5 ms·

The spot size is proportional to the phased array antenna size, right? Could SpaceX just keep scaling up their phased array antennas on the satellites as Starsh

by rlt 4y ago

The spot size is proportional to the phased array antenna size, right? Could SpaceX just keep scaling up their phased array antennas on the satellites as Starship reduces launch cost?

paranoidrobot 4y ago
There's multiple physics problems involved. Spot size is one physics problem. Antenna size is a different one. They're orbiting low enough that atmospheric effects become a real issue. See the deployment in February[1] where they lost ~40 of the 49 satellites in a launch batch, because a Geomagnetic storm increased the atmospheric density at their initial parking orbit, overwhelming the ability for their control system to maintain orbit. So, yeah, they could then scale up the control system, too - but that makes them even heavier, increases fuel expenditure, reduces lifetime, etc. [1] https://www.space.com/spacex-starlink-satellites-lost-geomagnetic-storm https://www.space.com/spacex-starlink-satellites-lost-geomag...
- londons_explore 4y ago
  The antenna array is parallel to the ground, so scaling it up has a minimal impact on drag. It increases mass, sure, but an increased mass to drag area ratio is usually considered a good thing (you'll be needing a better rocket to launch it tho).
  - paranoidrobot 4y ago
    It's not always perfectly edge on, and it requires a control system to keep it oriented that way. This is why from the linked article: > The satellites were then placed in a protective "safe mode" and commanded to fly edge-on "like a sheet of paper" to minimize drag effects as the company worked with the U.S. Space Force and the company LeoLabs to track them with ground-based radar, it added.
  - SEJeff 4y ago
    The v2 satellites they're testing out to launch on starship are in fact scaled up. The v1.5 they're launching now are larger than the originals which is why they've not launched 60 starlink satellites on a single falcon 9 launch in sometime.
- gridspy 4y ago
  I thought they start at about 250 km after launch and then boost up to around 400 km. They don't have very powerful thrusters or gyros so they couldn't boost faster then they fell out of orbit. It seems that they are far safer once they have boosted up into operational orbit - that's why only the recently launched Starlinks fell back to earth.
rkangel 4y ago
Phased array antennas do not put all of the power into a spot and then no power elsewhere. It's all about the ratios and the distribution. This example is from the wikipedia article on directional antennas: https://en.wikipedia.org/wiki/Directional_antenna#/media/File:Patch_antenna_pattern.gif https://en.wikipedia.org/wiki/Directional_antenna#/media/Fil... This is the standard way of showing the performance of antennas - it shows the varying amount of power that is sent out from the antenna in different directions. Note that (a) it doesn't roll off immediately outside the target zone (usually the 3dB threshold as shown here) and (b) there is measurable power in completely off-axis directions. In fact the tighter you beam steer, the more lobes you get in weird places. Both of these mean that you can't just parcel up physical space into perfect beam spots and just keep adding satellites. There are all sorts of things you can do to help as you go up, but it gets harder and harder.
- Retric 4y ago
  That's true, but assuming maximum possible density both the sender and receiver should be using directional antenna on top of this the closest satellite gets the strongest signal simply due to distance. That distance effect becomes less meaningful as the density increases but stays relevant for practical networks.
  - rkangel 4y ago
    Both ends of the starling link are already using directional antennas. The difference in power die to distance is already pretty irrelevant - the satellite is 400km up and we're talking about offsets of 10km on the ground. The 400km part of that is far more dominant making the power difference to the 'next satellite over' be very small.
    - Retric 4y ago
      “Both ends of the starlink are already using directional antennas” Sure, but it’s critical to understand the associated benefits when doing this kind of analysis. If you’re talking average 10km separation on the ground that would mean stupid high constilation densities. A lower densities it’s more meaningful. PS: I got to 1 billion customers assuming different hardware and more satellites not just more satellites. The only way 100,000 customers “works” is a none viable and lower bandwidth per customer business model, but the goal was to illustrate a point. Space X is aiming for an economic sweet spot, not the outer edge of what’s technically possible.

by rlt 4y ago

The spot size is proportional to the phased array antenna size, right? Could SpaceX just keep scaling up their phased array antennas on the satellites as Starship reduces launch cost?

paranoidrobot 4y ago

There's multiple physics problems involved. Spot size is one physics problem. Antenna size is a different one. They're orbiting low enough that atmospheric effects become a real issue. See the deployment in February[1] where they lost ~40 of the 49 satellites in a launch batch, because a Geomagnetic storm increased the atmospheric density at their initial parking orbit, overwhelming the ability for their control system to maintain orbit. So, yeah, they could then scale up the control system, too - but that makes them even heavier, increases fuel expenditure, reduces lifetime, etc. [1] https://www.space.com/spacex-starlink-satellites-lost-geomagnetic-storm https://www.space.com/spacex-starlink-satellites-lost-geomag...

londons_explore 4y ago
The antenna array is parallel to the ground, so scaling it up has a minimal impact on drag. It increases mass, sure, but an increased mass to drag area ratio is usually considered a good thing (you'll be needing a better rocket to launch it tho).
- paranoidrobot 4y ago
  It's not always perfectly edge on, and it requires a control system to keep it oriented that way. This is why from the linked article: > The satellites were then placed in a protective "safe mode" and commanded to fly edge-on "like a sheet of paper" to minimize drag effects as the company worked with the U.S. Space Force and the company LeoLabs to track them with ground-based radar, it added.
- SEJeff 4y ago
  The v2 satellites they're testing out to launch on starship are in fact scaled up. The v1.5 they're launching now are larger than the originals which is why they've not launched 60 starlink satellites on a single falcon 9 launch in sometime.
gridspy 4y ago
I thought they start at about 250 km after launch and then boost up to around 400 km. They don't have very powerful thrusters or gyros so they couldn't boost faster then they fell out of orbit. It seems that they are far safer once they have boosted up into operational orbit - that's why only the recently launched Starlinks fell back to earth.

rkangel 4y ago

Phased array antennas do not put all of the power into a spot and then no power elsewhere. It's all about the ratios and the distribution. This example is from the wikipedia article on directional antennas: https://en.wikipedia.org/wiki/Directional_antenna#/media/File:Patch_antenna_pattern.gif https://en.wikipedia.org/wiki/Directional_antenna#/media/Fil... This is the standard way of showing the performance of antennas - it shows the varying amount of power that is sent out from the antenna in different directions. Note that (a) it doesn't roll off immediately outside the target zone (usually the 3dB threshold as shown here) and (b) there is measurable power in completely off-axis directions. In fact the tighter you beam steer, the more lobes you get in weird places. Both of these mean that you can't just parcel up physical space into perfect beam spots and just keep adding satellites. There are all sorts of things you can do to help as you go up, but it gets harder and harder.

Retric 4y ago
That's true, but assuming maximum possible density both the sender and receiver should be using directional antenna on top of this the closest satellite gets the strongest signal simply due to distance. That distance effect becomes less meaningful as the density increases but stays relevant for practical networks.
- rkangel 4y ago
  Both ends of the starling link are already using directional antennas. The difference in power die to distance is already pretty irrelevant - the satellite is 400km up and we're talking about offsets of 10km on the ground. The 400km part of that is far more dominant making the power difference to the 'next satellite over' be very small.
  - Retric 4y ago
    “Both ends of the starlink are already using directional antennas” Sure, but it’s critical to understand the associated benefits when doing this kind of analysis. If you’re talking average 10km separation on the ground that would mean stupid high constilation densities. A lower densities it’s more meaningful. PS: I got to 1 billion customers assuming different hardware and more satellites not just more satellites. The only way 100,000 customers “works” is a none viable and lower bandwidth per customer business model, but the goal was to illustrate a point. Space X is aiming for an economic sweet spot, not the outer edge of what’s technically possible.