Constellations of satellites are being proposed in large numbers; most of them are expected to be in orbit within the next decade. They will provide communication to unserved and underserved communities, enable global monitoring of Earth and enhance space observation.
In a Low Earth Orbit (LEO), a satellite completes about 14 or 15 polar orbits every day, but not always over the same area (this only happens in equatorial orbits). The rotation movement of the Earth makes the satellite complete a ‘sweep’ over the entire surface of the Earth, so each step is different from the previous one. At the same time, there are cycles of orbits that are repeated at regular intervals.
The small satellites launched to date are almost always placed in LEO, most of which fall into Sun-Synchronous and Non-Polar Inclined orbits. A Sun Synchronous orbit is the most popular for small satellites as it places the satellite in
constant sunlight. This is convenient for satellites that image earth in visible wavelengths and allows for simpler power subsystems. Additionally, LEO provides the lowest $/kg delivered to orbit.
When designing the Earth observation missions, the first step is to know the requirements of the project, with some basic questions:
- Which geographical areas will the constellation serve?
- How often will the satellites receive and transmit information? for example, a satellite can be expected to pass through the same region every seven days. In that case, the satellite would repeat this cycle of orbits every seven days, allowing the same area to be observed on a regular basis.
If we consider uses such as monitoring crops, snow melt, or desertification, if the orbit cycle is seven days, every week you will obtain accurate and comparable images of each location through which the satellite passes. For other services, such as port traffic control (to mention another Earth observation mission), which requires more frequent images, this waiting period is too long, so it would not be a viable mission with only one satellite.
For communication to take place, two parts are necessary. On the one hand, we have the space segment, with the satellites in orbit, but it is important to also take into account the ground segment. At this point, it is necessary to determine how many stations will be needed to collect information and where they will be located. This decision will depend on the type of service to be provided
When selecting any piece of technology for your mission it is important to be aware of costs, lead times, integration and testing requirements, as well as the physical requirements of your system.
In addition, here are some of the key performance criteria to consider when assessing which payload is right for your needs:
- Spatial resolution – a measure of the smallest object that can be resolved by the optical payload.
- Spectral resolution – the number and width of spectral bands that the sensor can collect from reflected radiance.
- Swath – the area imaged by the optical payload.
- Radiometric resolution – this indicates how much information is in a pixel and is expressed in units of bits.
- SWaP factor – the size, weight and power of the satellite or sub-system.