Archive for February, 2008

Assessed Navigation Accuracy – part 1

February 26th, 2008 | Category: Navigation Accuracy

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Assessed navigation accuracy is a term AGI uses to define a navigation accuracy calculation that uses actual navigation constellation error data. When an accuracy calculation is performed, many different types of data can be used. In this article, I'll focus on the errors inherent in the control and space segments of GPS.

One of the larger error sources in you position estimation (now that Selective Availability is turned off) are the errors induced by inaccuracies in the navigation satellite's ephemeris and clock states. These errors are uncorrectable by the user who does not have a differential system such as the USCG DGPS network or WAAS. With a differential system, a base station or stations can observe the satellites in view and determine the ephemeris and clock errors. Once calculated, these base stations will communicate what these errors are to an appropriately equipped receiver. Details of how DGPS and WAAS work will be covered in a future article.

Navigation satellite ephemeris errors result as a difference from the broadcast satellite position and the actual satellite position. As part of the required data for a navigation solution, the navigation satellite broadcasts its position to your receiver. The receiver takes this position as truth and uses it in the navigation equations to calculate its position. The navigation satellite gets its position data from the control segment, which predicts it, typically for 24 hours in advance. These control segment predictions are very accurate at the time of prediction, but as time progresses (as the Age Of Data increases, as they say) the predictions are further and further off.

The same type of process results in the clock state for the satellite. Just as satellite orbit positions need to be predicted, so do atomic clock phases and frequencies. Satellite orbits are well understood, but there are still small, unmodeled perturbations which affect the orbit that predictions don't take into account. For atomic clocks the errors are induced by quantum behavior in the device itself. No amount of modeling can predict the clock state exactly, we typically rely on 2nd order fits of the clock state to make the predictions that are uploaded to the satellites.

So now we know where the constellation errors come from - how are they applied to a navigation accuracy assessment?

Before the satellite ephemeris and clock errors can be used, they must first be determined - and that will be the topic of the next article.

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A good place to start…

February 19th, 2008 | Category: Uncategorized

I thought a good place to start would be to create some pages on GPS Basics.  You'll see the page links in the upper right corner above the cool nav banner.  On these pages, I'll describe some of the basics of how GPS works, and eventually, more detailed information about GPS performance and processing.  Treat these pages as you would a reference, I'll update and add to them as I get time.

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Welcome to the AGI nav blog!

February 11th, 2008 | Category: General Navigation

Welcome to my new blog! The blog topics here will all be navigation related - somehow. My name is Ted Driver and I'm a nav engineer here at AGI. I've been here since 2004, helping develop the Navigation Tool Kit first and now plying my skills on our new AGI Component Technology team. We've been working hard at creating what we think is one of the most useful and versatile analysis packages around. In this blog, I'll spend time writing about all the nav issues I can think of, as well as some of the issues you think of. I can spend a lot of time writing about what I'm doing (never really being at a loss for words), but I'd really like to hear about what you're doing and talk about problems you may have run into. Let me know what topics you'd like to see entries on - I'm all ears! Additionally, if you'd like to appear as a guest on this blog, excellent- the more the merrier!

Now, the term nav blog sounds like a mouth full - like something that your 1 year old might say. From now on, I'm calling this blog the nog. While initially there is no relation to the lively holiday drink, there may yet be a use for that analogy - after all when a sailor has spent a long day navigating across the south pacific by chart and compass, what else is there to do when the moon lays low, the waters are calm and the stars begin appear except - wonder where the hell his GPS is?

Right, GPS. The Global Positioning System - the United States' contribution to the GNSS community. GNSS means Global Navigation Satellite System. A term someone made up to allow other countries to have GPS systems too....and that's a good thing. The more navigation satellites you have available to you, the better your navigation solution. More on this in future nogs.

Navigation spans a wide variety of techniques, from positioning based on the noon Sun position, orienteering, dead reckoning, doppler measurement, and finally the radio navigation methods we use today. The tools AGI develops are centered around the radio navigation methods that GNSS' employ. While the other methods are certainly valid, AGI focuses on GNSS type navigation technology because that's where more navigation users are and because, well, because it's so cool. There's more to navigation than GNSS though - lunar navigation and inter-planetary navigation are two notable exceptions.

Enough of a welcome - really glad you're here. Check out the RSS link in the upper right - when I make an update, you'll see it in your feed reader. My reader of choice is the Sage plugin for Firefox but everyone has their favorite, shop around.

So, here's to smooth sailing and clear horizons!

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