Wherever You Go, There You Are.
That just happens to be the title
of Jon Kabat-Zinn's 1994 book
on Buddhist meditation. However, you could also apply that
description to the U.S. Air
Force's Global Positioning System (GPS). This technology
knows exactly where you are, even if you don't.
Also referred to as Navstar, GPS has been in operation
since the early 1990s. It proved invaluable initially in the
Gulf War and in all military endeavors since then many times
over. Not only that, GPS has become an essential part of
many commercial and personal products that rely on location, position, and navigation.
Every year, GPS goes through constant updates, making it
more accurate than ever before. And many low-cost GPS
radio chips and navigation receivers have become available
to everyone. In fact, one standout trend is to add GPS to as
many handheld products as possible.
GPS IN ACTION
The GPS system comprises a constellation of 24 operational satellites, plus at least three spares,
that orbit the earth at 12,548 miles or 10,898 nautical miles
(20,200 km) up with an inclination of 55° to the equator.
There are six orbits with four satellites each. The rotational
period is just two minutes short of 12 hours per orbit.
With this arrangement, at least five to eight satellites are
always "in view" anywhere on earth. On the ground in Colorado, the USAF maintains a station that monitors and controls the satellites. It ensures that each satellite maintains its
position in the constellation and gets the correct position
data it needs to transmit back to earth.
Each satellite carries four atomic clocks (two cesium-based
and two rubidium-based) that generate dead-on accurate
timing pulses. These are used as the basis for generating the
signals sent to receivers on earth. Each satellite contains its
own unique pseudorandom code (PRC) for differentiating
itself from its neighbors.
Also, each satellite transmits what is called ephemeris information, which defines precisely where it is in orbit. Such information is translated into a ground track on earth that will
identify its latitude and longitude, providing the requested
location. The earth station updates the ephemeris data daily.
Each satellite transmits its PRC and ephemeris data in the
microwave L band at 1575.42 MHz. This is called the L1 signal. The receiver can recognize each individual satellite by its
unique PRC, just as in other direct-sequence spread-spectrum
systems. The PRC is transmitted at a 1-Mbit/s rate using
binary phase-shift keying (BPSK). Repeating every 1023 bits,
this is called the coarse-acquisition (C/A) code.
shows how this code is used as the chipping code for
the navigation data, which occurs at a 50-bit/s rate—yes, 50
bits per second! The navigation code contains the ephemeris data. The overall L1 signal
occupies a bandwidth of about
1 MHz.
Each satellite additionally
transmits an L2 signal at
1227.6 MHz. The L2 uses
another 1023-bit PRC called
the P-code. It occurs at a 10.23-Mbit/s rate and is used
to chip the 50-bit/s navigation
data. The P-code also may be
encrypted, in which case it's
called the Y-code. The resulting signal then modulates the
1227.6-MHz carrier and the
L1 signal as well. The L2 signal is strictly for military use.
Back on Earth, a receiver picks up the signals, does a
tricky triangulation calculation, and spits out time, altitude,
and position data. The position information is in the form
of latitude and longitude. Since time is available, it also can
figure velocity. Receiver manufacturers call it PVT, or position-velocity-time output. Using fancy software and map
overlays, you can generate a display that shows where you
are on a detailed map, much like that used in those 1960s
James Bond movies.
The most important issue in getting a GPS fix is being able
to "see" the satellites. Given that they're over 12,000 miles
away, you need all the signal you can get plus a good antenna
and a super-sensitive radio. The only real way to get a signal
is to have the antenna in clear view of the satellites. If you go
inside, you'll lose the signal. That's why GPS radios only
work outside or in a vehicle with a window.
Once you have a good view of the satellites, the receiver
takes several minutes to lock on to one of the satellites. It
then extracts the data and is passed off to another satellite
in view, and again the data is taken. Next, it locks onto a
third satellite, and so on. Latitude and longitude data
requires three satellites. Altitude and speed calculations
require four satellites.
The receiver measures the signal's time of travel from the
satellite to the receiver. Knowing the speed of light or radio
waves in space (slightly less than 300 million meters/s) and
the precise time, it can calculate the distance to the satellite.
That distance value is used in the calculations along with the
other data from the satellite.
The receiver itself is the usual superhet or direct-conversion type with DSP demodulation and baseband recovery in
an on-chip or external CPU. The processor, usually quite
powerful, typically is a 32-bit CPU with floating point so it
gets the required accuracy.