Archive for October, 2008

Assisted GPS – How does it work?

October 14th, 2008 | Category: Navigation Accuracy,Orbit Determination Tool Kit

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What exactly is A-GPS?  A-GPS for your GPS device is like a set of training wheels for your bike.  When you need help learning to ride, the training wheels keep you from falling flat on your face.  A-GPS helps your receiver find its position.  Unlike with training wheels however, you may need A-GPS over and over again.  Why do you need it?  What's it good for?  Let me explain...(grab a comfy chair - easier to do if you're reading this on your GPS enabled wireless device!)

When you use a typical GPS receiver, out in the open with lots sky available, and you don't really care how long it takes to get a GPS position fix, your receiver downloads almanac data for all satellites in the GPS constellation, then determines which satellites are above the horizon at your approximate position.  Once it knows this, it then proceeds to download the precise ephemeris broadcast from those satellites you actually have a shot at tracking.  It doesn't bother spending the time to download the precise ephemeris from the satellites on the other side of the Earth, you won't need that for hours yet.  Once it has the precise ephemeris (and a bit of other information it downloads), it can calculate your position within a few meters.  This is close enough to find your car in a parking lot - or for that matter, for anyone to locate you easily (you're the one holding the GPS receiver - doing the GPS walk).  Ok got slightly off topic.  So when do you not have the luxuries described above?  Turns out - most of the time.

Hikers, hunters, treasure seekers and other outdoor folk usually don't have an open sky issue, but urban dwellers often find themselves in a urban canyonIn this situation, your GPS receiver can't readily lock on to the satellites it needs, delaying your position fix.  Not good - how can you expect your friend finder application on your new iPhone to work if your iPhone can't figure out where you are?

Additionally, there is a mandate for enhanced 911 (e911) services, that allow emergency personnel to know your calling location - even if you call from a cell phone.  This smacks of GPS-enabled cell technology - as long as you can get a GPS fix before you choke to death!  Not a pleasant thought.

Finally, I can get to the point.  A-GPS lets your receiver bootstrap it's positioning fix by delivering the precise satellite ephemeris over the cell network rather than trying to download it from each satellite.  Getting this data takes time and having it delivered right to your device helps find you faster.  Be aware, your GPS device still needs to have at least 4 satellites in view for a small amount of time to get a precise position for you.  There are other methods that your cell device can use to help position you, such as using cell tower locations to approximate your position (and provide a good initial guess to your receiver as your approximate position), so the emergency personnel then only have to look for the crowd surrounding you.

Another improvement can be made by not just passing the precise ephemeris received at the cell tower on the GPS devices in the area, but to actually improve on the quality of the ephemeris.  Remember the 'precise' ephemeris still has several meters of error in it.  It's possible using AGI's Orbit Determination Tool Kit, to get these errors down using a predicted ephemeris technique.  Contact me if your interested in this and I'll put in touch with the right people.

So, using your GPS in harsh environments (like the umbrella covered patio at Starbuck's), is now easier, mostly hidden from view of the casual user and more efficient thanks to the engineers who thought up this stuff.

Until next time, happy shopping.

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GPS accuracy data – a new ‘generation’

October 01st, 2008 | Category: General Navigation,Utilities and Tools

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Ok, I like puns.  You'll see how this one fits in a minute.  Soon, a new version of our desktop navigation accuracy tool - NavTK version 2.3.4 - will be released.  One major change coinciding with this release is that we now have a new process for generating our GPS accuracy data - better known as Performance Assessment Files (PAFs) and Prediction Support Files (PSFs).  Previously, these files contained accurate estimates of GPS performance.  Now, with our new process these files contain the actual GPS ephemeris and clock errors.  Why are these errors important?  Read these Nog's of the past. The newest release of NavTK, as well as the AGI Components Navigation Accuracy Library, read in these new files and in fact, NavTK will go and find the correct file for your analysis situation - no need to hunt through the myriad FTP sites or cryptic download sites any longer.

So, we now use a new 'generation' scheme for our PAF and PSF files for our users (available for free) that include a new 'generation' of data quality.  To help visualize how well GPS satellites are performing, I developed a web application that allows you to visualize PAF and PSF data.  This application will allow you to view any PAF or PSF we have on our FTP site.  Here's the explanation from the web application - defining how the data is produced:

The PAF data is created by gathering the truth ephemeris and clock information (in .SP3 format) for a given day and differencing that data from the propagated broadcast ephemeris and clock information provided in the RINEX navigation file for the same day.  Both the SP3 and the RINEX navigation files are produced by the National Geospatial-Intelligence Agency (NGA) and available here.  The antenna phase center SP3 file is used to calculate the ephemeris and clock truth data.

The plotted values in the PSF graph are seven-day RMS's of the user range error (URE). The UREs are calculated using the Signal-In-Space (SIS) GOSPAR global URE formula, utilizing the data from each PAF file.  The seven-day RMS UREs for a specific PRN are constant for a given day, so the RMS URE values are plotted in a bar graph showing a three-day history of these values for each PRN.

The GOSPAR formula for user range errors (UREs, the quantity that affects your positioning error in GNSS systems) was defined in the late 1990's during the GPS Operational Control Segment (OCS) Performance Analysis and Reporting (GOSPAR) project.  This formula for the URE, sometimes called the global URE, is generated by integrating the UREs over the surface of the Earth.  The integrated formula follows:Global URE

where S is the surface of the Earth being integrated over, delta r is the ephemeris error vector for the GPS satellite and delta s is the clock error of the satellite.  The vector L is the line of sight vector between the GPS receiver and the GPS satellite.

The PAFs and PSFs have a 3-day latency, because the truth data produced by the NGA takes that long to produce.  Each day, the NGA data files are automatically retrieved, the PAF and PSFs are built and placed on our FTP site.  So, when we have the latest data - you have the latest data.

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