DGPS: Everything you need to know

DGPS: Like the popular saying, before Abraham, Jesus was; which goes down to DGPS and GPS.

But there is no comparison between the two because in this case, DGPS is far more better than GPS.

Lets take you down the memory lane.

What is a GPS?

gps navigation

GPS as an acronym stands for Global Positioning System which is a satellite navigation system used to determine the ground position of an object.

The ground position of an object which reflects in digit form is generally known as a “Coordinate”.

In the 1960s, the first GPS technology was used by the United States military and expanded into civilian use over the next few decades.

Today, GPS receivers are included in many commercial products, such as automobiles, smartphones, exercise watches, and GIS devices.

24 satellites are included in the GPS system which is deployed in space about 12,000 miles (19,300 kilometers) above the earth’s surface.

They orbit the earth once every 12 hours at an extremely fast pace of 7,000 miles per hour (11,200 kilometers per hour).

The satellites are evenly spread out so that four satellites are accessible via direct line-of-sight from anywhere on the globe.

Each GPS satellite broadcasts a message that includes the satellite’s current position, orbit, and exact time.

Triangulation is a process used whereby a GPS receiver combines the broadcasts from multiple satellites to calculate its exact position.

Three satellites are required in order to determine a receiver’s location,

though a connection to four or more satellites is ideal since it provides greater accuracy.

GPS devices works accurately when it must first establish a connection to the required number of satellites.

Depending on the strength of the receiver, the process can take anywhere from a few seconds to minutes.

Elements of GPS

GPS consists of three segments, which is the Space, User and the Control segment.

The Space Segment:

The Space Segment

The space segment comprises the satellite constellation, up-link and down-link satellite links which transmits radio signal to users.

The United States of America decided to be in-charge for the

maintenance of the availability minimum of 24 operational GPS satellites, and 95% of the time.

The Air Force has been flying 31 operational GPS satellites for the past few years.

GPS satellites fly in medium Earth orbit (MEO) at an altitude of approximately 20,200 km (12,550 miles).

Each satellite rotates round the Earth twice a day.

The satellites in the GPS constellation are arranged into six equally-spaced orbital planes surrounding the Earth.

User Segment:

User Segment:

The user segment consists of the GPS receiver equipment (handheld GPS),

which receives the signals from the GPS satellites and uses the transmitted information to calculate the user’s three-dimensional position and time.

GPS receivers are composed of an antenna, tuned to the frequencies

transmitted by the satellites, receiver-processors, and a highly-stable clock (crystal oscillator).

They might include a display for providing location and speed information to the user.

A receiver is often described by its number of channels, signifying how many satellites it can monitor simultaneously.

Control Segment:

control segment

The GPS control segment consists of a global network of ground facilities

that track the GPS satellites, monitor their transmissions, perform analyses, and send commands and data to the constellation.

The current Operational Control Segment (OCS) includes a master control

station, an alternate master control station, 11 command and control antennas, and 16 monitoring sites.

Constellation Arrangement

Each plane contains four “slots” occupied by baseline satellites.

This 24-slot arrangement is the reason why users from any point on the planet can virtually view at least four satellites.

The GPS constellation delivers consistently high performance thanks to the

dedicated efforts of its operators of the U.S. Air Force’s 2nd Space Operations Squadron (2SOPS)

and the Air Force Reserve’s 19th Space Operations Squadron (19SOPS) at Schriever Air Force Base, Colorado.

Functions of GPS

Generally, there are five(5) main functions of a GPS, which are

Location: — A GPS helps in the determining of a position on the earth surface. It helps user to be able to know its location on ground.

Navigation —GPS can direct you to get to any place located on earth. Most smartphones contains GPS receiver and helps them to drive easily from one unknown position to the other.

Tracking — It helps in the monitoring of any object or personal movement. The GPS provides a tracking systems where you can see what you are looking for on it.

Mapping — It also helps in the creation of maps of any or all part of the world.

Timing — The GPS makes it possible to take precise time measurements.

The most accurate handheld GPS for surveying

The most accurate unit which we have tested was the Garmin GPSMAP 66st.

Garmin GPSMAP 66st.

Garmin GPSMAP 66st is the only unit that was able to access all three of those satellite networks, and was able to get within 10 feet of accuracy.

The handheld hiking GPS is with 3” Color Display, TOPO Maps and GPS/GLONASS/GALILEO Support.

Its features access to BirdsEye Satellite Imagery subscription with direct-to-device downloads to help you find your way plus preloaded TOPO U.S. and Canada maps.

Garmin GPSMAP 66st. Specifications and price

General

Physical dimensions2.5″ x 6.4″ x 1.4″ (6.2 x 16.3 x 3.5 cm)
Display size240 x 400 pixels
Display typeTransflective color TFT
Weight8.1 oz (230 g) with batteries
Battery type2 AA batteries (not included); NiMH
or Lithium recommended
Battery lifeUp to 16 hours
Up to 170 hours in expedition mode
Water RatingIPX7
MIL-STD-810yes (thermal, shock, water)
High-sensitivity receivergood
Interfacehigh speed micro USB and
NMEA 0183 compatible
Memory/History16 GB (user space varies based
on included mapping)
ColorsBlack and dark ash

Click here for more Specifications and price

Accuracy of handheld GPS in surveying.

Commercial grade handheld GPS units are able to obtain coordinates with a

horizontal accuracy of approximately 3 meters if the unit can receive a wide area augmentation system (WAAS) signal;

otherwise, the accuracy is approximately 10 meters. This type of GPS handheld unit provides elevation data with poor accuracy.

GPS Error Source

A thought experiment (Wormley, 2004): Attach your GPS receiver to a tripod.

Turn it on, and record its position every ten minutes for 24 hours. Next day, plot the 144 coordinates your receiver calculated.

What do you suppose the plot would look like?

Do you imagine a cloud of points scattered around the actual location? That’s a reasonable expectation.

Now, imagine drawing a circle or ellipse that encompasses about 95 percent of the points.

What would the radius of that circle or ellipse be? (In other words, what is your receiver’s positioning error?)

The answer depends in part on your receiver.

If you used a hundred-dollar receiver, the radius of the circle you drew might be as much as ten meters to capture 95 percent of the points.

If you used a WAAS-enabled, single frequency receiver that cost a few

hundred dollars, your error ellipse might shrink to one to three meters or so.

But if you had spent a few thousand dollars on a dual frequency, survey-grade receiver, your error circle radius might be as small as a centimeter or less.

As the market for GPS positioning grows, receivers are becoming cheaper.

Still, there are lots of mapping applications for which it’s not practical to use a survey-grade unit.

For example, if your assignment was to GPS 1,000 manholes for your

municipality, you probably wouldn’t want to set up and calibrate a survey-grade receiver 1,000 times.

How, then, can you minimize errors associated with mapping-grade receivers? A sensible start is to understand the sources of GPS error.

GPS Error Correction

The quality of GPS coordinates is reduced by a variety of factors, which

includes the clocks in satellites and receivers,

the atmosphere, satellite orbits, and reflective surfaces near the receiver.

The arrangement of satellites in the sky can make matters worse (a condition called dilution of precision).

Random errors can be partially overcome by simply averaging repeated

fixes at the same location, although this is often not a very efficient solution.

Systematic errors can be compensated for by modeling the phenomenon

that causes the error and predicting the amount of offset.

Some errors, like multi-path, are caused when GPS signals are reflected

from roads, buildings, and trees, vary in magnitude and direction from place to place.

Other factors, including clocks, the atmosphere, and orbit eccentricities,

tend to produce similar errors over large areas of the Earth’s surface at the same time.

This type of error will correct using a method known as a differential correction.

Differential correction is a class of techniques for improving the accuracy of

GPS positioning by comparing measurements taken by two or more receiver

DGPS

dgps instrument

A Differential Global Positioning System (DGPS) is an enhancement to the Global Positioning System (GPS) which provides improved location

accuracy, in the range of operations of each system, from the 15-meter nominal GPS accuracy to about 1-3 cm in case of the best implementations.

It is essentially a system to provide positional corrections to GPS signals.

DGPS uses a fixed, known position to adjust real time GPS signals to eliminate pseudorange errors.

An important point to note is that DGPS corrections improve the accuracy of position data only.

What is the principle of DGPS?

Differential GPS (DGPS) works by placing a master GPS receiver that is a reference station at a known location.

The station measures the ranges to each satellite. Then it uses the measured ranges and the actual ranges calculated from its known position.

It will then use the measured ranges and its actual ranges calculated from its known position.


Measured ranges can contain errors such as ephemeris data errors or internal receiver noise.

In astronomy and celestial navigation, an ephemeris (plural: ephemerides) gives the trajectory of naturally occurring astronomical objects as well as

artificial satellites in the sky, which is the position (and possibly velocity) over time.

The difference between measured and calculated ranges becomes a “differential correction”.

Differential correction is a class of techniques for improving the accuracy of GPS positioning by comparing measurements taken by two or more receivers.

The differential correction will then transmitted to the DGPS receivers.

What is the difference between GPS and DGPS?

S/NGPSDGPS
1Accuracy = 15metresAccuracy = 10 cm
2 The instrument can be used globallyThe instrument are used locally within 100km
3The accuracy reduces when there is an atmospheric blockage like trees or roofs or clouds.The accuracy will start to degrade once the instrument distance from ground based transmitters start to increase.
4Its a collection of number of satellites in the space sending the precise location details in the space back to EarthIts an enhancement to the GPS (Global Position System).
5It rely on at-least 4 to 8 satellites.It rely on two stations one is base station and next is rover.
6The handheld device receive signal from the satellite to obtain the ground positionThe hand held device (rover) receives calibrated signal from the ground based transmitter.
7GPS system is affordable which is why all smart phones have built-in GPS system.DGPS is times 15 (x15) much expensive than GPS.
8In GPS, satellite transmit signal in frequency ranging from 1.1 to 1.5 GHz.In DGPS frequency varies by agencies, here is the list of frequency used by different agency.
9GPS accuracy is highly depend upon the number of satellites used for the calculationDGPS accuracy is not affected by these variables, it might be affected by the distance between transmitters and the instrument (rover).

DGPS: What is the accuracy of GNSS receivers in smartphones?

What is GNSS?

DGPS: GNSS

GNSS stands for Global Navigation Satellite System, and is an umbrella term that encompasses all global satellite positioning systems.

This includes constellations of satellites orbiting over the earth’s surface and continuously transmitting signals that enable users to determine their position.

DGPS: What is the accuracy of GNSS receivers in smartphones?

There are generally two cases that are of interest: Code measurements and phase carrier measurements.

Due to the low-quality GNSS antenna in the smartphone, and because of poor multi-path mitigation, the noise on the code has an RMS

(average value) of about 5 meters, compared to an approximate RMS of 0.20 m for geodetic receivers.

This, of course, also depends on the number of satellites available.

By averaging several measurements, the accuracy can reach about 1.5-2 meters horizontally and 3 meters vertically.

The height can be better estimated by using more accurate ionosphere

estimates (e.g. IONEX or local estimates from two-frequency receiver).

For the phase, I managed to produce a 3D precision of 5 mm by getting fix-solutions from the static-relative method.

I measured statically for two hours for two points and got a 5 mm precision for both.

This, however, was hard to produce with the kinematic method, even while

the smartphone was lying still, some intervals of a fix were achieved.

Hope this gives some insight into the matter.

dgps: accuracy of gnss
DGPS: accuracy of gnss

What is the difference between terms DGPS and Relative GPS Positioning (in real time)?

“Some people think that DGPS only applies to GPS code measurements techniques. 

There are two concepts to obtain positions (and other estimates) out of GPS/GNSS observations:

  • Absolute positioning
  • Relative positioning

Differential GPS techniques uses code observations only, the accuracy is around 0.5 – 1.0m depending on the equipment used in the application.

With this technique, it is possible to achieve respectable estimates by using single-frequency receivers only.

You are right by the term of relative positioning is of use in the combination with the phase measurements,

for example during coordinate estimation in networks (dual frequency, code+carrier, single/double differentiating).

One good example for real time applications is RTK (real time kinematic), which is also a relative method,

but uses the combination of code and phase and is applied to

dual-frequency equipment/observations.

I believe there is always the need to name the concept and the method/technique:

  • concept: absolute positioning: (method: single point positioning (SPP) for code; precise point positioning (PPP) for code+phase observations)
  • concept: relative positioning (method: Code – DGPS/DGNSS, Code+Carrier: Double differentiating (DD), single differentiating (SD), real time kinematic (RTK), etc.)

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