Measurements from a satellite - Page 1
Forum section: electronics / general

author: Circlotron
author_rank: Super Contributor
timestamp: August 23, 2016, 03:12:26 am
content: Not sure what category to put this one in.How do satellites measure say the difference in ocean or land level down to the millimetre? If they are 100km above the surface then that is one part in 100,000,000. That's more than 26 bit resolution. What are they doing to get this level of analogue accuracy / resolution. The cost must indeed be 'astronomical'.

author: Rick Law
author_rank: Super Contributor
timestamp: August 23, 2016, 03:32:07 am
content: re:"Not sure what category to put this one in."Answer: Exaggeration would be what I put it in.First, the ocean is not calm enough to have millimeter level surface flatness. A of sand would mess up the land-surface at +-1 millimeter as well.Second, 100km is not really high enough. You are just in the stratosphere/mesosphere area at 100km and not nearly high enough for a satellite to stay for long. Geo-sync satellites are around 22,000 miles give or take.22,000 miles = 35,200Km = 35,200,000 meter = 35,200,000,000mmForget part-per-million, you are in the ballpark of parts in 0.04 trillion!36 bit resolution is more like it.

author: Fungus
author_rank: Super Contributor
timestamp: August 23, 2016, 03:59:44 am
content: Quote from: Rick Law on August 23, 2016, 03:32:07 amIf it costs you $millions just to deploy it then you can afford a decent ADC.Do they actually measure height to that resolution? I thought sea height measurements were done from land (dipstick) and land height measurements were done from aircraft (LIDAR).

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: August 23, 2016, 05:10:19 am
content: Quote from: Circlotron on August 23, 2016, 03:12:26 amHow about 'General Discussion'? It's not really related to test gear so this is the wrong place.QuoteMaybe you should first clarify as to which satellites you're talking about because many satellites don't even measure altitude at all orthometric height, and of those that do I'm not aware of any that has a resolution in mm.

author: hendorog
author_rank: Super Contributor
timestamp: August 23, 2016, 07:14:01 am
content: This page has some pics showing global sea levels to 5mm measured by satellite:http://www.nasa.gov/mission_pages/ostm/multimedia/sla-20080730.html#.V7v1-ZOF6Hohttp://www.nasa.gov/sites/default/files/sla_0.jpgSatellites are called Jason-1 and Jason-2 apparently.

author: daqq
author_rank: Super Contributor
timestamp: August 23, 2016, 08:02:11 am
content: Possibly not so much ADCs as precise timing? Send a pulse, receive it and precisely measure the time between? Radio interferometry? All of the above?Either way, it's five parts amazing engineering and one part black magic...

author: rob77
author_rank: Super Contributor
timestamp: August 23, 2016, 08:35:33 am
content: Quote from: daqq on August 23, 2016, 08:02:11 amand probably one part of "creative data processing/interpretation"

author: dannyf
author_rank: Super Contributor
timestamp: August 23, 2016, 12:49:32 pm
content: Altimeter radar.1. They measure changes in sea levels.2. They oversame vua averaging.It is not without failures, however.Aka there is a false sense of precision.

author: dannyf
author_rank: Super Contributor
timestamp: August 23, 2016, 12:51:01 pm
content: "Geo-sync satellites are around 22,000 miles give or take."The birds in discussion are Leo.

author: Brumby
author_rank: Unknown
timestamp: August 23, 2016, 01:34:19 pm
content: Geostationary orbits would be useless for global surveying.Jason 1 and 2 orbit around 1300km (~830 miles).For sea level measurements - aside from the raw distance measurement, consideration needs to be taken for waves, chop, swell and tides.It's an impressive exercise.

author: ajb
author_rank: Super Contributor
timestamp: August 24, 2016, 03:45:25 am
content: I had to do some paperwork recently that required lat/lon/alt in terms of a specific map datum, which lead to several hours of learning far more about computational geography than I ever anticipated. It turns out that there's a massive amount of number crunching that goes into simply coming up with the reference geometry against which sea level (and latitude and longitude) is measured. There are a number of different reference "geoids" in use, which are approximations of the shape of the earth's surface to varying precision, which determine how latitude and longitude get projected onto the surface. Establishing a mean sea level on top of that requires taking measurements over something like a sixteen year cycle to adequately average out variations due to weather and tidal cycles, but even then local mean sea level will vary from any reference geoid due to variation in the local gravity as a result of the earth's mass being unevenly distributed. Oh, and let's not forget that your local datum may not be in exactly the same place as it was last year because plate tectonics are a thing.So on top of some extremely careful engineering, much of the magic is in taking a vast amount of data that consists overwhelmingly of noise and doing some tremendous amounts of computation to extract a useful signal from it. It's really amazing how detailed our physical knowledge of the world is in light of all of the confounding factors inherent in the data.

author: Rick Law
author_rank: Super Contributor
timestamp: August 24, 2016, 04:53:39 pm
content: Quote from: Brumby on August 23, 2016, 01:34:19 pmThe effort to measure land/sea level to 1mm is not so much impressive exercise as it is pointless. It is like measuring an flashlight LED's current draw to 6 decimals. It is not meaningful. A good size fish swimming by will change it by > 1mm at sea. A gentle wind can blow in a grain of sand and change it by > 1mm on land.That said, there is some value to work out methods so you can have 1ppm measurement. I see it as a solution waiting for a problem. In the future, such problem may just arise.For now, it would be more valuable to be able to measure distance to the moon +- 1mm where neither fish nor strong wind exists.

author: dannyf
author_rank: Super Contributor
timestamp: August 24, 2016, 06:55:09 pm
content: "The effort to measure land/sea level to 1mm is not so much impressive exercise as it is pointless. "That's precisely they don't publish the confidence interval of their measurement, something they must have, as well as the data cleansing process.If they did, I suspect that you will find a wide band / variability so that the changes in sea level may not be statistically significant.Going back to a point I made earlier that few picked up, for the study to be valid, they don't need the measurements to be accurate in absolute terms, as they just want the changes of sea levels.So wrong measurements are perfectly OK here, as long as the measurements are consistently wrong.Also, filtering out tidal waves is quite trivial, as those "noises" are periodical.