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====Beidou, the chinese satnav system====

Since early 2013 the Chinese 北斗 (Beidou) satellite navigation system
is operational. The coverage of the satellites still limits use to
China itself. Even though a number of 北斗 satellites have orbits
going over the whole Earth the system is of little use outside China
since China's ground stations are mostly inside China itself. So
corrections which are broadcast in modern satellite navigation systems
are simply not available in the rest of the world.

{{fotos:beidou:20150424_skyplot.png?200 }} 北斗 consists of a number
of geostationary satellites, inclined geostationary satellites and
medium-Earth orbit satellites. The medium-Earth orbits satellites are
in comparable orbits to GPS and Glonass satellites. So these are
visible all over Earth. But since there are only 4 operational now
(2015) the chance that one is in view is limited. You need about 30
medium-Earth satellites to cover the Earth. The other satellites cover
China itself. The four geostationary satellites are all visible from
all of China. The inclined geosationary satellites make long
figure-eight loops over the area. So China itself is pretty well
covered. They have succeeded in building an operational system with a
limited number of satellites.

Even though GPS is still widely used even in China 北斗 is being used
more and more. Many of its users will probaly not even notice they are
using 北斗 since it is simply a feature of the navigation chip in
their mobile phone (more about that later).  You can receive 北斗
satellites fine, even from northern Europe. There is one geostationary
satellite just high enough over the horizon, the inclined
geostationary ones come into view when they're in the northern curve
of their figure-eight loop and every now and then one of the global 北
斗 satellites come into view. The second image in this article shows
how the satellites moved over the sky over the course of an
evening. As you can see all 北斗 satellites (in red) are sparse
compared to the GPS constellation (in blue) and they are pretty low in
the sky. Nevertheless, there are enough in view.

To get a receiver you will need to order some equipment from China. I
ordered a few small GPS/北斗 receivers from Navspark. Their receivers
are not just attractive because they are small and use little power,
they are actually Arduino programmable! So you can just run your own
software on it and connect pressure sensors, a compass, inertial
sensors, LEDs, buttons, whatever you like.  The receiver comes with a
good viewer application. You can just connect the receiver board to
your USB port, start the viewer and see whether there are 北斗birds
overhead (see first image). The Navspark does a pretty good job
computing a combined solution. Which is not straightforward since the
pseudoranges of 北斗 can be off (no corrections over Europe...) I was
impressed what the little Navspark could do. You can see it has
computed a location using two GPS and two 北斗 satellites. Cool!  The
next step (still a work in progress) is putting the little Navspark
into a receiver i can take out and about. The third image shows the
Navspark on a prototype board with a Nokia LCD. The Nokia uses little
power as well so it fits nicely with the Navspark. Next step is
putting it al a bit more neatly in a receiver box, add some buttons,
add the Navspark SD card adapter for logging and i've got my own
homebuilt 北斗receiver.

{{ fotos:beidou:20150609_wolfgang.gif?160}} Then i found out my cheap
android phone already uses 北斗 for navigation! I got this phone, a
Wolfgang, from the Aldi supermarket chain. Turns out it has this
modern navigation chip in it. Since the navigation chip is obviously
intended for the chinese market it supports 北斗, together with GPS
and Glonass. In last image shows the sky view of all used navigation
satellites. Those with PRN numbers of 200 and higher are 北斗. So if
you are not into soldering your own Navspark receiver, just get the
phone from the Aldi.

{{tag>en:beidou en:geography en:satellite_navigation en:map}}