Albrecht GP 1, Instruction Manual, Page: 7 / 38

User Manual Albrecht GP 1
7
2.4. The Global Positioning System (GPS) network
GPS is a world-wide radio navigation system formed by a group of 24 satellites (21 operating and 3
spares) and their associated ground stations. GPS uses these satellites, appropriately called
NAVSTAR (Navigation Satellite Timing and Ranging), to calculate ground positions. The basis of GPS
operation is the use of triangulation from the satellites. To triangulate, a GPS receiver measures
distance using the travel time of radio signals. However, to measure travel time, GPS needs very
accurate timing, plus it needs to know exactly where the satellites are in space. To solve this problem,
each of the 24 satellites is inserted into a high enough orbit (12,000 miles) to preclude interference
from other objects, both man-made and natural, and to insure overlapping coverage on the ground so
that a GPS receiver can always receive from at least four of them at any given time. In addition,
compensation is inserted for any delay the signal experiences as it travels through the atmosphere to
the receiver.
The GPS network was originally conceived by the U.S. Department of Defense (DOD) to aid
navigation.
How the system works
With the satellites operating at 12,000 miles above the earth’s surface, they are arranged in a strategic
positions and orbit the earth at a speed of 17,000 miles-per-hour, thereby completing an earth orbit
every 12 hours. Each is powered by solar energy; if that fails, they are equipped with on-board backup
batteries to maintain operational GPS integrity, and with small rocket boosters to keep them flying
along the correct path.
Satellite Frequency and control signals
Each satellite transmits a low-power radio signal in the UHF frequency range; the frequencies used
are designated as L1, L2, etc. GPS receivers, such as the PMR-GPS unit,
listen on the L1 frequency
of 1575.42 MHz. This signal, since it is line-of-sight, will reach the ground receiver unless it is
obstructed by solid objects, such as buildings and mountains.
The L1 signal is accompanied by a pair of pseudo-random signals (referred to as a pseudo-random
code) which is unique to each satellite. These codes are identified by the GPS receiver and allow for
the calculation of the travel time from the satellite to the ground. If this travel time is multiplied by the
speed of light, the result is the satellite range (distance from satellite to receiver). The navigation
information provided by each satellite consists of orbital and clock data, plus delay information based
on an ionospheric model. Signal timing is provided by highly accurate atomic clocks.
Ground control
There are five GPS ground control stations ----- Hawaii, Ascension Island, Diego Garcia, Kwajalein
and Colorado Springs ---- that control the satellites by checking their operational disposition and exact
position in space. Four of these stations are unmanned, and the fifth -- Colorado Springs – is the
Master station. The four unmanned stations constantly receive data and send it to the Master station.
The Master station then provides corrections for satellite Ephemeris constants and clock offsets and,
in conjunction with two other antenna sites, uplinks this information to the satellites.
2.5. The GPS receiver
The GPS receiver, (in this case, the
PM
R-GPS unit), uses NAVSTAR satellite signals as a way of
determining exact position on earth. Mathematically, you need four satellite ranges to accomplish
these coordinates. Although three ranges are enough, an additional range is required for technical
purposes.
So, our position is based on how long it takes for a signal sent from the satellite to arrive at our
receiver. Since timing is everything, the satellite signal is almost perfect since it has an atomic clock on
board. But, what about our GPS receiver timing? Our receiver certainly contains no atomic clock; if it
did, its cost would be prohibitive -- nobody could afford it. To get around this problem, our receiver
must take an additional satellite measurement. Hence, it really needs four satellite signals to insure
our correct position. Since this fourth measurement, done as a cross check, will not intersect with the
first three, our receiver’s computer says, in effect, there is a discrepancy in my measurements, and I
must not be synchronized with universal time. Since any offset from universal time will affect all of our
measurements, the receiver looks for a single correction factor that it can subtract from all its timing
measurements that would cause them all to intersect at a single point.
That correction brings the receiver’s clock back into sync with universal time and, in this way, atomic
timing accuracy is in the palm of your hand.