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Lightning Talk: Where the Heck Am I? Understanding Location in your Application
By
Richard Sueselbeck
droidcon London 2019
Transcript
English
00:00
hi everyone I've been told to just kick
00:02
off so kick off I shall hi my name is
00:05
Richard I'm a developer evangelist for
00:08
here technologies and we are the world's
00:11
leading location technology company and
00:13
that means that location matters to me
00:17
here's a fun fact I submitted this as a
00:19
full hour talking because it got
00:21
accepted as a 15 minute lightning talk
00:23
so I have an interesting challenge ahead
00:24
of me I wanted to do a talk about
00:27
location where does location come from
00:30
when we use location in our app we
00:32
usually have something like this we have
00:33
a latitude and a longitude but we don't
00:36
necessarily understand really what that
00:38
means so where it comes from we use that
00:40
maybe to start a routing API or to
00:44
Center a map all these kind of things
00:46
what we don't necessarily understand
00:48
what that means so I had a wonderful
00:50
one-hour talk there were there nice
00:52
stories about the Cold War and Ronald
00:54
Reagan and fighter jets and space and
00:56
dinosaurs and all that kind of thing for
00:59
a 15 minute talk I had to cut that down
01:01
a little bit and I picked out kind of
01:03
one thing from that talk and that really
01:06
is the question where does that location
01:08
come from and the answer is usually GPS
01:12
again that's only partially true and if
01:14
we had the full talk we could talk about
01:16
that as well but really what I want you
01:18
to take away from the next 15 minutes is
01:21
the idea of a how does GPS actually work
01:25
because we use that every day as
01:27
developers often we use location in our
01:30
apps on on our Android phone that will
01:34
usually come from GPS but do we actually
01:36
know how that works we have an idea
01:38
there's satellites in space and they
01:40
sort of we talk to them and then they we
01:42
know where we are but how does that work
01:44
and so that's what I'm going to talk
01:46
about in 15 minutes kind of try to get
01:48
the idea across of how does that work
01:51
how do we actually get a location from
01:53
GPS and then also if we have a little
01:55
time left we can talk a little bit about
01:57
why is that location always awful why is
01:59
it always oh my you're uber driver is
02:01
gonna be here in four minutes no
02:03
actually 15 minutes no actually he's
02:04
already there so where do these problems
02:07
come from and maybe if we understand
02:09
that we can deal with that a little bit
02:10
better right up
02:11
our applications okay so GPS is a system
02:15
of 24 satellites that orbit the Earth at
02:19
20,000 kilometers and is a system
02:22
designed so that at least four of those
02:24
satellites are visible at any given time
02:28
okay that's nice so we shoot satellites
02:31
into space so how do we determine our
02:34
position using these satellites and
02:36
that's basically just four steps one is
02:39
we have to find satellites at least four
02:42
of them hence the system being designed
02:45
to show it at a minimum four satellites
02:47
at any given time second we need to find
02:51
out where they are third we need to find
02:55
out how far away we are from them and
02:57
then we use the magic of trilateration
03:00
to calculate where we are and I actually
03:03
want to start with step four with
03:04
trilateration because when we understand
03:07
trilateration we understand why these
03:08
other three steps are necessary so
03:11
simple example of trilateration I'm
03:13
gonna do this in 2d because 2 D is much
03:16
more much more easy to understand than
03:18
3d so let's say I want to know where I
03:20
am and I'm calling a friend of mine who
03:24
lives in Mach 2 book and I'm asking him
03:26
hey do you know where I am and you can
03:28
already see this a very practical
03:29
real-life example and he's telling me I
03:32
don't know where you are but I do for
03:34
some reason know that you are 128
03:36
kilometers away from me I mean we've all
03:39
had those phone calls right what that
03:41
means and at that point is I don't know
03:43
still don't know where I am but I know I
03:45
must be somewhere on that red circle
03:47
because those are the only places in the
03:49
world that are 128 kilometers away from
03:51
octa book okay so that helps me somewhat
03:54
but not enough so I call another friend
03:55
he leaves them sniffing and I ask him do
03:58
you know where I am and he says no but I
04:01
happen to know you're 127 kilometers
04:03
away from me and that actually helps a
04:06
lot because now we can only be in two
04:08
spots anymore the two spots were those
04:11
two circles intersect so I call another
04:14
friend he lives in Corpus and once again
04:16
I asked him the question do you know
04:17
where I am and he says no but I happen
04:19
to know you are 107 kilometers away from
04:21
me and so I draw another circle
04:24
and now very very conveniently there's
04:27
only one spot where those three circles
04:29
intersect and in that case I would be in
04:31
Berlin which is where I normally live
04:33
now if you imagine this with an added
04:37
dimension and you imagine our friends as
04:40
being satellites and you imagine those
04:43
circle those circles actually being
04:45
spheres around the satellites and then
04:48
you can also see one satellite another
04:50
satellite another satellite another
04:52
satellite with the added dimension we
04:54
obviously need another satellite then
04:56
those spheres would only intersect in
04:58
one point so here's the thing though
05:01
there's only three satellites on this
05:03
picture and you actually only need three
05:05
any idea why
05:09
coming in yes so there I don't know if I
05:14
understood it correctly but the answer
05:15
is of course you get two positions based
05:17
on this but one of them is out in space
05:19
so unless you're an astronaut like this
05:21
fellow up there it's probably the one
05:24
that's on earth or another way to look
05:26
at it the earth is conveniently round
05:28
you can just use the earth as the fourth
05:30
sphere we actually do need four
05:32
satellites but we need them later and
05:34
we'll see why okay so we understand
05:37
trilateration basically just circles and
05:39
spheres and intersections and everything
05:41
is fine so now we kind of understand
05:44
what we need for satellites so we can do
05:46
those spheres we need to figure out
05:48
where they are so we know where to draw
05:50
the spheres around we need to figure out
05:51
they're in Mach Tabuk or called puss or
05:53
whatever or wherever and we need to
05:55
figure out how far away they are from us
05:57
so we know what the diameter of the
05:59
sphere is so let's figure out how we can
06:01
do those three things first we have to
06:05
find the four satellites and very
06:06
conveniently those satellites
06:07
permanently send out a signal so they're
06:11
saying hello to us all the time all we
06:12
need is a receiver that can read and
06:14
understand that signal and in our
06:16
Android phones very conveniently that
06:17
receivers already provide it so easy
06:21
checkmark also very good when you only
06:23
have 15 minutes and need to go through
06:25
it quickly it's always nice when some of
06:26
those points are easy to explain
06:28
determine their position it's a little
06:32
bit more tricky but again fortunately
06:34
the satellites like my friends actually
06:36
know where they are the GPS signal
06:38
that they're sending out contains a
06:40
whole bunch of information like date and
06:41
time and if the how well the satellites
06:44
feeling if you should trust it or not
06:46
and it contains two other things one is
06:48
called ephemeris which is the Greek word
06:50
for diary and the other is called
06:52
Almanac which is basically a a yearbook
06:57
and the ephemeris data essentially is
06:60
contains the location of where the
07:02
satellite is they've got pretty stable
07:04
orbits so they generally know where they
07:05
are anyway and they're monitored by
07:07
ground stations and the ground stations
07:08
can - can give them information as well
07:11
so the satellite actually knows where it
07:14
is the Almanac also has information
07:17
course location on all of the other
07:19
satellites and a bunch of other
07:21
information including an ionospheric
07:23
model which will become useful later
07:25
here's a really interesting thing about
07:27
this though it takes forever to transmit
07:29
that data it's sent at a really slow
07:32
data rate to get this whole data takes
07:35
12 minutes and you if you have a GPS
07:38
receiver that's cold which is it has no
07:42
idea where it is
07:42
I had as no information about where
07:44
satellites are it literally would take
07:47
12 minutes to get a fix in the worst
07:50
case now the reason why our Android
07:52
device always gets a quick fix is you
07:57
can just download that information over
07:58
the Internet that's called a GPS or a
08:00
GNSS so you don't actually need that
08:03
information from the satellites you can
08:04
just download it off the internet if you
08:06
happen to have an internet connection
08:07
which again conveniently our Android
08:09
phones usually have okay very nice
08:12
another check mark we can determine
08:14
their position now comes the harder part
08:16
figuring out how far away they are from
08:19
us and we need to do this very very
08:21
precisely now
08:24
satellites GPS satellites are really
08:27
just atomic clocks we've shot into space
08:29
so they know what time it is very very
08:32
precisely and they send a signal to us
08:36
and so what we could do is we could
08:40
figure out what time it was when the
08:41
signal was sent what time it is when I
08:43
receive the signal and then just you
08:46
know single get send at the speed of
08:47
light distance is time to unbar we can
08:51
just figure
08:52
out how long that single was traveling
08:54
and then we know the distance because we
08:56
know how fast it is traveling very cool
08:59
right done anyone see the problem that
09:04
is correct
09:05
our Android phone unfortunately does not
09:07
happen to Tommy clock built in because
09:09
if it did it would be much bigger and
09:10
much more expensive so this
09:13
unfortunately doesn't work it's a good
09:16
idea though and there's one thing you
09:18
might notice one thing is that all of
09:22
the signals and getting all of the
09:23
distances I'm measuring this way to the
09:26
signals are all off by the same way like
09:29
my clock on my phone is off it's not
09:31
precise it's not synchronized with the
09:33
satellites but the satellites are all
09:35
very very precise and therefore
09:36
synchronized and that means it is all
09:39
off by the same amount that's what's
09:42
called a pseudo range I don't exactly
09:44
know what the range is but it's wrong in
09:47
the same way which is really convenient
09:49
because if I can determine what that
09:51
offset is that that the way in which it
09:54
is wrong is I would get the correct
09:56
distance to all satellites and this is
09:59
actually where the fourth satellite
10:00
comes in because as it happens when I
10:03
use four satellites those when I pick
10:06
the right offset only then do those four
10:09
spheres align in one spot now this ends
10:13
up being a huge mathematical equation
10:15
and in fact that mathematical equation
10:18
is so long that it's also not on my
10:20
one-hour talk slides so we'll just skip
10:23
over this conveniently not bother with
10:24
the math too much and just realize that
10:26
that actually works you can solve this
10:28
with math and that is why you need four
10:31
satellites because you can use the earth
10:32
for that cool so we actually have we
10:36
found four satellites we've determined
10:38
their position we know how far away they
10:40
are from us and now I just do
10:42
trilateration like we did earlier and
10:45
that calculation is actually fairly
10:46
simple and I can get a position now if
10:51
it is so easy why does my uber driver
10:53
still can constantly hop around the map
10:56
and I get a wrong wrong location why is
10:58
this also imprecise
10:59
well by its nature GPS is only accurate
11:02
to be about five meters in the
11:05
stiff conditions and that usually the
11:09
best of conditions you don't have so
11:12
let's look at what not non-ideal
11:15
conditions are what are some things that
11:17
affect GPS I need to hurry up because
11:19
I've got less than five minutes but this
11:21
is really cool I left this in because
11:23
GPS is effect actually affected by
11:25
relativity theory because as it turns
11:29
out both by special and by general
11:31
relativity because it turns out the
11:33
satellites move really really quickly
11:34
fourteen thousand kilometers an hour and
11:37
that means they actually move faster
11:39
than the clocks on the ground and that
11:41
means they actually run slower by seven
11:44
microseconds every day and gravity also
11:47
alters time and they're further away
11:49
from the planet so there's less gravity
11:51
so actually run faster by around 45
11:53
microseconds a day which means they're
11:55
off by I bunt do the math in my head
11:59
because I forgot what the numbers work
11:60
but those couple of microseconds that it
12:03
that it ends up running wrong adds up to
12:05
11 kilometers a day so the clocks act
12:09
are actually set to run slower than
12:12
they're supposed to be to compensate for
12:15
that that won't really affect you every
12:17
day because the clocks are set to
12:19
compensate there but it's so cool I just
12:20
have to talk about it because and and
12:22
your your physics teacher will probably
12:24
really enjoy the fact that you're using
12:26
relativity theory every day in your life
12:28
another thing that affects it is the
12:30
ionosphere there is a charged layer of
12:34
the atmosphere that actually bends the
12:36
the radio signal which is also why the
12:39
satellite has some ionosphere weather
12:41
reports so you can compensate for that a
12:43
little bit but it's kind of an imprecise
12:45
science and so depending on how the
12:47
weather is in the ionosphere the signal
12:49
will be worse or better not much you can
12:51
do about it except maybe to get a better
12:53
ionosphere weather report but the thing
12:56
that very often really affects you on an
12:58
everyday basis particularly when you're
13:00
calling a cab in Manhattan our multipath
13:03
and reflection errors turns out that
13:07
first satellites are easily blocked we
13:10
all know in a building we don't get GPS
13:12
so if there's a satellite I would see in
13:14
the sky but there's a building in front
13:15
of it I don't see it and so suddenly I
13:17
have fewer satellites and the fewer
13:18
satellites I have
13:19
the more difficult that equation gets to
13:21
solve and the the less precise it is and
13:23
sometimes I might not even see the four
13:25
satellites or more satellites because
13:27
there's so much obstruction but it gets
13:29
worse if I have a big glass building the
13:32
signal might actually bounce off the
13:33
building so I get a single but it
13:35
traveled a longer path than I thought it
13:37
did and that also means I get a wrong
13:40
position and there's a lot to talk about
13:43
for all of these kinds of things but my
13:45
time I think is about up so I just hope
13:48
this was kind of interesting for you you
13:49
learned one interesting thing today
13:51
which is how GPS which you use every day
13:53
which you probably all used to get here
13:55
today how that actually works in
13:57
principle and maybe a little bit about
13:59
the problem so when you use it in your
14:01
apps you or have a little bit better
14:02
understanding of what goes wrong even if
14:04
it's not necessarily a solution and
14:06
here's a picture of a dinosaur thank you
14:08
very much
14:09
[Applause]
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