About Swarm Satellites
The Swarm Satellite network is a constellation of small pico-satellites which operate at Low Earth Orbit (LEO). LEO is the spherical atmosphere with an altitude of up to 2000 kilometres above the Earth surface. Swarm satellite typically operate at approx. 550 kms above the earth.
Unlike the large and higher-bandwidth satellites, such as Iridium are situated on polar orbits around the Earth. This means they orbit the earth up to 11 times per day. Satellites like Iridium are geostationary (in a fixed position above the Earth).
The Swarm satellites are generally but not all, on a North-South orbit and these satellite are launched in different locations above the Earth to attempt to cover more of the Earth's surface. You can see in the picture below an example of one of the Swarm satellite's path around the poles.
Swarm Window Lengths
Due to their low earth orbit, each satellite can only cover a small area of the Earth at a time. These are known as Swarm Windows. Below you can see a Swarm satellite and its coverage area.
The above picture generally means that any Swarm modem in the white circle can initiate a Swarm connection with that satellite.
However, it also means, the devices closer to the centre of the white circle have a longer window in which to get a connection. Below is an example of the effect of the devices distance from the centre of the circle.
Distance from centre and elevation considerations
Comparatively to a device in the middle, when a device is near to the edge (red line above) - Not only does it have a shorter window but also, the position in the sky concerning your device is a lot closer to the horizon.
When devices are lower in the sky as compared to higher in the sky, natural and man made obstructions near and around the device contribute to whether or not a successful connection can be made.
The below diagram shows that if a Swarm satellite is only near 20 degrees above the horizon and you're near a building, you won't receive a connection. The satellite requires a higher elevation to make the connection.
Checking Swarm Windows
Swarm provide a tool to check the the Swarm passes for a given location on the Earth.
The tool can be found here: Swarm Pass Checker
Below is an example Swarm Window.
The above image indicates that the window is available to that location between those two times. It mentions the duration and also shows the elevation from the horizon. Also, it indicates the location specified on Earth is close to the centre of the Swarm white circle.
The above window is a lot shorter and only 17 degrees above the horizon. Anything blocking the path from the satellite (building, trees, etc.) will interrupt the formation of a successful connection.
Swarm Windows and Message Queueing
ModuSense IIoT Gateway is by default configured to a sample interval (default is 15 minutes). So, every 15 minutes, the IIoT Gateway will collect measurements from connected devices. The IIoT Gateway then packages these measurements together. If the message size of the measurements collected is over 192 bytes, the IIoT Gateway will split the message into smaller messages. Once these messages are collected, they are placed on the Swarm tile by the IIoT Gateway firmware for transmission.
When there are larger gaps between the window period - the Swarm tile will queue up these messages to be sent over in the next 15 minutes.
If the IIoT Gateway makes a connection during a small length window, it may not have enough time to transmit all the messages across. If the IIoT Gateway detects any incomplete messages, it will re-queue the messages for the next window back on the tile.
You can view the incoming Swarm messages, transmit time and the message time here: Receiving Swarm messages via the Modusense Portal – ModuSense