A technique for reducing the error in GNSS-derived positions by using additional data from a reference GNSS receiver at a known position. The most common form of DGNSS involves determining the combined effects of navigation message ephemeris and satellite clock errors (including the effects of SA) at a reference station and transmitting pseudorange corrections, in real time, to a user's receiver, which applies the corrections in the process of determining its position.


DGPS is a technique for reducing the error in GPS-derived positions by using additional data from a reference GPS receiver at a known position. The most common form of DGPS involves determining the combined effects of navigation message ephemeris and satellite clock errors at a reference station and transmitting pseudorange corrections, in real time, to a user's receiver, which applies the corrections in the process of determining its position.
More recently, Precise Point Positioning (PPP) augmentation methods are used, where very precise satellite orbit and clock parameters are calculated, based on observations from a global reference network, and broadcast to users.


DOP is a dimensionless number that accounts for the contribution of relative satellite geometry to errors in position determination. DOP has a multiplicative effect on the range errors. Generally, the wider the spacing between the satellites being tracked by a GPS receiver, the smaller the position error. The most common quantification of DOP is through the position dilution of precision (PDOP) parameter. PDOP is the number that, when multiplied by the root mean square range error, gives the rms position error. Other DOPs include the geometric dilution of precision (GDOP), horizontal dilution of precision (HDOP), and vertical dilution of precision (VDOP).


GLONASS is a space-based satellite navigation system operated by the Russian Aerospace Defence Forces. It provides an alternative to the United States Global Positioning System (GPS) and is currently the only alternative navigational system in operation with global coverage and of comparable precision, although the European Galileo and the Chinese Beidou systems will shortly provide two more systems.
All GLONASS satellites transmit the same code as their standard-precision signal; however each transmits on a different frequency using a 15-channel frequency division multiple access (FDMA) technique, as compared to the GPS code division multiple access(CDMA) technique.
Many of the more professional higher performance GNSS receivers now available can integrate both GPS and GLONASS observations in their position calculations, taking advantage of a combined constellation of around 50 satellites. This large constellation is particularly valuable in built up or otherwise obstructed areas providing a greater opportunity for range observations.
TerraStar uses both GPS and GLONASS data as standard.


GNSS is the generic term for any satellite system that is used to pinpoint the geographic location of a user's receiver anywhere in the world. Two GNSS systems are currently in operation: the United States' Global Positioning System (GPS) and the Russian Federation's Global Orbiting Navigation Satellite System (GLONASS).
Both of these employ a constellation of orbiting satellites working in conjunction with a network of ground stations. The accuracy and integrity of GNSS can be improved by augmenting it with correction data to minimise the system errors. Such augmentation is usually based upon a concept referred to as differential in which case GNSS becomes DGNSS (differential global navigation satellite system).
In future, two additional systems will be added to the GNSS family with the introduction of the European Galileo system and the Chinese COMPASS (CNSS) or BEIDOU system.


The Global Positioning System is a space-based satellite navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. The system provides critical capabilities to military, civil and commercial users around the world. It is maintained by the United States government and is freely accessible to anyone with a GPS receiver.
The GPS project was developed in 1973 to overcome the limitations of previous navigation systems, integrating ideas from several predecessors, including a number of classified engineering design studies from the 1960s. GPS was created and realized by the U.S. Department of Defense and was originally run with 24 satellites. It became fully operational in 1995.
The inherent accuracy of GPS was such a great improvement on all medium and long range terrestrial navigation systems, and it is so cheap and convenient to use, that it now forms the backbone for virtually all navigation in the world today.
Even before it became fully operational, it became clear that augmentation systems, whether governmental or commercial, could enhance the accuracy of GPS, whilst providing systems combining that accuracy with integrity, reliability, availability and continuity of service. Such qualities are valuable to a broad spectrum of professional positioning users.
As GPS is basically a very accurate clock system, it is also used for many timing applications, such as the synchronization of mobile telephony networks.


Inmarsat is an international consortium chartered in the mid-1970s to provide improved maritime public correspondence and radio determination capabilities. Now a private organisation, Inmarsat's primary asset is a global network of geostationary L-Band communications satellites which provide voice and data communications to users anywhere on earth within the footprint coverage of its satellites.
TerraStar uses channels on the Inmarsat satellites to broadcast the TerraStar augmentation data.


L-Band frequencies represent the segment of the microwave portion of the radio spectrum nominally between 1 and 2 GHz. L-Band frequencies are commonly used in mobile satellite communications systems because of the robust propagation performance characteristics in poor weather conditions, such as heavy precipitation.


The Network Control Centre is at the heart of the TerraStar system. Data from all the reference stations is gathered via the internet into the Network Control Centre, validated, and presented to the Orbit and Clock Determination System which calculates the correction data to be broadcast to users.
The Network Control Centre also performs all the monitoring, management and control functions for the complete system infrastructure, which are key to the overall quality of the system, assuring high levels of accuracy, availability, integrity, and continuity of service.


This committee of the National Marine Electronics Association developed a standard for interfacing marine electronic devices. The standard is now widely used for interfacing GPS receivers to other systems in all environments, and most GPS receivers have adopted this standard.

The standard defines a number of messages with varying combinations of data. They are in ASCII format, in the form of comma delimited strings. String lengths vary from 30 to 100 characters and are output at the rate chosen, which depends on application, but typically 5 times per second. The most common string (or sentence) is called the "GGA" string. It contains the time of the fix, latitude, longitude, and height, number of satellites used in the fix, DOP, differential status, and the age of the differential correction.


The Orbit and Clock Determination system uses the observed data from the TerraStar global network of GNSS reference stations to derive very accurate orbit and clock parameters for each of the GNSS satellites. Given that the orbit and clock are so precisely known, a user's receiver will then provide a highly accurate position.



Precise Point Positioning is a Global Navigation Satellite System (GNSS) positioning technique which calculates very precise positions at a few centimeters accuracy level using a single receiver. PPP methods are different from DGNSS positioning methods in that the PPP approach combines precise clocks and orbits calculated from a global network to calculate a precise position with a single receiver, normally a dual frequency unit.
Typically, accuracies of some 4-8cm are achievable on a global basis.


The Special Committee 104 of the Radio Technical Commission for Maritime Services that developed recommended standards for DGPS augmentation data transfer. The latest Standard includes message structures for DGPS, DGLONASS, RTK, and PPP based augmentations. Virtually all DGNSS-capable receivers use this standard.

As with many Standards, RTCM SC 104 is evolving with the technology and markets, and the latest version is RTCM SC104 V3.


Real Time Kinematic satellite navigation is a technique used to enhance the precision of position data derived from satellite-based positioning systems, being usable in conjunction with GPS, GLONASS and/or Galileo. It uses measurements of the phase of the signal′s carrier wave, rather than the information content of the signal, and relies on a single reference station to provide real-time corrections, providing up to centimetre-level accuracy.

Normally, satellite navigation receivers must align signals sent from the satellite to a pseudorandom binary sequence, also contained in the signal. Since the satellite signal takes time to reach the receiver, the two sequences do not initially coincide; the satellite's copy is delayed in relation to the local copy. By increasingly delaying the local copy, the two copies can eventually be aligned. That aligned delay represents the travel time needed for the signal to reach the receiver, and hence range.
The accuracy of the range measurement is a function of the ability of the receiver to accurately process signals from the satellite. In general receivers are able to align the signals to about 1% of one code bit-width, which represents about 3 metres on GPS C/A code and 30 cm on the GPS military P(Y) code.
RTK follows the same general concept, but uses the satellite signal's carrier wave as its signal. The improvement possible using this signal is potentially very high if one continues to assume a 1% accuracy in carrier locking, corresponding to a ±1.9 mm error in baseline estimation.
The difficulty in making an RTK system is properly aligning the signals. The coded navigation signals are deliberately encoded in order to allow them to be aligned easily, whereas every cycle of the carrier is similar to every other. This makes it extremely difficult to know if you have properly aligned the signals or if they are "off by one" and are thus introducing an error of 20 cm, or a larger multiple of 20 cm. This integer ambiguity problem can be addressed to some degree with sophisticated statistical methods that compare the measurements from the C/A signals and by comparing the resulting ranges between multiple satellites. However, none of these methods can reduce this error to zero.


Based on the latest technology provided by the IBM webSphere suite of products, the TerraStar eCommerce System supports the growth of TerraStar's global service and enhances the customer experience.
This solution allows customers to easily subscribe online to the TerraStar services, utilising the automated over the air activations built into the data broadcast. Users have the ability to choose the exact levels of service and work location to tailor the solution to their needs, and can activate immediately or schedule the service as best suits their application.


The Up Link Station is the ground facility used to provide uplink services for satellite data prior to broadcast to users.


Wide Area DGPS (or Wide Area DGNSS) is a system that supports wide-area or regional augmentation through the use of additional satellite-broadcast messages. Such systems are commonly composed of multiple ground stations, located at accurately-surveyed points. The ground stations take measurements of one or more of the GNSS satellites, the satellite signals, or other environmental factors which may impact the signal received by the users. Using these measurements, information messages are created and sent to one or more satellites for broadcast to the end users. WADGPS is sometimes synonymous with SBAS, Satellite Based Augmentation Service.


WGS 84 is a set of parameters, established by the U.S. Defence Mapping Agency, for determining geometric and physical geodetic relationships on a global scale. The system includes a geocentric reference ellipsoid; a coordinate system; and a gravity field model. The ellipsoid is essentially that of the International Union of Geodesy and Geophysics Geodetic Reference System 1980. The coordinate system is a realization of the conventional terrestrial system, as established by the International Earth Rotation Service. The descriptions of the GPS satellite orbits in the navigation message are referenced to WGS 84.