AUSPOS FAQ-3. How it works

Online GPS Processing

• Question 3.1 - How does the system work?
• Question 3.2 - How are observational errors accounted for?
• Question 3.3 - What Reference Frame is used?
• Question 3.4 - How does the file upload (BROWSE) button work?
• Question 3.5 - Does the system work with data collected anywhere on Earth?

[back to top]

Question 3.1 - How does the system work?

All computations are undertaken using the MicroCosm Software System. MicroCosm is a high precision orbit and geodetic parameter determination software system. MicroCosm is fully IGS compatible. For more information see MicroCosm. MicroCosm is a US registered trademark of Van Martin Systems, Inc.

The International GPS Service (IGS) product range is used to compute coordinates. Precise Orbit, Earth Orientation and Coordinate solution IGS products are used. This means the latest available product will be used. The IGS final orbit product is not available for around two weeks after observation, while the rapid orbit product is available two days after observation. If both the final and rapid orbit products are unavailable, then the IGS ultra-rapid orbit product will be used.

Once a user has submitted a RINEX file, a set of the nearest IGS stations are determined, and their data retrieved from an IGS data centre. A precise solution using a 'double difference' technique is then computed, using these IGS stations. The coordinates of the IGS stations are held fixed at the value of the cumulative IGS SSC (coordinate solution).

[back to top]

Question 3.2 - How are observational errors accounted for?

The effects of observation error sources, such as receiver clocks, troposphere and ionosphere are taken into account, either through modelling, or estimation of related parameters. All the computations are undertaken to the IERS96 conventions.

A summary of the computation standards are provided as an appendix to all processing reports.

[back to top]

Question 3.3 - What Reference Frame is used?

All coordinates are computed using the IGS cumulative SSC, this means they are effectively ITRF2000 coordinates.

For Australian users, Geocentric Datum of Australia (GDA) coordinates are also provided. GDA coordinates are determined by an AUSPOS derived ITRF to GDA transformation model. The accuracy of this transformation is sub-centimetre.

[back to top]

Question 3.4 - How does the file upload (BROWSE) button work?

This service takes advantage of the file upload functionality of modern web-browsers. It is not a security risk for users.

[back to top]

Question 3.5 - Does the system work with data collected anywhere on Earth?

Yes, the system was designed to work anywhere on Earth. Because the IGS network has great global coverage, you can go anywhere, and be confident that an IGS GPS receiver is closer than 1000km away.

We don't download all the IGS data for every site in the world every day. We first check the approximate location of the user data, we then collect close by IGS data from an International archive, say Crustal Dynamics Data Information System (CDDIS). The downside is that if CDDIS is 'down', and we don't already have the required IGS data in the AUSPOS data archive then our system is down. Fortunately, the CDDIS archive is rarely down. We can also manually switch our software to other International data archives if required.

[back to top]