Table of Contents
1. Introduction
1.1. What is a picosatellite?
1.2. What is a buoy?
1.3. What does it do?
1.4. Functional Specs.
2. Overall Block Diagram
2.1. PIC
2.2. Power System
2.3. Batteries
2.4. Solar Cells
2.5. Sensors
2.6. GPS
2.7. Flash memory
2.8. TNC
2.9. Radio
2.10. Mechanical
3. Preliminary Design
3.1. Creating Excel Model
3.1.1. DataàTX time à TX power à Power Reg
3.1.2. Assumptions
3.2. Sat Access Schemes
3.2.1. Intelligent Buoy
3.2.2. Beacon Buoy
3.2.3. Waiting Buoy
3.3. Conclusions
3.3.1. Based on model predictions
4. Subsystems
4.1. Communications
4.1.1. Alternatives / Tradeoffs
4.1.1.1. Antenna Theory
4.1.1.2. Radio
4.1.2. Feasibility
4.1.2.1. Testing
4.1.2.1.1. Procedure
4.1.2.1.2. Hardware
4.1.2.2. Data
4.1.2.3. Conclusion
4.1.3. Design Document
4.1.3.1. Explain
4.1.3.2. Schematic
4.2. Power
4.2.1. Alternatives / Tradeoffs
4.2.1.1. Batteries
4.2.1.1.1. Capacity, size, weight
4.2.1.1.2. Excel
4.2.1.2. Panels + Batteries
4.2.1.2.1. Theory (angles)
4.2.1.2.2. Excel
4.2.1.3. Conclusion
4.2.1.3.1. Choice (Justify using Excel numbers)
4.2.2. Feasibility
4.2.2.1. Testing
4.2.2.1.1. Battery Charging
4.2.2.1.2. Procedure / Hardware
4.2.2.1.3. Peak Power Point
4.2.2.2. Data
4.2.2.3. Conclusions
4.2.3. Design Document
4.2.3.1. Explain what we want / How it works?
4.2.3.2. Schematic
4.3. Sensors
4.3.1. Alt. & Tradeoffs
4.3.1.1. Air Temperature
4.3.1.2. Water Temperature
4.3.1.3. Pressure
4.3.1.4. Wind speed
4.3.1.5. Salinity
4.3.1.6. Humidity
4.3.2. Feasibility
4.3.2.1. Cost
4.3.2.2. Time
4.3.3. Design Documents
4.3.3.1. Testing
4.3.3.2. Schematic
4.4. Command & Data Handling
4.4.1. Alternatives & Tradeoffs
4.4.1.1. PIC -> 16 series vs. 18 series
4.4.1.2. AVR
4.4.2. Feasibility
4.4.2.1. Test Code
4.4.2.2. Results
4.4.2.3. Conclusion
4.4.3. Design Documents
4.4.3.1. Explanation
4.4.3.2. Schematic
4.4.3.3. Flowcharts
4.4.3.4. Code
4.5. Mechanical
4.5.1. Alternatives & Tradeoffs
4.5.1.1. Discus Buoy
4.5.1.2. Spar Buoy
4.5.2. Feasibility
4.5.2.1. Flotation
4.5.3. Design Documents
4.5.3.1. CAD drawing
- Prototype Construction
5.1. Breadboard
5.1.1. PIC
5.1.2. GPS
5.1.3. Radio / TNC Box
5.2. Buoy Structure
5.2.1. PVC
5.2.1.1. Payload Chamber
5.2.2. Mounting Solar Panels
5.2.3. Antenna ½ Wave
- Future Plans
6.1. Communications
6.1.1. Switch from handheld Radio to 3V custom low power radio.
6.1.2. Switch from pico packet TNC to TNC-X
6.2. Power
6.2.1. More efficient solar panels.
6.2.2. Research possibility of an entire system running at 3 volts
6.3. Sensors
6.3.1. Better design for salinity sensor
6.3.2. Design and build wind speed and direction sensor.
6.4. C&DH
6.4.1. Upgrade to PIC18F6722
6.4.2. Create interface for 3rd party devices.
6.5. Mechanical
6.5.1. Improve stability
6.5.1.1. Static and dynamics calculations
6.5.1.2. Placement of sensors
- Appendix
7.1. Website Wiki
7.2. Source Code
7.3. Data Sheets
1. Introduction
Scientists, meteorologists, mariners, offshore companies, and countless other groups depend on timely reports of current oceanic conditions. However, there are considerable challenges associated with gathering oceanic data. It is impractical and perhaps dangerous to station humans at remote offshore locations in potentially harsh weather conditions. Furthermore, unacceptable inefficiencies arise with manual data collection. Therefore, scientists and engineers prefer to design autonomous data collection devices with remote communication capabilities.
A remote data collection device possesses an inherent design challenge; namely, how to establish a reliable communication link. If an offshore device attempts to communicate with terrestrial stations, distance and line-of-sight limitations can pose significant obstacles to reliable communication.
To overcome these obstacles, engineers have chosen satellites for telecommunications that would be impractical or impossible with land-based solutions. Typically, for oceanic environmental data collection, scientists and engineers have designed floating or moored buoys which relay their data via satellites back to a ground station.
This design projects aims to create a similar satellite communications buoy with one main difference. This buoy will communicate with a Low Earth Orbiting (LEO) CubeSat; namely, the C.A.P.E. II (Cajun Advanced Picosatellite Experiment) satellite. With a LEO CubeSat, a constant communications link is impossible unlike geostationary satellites. However, as an advantage over geostationary satellites, a LEO CubeSat is much closer to Earth, which dramatically reduces the necessary signal strength for communications. Therefore, this project will combine the desirable features of satellite communications and low power requirements for future buoys and other remote applications.
1.1 What is a CubeSat?
A CubeSat is a miniaturized satellite with 10cm X 10cm X 10cm dimensions and a 1-kilogram weight. Generally, CubeSats are built with commercial off-the-shelf components for research purposes. CubeSats can be built and launched for approximately $60,000 to $80,000. Due to these satellites’ relative low costs, universities and companies find CubeSats to be an attractive means to enter the previously high-cost fields of space science and space exploration.
1.2 What is a weather buoy?
A weather buoy is a floating device that gathers environmental data while positioned in a large body of water, usually the ocean. Weather buoys monitor atmospheric parameters such as temperature, pressure, humidity, rainfall, wind speed, and wind direction. However, they can also monitor oceanic parameters such as salinity, wave height, and wave period. Weather buoys normally link with weather stations via satellite communications with geostationary satellites.
1.3 What is the purpose of the C.A.P.E. II weather buoy?
This design will demonstrate the usefulness of CubeSats in the area of data collection and forwarding. The buoy will gather environmental data and then upload it to the C.A.P.E. II satellite as it passes over the buoy’s location. This design’s success will create a new arena for opportunities and applications within the CubeSat community. Futhermore, future University of Louisiana at Lafayette senior design teams could develop other terrestrial applications with satellite link functionality.
1.4 Functional Specifications
Description:
This project is to design and build an expendable weather buoy with satellite communication abilities. The weather buoy will collect environmental conditions, store them on onboard memory, and relay them to the CAPE II satellite. This buoy system will ether have batteries that will keep the buoy active for 3 years or have a combination of solar panels and batteries to achieve a three year life. The cost of this buoy should be less that $150 including a GPS receiver. This buoy will have a data interface such that other devices may store data and transmit data to the satellite.
Specifications:
(1) Compatibility with the Naval Academy buoy.
(2) Contain an on board GPS receiver.
(3) Report its location.
(4) Collect water temperature.
(5) Time stamp all data samples.
(6) Memory storage > 100 Meg bits.
(7) Transmitter 144 MHz and or 435 MHz.
(8) Transmitter power sufficient to close a link to CAPE II Cubesat.
(9) Satellite access scheme TBD.
(10) Life of the buoy three years.
(11) Cost of the buoy $150.
(12) Data interface TBD probably I2C.
Deliverables:
(1) Produce one operational and fully tested buoy.
(2) Documents that the buoy has been tested with on an orbit satellites.
(3) Likely to be the International Space Station.
(4) A survey of likely originations that might want to collect data.
(5) Software for the ground station to feed the data collected to the
Internet.
(6) Software that will organize the data in to a spreadsheet interface.
Figure 1. Functional Specifications Diagram
Power Options
Inputs Outputs
Solar/Batteries
(1) Environmental Data
(2) Location
(3) Time
(4) Sunlight
(5) Satellite Commands
Environment
Large Body Of Water
Potentially Harsh Weather
Final Products
1 Operational and Fully-Tested Buoy
Documentation On Testing With An Orbiting Satellite (e.g. International Space Station)
Ground Station Software To Upload Buoy Data To Internet
Software To Organize Buoy Data Into A Spreadsheet Format
2. Overall Block Diagram
Our solution is to design a system which will utilize a microcontroller to manage the gathering and communication of environmental data. The system will awaken every hour and gather environmental data via a collection of sensors. Then, a GPS unit will provide time, date, and location information to accompany the environmental data. Next, the system will format and store the current batch of data. Following this, the system will broadcast its most recent data to the satellite when the satellite passes over its location. In order to broadcast data, a terminal node controller (TNC) will format the data into AX.25 packets and then modulate the packets into an analog signal. Finally, the radio will transmit the modulated signal.
See the overall block diagram in Figure 2.
Figure 2. Buoy Functional Block Diagram
2. Overall Block Diagram
2.1 PIC Block
This block represents the microcontroller. This microcontroller will most likely belong to the Microchip PIC18 family. The microcontroller will control all other buoy subsystems. The microcontroller will retrieve data from the sensor array and the GPS unit. After formatting and storing the data, the microcontroller will send the data to the communication subsystem for transmission. The microcontroller will also accept commands from the satellite which may alter its task schedule or task list.
2.2 Power System Block
The power system directs power from the batteries (and possibly solar cells) to the rest of the system. If solar cells are used, the power system will handle the recharging of the batteries. The power system will step down voltage to the appropriate level for each component. Also, the power system will protect against destructive currents.
2.3 Batteries Block
The batteries will store power for the system.
2.4 Solar Cells Block
Solar cells will absorb solar energy and convert it into electrical energy. Then, the power system will store that electrical energy in the batteries.
2.5 Sensors Block
Sensors will measure analog environmental values and convert these values into electrical signals which the microcontroller will read.
2.6 GPS Block
The GPS (Global Positioning System) receiver compares signals from several GPS satellites to determine its location, time, speed, and direction. The GPS receiver will forward this information to the microcontroller. Using this information, the microcontroller will associate data with specific time and location details.
2.7 Flash Memory Block
Flash memory will provide long-term storage for environmental data.
2.8 TNC Block
As part of the communication subsystem, the TNC formats digital data and then converts the data into an analog signal. Likewise, when receiving a signal from the radio, the TNC will demodulate the signal and transmit the digital data to the microcontroller.
2.9 Radio Block
The radio and its antenna are key components of the communication subsystem. The radio will transmit a modulated signal from the TNC over its antenna. Also, the radio will receive signals from its antenna and direct them to the TNC.
2.10 Mechanical Block
This block represents the physical aspects of the buoy, such as its shape, size, and weight. We will address the buoy mechanical issues later in the design process.
3. Preliminary Design
After the block diagram was created, the design process continued with a quantitative buoy system model which included a data budget, a power budget, and a link budget. Using Microsoft Excel, this system model was drafted as a spreadsheet with preset functions that expressed the mathematical relations between design parameters. As potential design choices were typed in, the spreadsheet buoy model would instantly calculate the system-wide consequences of those choices. Therefore, the spreadsheet model was a powerful development tool which informed the team on the interrelationships of system characteristics.
3.1 Creating The Buoy System Model
The buoy is essentially a data collection and transmission system. The buoy system model began as a link budget to ensure that the buoy could indeed successfully transmit its data to the satellite. Once the necessary transmission power was calculated from the link budget, the buoy system model’s development continued with the creation of a data budget.
The data budget started with an assumption about the quantity of data gathered during each collection period. Then, assuming one data collection every hour, a total data amount per day was calculated.
Next, the buoy system model was expanded to include the underlying calculations and system requirements of a satellite access scheme. A satellite access scheme is a means by which the buoy will establish a reliable communications link with the satellite. The satellite access scheme will be discussed in further detail in a later section.
With a data budget, a link budget, and a communications schedule, the buoy system model was ready to add a power budget. The power budget first calculated the total system power needed from the two major draws of power, the communications system and the command and data handling system. Next, given our power system design, the power budget calculated the power generated from the buoy’s solar panel array. Finally, the power budget demonstrated that the buoy harnessed more solar power than it consumed each day.
Before summarizing the buoy system model conclusions, this report will outline the design and choice of the satellite access scheme.
3.2 Satellite Access Scheme Design
The satellite access scheme describes the algorithm for establishing contact between the buoy and the satellite. The satellite access scheme poses a crucial engineering challenge. With a drifting buoy and an orbiting satellite, when will a line-of-sight transmission opportunity become available? There are three major approaches to a satellite access solution:
(1) Intelligent Buoy: The buoy uses sophisticated software to predict satellite passes based on orbital data.
Advantages: The buoy will only transmit during actual satellite passes and thus will save power because it can power down until the predicted satellite pass.
Disadvantages: The buoy will require a more powerful microcontroller and a more complex operating system. These two features could drastically increase system power requirements. Also, the implementation of a complex operating system could introduce further engineering challenges and consequently delay project completion.
(2) Beacon Buoy: The buoy transmits on a set schedule such that, inevitably, it will establish a link with the satellite.
Advantages: A software implementation of this algorithm is relatively straightforward and therefore reliable.
Disadvantages: The buoy will consume more power as it broadcasts to “deaf ears” when the satellite is not in the line-of-sight.
(3) Waiting Buoy: The buoy remains in a receive mode as it waits for the satellite to make first contact. The satellite can then choose when it wishes to extract data from the buoy. The buoy functions as a passive data repository that the satellite can access at will.
Advantages: The software implementation of this algorithm is even simpler than the two previous algorithms. The buoy adopts a more passive, slave role with respect to the satellite. Perhaps more software development time could be spent adding programmable features to the buoy such that the satellite may program the buoy’s behavior. This would further designate the buoy as a slave device of the satellite.