-------[ Phrack Magazine --- Vol. 9 | Issue 55 --- 09.09.99 --- 14 of 19 ]
-------------------------[ A Global Positioning System Primer ]
--------[ e5 ]
----[ 1] Abstract
Satellite navigation systems are now one of the most important communication
tools around today. Everything from Intercontinental Ballistic Missiles
to fishing ships benefit from highly accurate position, velocity, and time
determination 24 hours a day from anywhere in the world. The most popular
satellite navigation system, GPS, is now so highly used that one can purchase
a user-friendly GPS receiver for under $200 at Radio Shack. This article will
provide an overview of satellite communications in general, and a more in-depth
look at GPS. I hope that this article will help readers understand this highly
interesting system which is growing more prevalent every day.
----[ 2] An Overview of Satellite Communications
Satellites have changed the telecommunications world as much, if not more,
than fiber optics. There are over 1,000 satellites in orbit today, and all
international telephone traffic which is not transmitted over fiber optic
trunks or buried cable is handled by satellites. Nearly all international
television transmissions are sent through satellites.
The first satellite which ever reached orbit was Sputnik 1, launched by the
Soviet Union on October 4, 1957. The first attempt at satellite communication
was the United State government's project Score, which launched a satellite
on December 18, 1958.
The first international satellite communication system originated when 11
countries agreed to form Intelsat in August 1964. Intelsat is responsible
for the maintenance, design, and development of this international system.
By the late 1980s the Intelsat system included over 400 Earth stations,
and provided well over 25,000 two-way telephone circuits between some 150
countries.
In all satellite communication, signals are transmitted from an Earth station
to the satellite, where they are amplified and rebroadcasted to another
station, or forwarded to another satellite which broadcasts the signal to a
station further away. Every satellite contains one or more transponders.
Each transponder includes a receiver, tuned to a frequency, or range of
frequencies, lying in the uplink (receive) region, and a transmitter tuned
to a downlink (transmit) frequency or range of frequencies. The number of
transponders, or channels, on a satellite determine its communication capacity.
When a satellite is launched, it may go into orbit at any height above the
earth. There are generally 3 different classifications for satellite orbit
heights, described below.
GEOS (Geosynchronous Earth Orbiting Satellite) - This type of orbit, also
referred to as geostationary orbit, is when a satellite is launched to an
altitude of precisely 22,300 miles above the Earth. At this altitude, the
satellite orbits the Earth every 24 hours. Thus, to an observer stationed on
the Earth, the satellite appears to be stationary. This is a tremendous
advantage, as it allows complete 24 hour communication within its huge
footprint (covering approximately 1/4 of the Earth). However, geosyncronous
satellites are not ideal for voice circuit transmission. Due to their
height above the it takes radio signals approximately .25 seconds to be
transmitted to the satellite and reflected back down to Earth, depending
on whether the signal is passed among satellites before it is transmitted
back down to Earth. This delay is quite noticeable, and you may notice
it when talking on international calls.
MEOS (Medium Earth Orbiting Satellite) - This type of orbit is within 6,000 -
12,000 miles above Earth. Approximately a dozen medium Earth orbiting
satellites are necessary to provide continuous global coverage 24 hours a
day. Several MEOS systems are now in development, most notably Bill Gates
and Craig McCaw's Teledesic project, which will ultimately attempt to provide
Internet access to all corners of the globe (all under Microsoft software, of
course :) ).
LEOS (Low Earth Orbiting Satellite) - This type of orbit is generally within
the 500 - 5,000 mile altitude range. Although the satellite footprint is
greatly reduced, global coverage can be accomplished through a network of
satellites, in which if an uplink is required to be transmitted to a location
outside of the footprint, the transmission is passed from satellite to
satellite until it reaches the satellite which has the location within its
footprint. As there is no noticeable delay for signal transmission, low Earth
orbiting satellites are becoming the preferable method of voice transmission,
with numerous companies currently attempting to establish LEO satellite
networks, most notably Motorola's Iridium project (see www.iridium.com)
----[ 3] The Global Positioning System
--[ 3.0] Overview
The Global Positioning System was originally designed for, and is still used
by the U.S. military. GPS is funded, controlled, and maintained by the
United States Department of Defense (DOD), although there are thousands of
civilian users of GPS worldwide. The GPS project was first initiated by the
DOD in 1973, and the first experimental GPS satellite was launched in February
1978. The GPS system achieved full operational capability (FOC) on July
17, 1995. The original scope of the GPS for military operation has been far
outgrown by civilian operations, and is provided free of charge or
restrictions (actually, it's paid for by our tax dollars). The system
provides continuous, highly accurate positioning anywhere on the planet (where
the radio signals are not impeded), 24 hours a day. The system is composed of
3 segments, described in the following sections: space, control, and user.
--[ 3.1] Accuracy
GPS currently provides two levels of point positioning accuracy, the Precise
Positioning Service (PPS) and the Standard Positioning Service (SPS). Civilian
users worldwide use the SPS without charge or restrictions, and most commercial
receivers are capable of receiving and using the SPS signal. Authorized
military users, however, in possession of cryptographic equipment and specially
equipped PPS receivers (military GPS receivers) may make use of the PPS. SPS
use is intentionally degraded by the DOD, by the use of Selective Availability.
The following table lists PPS and SPS approximate accuracy levels. However,
highly accurate commercial service is possible by using a number of corrective
methods.
PPS SPS
+---------------------+-----------------+-----------------+
| Horizontal Accuracy | 17.8 meters | 100 meters |
+---------------------+-----------------+-----------------+
| Vertical Accuracy | 27.7 meters | 156 meters |
+---------------------+-----------------+-----------------+
| Time Accuracy | 100 nanoseconds | 167 nanoseconds |
+---------------------+-----------------+-----------------+
--[ 3.2] The Space Segment
The Space Segment consists of the actual constellation of GPS satellites. The
GPS Operational Constellation is 24 satellites, orbiting at roughly 12,000
miles above the Earth, and circling the Earth once every 12 hours. The GPS
constellation is placed so that from 5 to 8 satellites are always visible from
everywhere on Earth. The 24 satellites are placed in 6 orbital planes, and
inclined at approximately 55 degrees to the equatorial plane. GPS operation
requires a clear line of sight, and the signals cannot penetrate soil, water,
or walls very well, so satellite visibility can be affected by those factors.
--[ 3.3] The Control Segment
The Control Segment of the GPS system is essentially the tracking and
maintenance section. The Control Segment consists of a large system of
tracking stations located around the world, of which 3 have uplink capability
with GPS satellites. All GPS data collected from these stations is sent to
the Master Control Center (MCS), located at Schriever Air Force Base in
Colorado, for analysis. The MCS then calculates the satellite's exact orbital
parameters (ephemeris), as well as clock corrections, and uploads them to GPS
satellites over an unknown frequency, at least once a day. Each satellite is
equipped with precise atomic clocks, allowing them all to maintain synchronous
GPS time until the next update.
--[ 3.4] The User Segment
The GPS User Segment is the wide collection of GPS receivers, and the entire
GPS user community (both civilian and military). A GPS receiver converts
input signals from the satellites into position, velocity, and time estimates.
The primary function of GPS, however, is navigation in three dimensions. In
effect, a GPS position calculation can be reduced to a simple trigonometry
problem, that of distance intersection. If one knows the distance from an
unknown point to three known points, it is possible to calculate the x, y,
and z coordinates of the unknown point. The GPS problem is complicated
slightly more by the fact that the radio signal travel time is unknown.
However, this simply means taking measurements from at least four satellites.
Usually multiple satellite signals are used, if possible, as redundant
measurements will add considerable strength to the solution.
--[ 3.5] Satellite Transmissions
GPS satellites transmit two microwave carrier signals, the L1 frequency at
1575.42 MHz, and the L2 frequency at 1227.60 MHz, although for SPS uses only
the L1 frequency is used. The L1 frequency carries the navigation message and
SPS code signals, and the L2 frequency is used to measure ionospheric delay
by PPS equipped receivers. Also UHF signals are used for intra-satellite
links.
--[ 3.6] GPS Packet Format
The navigation message is a continuous 50 BPS date stream modulated onto the
carrier signal of every satellite. The data is transmitted in frames of 1500
bits each, and thus each frame takes 30 seconds to transmit. Each frame is
divided into subframes of 300 bits each. Each subframe is divided into 10
words of 30 bits each, of which 6 bits in each is for parity, and the rest
is for data content. Words one and two of every subframe have the same
format, as shown in the picture. The first word, called the telemetry word,
is composed of an 8-bit preamble used by the GPS receiver to correctly decode
the data, 16 bits of data, and a final 6 bits for parity. Word two, known as
the handover word, contains 17 bits indicating the time of week according to
the satellite's clock when the end of the subframe will be transmitted, known
as the Z-count.
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
+---------------+-------------------------------+-----------+
| 8-bit preamble| Data Content | Parity |
+---------------+-------------------------------+-----------+
Telemetry Word
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
+---------------------------------+-------------+-----------+
| 17-bit Time of Week Message | Data | Parity |
+---------------------------------+-------------+-----------+
Handover Word
Subframes 1, 2, and 3 contain the high accuracy ephemeris and clock offset
data, and the data in these frames can remain constant for hours at times.
Subframes 4 and 5 contain the almanac data and some related configuration data.
An entire set of twenty five frames (125 subframes) makes up the complete
Navigation Message which is sent over a 12.5 minute period.
.____.____.________________________________________.
Subframe 1 | TW | HOW| Clock Offset Data |
`----'----'----------------------------------------'
.____.____.________________________________________.
Subframe 2 | TW | HOW| Orbital Data Set I |
`----'----'----------------------------------------'
.____.____.________________________________________.
Subframe 3 | TW | HOW| Orbital Data Set II |
`----'----'----------------------------------------'
.____.____.________________________________________.
Subframe 4 | TW | HOW| Other Data (configuration data, etc.) |
`----'----'----------------------------------------'
.____.____.________________________________________.
Subframe 5 | TW | HOW| Almanac Data |
`----'----'----------------------------------------'
4 Glossary
----------
Note that many of these acronyms are not used in this article, but are included
to allow the reader to understand other technical GPS documents.
DPGS - Differential GPS
Ephemeris - Precise orbital parameters
GDOP - Geometric Dilution of Precision
GLONASS - The Russian Equivalent of GPS
GPS - Global Navigation System
MCS - Master Control Station
PPS - Precise Positioning Service
PRN - Pseudo Random Noise
RMS - Root Mean Square
SEP - Spherical Error Probable
SPS - Standard Positioning Service
SV - Space Vehicle
UTC - Universal Coordinated Time
----[ 5] Conclusion
I apologize for the extreme brevity of this article, but there is somewhat of
a lack of information regarding technical aspects of the GPS system. Don't
worry, though, I will be submitting some cool telco stuff to phrack later :).
Until, next time, visit the following websites for more information on
telecommunications in general:
http://www.internettrash.com/users/e5/ [My page]
[No Satellite Info yet]
http://www.internettrash.com/users/bft/ [BFT]
----[ EOF