About Satellite Navigation

Global satellite navigation is an exciting technology, providing enhanced productivity and accuracy in a vast number of industries. It adds a new level of enjoyment and safety to a wide range of navigation, sports and recreational activities.

A Global Navigation Satellite System (GNSS) is a network of satellites that transmits high-frequency radio signals containing time and distance data that can be picked up by a receiver, allowing the user to pinpoint their precise location anywhere around the globe.

There are two Global Navigation Satellite Systems currently in operation; the U.S. Global Positioning System (GPS) and the Russian GLObal NAvigation Satellite System (GLONASS). These systems are constantly being upgraded to meet higher standards of reliability. A third GNSS named GALILEO, after the Italian astronomer of the early 1600s, is currently being developed in Europe to specifically provide a higher standard of integrity and reliability, required to ensure the safety of lives during transport by air, land and sea without the use of additional augmentation systems.

While the GPS and GLONASS satellite networks are being developed to achieve maximum performance, Satellite-Based Augmentation Systems (SBAS) have been established to provide improved accuracy. SBAS provides differential signal corrections for GPS and GLONASS transmissions with the use of ground stations and geostationary satellites in specific regions.

This is GNSS-1, the first phase in establishing the required integrity for high-precision satellite navigation. GNSS-2 requires the launching of new satellites into orbit and a complete upgrade of the existing satellite systems. This second phase is already well underway. GALILEO, scheduled to begin service in 2008, is being developed to meet the standards of GNSS-2 for rapid and reliable certified precision positioning.

Source: http://corp.magellangps.com

Data Link

Data links are Differential Global Positioning System (DGPS) radio signals transmitted from a reference station or relay antenna at a known location to one or more rover receivers, to provide the positioning correction data that gives Magellan GPS systems maximum accuracy, up to within a centimeter. Data links allow reference stations, relay transmitters and rover receivers to communicate calculations performed at each location to build a preponderance of data that is cross-checked at every point for verified precision.

These data links are critical to collecting reliable GPS data. Magellan has developed data link solutions for optimum system integration, and hardware and software compatibility with the highest market standards, to ensure superior performance and absolute reliability.

A number of factors including range, update rate and error correction rate determine the quality and specific application of a data link.

Magellan (also Pacific Crest and Satel) has developed data link solutions that are readily adaptable to any application, overcoming the limitations to accuracy associated with vehicle tracking, DGPS, Local Area Differential GPS (LADGPS) and general data transfer.

There are three types of data links deployed in Magellan GPS systems for high-performance accuracy:

1) HF data link operates in the HF frequency between 1.6 MHz and 3.5 MHz. Primarily designed for LADGPS marine applications, HF data link is optimized for a meter-accurate update rate and easy government frequency allocation. Using a BCPSK modulation, advanced data compression, and high-performance GPS signal correction algorithms, HF data link offers superior accuracy for marine applications. With an operating range of 100 to 800 kilometer, HF data link is integrated into a number of Ashtech Marine Survey & Navigation GPS solutions.

2) UHF data link operates in the UHF frequency between 410 MHz and 470 MHz. Optimized for LADGPS or Real-Time Kinematic (RTK) marine and land survey applications, UHF data link is available in a number of configurations for maximum performance according to the requirements of specific Magellan GPS systems. UHF data link can be configured with a DQPSK modulation, operating at 1200 baud. This configuration is compatible with earlier DGPS and Kinematic Applications in Real Time (KART) applications and uses NDS100MkII signal format. When configured with a GMSK modulation and Long Range Kinematic (LRK) signal format, UHF data link is optimized for LRK or KART kinematic applications. A third option uses a common signal format known as RTCM. UHF data link has a range of 80 kilometers and is integrated into a number of Ashtech Marine Survey & Navigation and Land Survey GPS solutions.

3) UHF bi-directional data link also operates in the UHF range and offers additional flexibility in configuring data processing functionality. Based on the Time Division Multiple Access (TDMA) principle, UHF bi-directional data link uses an MSK modulation optimized for relative positioning systems, used in marine seismic exploration and fleet management.

Source: http://corp.magellangps.com

DGPS (Differential Global Positioning System)

A Differential Global Positioning System (DGPS) is a system designed to improve the accuracy of Global Navigation Satellite Systems (GNSS) by measuring infinitesimal changes in variables to provide satellite positioning corrections.

Two or more receivers observe the same set of satellites, taking similar measurements that produce similar errors when positioned closely together. A reference receiver, placed at a known location, calculates its theoretical position and compares it to the measurements provided by the navigation satellite signals. The difference between the two values reveals the measurement error. The reference receiver then transmits a corrected signal to any number of receivers at unknown positions within the area covered by the DGPS. Accuracy of global satellite positioning is thereby increased from 15 meters to within a few meters. This technique compensates for errors in the satellite navigation system itself but may not always correct errors caused by the local environment when satellite navigation signals are reflected off of tall buildings or nearby mountains, creating multi-path signals. The accuracy of DGPS decreases with asynchronous measurement caused by spatial and temporal error decorrelation when the system receivers are set at greater distances apart.

More sophisticated DGPS techniques can increase positioning accuracy to within a few millimeters. Raw measurements recorded by the reference receiver and one or more roving receivers can be processed using specially designed software that calculates the errors. The corrections may then be transmitted in real time or after the fact (post-processing). By applying the corrections and recalculating the position, accuracy from within several meters to within a few millimeters is achieved, depending on the specific methodology used and the quality of the real-time data link.

DGPS Methods
Satellite navigation receivers calculate position by measuring pseudo distances from the positioning satellites. These measurements are taken in several different ways. The most common method, used by all receivers, is to calculate the difference between the time a signal is transmitted from a satellite and the time it is recorded by the receiver, using the code embedded in the satellite's signal. This measurement is called code phase, and produces non-ambiguous meter-level results.

There are three types of DGPS using code phase measurement methods. DGPS and LADGPS (Local Area DGPS) typically cover an area up to several tens of kilometers. The coverage area is greatly increased up to several thousand kilometers by a more sophisticated method known as WADGPS (Wide Area DGPS). WADGPS classifies errors into position-dependent and position-independent components creating a secondary set of measurements that are transmitted to the rover receivers. The rover receivers are then able to reconstruct the pseudo range correction most applicable to their actual position and compute an accurate differential position.

A second method serves to compliment code phase measurement by measuring the carrier phase of the satellite carrier wave. This method provides millimeter-level resolution with measurements that are ambiguous to about 19 centimeters.

DGPS using the carrier phase achieves maximum accuracy only when measurement ambiguities are resolved in some way. The static method of ambiguity resolution is related to stationary receivers, with rover receiver point occupation from 30 minutes to several hours or even several days. The rapid static method reduces occupation periods to several minutes, while the kinematic method allows rover receivers to move without constraint.

Source: http://corp.magellangps.com

RTK (Real Time Kinematic)

RTK is a process where GPS signal corrections are transmitted in real time from a reference receiver at a known location to one or more remote rover receivers. The use of an RTK capable GPS system can compensate for atmospheric delay, orbital errors and other variables in GPS geometry, increasing positioning accuracy up to within a centimeter. Used by engineers, topographers, surveyors and other professionals, RTK is a technique employed in applications where precision is paramount. RTK is used, not only as a precision positioning instrument, but also as a core for navigation systems or automatic machine guidance, in applications such as civil engineering and dredging. It provides advantages over other traditional positioning and tracking methods, increasing productivity and accuracy.

Using the code phase of GPS signals, as well as the carrier phase, which delivers the most accurate GPS information, RTK provides differential corrections to produce the most precise GPS positioning.

The RTK process begins with a preliminary ambiguity resolution. This is a crucial aspect of any kinematic system, particularly in real-time where the velocity of a rover receiver should not degrade either the achievable performance or the system's overall reliability.

Magellan has developed two innovative, high-performance technology solutions for RTK ambiguity resolution; KART (Kinematic Applications in Real Time) for single-frequency receivers and LRK® (Long Range Kinematic) for dual-frequency receivers.

KART (Kinematic Applications in Real Time)
KART is a kinematic method that allows any initialization mode, from a static or fixed reference point, to On The Fly (OTF) ambiguity resolution, when performing single-frequency GPS positioning.

This unique technology introduces a fundamentally different strategy than that of other traditional methods, and delivers results that are substantially more reliable with single-frequency GPS L1 receivers.

Conventional ambiguity resolution follows this procedure:

1) Define a search area based on an approached solution and its uncertainty.
2) Test all potential solutions within the area statistically.
3) Select the most likely solution among the possible solutions, according to a minimal variance criterion.
4) Validate the chosen solution according to statistical criteria or by comparison with the second best candidate.

In theory, this process is correct and perhaps reasonable under certain circumstances. However, Magellan has found it to be inadequate when applied to single-frequency operations in the field for two fundamental reasons:

1) When only GPS L1 frequency data is available, the search area contains numerous potential solutions positioned closely together. The actual position may appear to have the minimal variance for some time.
2) The statistical tests and criteria that are used cannot provide reliable results unless the functional and a priori stochastic models used can correctly represent reality, which in practice is not the case.

Any reliable ambiguity resolution in real-time using this approach with a single-frequency receiver is nearly impossible. That is why Magellan has created a new approach.

KART operates by a simple and reliable method:

1) Calculate approximate position using the EDGPS process, which offers no ambiguity, is unbiased, and tends towards the true position.
2) Re-calculate the EDGPS result from the unambiguous pure phase solution, providing the minimal risk of error in ambiguity resolution.
3) Validate the solution position using the residuals of the least-squares adjustment.
4) Confirm the solution by observing its stability over time. Resolutions of the conventional method will tend to drift.

KART technology does not depend on an a priori stochastic model to achieve and confirm a kinematic solution, making it a faster, easier, more reliable and cost-effective method. KART makes real-time kinematic operation possible with single-frequency receivers in applications otherwise impossible without dual-frequency measurements.

LRK (Long Range Kinematic)
Magellan's LRK technology is a sophisticated kinematic method that optimizes the advantages of dual-frequency GPS operation. Other conventional methods use the dual-frequency only during initialization. LRK makes solving ambiguities during initialization easy and continuous dual-frequency kinematic operation possible at distances up to 40 kilometers.

Conventional dual-frequency kinematic operation is limited to about 10 kilometers, using a combined observation on GPS L1 and L2 frequencies to produce an initial wide lane solution, ambiguous to around 86 centimeters. During a second phase, the conventional kinematic method uses measurements from the L1 frequency only. This method only allows for kinematic operation as long as the de-correlation of atmospheric errors is compatible with a pure phase single-frequency solution.

Similar to the KART process, LRK is a simple and reliable method that allows any initialization mode, from a static or fixed reference point, to On The Fly ambiguity resolution, when performing dual-frequency GPS positioning. LRK technology reduces initialization times to a few seconds by efficiently using L2 measurements in every mode of operation. LRK maintains optimal real-time positioning accuracy to within a centimeter at a range up to 40 kilometers, even with a reduced number of visible satellites.

Source: http://corp.magellangps.com

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