain. Figure 8 shows that transmitting to the orbiter gives us an astounding 277MW of EIRP, or 180.66dB of gain.

Communications between VIRGILs will be handled by in an ad-hoc way. Each VIRGIL will have a monopole antenna for communicating with other VIRGILs. Realizing that our data packets are small - each VIRGIL will only generate 3 bytes of data per seismic event - we can choose a lower frequency and have less path loss. From Figure 9, the VIRGILs will have an estimated loss factor of 5.5. From this loss factor, we can calculate that the VIRGILs will have a path loss of roughly 5.5/λ^2 dB/km, which for our which for our 300MHz frequency works out to be 5.5dB/km [1]. The SiCLOPS mission can draw the following conclusions:

• Transmit small, easily encodable/modulated data. The less the better. SiCLOPS' processor has a RS-232 output that will be modulated using BPSK, and each data packet will be 3bytes long (24 bits), plus our CRC.
• Employ forward error correction.

VIRGILs will use a the Low Energy Adaptive Clustering Hierarchy (LEACH) Protocol for determining which VIRGIL will assume the role of uplink to the orbiter. LEACH is a variant of a TDMA protocol in which the VIRGILs will decide, based on a 'ping' from the orbiter and reading from their RSSI, which VIRGIL will assume the role of uplink [4]. Because of the extreme attenuation of the Venusian atmosphere (around 3100dB/km), SiCLOPS will set a minimum received value that a VIRGIL will have to receive on it's RSSI to assume the role of uplink. A VIRGIL that is even a few meters away from the uplink VIRGIL will have attenuation of 50-60dB more than the uplink VIRGIL, meaning only one VIRGIL will ever be uplink at a time.

Using an ad-hoc network of VIRGILs allows the SiCLOPS program a few extra degrees of freedom not normally associated with planetary exploration:

• SiCLOPS can leverage fully documented, well supported, and most importantly, reliable routing algorithms to ensure that the VIRGIL in communication with the orbital satellite will always have the latest and most up-to-date activity from the VIRGILs. Dijkstra's Algorithm is the optimum choice for this, as it has the shortest path tree to a particular point. VIRGILs are most concerned with the shortest PHYSICAL distance and will be able to calculate paths to the transmitting VIRGIL based on Received Signal Strength Indicators (RSSI).
• The loss of any finite number of VIRGILs will not affect the outcome of the mission. Although possibly making a suboptimal configuration of VIRGILs, losing a VIRGIL will not be a mission failure as it would be on a single rover type mission.

The VIRGIL will use scheme for RF communication. This will enable us to employ BPSK. From VIRGIL to VIRGIL, we can expect roughly 24kW EIRP transmitted (205dB gain), with a receive gain of 85dB. Figure 10 shows our receive gain of 85dB (9dBi antenna + 76dB amplification). Figure 11 shows our transmit chain with 120.66B of gain.

Data Compression Seismic data has different characteristics from the standard communicated data of video, pictures, etc. Seismic measurements are typically composed of a large amount of low frequency content, and the statistics of previously measured seismic occurrences on Earth can be leveraged to maximize the compression ratio [5,6]. The Steim 2 compression method is frequently used for seismic data compression due to its low complexity in implementation which lends itself towards use at the sensors in a distributed sensor system [7].

Steim 2 is a first difference technique where frames of data are made by giving the absolute value at the first point, and expressing the next data points as the difference between the given value and the following data points. The method expresses small differences using fewer bits than larger differences. For example, one byte is used to represent differences between -128 and +127, and two bytes are used to represent differences between -32768 and +32767. Bits in the header are used to indicate how many differences are stored in each word which allows the decoder to determine how many data points are in each word in the frame despite the variable bit length representation of data value differences. With its overhead and data frame size of 512 bits, the Steim 2's best possible compression ratio is 6.74 with the better compression ratios during periods of low seismic activity [7].

In evaluating the compression ratio for seismic data, the testing can be separated into time periods of calm and periods of seismic activity. Due to the steadiness of calm periods, the differences are smaller, and the Steim 2 compression technique is more effective. This means the compression ratio is dependent on the input data. The actual compression ratio for measured seismic data on Earth is around 2 for periods of seismic activity and 5 for calm periods [5]. The seismic data characteristics will be roughly the same on Venus, but Venus is assumed to be calmer than Earth. So, Steim 2 compression is conservatively expected to provide an average compression ratio of 4, although the sensor communication system will have to be designed to operate with a compression ratio as low as 2 for times of high seismic activity.

Cyclic Redundancy Check: Transmitter calculates R based on a previously determined n+1 bit G and the following equation

Transmitter sends the following packet

Receiver divides received [D R] by G, and an outcome of 0 indicates error free transmission [9].

We elected to handle bit errors by packet retransmission instead of forward error correction due to the limited computational ability and sufficient memory on the chosen processing systems. Cyclic redundancy check will be used for error detection since it is easily implemented in hardware which will help with the limited computational resources of the sensor. For our maximum packet length of 240000, an r of 64 was chosen as being appropriately large without introducing too many redundant bits.