Why do AST's Space Mobile satellites have antennas much larger than Starlink's satellites, whereas they both aim at providing network coverage anywhere on earth?
Physics. "ultra-powerful SpaceMobile satellites will provide 2G/3G/4G LTE/5G and NB-IoT connectivity (directly) to standard mobile phones and IoT devices." (sources below)
Starlink satellites transmit to essentially ground stations, small dish antennas with motors that move around and track one satellite at a time.
Whereas these are "up to 10 times bigger than the norm because they are, essentially, cell towers in space. They are fitted with 900 square metres phased-array antennas and moving the satellites around will be complex in the extreme. NASA says..." (source below)
Complex DSP to the antenna will likely provide narrow beam-forming to individual or clusters of cellular devices on the ground. 30 meters by 30 meters at 2 GHz (λ=0.15 meters) means beams can have an angular width of roughly say 0.3 degrees. At 720 km (near the A-train) this is a few kilometers on earth, roughly the same size of coverage of an actual rural cell tower.
The concept seems to have ruffled some feathers.
Objections (see below) are that with large and likely delicate 900 square meter antennas these will be less maneuverable, requiring more gentle accelerations, and have much larger cross-sections than "normal" spacecraft at this altitude, and so pose more of a threat to other spacecraft and to the production of space debris if something happens.
To power these "cell towers in space" which will be 700 to 1000 km away from their targets within each satellite's instantaneous footprint, they will need a substantial amount of electrical power, which means big solar panels like the ones used in GEO, that will also make nasty space debris if they hit something in LEO.
Think ViaSat's 18 kW solar array - largest ever for a commercial telecom satellite? but in this case a thousand or so of these in LEO!
Source
From https://ast-science.com/spacemobile/faqs/
Q: How is SpaceMobile different from other LEO satellite ventures?
A: We are not aware of any other company that can deliver broadband from space to the mobile phones everyone already has. Other LEO satellites – current and planned – require costly and often network-specific satellite phones, terminals or antennas. With SpaceMobile, we anticipate that wireless subscribers will be able to connect to SpaceMobile with their current mobile phone or IoT device.
Q: How does the technology work?
A: With over 750 patent claims, the technology is highly proprietary, and exactly how it works cannot be disclosed. We can say that our engineers have designed an entirely new form factor and deployment method that we anticipate will significantly reduce the time and costs associated with manufacturing, launching and operating satellites.
Leveraging proven technologies, ultra-powerful SpaceMobile satellites will provide 2G/3G/4G LTE/5G and NB-IoT connectivity to standard mobile phones and IoT devices.
Our team also has worked – and continues to work – closely with mobile network operators and regulators to ensure compatibility with today’s wireless networks.
Q: Do users need a satellite phone or special antenna?
A: No, SpaceMobile is being designed to work with standard mobile phones. No separate or specialized satellite hardware will be required.
From Telecom TV's New 243-strong satellite system will bring 4G and 5G to equatorial regions:
That said, AST is being extremely coy on exactly how the new technology works, saying only that "our engineers have designed an entirely new form factor and deployment method that significantly reduce the time and costs associated with manufacturing, launching and operating satellites. Leveraging proven technologies, ultra-powerful SpaceMobile satellites will provide 2G/3G/4G LTE/5G and NB-IoT connectivity to standard mobile phones and IoT devices." Narrowband Internet of Things (NB-IoT) is a low power wide area network radio technology standard developed by 3GPP to enable a wide range of cellular devices and services.
The SpaceMobile satellites are unusually large for LEO systems, they weigh more than a tonne each and are up to 10 times bigger than the norm because they are, essentially, cell towers in space. They are fitted with 900 square metres phased-array antennas and moving the satellites around will be complex in the extreme. NASA says, "For the completed constellation of 243 satellites, one can expect 1,500 mitigation actions per year and perhaps 15,000 planning activities. This would equate to four manoeuvres and 40 active planning activities on any given day."
and
The constellation will orbit at a height of 720 kilometres but the company is keeping schtum about whose and which rockets will be used to loft the satellites into space, and the satellites are big. So big in fact that a couple of months ago the US space agency NASA, having looked at SpaceMobile's plans, complained the "extremely large satellites when in orbit will engender an unacceptably high risk of a catastrophic debris-producing collision."
The altitude and orbits of the SpaceMobile satellites are uncomfortably close to the "A-Train" line of 10 earth-science monitoring satellites operated by NASA, the US Geological Survey and scientific bodies in both France and Japan. A NASA statement says. "Historical experience with the A-Train constellation has shown that this particular region of space tends to produce a large number of conjunctions between space objects". In other words, a series of accidents waiting to happen and a massive threat that will need to be completely mitigated if the new constellation is ever to fulfil its undoubted potential. Expect serious regulatory oversight.
