European data relay satellite system

[PainterGuy]

Hi,

I was watching the following video.

So, a low Earth orbit continuously transmits data to a geosynchronous satellite via a laser link. The geosynchronous satellite relays the data to a ground station on Earth via a radio link. In case of European Data Relay System (EDRS) laser communication is 30 times faster than the radio waves.

Note that low Earth orbit is normally at an altitude of less than 1000 km but could be as low as 160 km above Earth. Also, a geosynchronous satellite occupy the orbit at almost 36,000 km above earth.

Now the question is a chain is as strong as the weakest link. If the geosynchronous satellite is still using radio waves to transmit the data to a ground station, the radio link is still the bottleneck. For example, 1000 MB is being received by geosynchronous satellite every second via laser link but it can only 33 MB of data back to Earth via a radio link. Could you please help me to understand how it drastically increase the data speed? I understand that it's an improvement since continuous data transmission can take place to a ground station.

Also, there is a lot of space debris, doesn't it disrupt the functioning of EDRS because material obstacle is one of the major problems with laser communication? How this problem is overcome?

Source: https://earthhow.com/space-junk/

Helpful link:
https://en.wikipedia.org/wiki/European_Data_Relay_System

 

[Borek]

Debris is probably the least important factor here - these are small pieces, traveling very fast and in a waste volume, they block LOS very rarely and for several microseconds at most (think 10 cm object traveling at 10 km/s).

 

[Baluncore]

And: the energy in the laser beam has diverged sufficiently to go around the majority of those obstacles before entering the optical aperture of the receiver.

PS
I would expect a minimum divergence of about 1 in a million.
At 1000 km the beam would be at least 1 metre diameter.

 

[tech99]

Hi,

I was watching the following video.

So, a low Earth orbit continuously transmits data to a geosynchronous satellite via a laser link. The geosynchronous satellite relays the data to a ground station on Earth via a radio link. In case of European Data Relay System (EDRS) laser communication is 30 times faster than the radio waves.

Note that low Earth orbit is normally at an altitude of less than 1000 km but could be as low as 160 km above Earth. Also, a geosynchronous satellite occupy the orbit at almost 36,000 km above earth.

Now the question is a chain is as strong as the weakest link. If the geosynchronous satellite is still using radio waves to transmit the data to a ground station, the radio link is still the bottleneck. For example, 1000 MB is being received by geosynchronous satellite every second via laser link but it can only 33 MB of data back to Earth via a radio link. Could you please help me to understand how it drastically increase the data speed? I understand that it's an improvement since continuous data transmission can take place to a ground station.

Also, there is a lot of space debris, doesn't it disrupt the functioning of EDRS because material obstacle is one of the major problems with laser communication? How this problem is overcome?

View attachment 290009
Source: https://earthhow.com/space-junk/View attachment 290010Helpful link:
https://en.wikipedia.org/wiki/European_Data_Relay_System

It looks as if the system is untended for relatively low speed data, not Internet to every home. You say the microwave link to/from the LEO operates at 33Mbit/s (presumably not Mbytes/sec). So why have a very fast laser link to the Geostationary satellite?

Here are a few reasons. First of all it avoids the radio licensing issue, as microwave spectrum to the geosynchronous orbit is already used up. This means that the satellites can be positioned anywhere without consideration of radio interference/co-existence. Second, the laser does not require a very big dish and a lot of transmitter power onboard the Leo. This is because at microwave frequencies the geo satellite is looking at a hot earth, so the uplink needs lots of ERP to be heard. Thirdly, the geo satellite will presumably operate to many Leos. so we might imagine the data from each being sent in very short high speed bursts.

 

[PainterGuy]

You say the microwave link to/from the LEO operates at 33Mbit/s (presumably not Mbytes/sec). So why have a very fast laser link to the Geostationary satellite?

I should have been more clear about it. I just made up those numbers.

 

[sophiecentaur]

So why have a very fast laser link to the Geostationary satellite?

It could be that the GS satellite is serving a lot of LEO satellites and they are using a common frequency. Hence they would be time multiplexed. That's a standard approach in many comms systems.
Edit: I re-read this and I am having second thoughts. I think it could be just that the LEO-GSO link to each satellite can be in short bursts at high data rate and the GSO station can arrange the data from all of them and perhaps prioritise the data. There is also the fact that each optical link is via the same laser and there will be dead time whilst the laser moves and settles its beam from source to source.