GPS Satellites
GPS Satellites all share two main components: a set of clocks,
and a radio transmitter. The satellites use the clocks and the
transmitters to continuously broadcast a series of time marked
codes on specific radio frequencies.
Satellite Clocks
Each GPS Satellite contains a number of different clocks. The
primary clocks are atomic clocks, which keep track of time by
measuring the oscillations of atoms of specific elements. The
two main types of atomic clocks used in the satellites are cesium
clocks and rubidium clocks. Which clock or clocks the satellites
contain depends on which block (or set of satellites) they came
from.
Although the satellites contain multiple clocks, they only use
one at a time. The other clocks are used as backups. The clocks
are monitored and controlled by the Master Control Station located
at Schriever AFB in Colorado. On January 6th, 1980 the satellite
clocks were initially synced to the UTC(USNO), which is the U.S.
Naval Observatory’s version of Coordinated Universal Time,
and which is the time standard for the U.S. Department of defense.
Since that time, UTC has been adjusted thirteen times with a leap
second. The leap seconds are used to keep atomic atomic better
aligned to the actual rotation of the earth, much in the same
way that leap years adjust the years to keep them better aligned
to the actual duration of the Earth's orbit. The GPS satellite
clocks have not been adjusted with the leap seconds, and GPS time
is currently thirteen ahead of UTC.
Radio Frequencies
All GPS satellites broadcast on the same two frequencies, known
as L1 AND L2. L1 and L2 are carrier waves for the specific codes
put out by each satellite. L1 carries the coarse/acquisition code
(C/A) and a P (or Y) code. L2 carries the L2 Civil code and another
and a P (or Y) code. The next block of GPS satellites (IIF) will
also be broadcasting on the new L5 frequency.
Fundamental
Frequency
10.23 MHz |
|
÷10 |
|
| x154 |
L1
1575.42 MHz |
C/A
Code
1.023 MHz |
P-Code
10.23 MHz |
| x120 |
L2
1227.60 MHz |
L2C
Code
1.023 MHz |
P-Code
10.23 MHz |
GPS Carrier Waves, Codes, and Frequencies
Standard Positioning Service
"The Standard Positioning Service (SPS) is a positioning
and timing service which will be available to all GPS users on
a continuous, worldwide basis with no direct charge. SPS will
be provided on the GPS L1 frequency which contains a coarse acquisition
(C/A) code and a navigation data message. SPS provides a predictable
positioning accuracy of 100 meters (95 percent) horizontally and
156 meters (95 percent) vertically and time transfer accuracy
to UTC within 340 nanoseconds (95 percent)." - USNO NAVSTAR
Global Positioning System Website
Most hand-held GPS do not require a high level of accuracy. They
provide the user with an approximate position that is accurate
enough to navigate from, and assuming there is a clear line of
sight, the user should be able to see the intended target from
the position provided by the GPS receiver. Since selective availability
was turned off in May of 2000, single point accuracies have been
in the 10 - 20 meter range when using the C/A code.
The Precise Positioning Service
"The Precise Positioning Service (PPS) is a highly accurate
military positioning, velocity and timing service which will be
available on a continuous, worldwide basis to users authorized
by the U.S. P(Y) code capable military user equipment provides
a predictable positioning accuracy of at least 22 meters (95 percent)
horizontally and 27.7 meters vertically and time transfer accuracy
to UTC within 200 nanoseconds (95 percent). PPS will be the data
transmitted on the GPS L1 and L2 frequencies. PPS was designed
primarily for U.S. military use. It will be denied to unauthorized
users by the use of cryptography. PPS will be made available to
U.S. and military and U.S. Federal Government users. Limited,
non-Federal Government, civil use of PPS, both domestic and foreign,
will be considered upon request and authorized on a case-by-case
basis, provided: it is in the U.S. national interest to do so,
specific GPS security requirements can be met by the applicant,
and a reasonable alternative to the use of PPS is not available."
- USNO NAVSTAR Global Positioning System Website
PPS is currently being made available to all users, but if the
U.S. Department of Defense deems it necessary they can implement
anti-spoofing and encrypt the P-codes on L1 and L2. Use of PPS
in combination with Continuously Operating Reference Stations
is what allows users to use GPS with the accuracy required of
most surveying applications.
Selective Availability
Selective availability refers to the practice of dithering the
satellite clock frequency or manipulating the satellite navigation
orbit data. The purpose of selective availability is to deny the
user the full level of accuracy of the GPS system.
The practice of using selective availability was discontinued
in May of 2000 by President Clinton. President Clinton’s
Statement on Discontinuing Selective Availability.
Anti-Spoofing
Anti-spoofing refers to guarding against the broadcasting of
fake GPS satellite transmission or “spoofing.” The
Department of Defense has the ability to encrypt the P-code, which
allows it to be differentiated from the fake code. The encrypted
P-code is referred to as the Y-code. Implementing anti-spoofing
will block civilian access to the Precise Positioning Service,
but Military GPS receivers are capable of unencrypting the Y-code
and using it to calculate a position.
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