The StarFire™ global subscription service provides real-time accuracy typically better than 10cm (4 inches). Its globally corrected signal is available virtually anywhere on the Earth's surface on land or sea, from 76ºN to 76ºS latitude.
To accomplish this, StarFire™ utilizes a network of more than 60 GPS reference stations around the world to compute GPS satellite orbit and clock corrections. Two completely redundant processing centers and multiple communication links ensure the continuous availability of StarFire™ GPS corrections. These corrections are broadcast via three geostationary satellites, providing worldwide coverage and enabling precise real-time navigation without the need for local ground base stations.
Methodology
The StarFire™ Network is a major advance from earlier ground-based augmentation systems because it considers each of the GPS satellite signal error sources independently. GPS satellite orbit and clock corrections are calculated from a global tracking network of dual-frequency receivers; these corrections are transmitted via Inmarsat satellite links directly to StarFire™ receivers, resulting in worldwide operation and minimal data latency. All StarFire™ dual-frequency GPS receivers measure the ionospheric delay for each satellite; tropospheric zenith delays are calculated from a multi-state time and position model aided by redundant satellite observables.
Reliability
Redundant data links, geographically separated processing hubs, and dual satellite uplink equipment ensure continuous reliable positioning. The system is inherently robust, with the ability to calculate a full set of corrections even if multiple reference stations were to become unavailable. With an uptime of 99.999%, customers can be confident of the global availability and accuracy of the system.
As a Global Satellite-Based Augmentation System (GSBAS), the StarFire™ Network holds a significant advantage over differential GPS (dGPS) systems. Since no base station is required, the StarFire™ user is free to roam, with no limitations on range. StarFire™ provides one set of corrections for the entire world, as opposed to dGPS systems’ baseline-dependent regional corrections, which are geographically limited.
Learn more about GSBAS vs. dGPS systems.
GSBAS vs. dGPS systems
In order to evaluate the satellite positioning options available, the primary error sources contributing to the corrections need to be understood.
Reference and rover errors include:
- Satellite Orbit
- Satellite Clock
- Ionosphere
- Troposphere
- Multipath
- Earth Tides
- Receiver Clock
- Receiver Biases
How these are handled can be placed into two broad classes: differential GPS (dGPS) systems and Global Satellite-Based Augmentation System (GSBAS) systems.
dGPS Systems
dGPS provides relative positioning; these positioning options determine corrections at the Earth’s surface, either at individual sites or averaged over a region. These corrections are an amalgam of primary reference error sources and are transmitted to the user via radio or satellite link. User coordinates are determined relative to the reference site(s). Accuracy degrades with distance from the individual reference site or the regional boundary, primarily because of differential ionospheric and tropospheric effects, satellite orbits, and clocks. Examples of correction services include:
- Beacon DGPS
- Commercial DGPS
- RTK / Network RTK
- Widelane RTK
Widelane RTK is a relative newcomer, which is only available in the few areas of the world that require high accuracy on a regular basis for oil and gas exploration surveys.
GSBAS
Importantly, GSBAS systems provide absolute positioning, meaning they do not determine position relative to some fixed point of the Earth’s surface. Instead, they determine a position within a space-based reference frame, thus they may also be considered to be within the Space Domain group.
Satellite communication links are used for some dGPS systems, so the term GSBAS does not relate to the communication link used but instead describes the nature of the corrections, i.e. that they are for the satellites. This class includes:
- Standalone GPS
- WAAS
- EGNOS
- MSAS
- StarFire™
Augmentation for Standalone GPS consists of satellite orbit and clock updates which are generated by the GPS Ground Control Segment and distributed to the user with an ionospheric model in the GPS signal structure. It is this basic principle of determining each error source instead of amalgamating them which distinguishes GSBAS from dGPS.
Figure 1 shows the GSBAS and StarFire™ service areas and the global reference stations that are used for the StarFire™ system. The reference network infrastructure, quality of hardware, algorithm sophistication, and the speed at which this is accomplished all contribute to the robust positioning accuracy attained by the user.
During the mid-1990’s, Deere and Company challenged NavCom with developing a proprietary, global, cost-efficient GPS correction network capable of providing greater accuracy than existing available systems.
In response, NavCom implemented a fully redundant, worldwide, dual-frequency GPS reference network that was free from regional boundaries. It also devised new processing algorithms to allow for optimal compression of GPS correction data, to reduce both satellite bandwidth needs and operating costs. Launched globally in 1999, the StarFire™ Network uses the GPS satellite system, L-band communication satellites, and a global network of reference stations to deliver real-time high precision positioning.
In 2001, NavCom formed an historic agreement with NASA's Jet Propulsion Laboratory (JPL). This partnership combined StarFire™ with JPL’s RTG (Real Time GISPSY) technology to further enhance the unique capabilities of this robust global network.
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