Each satellite’s flight path is monitored
by a network of six U.S. Air Force stations
located around the world, which record
any deviations in the satellites’ orbits (slight
changes are usually caused by the pull of
the moon and the sun). Each station for-
wards the information it receives from the
satellites to a master control station located
in Colorado Springs. The master control
station synchronizes the atomic clocks
carried on each satellite and uploads any
orbit changes, which are in turn sent to GPS
receivers as part of the satellites’ signals.
If a GPS satellite’s orbit ever needs to
be adjusted (or if the satellite is otherwise
determined to be unreliable), the master
controller labels it as “unhealthy,” so GPS
receivers won’t use it in their calculations.
Once the problem has been resolved (fol-
lowing an orbit correction, for example), the
master controller uploads the satellite’s new
ephemeris data and tags it as healthy again.
WHERE AM I?
In order to calculate its position, a GPS
receiver compares the time the satellites’
signals arrive to the time at which the satel-
lites initiated their transmissions. It then
multiplies these differences by the speed
of light to determine the distance that each
signal has traveled.
Considering the great distances
involved, making a precise calculation
requires that the clocks on the GPS receiver
and each of the satellites be synchronized
to the nanosecond, which typically can be
achieved only with costly atomic clocks.
It’s not feasible to put an atomic clock in a
consumer GPS receiver, but there is a clever
solution: Since the master control station
synchronizes the atomic clocks on all the
satellites, the receiver constantly resets its
inexpensive quartz clock to match the time
that the satellites are reporting.
The receiver uses trilateration to deter-
mine its location on a 2D plane. Trilateration
is similar to triangulation, but where the lat-
ter method uses angle measurements and at
least one known reference point to determine
the coordinates of a specifi c location, the
former uses the known locations of three
reference points and the calculated distance
between the object and those known refer-
ence points (see diagram). Using a fourth sat-
ellite enables the GPS receiver to determine
its current altitude.
Once the receiver has determined its exact
position on Earth, it translates this information
into latitude and longitude and plugs that data
into a map fi le stored in its memory.
AUTOPSYJabra BT5010 Bluetooth
Headset
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SUBMIT YOUR IDEA Ever wonder what the inside of a power supply looks like?
Don’t take a chance on destroying your own rig; instead, let us do the dirty
work. Tell us what we should crack open for a future autopsy by writing to
[email protected].Ever wanted to rip one of these headsets out of an annoying chatterer’s ear
and smash it into the ground? We did just that... to show you what’s inside!SPEAKER
The BT5010’s speaker at-
taches to its enclosure here.
Inside, a tiny electromagnet
wrapped in coils is surround-
ed by a larger magnet. When
electric current passes into
the coils, the smaller magnet
quickly bounces up and down,
creating sound waves.PROCESSOR
CSR’s Bluecore single-
chip microcontroller is
the brains of the BT5010.
It hosts anywhere from 4
to 6 megabits of included
read-only memory.
Depending on the spe-
cifi c chipset, it can also
feature an integrated FM
tuner and GPS receiver.MICROPHONE
Underneath the
BT5010’s sliding front
panel is a 4mm micro-
phone. It’s omnidirec-
tional, meaning it can
pick up sound from any
direction. A unidirec-
tional microphone
requires you to speak
directly into its center
for your voice to be
picked up.BATTERY
The BT5010’s circuit board
features an on/off button on
its underside, but you’re un-
likely to use it, as the headset’s
rechargeable lithium-ion poly-
mer battery boasts 12.5 days of
standby time.VIBRATOR
This spinning device
warns your ear that a call
is incoming. That is its
sole purpose—keeping
others from knowing
when your friends hit up
your digits.