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Introduction:
The Global Positioning System (GPS), originally NAVSTAR GPS, (Navigational System using Time and Ranging) a satellite-based radio navigation system owned by the United States Space Force and operated by Mission Delta. It is one of the global navigation satellite systems (GNSS) that provide geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.
It is owned and operated by The United States Space Force. It consisted of at least 24 operational satellites. The GPS receivers use signals from multiple satellites to calculate their precise location (latitude, longitude, and altitude) and the current time.
It has a wide range of applications, including navigation, surveying, mapping, and timing. It's also used in many everyday devices, such as smartphones, cars, and watches. GPS can provide location accuracy down to a few meters. GPS is one of several Global Navigation Satellite Systems (GNSS). Other GNSS include Russia's GLONASS, Europe's Galileo, and China's BeiDou Navigation Satellite System.
There are no subscription fees or setup charges to use GPS data. GPS works in any weather conditions: It's a reliable and robust system.
The space segment of GPS
It is the backbone of the entire system, consisting of a constellation of satellites that transmit radio signals to GPS receivers on Earth.
The GPS constellation typically consists of 24 to 32 operational satellites. These satellites orbit the Earth in six orbital planes, each inclined at an angle of 55 degrees to the equator. They orbit at an altitude of approximately 20,200 kilometres (12,500 miles) This arrangement ensures that at least four satellites are visible from any point on Earth at any given time.
Satellite Components:
Atomic Clocks: Each satellite carries multiple atomic clocks, which provide extremely accurate timekeeping. This precision is crucial for calculating precise positioning.
Radio Transmitters: These transmitters broadcast radio signals containing information about the satellite's position, time, and other data.
Solar Panels: These provide power to the satellite's systems.
Antennas: These transmit and receive signals to and from the Earth
Functions of the Space Segment:
Signal Transmission: The satellites continuously transmit radio signals to Earth.
Timekeeping: The highly accurate atomic clocks ensure precise timing for accurate positioning calculations.
Position Information: The satellites transmit information about their current position in orbit.
Navigation Message: This message contains information about the satellite health, clock corrections, and other data necessary for GPS receivers to calculate their position.
The space segment is the most critical component of the GPS system, enabling global positioning and navigation. The satellite constellation ensures continuous coverage and reliable signal reception. The highly accurate atomic clocks on the satellites are essential for precise positioning calculations. The space segment is constantly monitored and maintained to ensure optimal performance.
The GPS Control Segment:
It is a network of ground stations responsible for monitoring and controlling the GPS satellites. It ensures the accurate positioning and timing signals that GPS receivers rely on.
The Master Control Station is located at Schriever Air Force Base in Colorado Springs, Colorado,USA. It oversees the entire control segment and processes data from monitor stations to calculate precise satellite orbits and clock corrections. The MCS uploads updated navigation data to the satellites and manages satellite health.
Monitoring Stations:
Monitoring stations are distributed globally. It tracks GPS satellites and collects data on their positions, velocities, and clock errors. It can transmit this data to the MCS for analysis. It also consists of Ground antennas located near monitor stations. It transmits commands and data to the satellites and receives telemetry data from the satellites.
Functions of the Control Segment:
Satellite Tracking: Monitors the orbits of GPS satellites to ensure they remain within specified parameters.
Data Processing: Processes data from monitor stations to calculate precise satellite orbits and clock corrections.
Navigation Data Updates: Uploads updated navigation data to the satellites, including ephemeris, almanac, and clock corrections.
Satellite Health Monitoring: Monitors the health of GPS satellites and takes corrective actions as needed.
Satellite maneuvering: Commands satellites to perform maneuvers to adjust their orbits or maintain their health.
The User segment of GPS
The user segment of GPS encompasses a wide range of individuals and industries who utilize GPS technology for various purposes. Here's a breakdown of the key user segments:
Types of GPS Devices and Their Usage
Portable GPS Devices:
Description: Handheld GPS units designed for outdoor enthusiasts like hikers, campers, and geocachers.
Usage: Navigation on trails, marking waypoints, and finding hidden geocaches.
In-Vehicle GPS Systems:
Description: GPS devices installed in cars and trucks.
Usage: Providing real-time directions, traffic updates, and optimized routes for drivers and logistics.
Smartphone GPS Applications:
Description: Built-in GPS functionality in smartphones, integrated with apps like Google Maps or Waze.
Usage: Everyday navigation, location sharing, and location-based services like food delivery.
Fitness and Wearable GPS Devices:
Description: Fitness trackers and smartwatches equipped with GPS.
Usage: Tracking routes, distance, and pace during activities like running, cycling, or swimming.
Marine GPS Systems:
Description: Specialized GPS devices for ships and boats.
Usage: Navigation, collision avoidance, and route optimization in maritime environments.
Aviation GPS Systems:
Description: GPS systems used in aircraft.
Usage: Supporting pilots in navigation, landing systems, and air traffic management.
Surveying and Mapping GPS:
Description: High-precision GPS devices used in surveying and mapping.
Usage: Creating accurate maps, conducting land surveys, and geospatial data collection.
Agricultural GPS Systems:
Description: GPS-enabled devices for farming equipment.
Usage: Precision planting, fertilization, and harvesting to optimize resources.
GPS Trackers:
Description: Small GPS devices for tracking people, pets, or assets.
Usage: Monitoring movements, ensuring safety, and managing assets in logistics.
Military and Tactical GPS Devices:
Description: Rugged, secure GPS units designed for defense operations.
Usage: Navigation, target acquisition, and mission planning in military settings
Image Source- https://euro-sd.com/2023/10/articles/34779/operations-in-denied-environments/
Working Functions of GPS
Generally, the functions of a GPS are completed with 5 steps.
Step -1: Triangulating from Satellites:
GPS operation is based on the concept of ranging and Trilateration from a group of satellites, which act as precise reference points. Each satellite broadcasts a Navigation Message that contains the following information. A pseudo-random code called a Course Acquisition (CA) code, which contains orbital information about the entire satellite constellation (Almanac). Detail of the individual satellite’s position (Ephemeris) that includes information used to correct the orbital data of satellites caused by small disturbances.
The GPS system time, derived from an atomic clock installed on the satellite, with clock correction parameters for the correction of satellite time due to differences between UTC and GPS time (the occasional ‘leap’ second added to a year) and delays (predicted by a mathematical ionospheric model) caused by the signal travelling through the ionosphere. A GPS health message that is used to exclude unhealthy satellites from the position solution.
The GPS receiver in the aircraft takes 12.5 minutes to receive all of the data frames in the navigational message. Once obtained, the receiver starts to match each satellite’s CA code with an identical copy of the code contained in the receiver’s database. By shifting its copy of the satellite’s code, in a matching process, and by comparing this shift with its internal clock, the receiver can calculate how long it took the signal to travel from the satellite to the receiver.
The distance derived from this method of computing distance is called a Pseudo-range because it is not a direct measure of distance, but a measurement based on time. Pseudo-range is subject to several error sources, including atmospheric delays and multipath errors, but also due to the initial differences between the GPS receiver and satellite time references. Using a process called Trilateration, the GPS receiver then mathematically determines its position by using the calculated pseudo-ranges and the satellite position information that has been supplied by the satellites.GPS_3D-Trilateration. If only one satellite is visible, position location is impossible as the receiver location can be anywhere on the surface of a sphere with the satellite at its center. If two satellites are visible the receiver location can be anywhere on a circle where the surfaces of the two spheres intercept. So, position location is also impossible.
Step-2: Measuring distance from a Satellite:
Normally distances are calculated on GPS is based on signals of a Satellite ranging.
The easy formula to calculate the distance is:
Distance (d) = Speed of satellite ranging (3 x 108 m/second) x time
Time (Δ t) = t2 – t1 where, t1 = sending time, t2 = receiving time
Step- 3: Getting Perfect Timing
If travel time measures through the radio signal are the basics of GPS, then stopwatch are very working instrument in this case. If their time is stopped for one thousandths of a second, then it will wrong at 200 miles. In terms of Satellites, timing is perfect because the Atomic clock is the compulsory element of Satellite systems. The key to accurate scheduling is to measure the distance to an extra satellite. If the three exact measurements can identify the three-dimensional position, then the fourth incorrect measure does the same thing.
Satellite is in Space:
We assume that we know the exact position of Satellites, for which we can use that satellites as a reference point. The satellites float in the space of 11,000 miles.
Step-5: Correcting Errors:
In reality, there are a lot of things which can disrupt the GPS signals. To get the accurate results, this error is likely to be corrected. For example, the ionosphere and atmosphere can delay the whole function. Some errors can be factored out by using arithmetic calculation and model. The relative position of the satellites in the sky can give rise to other errors.
Signals of GPS
The GPS system sends their information through microwave signals.
The signal systems are as below:
Pseudo Random Code (PRC):It is the prime part of GPS. It is a physically complicated digital number or complicated sequence of ‘on’ and ‘off’ pulse. There are 2 types of PRC signals generally found.
Coarse Acquisition Code (C/A): (a) This contains L1 signals. (b) It repeats every 1023 bits & modulates at a 1 MHz rate. (c) C/A code is the basis for civilian GPS uses.
Precise Code (P):
(a) Modulate both L1 & L2 carries at a 10 MHz rate.
(b) Used for Military purposes.
(c) It is more complicated than C/A code.
There are 2 types of Signals: L1 & L2
L1 carries:
L1 carries 1575.42 MHz.
L1 carries both the status message and a pseudo random code for timing.
L2 carries:
L2 carries 1227.60 MHz. (b) Use for the more precise military pseudo random code
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Measurement of Errors of GPS
There are a number of sources of error that corrupt these measurements. An examination of these error sources is presented within this section.
Satellite Clock Error:
The satellites contain atomic clocks that control all on board timing operations, including broadcast signal generation. Although these clocks are highly stable, the clock correction fields in the navigation data message are sized such that the deviation between SV time and GPS time may be as large as 1 ms. (An offset of 1 ms translates to a 300-km pseudo range error.) The MCS determines and transmits clock correction parameters to the satellites for rebroadcast in the navigation message. These correction parameters are implemented by the receiver using the second-order polynomial since these parameters are computed using a curve-fit to predicted estimates of the actual satellite clock errors, some residual error remains.
This residual clock error, δt, results in ranging errors that typically vary from 0.3 -- 4m, depending on the type of satellite and age of the broadcast data. Range errors due to residual clock errors are generally the smallest following a control segment uploaded to a satellite, and they slowly degrade over time until the next upload (typically daily). At zero age of data (ZAOD), clock errors for a typical satellite are on the order of 0.8m. Errors 24 hours after an upload are generally within the range of 1–4m. It is expected that residual clock errors will continue to decrease as newer satellites are launched with better performing clocks and as improvements are made to the control segment. Errors were observed to be statistically independent from satellite to satellite with significant correlation over time.
Ephemeris Error
Estimates of ephemerides for all satellites are computed and uplinked to the satellites with other navigation data message parameters for rebroadcast to the user. As in the case of the satellite clock corrections, these corrections are generated using a curve fit of the control segment’s best prediction of each satellite’s position at the time of upload.
The residual satellite position error is a vector with typical magnitudes in the range of 1–6m. The effective pseudo range and carrier-phase errors due to ephemeris prediction errors can be computed by projecting the satellite position error vector onto the satellite- to-user LOS vector. Ephemeris errors are generally smallest in the radial (from the satellite toward the center of the Earth) direction. The components of ephemeris errors in the along-track (the instantaneous direction of travel of the satellite) and cross track (perpendicular to the along-track and radial) directions are much larger. Along-track and cross-track components are more difficult for the control segment to observe through its monitors on the surface of the Earth, since these components do not project significantly onto LOSs toward the Earth. Fortunately, the user does not experience large measurement errors due to the largest ephemeris error components for the same reason.
Relativistic Effects
Both Einstein’s general and special theories of relativity are factors in the pseudo range and carrier-phase measurement process. The need for Special Relativity (SR) relativistic corrections arises any time the signal source (in this case, GPS satellites) or the signal receiver (GPS receiver) is moving with respect to the chosen isotropic light speed frame, which in the GPS system is the ECI frame. The need for general relativity (GR) relativistic corrections arises any time the signal source and signal receiver are located at different gravitational potentials.
The satellite clock is affected by both SR and GR. In order to compensate for both of these effects, the satellite clock frequency is adjusted to 10.22999999543 MHz prior to launch. The frequency observed by the user at sea level will be 10.23 MHz; hence, the user does not have to correct for this effect. The user does have to make a correction for another relativistic periodic effect that arises because of the slight eccentricity of the satellite orbit. Exactly half of the periodic effect is caused by the periodic change in the speed of the satellite relative to the ECI frame and half is caused by the satellite’s periodic change in its gravitational potential. Due to rotation of the Earth during the time of signal transmission, a relativistic error is introduced, known as the Sagnac Effect, when computations for the satellite positions are made in an ECEF coordinate system. During the propagation time of the SV signal transmission, a clock on the surface of the Earth will experience a finite rotation with respect to an ECI coordinate system. Figure illustrates clearly, if the user experiences a net rotation away from the SV, the propagation time will increase, and vice versa. If left uncorrected, the Sagnac Effect can lead to position errors on the order of 30m. Corrections for the Sagnac Effect are often referred to as Earth rotation corrections.
Atmospheric Effects
The propagation speed of a wave in a medium can be expressed in terms of the index of refraction for the medium.
The index of refraction is defined as the ratio of the wave’s propagation speed in free space to that in the medium by the formula n= c/v Where c is the speed of light equal to 299,792,458 m/s as defined within the WGS84 system. The medium is dispersive if the propagation speed (or, equivalently, the index of refraction) is a function of the wave’s frequency.
Ionospheric Effects
The ionosphere is a dispersive medium located primarily in the region of the atmosphere between about 70 km and 1,000 km above the Earth’s surface.
Within this region, ultraviolet rays from the sun ionize a portion of gas molecules and release free electrons. These free electrons influence electromagnetic wave propagation, including the GPS satellite signal broadcasts.
Tropospheric Delay
The troposphere is the lower part of the atmosphere that is non dispersive for frequencies up to 15 GHz. Within this medium, the phase and group velocities associated with the GPS carrier and signal information (PRN code and navigation data) on both L1 and L2 are equally delayed with respect to free-space propagation.
This delay is a function of the tropospheric refractive index, which is dependent on the local temperature, pressure, and relative humidity. Left uncompensated, the range equivalent of this delay can vary from about 2.4m for a satellite at the zenith and the user at sea level to about 25m for a satellite at an elevation angle of approximately 5º.
Receiver Noise and Resolution
Measurement errors are also induced by the receiver tracking loops. In terms of the DLL, dominant sources of pseudo range measurement error (excluding multipath) are thermal noise jitter and the effects of interference.
The C/A code composite receiver noise and resolution error contribution will be slightly larger than that for P(Y) code because the C/A code signal has a smaller RMS bandwidth than the P(Y) code. Typical modern receiver 1σ values for the noise and resolution error are on the order of a decimeter or less in nominal conditions (i.e., without external interference) and negligible compared to errors induced by multipath. Receiver noise and resolution errors affect carrier phase measurements made by a PLL.
Multipath and Shadowing Effects
One of the most significant errors incurred in the receiver measurement process is multipath.
Multipath errors vary significantly in magnitude depending on the environment within which the receiver is located, satellite elevation angle, receiver signal processing, antenna gain pattern, and signal characteristics.
Other Applications
Surveying and Mapping: GPS is used to create accurate maps and conduct precise land surveys.
Timekeeping: GPS provides highly accurate time signals, used in various applications like telecommunications and financial systems.
Augmented Reality: GPS integrates with AR technology to overlay digital information onto the real world, enhancing user experiences.
CONCLUSION
These are many of the many applications of GPS technology. As technology continues to advance, we can expect even more innovative and impactful uses of GPS in the future.
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