What is GPS System & Its Working

The navigation system based on satellite like Global Positioning System (GPS) is made up of a 24 satellite network located into orbit through the U.S. DoD (Department of Defense). This system is mainly designed for military applications; however, the government made the system accessible in the year 1980 for civilian use. This system performs in any kind of environment around the world for 365 days at any time. The GPS includes 24 satellites that rotate around the sphere one time for every 12 hours to offer worldwide time, position & velocity information. The main function of GPS is to identify the locations on the globe precisely by determining the distance from the satellites. This system lets you create otherwise record exact locations on the globe & assist you to navigate from those locations. Basically, this system was mainly designed for military applications but in the year 1980, it was made accessible for civilian use. This article discusses an overview of the GPS system and its working & uses.

What is GPS System?

Definition: The term GPS full form is “Global Positioning System” which is a satellite navigation system that furnishes location and time information in all climate conditions to the user. GPS is used for navigation in planes, ships, cars, and trucks also. The system gives critical abilities to military and civilian users around the globe. GPS provides continuous real-time, 3-dimensional positioning, navigation, and timing worldwide.

How does GPS System Work?

The GPS consists of three segments:

• The space segment: the GPS satellites
• The control system, operated by the U.S. military,
• The user segment, which includes both military and civilian users and their GPS equipment.

Space Segment

The space segment is the number of satellites in the constellation. It comprises 29 satellites circling the earth every 12 hours at 12,000 miles in altitude. The function of the space segment is utilized to route/navigation signals and to store and retransmit the route/navigation message sent by the control segment. These transmissions are controlled by highly stable atomic clocks on the satellites. The GPS Space Segment is formed by a satellite constellation with enough satellites to ensure that the users will have, at least, 4 simultaneous satellites in view from any point at the Earth’s surface at any time.

Control Segment

The control segment comprises a master control station and five monitor stations outfitted with atomic clocks that are spread around the globe. The five monitor stations monitor the GPS satellite signals and then send that qualified information to the master control station where abnormalities are revised and sent back to the GPS satellites through ground antennas. The control segment also referred to as a monitor station.

User Segment

The user segment comprises the GPS receiver, which receives the signals from the GPS satellites and determines how far away it is from each satellite. Mainly this segment is used for the U.S military, missile guidance systems, civilian applications for GPS in almost every field. Most of the civilians use this from survey to transportation to natural resources and from there to agriculture purpose and mapping too.

How Accurate GPS is?

At present, GPS receivers are very accurate and their accuracy mainly depends on numerous variables which include the ionosphere, the available satellites, the urban environment, etc. There are some factors that obstruct GPS accuracy like the following.

Physical Obstructions

The measurements of arrival time can be skewed through large masses such as buildings, mountains, trees, etc.

Atmospheric Effects

GPS devices mainly affected by solar storms, heavy storm cover, Ionospheric delays, etc.

Ephemeris

In a satellite, the orbital model could be inaccurate otherwise outdated, even though this is becoming increasingly rare.

Numerical Miscalculations

This might be a feature once the hardware of the device is not planned to conditions.

Artificial Interference

Artificial interference mainly comprises spoofs or GPS jamming devices. In open places, the accuracy of the device is high with no contiguous big buildings that can obstruct signals. So, this effect is called an urban canyon. Once a device is enclosed through large buildings, first the satellite signal can be blocked, after that bounced off a tall building, wherever it is lastly read through the device to result in faults of the satellite distance.

Fortunately, the issues faced by GPS technology have been recognized & are nearing resolution. Here, the accuracy provided by the receivers with high quality is better than 2.2-meter level accuracy within 95% of cases & superior as compared to 3-meter accuracy within 99% of cases.

How GPS Determines a Position

The working/operation of the Global positioning system is based on the ‘trilateration’ mathematical principle. The position is determined from the distance measurements to satellites. From the figure, the four satellites are used to determine the position of the receiver on the earth. The target location is confirmed by the 4^th satellite. And three satellites are used to trace the location place.

A fourth satellite is used to confirm the target location of each of those space vehicles. The global positioning system consists of a satellite, control station, and monitor station, and receiver. The GPS receiver takes the information from the satellite and uses the method of triangulation to determine a user’s exact position.

GPS is used on some incidents in several ways, such as:

• To determine position locations; for example, you need to radio a helicopter pilot the coordinates of your position location so the pilot can pick you up.
• To navigate from one location to another; for example, you need to travel from a lookout to the fire perimeter.
• To create digitized maps; for example, you are assigned to plot the fire perimeter and hot spots.
• To determine the distance between two different points.

Other GPS Systems

There are different GPS systems available around the world like GNSS (Global Navigation Satellite System. The GNSS system is classified into four types like GPS by the US, GLONASS by Russia, Galileo by EU, BeiDou by China. In addition, there are two regional systems like QZSS by Japan & IRNSS/NavIC by India.

GPS System Trackers

Generally, there are four kinds of GPS trackers available where some trackers are used for tracking vehicles and others are used for monitoring people.

Personal Trackers

These trackers are mainly used to monitor people/pets. Generally, these trackers use a personal device namely a pocket chip otherwise bracelet to work. After that, the devices will be turned ON. Once they turned ON, operators can remotely place & track the device.

The GPS tracking devices used for dogs are called GPS-equipped collars. These devices play a key role to track pets like dogs. So these collars give peace of mind as owners can continually track & place their dogs.

Asset Trackers

Asset trackers such as tiny radio chips to big satellite tags are used for non-vehicular items like personal trackers. This kind of tracking is used in supermarkets in order to stop a burglary. So a new solution for this is asset tracking.

Once using these trackers, cart theft in supermarkets can be decreased. In addition, some trackers will increase by recognizing items within the cart, matching them to the loyalty card of the buyer & sharing that through the advertising team!

Cell-based Vehicle Tracking

This kind of tracking can be done through either satellite/cellular networks which are almost certainly one of the most common kinds of GPS tracking. This kind of tracking is frequently used as compared to satellite tracking.

This kind of system utilizes a device to capture data from the vehicle & after that reports the data by using cell towers. As compared to satellite tracking, this kind of vehicle tracking is less costly & reports quicker. Generally, delivery companies utilize GPS tracking to make simpler the workflow of customer service like calling their vehicle & ask for their place to inform their customers.

Satellite-based Vehicle Tracking

As compared to satellite-based vehicle tracking, cell-based tracking systems will not work properly as the towers of cells are occupied. Satellite tracking provides the best solution to this trouble because these networks can obtain updates from the most remote areas.

An Alaskan trucking corporation mainly covers distant areas. The main GPS tracking challenge is to get updates even once cell towers are not accessible. These trackers provide stable updates which were utilized by vehicle drivers to call for help once their cell phones didn’t function.

Sources of GPS Signal Errors

GPS signal & its accuracy can be effected through the following factors

Ionosphere & Troposphere Delays

Satellite signals get slow because they flow throughout the atmosphere. So this system utilizes a fixed model to partially correct the error.

Signal Multipath

The GPS signal may reflect off objects like big rock surfaces, tall buildings before it arrives at the receiver to enhance the signal’s travel time & cause mistakes.

Receiver CLK Errors

An in-built GPS clock in the receiver may include small timing mistakes as it is low accurate as compared to atomic clocks over GPS satellites.

Orbital Errors

The reported place of the satellite may not be correct.

Number of Satellites Noticeable

The accuracy mainly depends on when a GPS receiver notices a number of satellites. Once a signal is blocked, then you may get location errors. Usually, GPS units will not work underground, however, new receivers with high-sensitivity are capable to follow some signals once in buildings otherwise under tree-cover.

Satellite Geometry or Shading

Satellite signals are very effective once satellites are placed at broad angles instead of in a tight grouping or line.

Selective Availability

Once the U.S. DoD is applied SA (Selective Availability) to satellites, then signals will be less accurate to maintain ‘enemies’ by using GPS signals which are extremely accurate. To enhance the accuracy for civilian GPS receivers, then the government turned off Selective Availability in the year 2000, which enhanced the civilian GPS receiver’s accuracy.

Codes & Services of GPS

Every GPS satellite is used to transmit two signals with different frequencies like L1 & L2. A simple technique like Trilateration is used to find the location like the Longitude, Latitude & Elevation of the GPS receiver. This technique is also used to measure the location of an unidentified point using three identified points

GPS Codes

GPS codes are available in two types like the following.

• C/A code or Coarse Acquisition Code
• P-Code or Precise Code

The C/A code can be defined as the signal with ‘L1’ frequency is changed through 1.023 Mbps pseudo-random bit series and it is utilized by the public. Similarly, the signal with ‘L2’ frequency can be changed with a 10.23 Mbps pseudo-random bit series, so this is known as precise code. This code is mainly utilized in military positioning systems. Usually, this type of code is transmitted within an encrypted format, called Y code.

The P-code provides superior measurement as compared to coarse acquisition code, as the bit rate of this code is higher as compared to the bit rate of Coarse Acquisition Code.

GPS Services

GPS system provides two kinds of services like the following.

• PPS or Precise Positioning Service
• SPS or Standard Positioning Service

The precise positioning service receivers always track the two codes like C/A code & P-code on both the signals with two frequencies like L1 & L2. At the receiver, the Y-code is decrypted to get P-code whereas, SPS receivers track simply coarse acquisition code on a signal with L1.

Using a GPS Receiver

In the GPS system, there exists simply one-way communication from satellite to consumers. So, each user does not require the transmitter, however simply a GPS receiver. It is mostly utilized to discover the precise location of an entity. It executes this task through the signals obtained from satellites. The GPS receiver’s block diagram is shown below where each block’s function is present within the receiver that is stated below.

Receiving Antenna

This antenna gets the satellite signals and it is mostly an antenna with circularly polarized.

LNA (Low Noise Amplifier)

This kind of amplifier amplifies the weak received signal

Down Converter

This kind of converter changes the signal’s frequency which is received to an IF (Intermediate Frequency) signal.

IF Amplifier

This kind of amplifier is used to change the IF (Intermediate Frequency) signal.

ADC

Analog to digital converter is used to perform the signal conversion from analog to digital. Analyze the two blocks namely the sampling as well as quantization which are present within Analog to Digital Converter.

DSP

The digital signal processor produces the coarse acquisition code.

Microprocessor

The microprocessor executes the computation of position & gives the timing of the signal to manage the process of adding digital blocks. It transmits the useful data toward the display unit to exhibit it on the display.

There are several different models and types of GPS receivers. While working with a GPS receiver it is important to have :

• A compass and a map.
• A downloaded GPS cable.
• Some extra batteries.
• Knowledge about the memory capacity of the GPS receiver to prevent loss of data, decrease inaccuracy of data, or other problems.
• An external antenna whenever possible, especially under the tree canopy, in canyons, or while driving.
• A setup GPS receiver according to incident or agency standard regulation; coordinate system.
• Notes that describe what you are saving in the receiver.

GPS Error

There are many sources of possible errors that will degrade the accuracy of positions computed by a GPS receiver. The travel time taken by the GPS satellite signals can be changed by atmospheric effects; when a GPS signal passes through the ionosphere and troposphere it is refracted, causing the speed of the signal to be different from the speed of a GPS signal in space. Another source of error is noise, or distortion of the signal which causes electrical interference or errors inherent in the GPS receiver itself.

The information about satellite orbits will also cause errors in determining the positions because the satellites are not really where the GPS receiver “thought” based on the information it received when they determine the positions. Small variations in the atomic clocks onboard the satellites can translate to large position errors; a clock error of 1 nanosecond translates to 1 foot or 3 meters user error on the ground.

A multipath effect occurs when signals transmitted from the satellites bounce off a reflective surface before getting to the receiver antenna.  During this process, the receiver gets the signal in a straight-line path as well as the delayed path (multiple paths). The effect is similar to a ghost or double image on a TV set.

Geometric Dilution of Precision (GDOP)

Satellite geometry can also affect the accuracy of GPS positioning. This effect is referred to as the Geometric Dilution of Precision (GDOP). Which is refers to where the satellites are about one another and is a measure of the quality of the satellite configuration. It can be able to modify other GPS errors. Most GPS receivers select the satellite constellation that will give the least uncertainty, the best satellite geometry.

GPS receivers usually report the quality of satellite geometry in terms of Position Dilution of Precision, or PDOP. PDOP is of two types, horizontal (HDOP) and vertical (VDOP) measurements (latitude, longitude, and altitude). We can check the quality of the satellite positioning the receiver is currently available by the PDOP value.

A low DOP indicates a higher probability of accuracy, and a high DOP indicates a lower probability of accuracy. Another term of PDOP is TDOP (Time Dilution of Precision). TDOP refers to the satellite clock offset. A GPS receiver can set a parameter known as the PDOP mask. This will cause the receiver to ignore satellite configurations that have a PDOP higher than the limit specified.

Selective Availability (SA)

Selective Availability occurs when the DOD intentionally degraded; the accuracy of GPS signals is introducing artificial clock and ephemeris errors. During the implementation of SA, it was the largest component of GPS error, causing an error of up to 100 meters. SA is a component of the Standard Positioning Service (SPS).

Advantages

The advantages of a GPS system include the following.

• GPS satellite-based navigation system is an important tool for military, civil and commercial, users
• Vehicle tracking systems GPS-based navigation systems can provide us with turn by turn directions
• Very high speed

Disadvantages

The disadvantages of a GPS system include the following.

• GPS satellite signals are too weak when compared to phone signals, so it doesn’t work as well indoors, underwater, under trees, etc.
• The highest accuracy requires line-of-sight from the receiver to the satellite, this is why GPS doesn’t work very well in an urban environment.

Uses of GPS System

GPS systems are very flexible & we can found this system in every industry sector. At present, GPS plays a key role to map forests; assist farmers in harvesting their fields & pilots uses airplanes to navigate the earth otherwise in the atmosphere. These systems are essential parts of military applications & for crisis crews to situate people in need of assistance. These technologies are frequently working in several regions that we do not usually consider.

Generally, GPS systems fall into five main categories which include the following.

• Location is used to determine a location
• Navigation is used to get from one place to another
• Tracking is used to monitor the object otherwise personal movement
• Mapping is used to create maps of the globe
• Timing is to bring an exact time to the globe

In each case, using a GPS is mainly depends on measuring the distance of individuals from numerous satellites. GPS is simply one of the different satellite groups which are used to decide a location. The four major satellite groups used worldwide are GPS, GLONASS, Galileo & BeiDou. These technologies mainly employ signals using these satellites to determine the distance of individuals from all these satellites. So, these measurements can recognize wherever an individual is within the world & how to find the way to another position.

Thus, this is all about an overview of the GPS system, working, components, advantages, disadvantages & its applications. GPS system is mainly used to determine extremely specific timing through rubidium clocks over every satellite. These clocks allow the satellites to recognize their exact location at very exact times. The information of time can be used in a variety of applications like precision agriculture, marine autonomous & vehicle hydrography.