Don't mean to alarm you – but NASA is about to pummel the planet with huge frikkin' space laser An orbital Earth sciences laser, NASA's ICEsat-2, is in the final stages of preparation ahead of next month's launch. With the bird about to fly, the space agency is touting the elevation accuracy Earth observation will get from the satellite's laser, the Advanced Topographic Laser Altimeter System (ATLAS). Firing 10,000 …
You know, I have one simple request. And that is to have sharks with frickin' laser beams attached to their heads! Now evidently my cycloptic colleague informs me that that cannot be done. Ah, would you remind me what I pay you people for, honestly? Throw me a bone here! What do we have?
Fantastic, triple green lasers firing 10K a second providing a mapping range as small as 70cm..... just the one question, just how accurate is this touted elevation accuracy, +- 1mm or 10Km.. unless I missed it (I’m in an liquorice and Heineken enhanced haze here), there’s perhaps absolutely no mention??
Raw return accuracy is 15cm for ice, about 1m on ground and 10m on steep ground.
One of the problems/features is that the measuring spot is quite big on the ground (100m) so you average out small scale variations but end up with low overall elevation accuracy compared to terrestrial LIDAR
So, now we know that the horizontal resolution is an impressive 70cm, but not so much about the vertical resolution.
Resolving round trip time to a billionth of a second gives a theoretical accuracy of 0.1mm, but when other factors such as the accuracy with which the orbital altitude is known, atmospheric interference, etc are included, the measurement accuracy will almost certainly not be +/- 0.1mm. Its disappointing that the expected error bars on this measurement weren't quoted.
"Resolving round trip time to a billionth of a second gives a theoretical accuracy of 0.1mm"
You sure?
Speed of light in air = 3.10^9 m/s
Divide by 1.10^9 = 3.1m
That's a round trip so can resolve to half that, so ~1.5m.
Having said that resolving to 1nS isn't much to boast about, is that correct?
Speed of light in air is 299,700 km/s (~3*10^8 m/s) very close to the speed of light in a vacuum so not sure where you get the 3.10^9 from (which according to my calculator is 26439.6)
Rule of thumb - light goes at about 1ft per nanosecond or more accurately 30cm/ns. Resolving to 1ns isn't too bad but better is easily achievable.
Measuring time interval to 0.1ps is easy, measuring a laser return isn't.
You are assuming you send out a perfect square wave and get back a perfect return. At this range you send out a pulse that is a few of micro-seconds wide and get back a tiny fraction of that energy smeared over the pulse width - you have no idea where (in time) in the outgoing pulse the peak return is. It is further complicated because the same outgoing pulse will reflect off different layers in the target and give multiple return pulses.
YAAC suggested, "...send out a perfect square wave..."
Hopefully they make use of a more-clever modulation technique than just square waves. And of course they'll employ repetition and signal processing techniques.
Now, excuse me... I've got to start gathering some large first-surface mirrors so that I can assemble my mediumly-huge corner reflector. Give them something to talk about...
My tea cosy is far superior. It protects my head from frost, mind control rays and physical damage, due to being padded.
Plus I can use it to keep my tea warm.
Oh and it's the same shape as the Pope's hat, so I can blend in if I'm ever required to hide in the Vatican City.
I see the satellite's orbit is being measured by star-tracker and GPS. Thats fine for Lat/lon determination to about a metre, but vertical GPS resolution is a lot worse, somewhere in the 3-5m range, so either there's another scheme thats not being talked about for measuring the orbital altitude, correcting for gravitational variation etc., or the +/- 3cm height resolution calculated from photon flight time and mentioned in the referenced graphic is somewhat irrelevant.
I always thought that the GPS height accuracy had more to do with the geoid model (which one you chose and how accurate it is for your local position) than absolute 3D positioning. Also, if you're using GPS to measure something on earth, you have the effects of the atmosphere to consider.
The other part to consider is that the absolute height of the satellite might not be critical, as long as the error is slow and predictable. If your measurements show that the ice cap dropped by 11cm and you also orbit over New York's Central Park and show that dropped by 11 cm, you can probably determine that your altitude has changed (I'm sure the orbital dynamics are more complicated, but you get the idea).
The height problem in GPS is because the satellites are much more similar in height than they are in distance to you.
Measuring the height of this satellite doesn't depend on GPS - it just depends on measuring the orbit time (easy and accurate) and knowing where the planet is and how much it weighs (trickier but we do this)
I'm guessing you can nail this to higher accuracy than the LIDAR system, so they will likely use passes over known flat fixed earth targets to calibrate the laser ranger assuming a known orbit - rather than use the LIDAR to measure the satellites' height.
22 August 2018: Europeans launch the British-built Aeolus weather satellite that will "...get its data by firing a powerful laser down into the atmosphere to trace the movement of air particles." [BBC]
23 August 2018: El Reg, a British-based Tech News / Comedy website seeks to distract from their own European/British frickin' laser-wielding planet-frying death-dealing "weather" Aeolus satellite by publishing something about a future ICEsat-2 satellite: "NASA is about to pummel the planet with huge frikkin' space laser..."
It's an amusing juxtaposition of reality versus the headline. One day in the gap!!!! GEESH! :-)
The Register 
Biting the hand that feeds IT
Copyright. All rights reserved © 1998–2024
