Can 2.4GHz even penetrate the atmosphere efficiently?


#1

can 2.4GHz even penetrate the atmosphere efficiently?
repost from a user on reddit, so this question doesn’t get buried: http://www.reddit.com/r/technology/comments/1wqgmh/outernet_wifi_for_the_world_from_outer_space/cf4uwoq

Assume cost isn’t an issue, and you get clearance to deploy and you somehow solve the problem of providing high power output from the LEO AP long-term, and don’t worry about overloading a ‘channel’ with traffic, noise, etc…

Even if you get a 50W output at 2.4Ghz (50dB) the FSPL even through JUST our atmosphere (being generous let’s say 15km), there will be nothing left, no signal, from a 50W/50dB output from the satellite to the ground. That’s NOTHING to say of the inability of a low power phone or laptop to be able to transmit back up. Even using high gain atennas, there’s just nothing left of that spectrum to use at 500km altitude. And -90 is a pipe dream at the 2.4Ghz spectrum, and while 5Ghz usually is ‘cleaner’, the propagation characteristics are half as much as the 2.4GHz (generally speaking). Finally, latency will kill/diminish any real use when you are using CSMA/CA for collision avoidance of the 802.11spec.

I applaud the effort, but it’s just not tenable IMHO without some specialized equipment on the ground, as well as overcoming some significant hurdles on the LEO AP side of the house.


#2

I can’t get into details just yet, but I can clarify a few issues. Since the goal is to provide a broadcast data service, an uplink from a handset will not be necessary. Also, as a point of reference, the world distance record for a two-way wifi link is 304km, and this is with commercial-grade components.

Yes, 2.4GHz can penetrate, many cubesats in the past five years have used S-band for downlinks. 5.8GHz is an option, but far fewer existing wifi devices have 5.8GHz radios.


#3

Long range low power wifi connections usually needs a stable and fixed setup of the receiver and transmitter. Often directional antennas are used on both ends. A clear line of sight between the satellite and the recipient might also help.

I can imagine the average Joe wants to use his wifi device indoors, especially in heavy censored countries. I can also imagine he does not want to put an antenna on his roof, for everyone to see.

Getting the mini satellite in a stable orbit and the antenna motion compensated pointing to an earth region seems a daunting task to me. Also I wonder how you guys want to keep the satellite reliable powered for extended period of time, say 10 years. An “android cokecan in space” seems to consumes a lot of power over the years.

The later goal for 2 way wifi communication using standard commercial smartphones seems impossible to me since it’s designed for short range communication by default. It could work if additional equipment is added to the smartphone.


#4

Unless you “daddy” is a BIG man in high places,if you know what i mean…


#6

The short answer is yes. The equipment that I currently utilise operates on UHF freqs and is able to talk to geostationary satellites with 5w output coming from an omnidirectional antenna. Having a cell phone or tablet try to transmit back to the satellite would be useless for a few reasons (RF hazards, battery consumption, frequency interference, etc.) but it would be just fine for them to be used as a receiver, given there is a clear line of sight to the satellite. The best bet would be to stick with UHF. EHF requires an antenna with much more directional gain than a personal device could ever feasibly have.


#7

Apologies I don’t know about the proposed specification here, but for the rea`l skeptics who aren’t even sure if the general population can receive packet data from space:

Such technology exists and has been in use for many years.

Just look here: https://en.wikipedia.org/wiki/Vizada

Several kbps bi-directional data connections covering Earth from pole-to-pole. And this is the old stuff, the new stuff cites Mbps speeds. Again, two-way. This project really is technically feasible.


#8

Also, Globalstar has already been developing this technology for military use with UHF.


#9

With a big enough antenna in LEO, anything is possible.
Big antennas look very powerful, but they don’t use any extra electricity.
For that matter, since they are fundimentally bi-directional, they make the
little cellphone transmitter on the ground look that much more powerful if you
wanted an uplink.

2.4GHz 20dBi gain (x100 power) in a 1U CubeSat volume seems pretty easy. I have seen 16dBi units flown.

I would be much more worried about what you need to do with a cellphone’s
wifi radio.


#10

Can you point me to the 16dBi antenna you’ve seen on a 1U cubesat?


#11

Aeneas 3U cubesat, (Didn’t remember the name yesterday).

I think I remember it flying in late 2010. It was going to track shipping containers across the pacific or something. I haven’t really found to much information on it recentrly. I have a few more pics/Pdf, but can’t upload.

I think I could do better with a less complex deployment system and highter gain in the same 1U package based on some old work I did.

-G.


#12

Ah yes, I visited with the program manager last year. They did some amazing work with the deployable antenna, but if I recall from a follow up conversation, Aeneus lost communication shortly after it was deployed.

Canopus sells a 2-meter deployable, which produces around 20 dBi of gain (Aneus was around 12 dBi, if I recall correctly). I believe the latter had a price tag of about $100,000; hopefully the Canopus version is a little more affordable.

http://canopus-us.com/#!/portfolio-view/uhf-through-s-band-parabolic-dish-cdpa-xxx-x


#13

I would be worried about the canopus system. a 2m 2.4GHz dish should be doing much better than
only 20 dBi. Boresight it should be closer to 30. ? (or maybe My calcs are way off)
Similarly the Aeneas 0.5 m should similarly be almost 20. They advertised 18, I had heard 16, and
you say only 12 ? Dishes dont seem to be performing as well as hoped.
Perhaps it is a beamwidth issue.

I would also suggest something a little less draggy on orbit.

Son’t forget to be circularly polarized.

What about an array ? Seems like feeding for multiple lobes with different doppler shifts could
solve two problems at once.

But then again, I’m suppoed to be a rocket guy.
-G.


#15

So this is surprising that you considered 2.4Ghz. Good analysis of why this wouldn’t work here: http://www.wirelesswaffle.com/comments.php?y=14&m=03&entry=entry140301-020313

It seems like you need a Mhz frequency to make this work long range.

What frequency did you decide on in the end? What is the radio used in the lanterns you are now selling?