Communication System Selection process
Table of Contents
Communication System Selection process
The four main sub-functions that were identified from the functional decomposition for the communication were, telemeter location, transmit video and sound to ground, tx sensor data to ground, and Relay commands from the ground to the rest of the system.
Various parts that are to be needed to meet the functionality in the communication system were benchmarked against each other. Previous high altitude balloons were researched for their functionality to determine how much could be reused and how much new functionality needed to be developed.
Past Communication System Benchmark
2m transceiver Benchmark
In the next phase, more aspects of the previous design will be tested to see if existing parts are in working order, or need to be replaced. From the benchmarking analysis, it was determined that the Tracksoar V1 APRS should be reused if possible since it did work previously and it is an expensive part. The same GPS(Ublox M8Q) that was used previously would be a good option as well for similar reasons. The Baofeng UV5RA will be a good option for the 2m transceiver. A similar model would work as well if we needed to purchase a different transceiver.
A link to the full document can be found here: Systems Level Design Documents/Comms/Comms_Benchmarking.xlsx
Sound RecordingA simple way to record sounds from the environment would be to use a microphone shown below that would integrate with a RP0 easily.
Feasibility: Prototyping, Analysis, Simulation
Transmission PowerOur balloon at most will travel up to 120,000ft in the atmosphere(~23 miles, 36.5km). The command uplink, video and and telemetry downlink must therefore provide enough power to maintain signal contact at all times.
For data and telemetry, we will be using the 2m frequency band which is a common Amateur radio band. This band uses a frequency of (144-148 MHz). We have a 2m Yagi antenna in the METEOR Lab (brand: C3i, model: FO12-144). It has a gain of 12.6 dB. The 2m antenna that we will most likely be using on our HAB has a gain of 1.5dB. Using the below equation, the free space loss for a 144MHz signal is approximately -106.9 dB, which is 9dB less than a 440MHz signal. The total gain in dB to achieve a 40db signal to noise ratio using the chart below is 12.6dB+1.5dB+9dB = 23.1dB. We will need a ~2W transmitter to achieve a 23 mile transmission.
Transmission: For data and telemetry, we will be using the 23cm frequency band. This band corresponds to a frequency of 1.28Ghz. The antenna on the bladeRF that will transmit the captured video has a gain of 6 dBm. The output of the BladeRF board is sent to an amplifier to transmit the signal at 0 dB (1 Watt). The video signal will then be output from a Diamond NR124 antenna that has a gain of 8dB.
Free Space Loss: Using the below equation, the free space loss at 120,000ft will be -126dB, which is nearly 20dB more than a 144MHz transmission.
Reception: The antenna in the METEOR Lab has a gain of 14.5 dB. The preamplifier that connects the antenna to the cable has a minimum gain of 15 dB. The cable loss will be approximately -7dB including the connectors.
The overall gain will be 8.4dB(tx) - 126dB(FSPL) + 14.5dB(Ground Antenna) + 15dB(preamp) - 7dB(cable loss) = -95dB
The minimum power needed to receive a good video signal is -65dBm or -95dB which is the video power loss assuming the balloon is directly overhead and at its maximum altitude. As the balloon drifts away from Rochester, the loss may increase enough to make the signal not strong enough to be received as viewable video.
Concept Development and Morphological Chart
Concept options were considered for each of the functionality needs of the system. Many of the functions that needed to be met did not have that many realistic options due to the environment that the balloon will operate in and the budget and time we have to work with.
A link to the full document can be found here: Systems Level Design Documents/Comms/Comms_Morph_Chart.xlsx
Concept SelectionThree different concepts called A, B, and C were developed. Only the functions that would possibly work were considered. Concept A was the concept that used the most recent solutions to all the needed functionality that has been developed by past RIT students. Concept B is similar except that it used the 70cm (144MHz) amateur radio band to transmit video and data. Concept C is similar to A except that it doesn't use APRS and also used the 70cm (144MHz) amateur radio band to transmitt data.
Using a Pugh chart, the three different options were weighed against each other. The selection criteria that were used were feasibility, robustness, power consumption, Hours to implement, cost and manufacturability.
Concept A is the concept that will be used at this point. This is a good choice because it builds upon previous designs and still delivers all the functionality desired at the lowest cost. A board that will receive and distribute the commands will most likely need to be built. The video processing/transmitting system will consist of the system that Steven LaPlant developed in the summer of 2018.
A link to the full document can be found here: Systems Level Design Documents/Comms/Comms_Concept_Selection_and_Pugh_Chart.xlsx
Designs and Flowcharts
The portion of the communication system that will transmit and receive on the 2m wavelength is shown below.
The portion of the communication system that will transmit digital video is shown below.
The rest of the Systems Design page will describe how this system will integrate with the other systems on SF1.