Tuesday, 8 December 2009

Interview with Steve Ault (Part 3 of 4: Starfire)

This is the third segment of a 4-part series of interview questions posted to Mr Steve Ault, the Technical Support Manager of Navcom Technology, Inc., who conducted training for our sales and field personnel in early November 2009.

PointsandPixels:s: What is StarFire?

Steve: In a traditional RTK system, corrections generated and applied are 'land-based'. In other words, correction measurements are based on singular ground antennae. While providing highly accurate correction data for very localised areas, the impact from spatial de-correlation makes systems such as StarFire very attractive.

StarFire is a 'space-based' correction format. This means that corrections are generated for actual satellite orbit position and satellite clock for all GPS (and, in the near future, all GLONASS) satellites. This is not the same for an RTK solution (as RTK base corrects pseudo-range measurements from the satellites under track).

There are two major elements as to how the StarFire process is accomplished.The First is how corrections are created and the second is how these corrections are applied by the navigation processor.Considering that a single satellite is tracked by multiple reference stations on the earth surface, each point sees exactly the same satellite signal but with the normal satellite position, satellite clock, pseurorange, ionospheric and other errors as a traditional base reference station. Each reference station provides its satellite measurement data and position to a central processing computer. The computer takes the data from all reference stations that 'see' the same satellites and computes corrections for the satellite orbit and clock. These can be calculated since the ionospheric differences and other range measurement errors can be filtered.

The second component is the navigation software or algorithms to properly apply the StarFire correction message. Since the processing computer generates a single global correction for each and every satellite, the receiver needs to 'understand' this unique message and how to properly employ it (some of which includes filtering techniques to achieve the final attainable accuracy levels of less than 10cm!).

PointsandPixels: What is the current performance of StarFire?

Steve: StarFire performance can be expressed in several way. These include convergence, accuracy, sustainability, QuickStart and RTK-Extend.

Convergence: This is the time it takes to achieve the published specifications. In our previous product lines(i.e. the Legacy code), this typically required between 30 to 45 minutes of uninterrupted operating time. While for the SF-3050(i.e. Sapphire code), the same specification exists with a noticeable improvement (~20minutes) as tabulated below.

Accuracy: It is determined by the number of satellites being tracked and used in the navigation algorithm. Their relative geometry and the length of time that StarFire corrections have been continuously applied to the navigation solution (as depicted above). The accuracy specifications of less than 10cm (horizontal) is a 24-hour 1-sigma calculation. StarFire typically produces results in the 6-8cm range quite reliably.

Sustainability: This is achieved via continuous GPS signal tracking (in much the same way as RTK performance requires continuous signal tracking). Once the receiver drops out of navigation (e.g., the receiver traverses under a bridge), the convergence period begins all over again because the receiver will have different satellite phase signal characteristics once the signals are re-acquired.

QuickStart: This is a method of attaining the less than 10cm accuracy without going through the full convergence period (as described above). The receiver can be within the performance specifications in less than 5 minutes if the QuickStart function is initiated from a precisely known point. Information on this can be found in the StarUtil-3000 User Guide on our website.

RTK-Extend: This is a unique Navcom patented technique that combines RTK and StarFire technologies to extend the 'footprint' in areas where RTK telemetry tracking proves difficult. When a user is performing an RTK survey, the receiver also tracks the StarFire satellites. If the RTK correction link is interrupted, the receiver can coast on the StarFire correction signal for up to 15 minutes from a Navcom proprietary correction and up to 7 minutes on CMR(+) or RTCM correction from a Navcom base.

Related posts/other resources:

1. Interview with Steve Ault Part 1/4: His experience with GNSS/RF, history and future trend of the industry
2. Interview with Steve Ault Part 2/4: Navcom Technology Inc and SF-3050
3. StarFire data sheet in pdf format.

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