-------[ 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