Radio Test
WindBorne’s Opportunity
The team is currently exploring radio solutions as a supplemental communication system to satellite communication for transmitting data from our global constellation. In the long run, our balloons will usually be closer to another balloon than to LEO, so a robust radio solution tailored to the specific needs of our platform would allow for significant reductions in power, cost, and complexity of comms hardware, as compared to what is needed for satellite comms. Further, a dedicated solution would improve reliability at low temperatures through the ability to use components specifically tailored to operation in extreme environments. In the nearterm, meanwhile, radio offers the possibility of relaying data from systems that are too small to equip satellite comms hardware, such as expendable deployable sensors that can be used to sample hurricanes, winter storms, or other extreme weather that is challenging to fly through for a balloon system. Further, even with far fewer balloons aloft now than in the coming years, radio can still serve as a failover communications method whenever a balloon is unable to connect to satellite, but is within line of sight to another that can.
Engineering Context
Our balloons travel with the atmospheric winds and can cover large distances quickly, so the radio solution has to be reliable over a significant range to communicate sensor data. Radio range is normally limited by the line or sight and obstructions within the Fresnel Zone. To achieve maximum range, antennas are typically placed high up on roofs in order to clear obstructions like buildings and hills, and tested in flat locations. Our balloons are a unique platform since they typically level off around 10km early in flight. This creates an ideal setup with an antenna at a significant height and minimal obstructions in the line of sight. Generally manufacturers do not provide guaranteed range numbers since the maximum range is physically difficult to validate and depends on setup. The particular test described below was designed to take advantage of the balloon platform and develop a better understanding of the limitations of a small radio module.
Testing Details
Setup
For this test a low cost, off-the-shelf radio module was integrated with the avionics. The balloon was configured as the transmitter, and a similar lightweight radio module was placed on the roof of the office as the receiver. Both transmitter and receiver used generic, omni-directional antennas for a more generalized setup, so there was no steering or pointing involved.
Results
During this test the receiver was able to detect messages from the balloon at a distance of ~320km (200mi), almost to Bakersfield!
Early Analysis
Even though significant range was achieved with a small radio module, the results do not match the theoretical range. Also the RSSI (received signal strength indicator) was low but still within the range of the receiver based on previous bench and field testing. So why did the test stop there?
Thinking back to the original limitations of radio range tests, the primary constraint is typically line of sight. And line of sight to the horizon is driven by the height of the antenna. At a 10 km antenna height the line of sight to the horizon is ~360 km (220 mi). The 320 km range achieved in testing is on a similar order to this limit, and taking into account the office is located in a valley the results seem to match up. Even with the balloon antenna at a high altitude (~10km), there is a limit given the curvature of the earth!
In the future, tests can keep pushing this limit by flying antennas on higher altitude flights or testing across two balloons.
Below is a plot of the RSSI over time as recorded by the office side receiver. The initial ascent of the balloon is clearly seen in the beginning where signal strength decreases rapidly. The purple markers show the distance of the balloon from the office. The spikes in the data are lost packets and tend to pick up around 100km into the test. This test used a very simple protocol for sending and receiving messages, in the future a more robust error checking and transmission protocol would be used to reduce these losses.
Radio Testing - Gallery
Below are some other photos that were part of this specific flight test, or were part of the validation testing leading up to this flight.
