GPS Module:
What is GPS?
The Global Positioning System, usually called GPS, and originally named NAVSTAR, is a satilight navigation system used for determining one's precise location almost anywhere on Earth . A GPS unit receives time signal transmissions from multiple satellites, and calculates its position by triangulating this data. The GPS was designed by and is controlled by the United States Department of Defence and can be used by anybody for free. The cost of maintaining the system is approximately $400 million per year.
Measurement uncertainty of the majority of commercial GPS receivers varies from 10-11 to 10-13 by the frequency scale, and from 100 ns to 50 ns by the time scale, being dependent on the receiver design. The main sources of uncertainty in GPS measurements are the GPS receiver position error, the orbital error, the satellite and receiver clock errors, the ionosphere and the troposphere delays, the receiver internal delay, the satellite antenna and cable delay, the receiver noise, and the multipath error. The frequency uncertainty for a GPS receiver is larger than that for Cs-standard by 2-3 orders within a short-time interval (1 – 1000 s), and by one order within a long-term interval of about one week. A GPS receiver’s time scale bias from the Universal Coordinated Time (UTC) can be in order of microseconds. Not every GPS receiver is suitable for use as a traceable primary frequency and/or time standard. Therefore, GPS receiver calibration against the primary time and frequency standard (the Cs-atomic clock traceable to UTC) is of great importance and implies the calibration of both output frequencies and time scale. The frequencies calibration is performed through comparison of one of the outputs (1 pps, 10 MHz, and/or 5 MHz) with the corresponding reference frequency of the cesium atomic standard. The user’s GPS receiver time scale calibration is fulfilled through comparison between 1 pps signals of the user’s GPS receiver and those of the Cs-atomic standard.
GPS is a system of satellites, ground control stations, and receivers that allows users to determine their position. By capturing and storing that position, GPS receivers “digitize” spatial data as they walk, drive, or otherwise traverse the land. For this reason the term “rover” receiver unit is often used to describe a GPS field receiver. For the sake of consistency, the term “GPS receiver” or “receiver” will be used to identify the GPS “rover” receiver from this point forward. Perhaps the most important characteristic that GIS data developers need to realize about GPS is that it is a highly dynamic system with new satellites being launched and old ones being retired. The constellation of satellites available to users throughout the day is constantly changing as the satellites move through their orbits. Occasionally, satellites are shifted into new orbits. Collecting GPS data in Vermont often adds the complications of vegetative cover, topography, and relatively northern latitude to the inherent variability in the system. Receivers differ in their ability to receive and process GPS signals and users can have a huge affect on accuracy depending on the methods used to collect and process data.
It may help to think of a GPS receiver as similar to a standard radio. Like the radio in your car, a GPS receiver is collecting radio signals from the “ether” and magically turning these signals into information we can use. In the case of a GPS receiver the “stations” are satellites broadcasting 11,000 miles away in space and the music is a binary code, but the antenna and radio hardware are subject to similar kinds of interference that affect your car radio’s ability to produce a clear sound. In your car speaker we hear this interference as “static”; in your GPS receiver the interference may result in positional “static”, i.e., degradation of accuracy. A better radio receiver and antenna system, fewer terrain obstructions, a stronger connection to the broadcasting station all result in better sound quality for your car radio and better positional quality for your GPS radio. A GPS receiver derives its location or positional “fix” with distance measurements (called pseudo ranges) from multiple satellites at precisely the same time, a “measurement epoch”. Attributes collected and stored with the position for each feature can be used in GIS map making and analysis. While there are only so many things you can do to improve your car radio’s performance, by contrast there are many more things users can do to influence GPS positional quality.
This document presumes the reader has only minimal GPS experience. Section II begins with a discussion of GPS receiver types and their appropriateness for different types of data collection. Section III covers other features of GPS that can affect accuracy and can be controlled by the user.
How does GPS work?
Satellites
The United States Global Positioning System (GPS) is the first fully operational Global Navigation Satellite System (GNSS). Each satellite broadcasts a signal that is used by receivers to determine precise position anywhere in the world. The receiver tracks multiple satellites and determines apseudo range measurement (a range measurement based on time) that is then used to determine the user location. A minimum of four satellites is necessary to establish an accurate three-dimensional position.
The Department of Defense (DOD) is responsible for operating the GPS satellite constellation and monitors the GPS satellites to ensure proper operation. Every satellite's orbital parameters (ephemeris data) are sent to each satellite for broadcast as part of the data message embedded in the GPS signal. The GPS coordinate system is the Cartesian earth-centered earth-fixed coordinates as specified in the World Geodetic System reference system 1984 (WGS-84).
24 GPS satellites are currently in orbit around the earth. the first was launched in 1972 and the latest satellite was launched in 2012. The maximum available at any time from a point in Oregon is generally between 4 to 11. The satellites send out radio signals that are collected and read by the GPS receiver.
List of GPS satellite launches
· In 1972, the USAF Central Inertial Guidance Test Facility (Holloman AFB), conducted developmental flight tests of two prototype GPS receivers over White Sands Missile Range, using ground-based pseudo-satellites.
· In 1978, the first experimental Block-I GPS satellite was launched.
· In 1983, after Soviet interceptor aircraft shot down the civilian airliner KAL 007 that strayed into prohibited airspace because of navigational errors, killing all 269 people on board, U.S. President Ronald Reagan announced that GPS would be made available for civilian uses once it was completed although it had been previously published [in Navigation magazine] that the CA code (Coarse Acquisition code) would be available to civilian users.
· By 1985, ten more experimental Block-I satellites had been launched to validate the concept. Command & Control of these satellites had moved from Onizuka AFS, CA and turned over to the 2nd Satellite Control Squadron (2SCS) located at Falcon Air Force Station in Colorado Springs, Colorado
· On February 14, 1989, the first modern Block-II satellite was launched.
· In 1992, the 2nd Space Wing, which originally managed the system, was de-activated and replaced by the50th Space Wing.
· By December 1993, GPS achieved initial operational capability (IOC), indicating a full constellation (24 satellites) was available and providing the Standard Positioning Service (SPS).
· Full Operational Capability (FOC) was declared by Air Force Space Command (AFSPC) in April 1995, signifying full availability of the military's secure Precise Positioning Service (PPS).
· In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset.
· In 1998, United States Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety and in 2000 the United States Congress authorized the effort, referring to it as GPS III.
· On May 2, 2000 "Selective Availability" was discontinued as a result of the 1996 executive order, allowing users to receive a non-degraded signal globally.
· In 2004, the United States Government signed an agreement with the European Community establishing cooperation related to GPS and Europe's planned Galileo system.
· In 2004, United States President George W. Bush updated the national policy and replaced the executive board with the National Executive Committee for Space-Based Positioning, Navigation, and Timing.
· In 2005, the first modernized GPS satellite was launched and began transmitting a second civilian signal (L2C) for enhanced user performance.
· On September 14, 2007, the aging mainframe-based Ground Segment Control System was transferred to the new Architecture Evolution Plan.
· On May 19, 2009, the United States Government Accountability Office issued a report warning that some GPS satellites could fail as soon as 2010.
· On May 21, 2009, the Air Force Space Command allayed fears of GPS failure saying "There's only a small risk we will not continue to exceed our performance standard."
· On January 11, 2010, an update of ground control systems caused a software incompatibility with 8000 to 10000 military receivers manufactured by a division of Trimble Navigation Limited of Sunnyvale, Calif.
· On February 25, 2010. the U.S. Air Force awarded the contract to develop the GPS Next Generation Operational Control System (OCX) to improve accuracy and availability of GPS navigation signals, and serve as a critical part of GPS modernization.
· A GPS satellite was launched on May 28, 2010. The oldest GPS satellite still in operation was launched on November 26, 1990, and became operational on December 10, 1990.
· The GPS satellite, GPS IIF-2, was launched on July 16, 2011 at 06:41 GMT from Space Launch Complex 37B at the Cape Canaveral Air Force Station.
· The GPS satellite, GPS IIF-3, was launched on October 4, 2012 at 12:10 GMT from Space Launch Complex 37B at the Cape Canaveral Air Force Station.
GPS receivers:
GPS receivers can calculate a position on the earth by measuring the travel time of radio signals from the satellites to the receiver. The calculations depend on highly accurate clocks. The satellites have atomic clocks that are accurate to a nanosecond but due to cost, the clocks in most GPS receivers are not that accurate. Using three satellites, each measurement of time generates a sphere. Where these three spheres intersect is a point that indicates a place on the earth. The fourth satellite can then be used to eliminate any clock errors in the ground-based receiver. Even a small clock error can create a large error in location.
When Global Navigation Satellite System (GNSS) equipment is not using integrity information from Wide Area Augmentation Systems (WAAS) orLocal Area Augmentation Systems (LAAS), the GPS navigation receiver usingReceiver Autonomous Integrity Monitoring (RAIM) provides GPS signal integrity monitoring. RAIM is necessary since delays of up to two hours can occur before an erroneous satellite transmission can be detected and corrected by the satellite control segment. The RAIM function is also referred to as fault detection. Another capability, fault exclusion, refers to the ability of the receiver to exclude a failed satellite from the position solution and is provided by some GPS receivers and by WAAS receivers.
The GPS receiver verifies the integrity (usability) of the signals received from the GPS constellation through Receiver Autonomous Integrity Monitoring(RAIM) to determine if a satellite is providing corrupted information. At least one satellite, in addition to those required for navigation, must be in view for the receiver to perform the RAIM function; thus, RAIM needs a minimum of five satellites in view, or four satellites and a barometric altimeter (baro-aiding) to detect an integrity anomaly.
For receivers capable of doing so, RAIM needs six satellites in view (or five satellites with baro-aiding) to isolate the corrupt satellite signal and remove it from the navigation solution. Baro-aiding is a method of augmenting the GPS integrity solution by using a no satellite input source. GPS derived altitude should not be relied upon to determine aircraft altitude since the vertical error can be quite large and no integrity is provided. To ensure that baro-aiding is available, the current altimeter setting must be entered into the receiver as described in the operating manual.
RAIM messages vary somewhat between receivers; however, generally there are two types. One type indicates that there are not enough satellites available to provide RAIM integrity monitoring and another type indicates that the RAIM integrity monitor has detected a potential error that exceeds the limit for the current phase of flight. Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position.
RAIM Capability
Many VFR GPS receivers and all hand-held units have no RAIM alerting capability. Loss of the required number of satellites in view, or the detection of a position error, cannot be displayed to the pilot by such receivers. In receivers with no RAIM capability, no alert would be provided to the pilot that the navigation solution had deteriorated, and an undetected navigation error could occur. Only a systematic cross-check with other navigation techniques would identify this failure, and prevent a serious deviation.
GPS receivers and software can be used to obtain positions with accuracies from centimeter to tens of meters. The mapping/resource grade receivers that are the focus of this document are generally able to obtain positions with accuracies from five decimeters to ten meters. Some mapping/resource grade receivers can also be used as remote base stations. There are estimated to be over 500 GPS receiver models available from over 100 different manufacturers around the world. Competition has improved the products and reduced the cost, but has also confused the buyer. The table, illustrated in Part D below, is offered as a generic guideline to available GPS products.
A civilian GPS receiver is generally categorized as (1) recreational grade, (2) mapping/resource grade, or (3) survey grade, based on its functionality. The characteristics of each of these GPS “grades” are briefly described below, and then listed in a table for easier comparison.
Recreational grade GPS receivers are the least expensive and the simplest to use, because they have less functionality (and less associated software and hardware) than the other grades. As the name implies, these “handheld” GPS receivers are intended primarily for recreational purposes. These are useful for general navigation and surveillance purposes because they can quickly collect the x-y coordinates of point features, and can be used to pre-plan routes and/or navigate to specific locations using waypoints. They are not, however, recommended for most data field collection or mapping activities. Though some of the more expensive recreational grade receivers come with a communication port that allows for the download and post-processing of data or come with a radio receiver that provides for real-time differential correction of data, most receivers in this class do not. This fact not only limits the accuracy of features collected but also prevents the user from downloading captured features digitally. The only recourse to creating spatial data with these receivers is to manually transcribe feature coordinates into a format that can be imported into a GIS.
Popular recreational receivers can be expected to produce horizontal positions with an accuracy of approximately 10 meters under clear tracking conditions. Positioning under canopy can reduce this accuracy to approximately 30 meters, or worse, depending on tracking conditions.
This guideline recommends that GPS data being collected for GIS utilize mapping/resource grade receivers. Mapping/resource GPS tools capture data of higher positional accuracy than recreational receivers, and all have post-processing differential correction capabilities. Unlike recreational GPS, these receivers also collect locations for features represented as points (e.g., sample point), lines (e.g., trail), and areas (e.g., field boundary), complimenting GIS database organization schemes. Mapping/resource GPS equipment required in the field ranges from “handheld” to “backpack” systems. The more expensive mapping/resource grade GPS receivers are designed to: (1) collect and store large volumes of data, (2) be used in extreme environmental conditions, (3) perform real-time differential correction of data; and 4) act as a field reference base station. The average accuracy for this grade of GPS receivers varies and changes as technology develops, but at this time accuracy is generally between .5-5 meters when data is “post-processed” or acquired “real time”, under typical data collection constraints.
Most professional-grade receivers differ from the lower-end resource or mapping grade receivers in the hardware and techniques used to process the GPS signal. Often called “narrow correlation” receivers, these “high-end” receivers provide better performance under difficult conditions (especially tree canopy) while reducing multipath interference (see section below on this subject for details). Both these narrow-correlation receivers and the so-called “standard-correlation” receivers are suitable for most resource grade mapping (2-5 meters), though the narrow-correlation receivers generally achieve these accuracies with less time spent at each point.
This document does not address survey grade activities. Survey grade GPS tools are only used for surveying-related activities requiring a high degree of accuracy. For example, licensed land surveyors use these GPS tools for geodetic surveys, and to measure elevations. These systems produce data of the highest horizontal and vertical positional accuracy, but are very expensive and complex. The use of a survey grade system requires specialized training, and one or more dedicated staff to oversee its use and maintenance. Survey grade GPS data are almost always post-processed to increase their accuracy.
Structure:
The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US). The U.S. Air Force develops, maintains, and operates the space and control segments. GPS satellites broadcast signals from space, and each GPS receiver uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time.
The space segment is composed of 24 to 32 satellites in medium Earth orbit and also includes the payload adapters to the boosters required to launch them into orbit. The control segment is composed of a master control station, an alternate master control station, and a host of dedicated and shared ground antennas and monitor stations. The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial, and scientific users of the Standard Positioning Service (see GPS navigation devices).
Space segment
The space segment (SS) is composed of the orbiting GPS satellites or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three approximately circular orbits, but this was modified to six orbital planes with four satellites each. The orbits are centered on the Earth, not rotating with the Earth, but instead fixed with respect to the distant stars. The six orbit planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection).[47] The orbital period is one-half a sidereal day, i.e., 11 hours and 58 minutes. The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface.[49] The result of this objective is that the four satellites are not evenly spaced (90 degrees) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30, 105, 120, and 105 degrees apart which, of course, sum to 360 degrees.
Orbiting at an altitude of approximately 20,200 km (12,600 mi); orbital radius of approximately 26,600 km (16,500 mi), each SV makes two complete orbits each sidereal day, repeating the same ground track each day. This was very helpful during development because even with only four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.
As of March 2008, there are 31 actively broadcasting satellites in the GPS constellation, and two older, retired from active service satellites kept in the constellation as orbital spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a non-uniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail. About nine satellites are visible from any point on the ground at any one time , ensuring considerable redundancy over the minimum four satellites needed for a position.
Control segment
The control segment is composed of
· a master control station (MCS),
· an alternate master control station,
· four dedicated ground antennas and
· six dedicated monitor stations
The MCS can also access U.S. Air Force Satellite Control Network (AFSCN) ground antennas (for additional command and control capability) and NGA (National Geospatial-Intelligence Agency) monitor stations. The flight paths of the satellites are tracked by dedicated U.S. Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs, Colorado and Cape Canaveral, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC.[53]The tracking information is sent to the Air Force Space Command MCS at Schriever Air Force Base 25 km (16 mi) ESE of Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the U.S. Air Force. Then 2 SOPS contacts each GPS satellite regularly with a navigational update using dedicated or shared (AFSCN) ground antennas (GPS dedicated ground antennas are located at Kwajalein, Ascension Island, Diego Garcia, and Cape Canaveral). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter that uses inputs from the ground monitoring stations, space weather information, and various other inputs.[54]
Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked unhealthy, so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again.
The Operation Control Segment (OCS) currently serves as the control segment of record. It provides the operational capability that supports global GPS users and keeps the GPS system operational and performing within specification. OCS successfully replaced the legacy 1970’s-era mainframe computer at Schriever Air Force Base in September 2007. After installation, the system helped enable upgrades and provide a foundation for a new security architecture that supported the U.S. armed forces. OCS will continue to be the ground control system of record until the new segment, Next Generation GPS Operation Control System [2] (OCX), is fully developed and functional.
The new capabilities provided by OCX will be the cornerstone for revolutionizing GPS’s mission capabilities, and enabling [55] Air Force Space Command to greatly enhance GPS operational services to U.S. combat forces, civil partners and myriad of domestic and international users. The GPS OCX program also will reduce cost, schedule and technical risk. It is designed to provide 50% [56] sustainment cost savings through efficient software architecture and Performance-Based Logistics. In addition, GPS OCX expected to cost millions less than the cost to upgrade OCS while providing four times the capability. The GPS OCX program represents a critical part of GPS modernization and provides significant information assurance improvements over the current GPS OCS program.
· OCX will have the ability to control and manage GPS legacy satellites as well as the next generation of GPS III satellites, while enabling the full array of military signals.
· Built on a flexible architecture that can rapidly adapt to the changing needs of today’s and future GPS users allowing immediate access to GPS data and constellations status through secure, accurate and reliable information.
· Empowers the warfighter with more secure, actionable and predictive information to enhance situational awareness.
· Enables new modernized signals (L1C, L2C, and L5) and has M-code capability, which the legacy system is unable to do.
· Provides significant information assurance improvements over the current program including detecting and preventing cyber attacks, while isolating, containing and operating during such attacks.
· Supports higher volume near real-time command and control capabilities.
User segment
The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often acrystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20 channels.
GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of an RS-232port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM[citation needed]. Receivers with internal DGPS receivers can outperform those using external RTCM data[citation needed]. As of 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers.
Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. Although this protocol is officially defined by the National Marine Electronics Association (NMEA), references to this protocol have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF and MTK protocols. Receivers can interface with other devices using methods including a serial connection, USB, or Bluetooth.
Factors that affect data quality:
Satellite geometry and PDOP
PDOP, or Position Dilution of Precision, is a measure of how well the satellites are distributed in the sky – the lower the number the more accurate the reading. A minimum of four GPS satellites are required to compute accurate three dimensional GPS positions. The best satellite geometry, and therefore the lowest PDOP, has at least four satellites, three evenly spaced around the horizon, but above 15 degrees, and one directly overhead. With the current number of satellites available, collecting data in 2D is never recommended. When “only three satellites” are available, an accurate “project elevation” must be determined and input into the receiver.
Signal to Noise Ratio (SNR)
Also called signal level or signal strength. Arbitrary strength units used to determine the strength of a satellite signal. SNR ranges from 0 (no signal) to around 35. Higher-elevation satellites have SNRs in the high teens to low 20’s.
Multipath
Another possible source of error is interference, similar to ghosts on a television screen, that occurs when GPS signals arrive at an antenna having traversed different paths. The signal traversing the longer path causes an error in the position fix. Multiple paths can arise from reflections off structures and other obstructions near the antenna.
Accuracy of the data under different conditions (PDOP and SNR)
Canopy cover can adversely effect accuracy in two ways. It can reduce the number of satellites the receiver can use and can cause the signal to bounce off of nearby surfaces before reaching the receiver, causing errors in location.Topographic position Steep canyons can limit the number of satellites the receiver can use. They can also limit the spread of satellites across the sky causing poor satellite geometry.
Ability to “turn off” data collection when predicted error is high.
On the Trimble Geo Explorers, Pro XRs and other higher end receivers there is a setting for PDOP that will cause the receiver to not log positions when the PDOP is too high due to poor satellite constellation or when SNR is poor. The recreational units do not have the ability to set these filters and thus will log positions regardless of their accuracy.
The use of Trimble Pathfinder Office Planning Software (which comes with the purchase of a Trimble GPS unit) to predict satellite availability is important for those using GPS receivers under less than ideal conditions, such as under tree canopy or in steep terrain. The number of satellites and PDOP available due to these canopy and terrain constraints can limit field data logging to specific times of day. This software is essential for planning when to log data. Though this planning software was designed for Trimble units, it could also be used to increase the accuracy of recreational units. Planning reduces “wasted time” by predicting exactly when the appropriate number of satellites will be available for each planning session, or data point.
Recommendations for GPS data collection:
Data to be used outside GIS
For data that will never be put into GIS, you can use whatever equipment or method gives you acceptable accuracy for your project (see above accuracy table).
Data to be incorporated into GIS
Project layers
For data that is not used in conjunction with any other corporate data, is used only at a very small scale, or is very time-sensitive, you may use whatever method will give you an acceptable accuracy. This would include things such as firelines, points for a small-scale map (such as the location of a timber sale on a forest-wide map), or for a personal project such as tracking your path of travel during a survey.
Corporate layers
Landlines or legal boundaries must be GPSed with survey-grade GPS units. For other corporate layers, if recreational grade GPS data is all that’s available, it may be incorporated into the corporate data, but must be designated as “>5 meter accuracy” in the metadata for the features you GPSed (see below). This will allow us to exclude this data from applications requiring highly accurate features, like PBS updates. If more accurate data is available from another source it should be used instead.
Metadata for GSPed features
Metadata is necessary to identify the source and accuracy of the data you are adding to GIS. Any GPS data that may eventually be added to a Primary Base Series quad map must have metadata attached. In addition, some GPS data, such as public land survey and ownership boundaries, must meet survey-grade requirements and have metadata to back it up.
National Map Accuracy Standards
The 2003 GIS Data Dictionary requires the horizontal accuracy of most layers to meet the National Standard for Spatial Data Accuracy. At the scale of 1:24,000, this standard is 40 feet or 12.2 meters.
Source Codes for GPS Data
Three source codes for GPS data have been specified in the Guidelines for Digital Base Map Updates. The Draft GPS Data Accuracy Standard (USFS) also refers to these same codes to use for GPS accuracy. CSA 2 has added a fourth code to address greater than 5 meter accuracy (such as data collected with a Garmin). A detailed description of the codes is attached to this document in Appendix A.
How to Record the Metadata
Record the source of your GPS data, using the codes from Appendix A, in the item listed in the table below. The item should be added to the coverage if it is not already there. You will need to coordinate this with a GIS person on your district or Forest (item_width 2, output width 2, item type C.) In addition, an item called source_code_memo can be added to record additional information or explanatory notes about the data, such as the model of the receiver used.
Satellite frequencies:
All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The satellite network uses a CDMA spread-spectrum technique where the low-bitrate message data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. The receiver must be aware of the PRN codes for each satellite to reconstruct the actual message data. The C/A code, for civilian use, transmits data at 1.023 million chips per second, whereas the P code, for U.S. military use, transmits at 10.23 million chips per second. The actual internal reference of the satellites is 10.22999999543 MHz to compensate for relativistic effects that make observers on Earth perceive a different time reference with respect to the transmitters in orbit.
The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code.[74] The P code can be encrypted as a so-called P(Y) code that is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user. The L3 signal at a frequency of 1.38105 GHz is used by the United States Nuclear Detonation (NUDET) Detection System (USNDS) to detect, locate, and report nuclear detonations (NUDETs) in the Earth's atmosphere and near space. One usage is the enforcement of nuclear test ban treaties.
The L4 band at 1.379913 GHz is being studied for additional ionospheric correction.[citation needed] The L5 frequency band at 1.17645 GHz was added in the process of GPS modernization. This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2009. The L5 consists of two carrier components that are in phase quadrature with each other. Each carrier component is bi-phase shift key (BPSK) modulated by a separate bit train. "L5, the third civil GPS signal, will eventually support safety-of-life applications for aviation and provide improved availability and accuracy."
A conditional waiver has recently been granted to LightSquared to operate a terrestrial broadband service near the L1 band. Although LightSquared had applied for a license to operate in the 1525 to 1559 band as early as 2003 and it was put out for public comment, the FCC asked LightSquared to form a study group with the GPS community to test GPS receivers and identify issue that might arise due to the larger signal power from the LightSquared terrestrial network. The GPS community had not objected to the LightSquared (formerly MSV and SkyTerra) applications until November 2010, when LightSquared applied for a modification to its Ancillary Terrestrial Component (ATC) authorization. This filing (SAT-MOD-20101118-00239) amounted to a request to run several orders of magnitude more power in the same frequency band for terrestrial base stations, essentially repurposing what was supposed to be a "quiet neighborhood" for signals from space as the equivalent of a cellular network. Testing in the first half of 2011 has demonstrated that the impact of the lower 10 MHz of spectrum is minimal to GPS devices (less than 1% of the total GPS devices are affected). The upper 10 MHz intended for use by LightSquared may have some impact on GPS devices. There is some concern that this will seriously degrade the GPS signal for many consumer uses. Aviation Week magazine reports that the latest testing (June 2011) confirms "significant jamming" of GPS by LightSquared's system.
Applications:
While originally a military project, GPS is considered a dual-use technology, meaning it has significant military and civilian applications. GPS has become a widely deployed and useful tool for commerce, scientific uses, tracking, and surveillance. GPS's accurate time facilitates everyday activities such as banking, mobile phone operations, and even the control of power grids by allowing well synchronized hand-off switching
Civilian
Many civilian applications use one or more of GPS's three basic components: absolute location, relative movement, and time transfer.
· Cellular telephony: Clock synchronization enables time transfer, which is critical for synchronizing its spreading codes with other base stations to facilitate inter-cell handoff and support hybrid GPS/cellular position detection for mobile emergency calls and other applications. The first handsets with integrated GPS launched in the late 1990s. The U.S. Federal Communications Commission (FCC) mandated the feature in either the handset or in the towers (for use in triangulation) in 2002 so emergency services could locate 911 callers. Third-party software developers later gained access to GPS APIs from Nextel upon launch, followed by Sprint in 2006, and Verizon soon thereafter.
· Clock synchronization: The accuracy of GPS time signals (±10 ns) is second only to the atomic clocks upon which they are based.
· Fleet Tracking: The use of GPS technology to identify, locate and maintain contact reports with one or more fleet vehicles in real-time.
· Geofencing: Vehicle tracking systems, person tracking systems, and pet tracking systems use GPS to locate a vehicle, person, or pet. These devices are attached to the vehicle, person, or the pet collar. The application provides continuous tracking and mobile or Internet updates should the target leave a designated area.
· Geotagging: Applying location coordinates to digital objects such as photographs and other documents for purposes such as creating map overlays.
· GPS tours: Location determines what content to display; for instance, information about an approaching point of interest.
· Phasor measurements: GPS enables highly accurate timestamping of power system measurements, making it possible to computephasors.
· Robotics: Self-navigating, autonomous robots using a GPS sensors, which calculate latitude, longitude, time, speed, and heading.
· Telematics: GPS technology integrated with computers and mobile communications technology in automotive navigation systems
Restrictions on civilian use
The U.S. Government controls the export of some civilian receivers. All GPS receivers capable of functioning above 18 kilometres (11 mi)altitude and 515 metres per second (1,001 kn) are classified as munitions (weapons) for which State Department export licenses are required. These limits attempt to prevent use of a receiver in a ballistic missile. They would not prevent use in a cruise missile because their altitudes and speeds are similar to those of ordinary aircraft. This rule applies even to otherwise purely civilian units that only receive the L1 frequency and the C/A (Coarse/Acquisition) code and cannot correct for Selective Availability (SA), etc. Disabling operation above these limits exempts the receiver from classification as a munition. Vendor interpretations differ. The rule refers to operation at both the target altitude and speed, but some receivers stop operating even when stationary. This has caused problems with some amateur radio balloon launches that regularly reach 30 kilometres (19 mi). These limits only apply to units exported from (or which have components exported from) the USA - there is a growing trade in various components, including GPS units, supplied by other countries, which are expressly sold as ITAR-free.
Military
As of 2009, military applications of GPS include:
· Navigation: GPS allows soldiers to find objectives, even in the dark or in unfamiliar territory, and to coordinate troop and supply movement. In the United States armed forces, commanders use the Commanders Digital Assistant and lower ranks use the Soldier Digital Assistant.
· Target tracking: Various military weapons systems use GPS to track potential ground and air targets before flagging them as hostile.[citation needed] These weapon systems pass target coordinates to precision-guided munitions to allow them to engage targets accurately. Military aircraft, particularly in air-to-ground roles, use GPS to find targets (for example, gun camera video fromAH-1 Cobras in Iraq show GPS co-ordinates that can be viewed with specialized software).
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Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles, precision-guided munitions and Artillery projectiles. Embedded GPS receivers able to withstand accelerations of 12,000 g or about 118 km/s2 have been developed for use in 155 millimeters (6.1 in) howitzers.
· Search and Rescue: Downed pilots can be located faster if their position is known.
· Reconnaissance: Patrol movement can be managed more closely.
· GPS satellites carry a set of nuclear detonation detectors consisting of an optical sensor (Y-sensor), an X-ray sensor, a dosimeter, and an electromagnetic pulse (EMP) sensor (W-sensor), that form a major portion of the United States Nuclear Detonation Detection System.
Conclusions:
Due to performance differences, two user’s GPS receivers can yield different frequency results even when connected to the same antenna at the same location. The frequency uncertainty for user’s GPS receiver is larger than that for Cs-standard by 2-3 orders for a short-time interval (1 – 1000 s), and by one order for a long-term interval of about two weeks.
The user’s GPS receiver time scale bias from the Universal Coordinated Time can be in order of microseconds. It depends on the time delay in GPS receiver, satellite antenna, antenna’s cable, ionosphere correction error, and software errors.
Commercial GPS receiver calibration service provided by INPL ensures measurement uncertainty of about 10-14 in the frequency scale, of about 5 ns in the time scale, as well as traceability to the Universal Coordinated Time.
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