GPS receivers

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GPS receivers

Postby chocolatethunder » Sun Dec 09, 2007 10:34 pm

My project is to determine the correlation between ionospheric storm activity and the accuracy of gps receivers. I need to obtain a gps receiver that best serves this purpose and I know that i want a single frequency one that can have waas disabled or enabled and most importantly at a reasonable price. Does anybody have any suggestions on which gps receiver to buy?
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Postby Craig_Bridge » Mon Dec 10, 2007 6:13 am

Your question tends to indicate that you need to first research
1) Hw GPS really works.
2) Hw the ionospheric storm activities affect signals passing through at different angles.

You first need to understand how the GPS units require signals from multiple satellites to calculate a position.

Hints: It takes a minimum of 4 signals for an aircraft GPS unit to determine a position. Ground units can assume that they are on the earth surface; however, the earth surface is not a sphere so this introduces additional inaccuracies.

You really need to read up on this and understand it better before you try to come up with an experiment. I personally haven't investigated any GPS units made for personal use to know what information they can display on satellite signal strength and number of satellites being received. Unless the GPS unit provides this information, I don't think it will be usable for this purpose. This probably means that you are going to have work with somebody like a surveyor who has a very expensive GPS that provides this data and is willing to setup on a benchmark point in bad weather. Surveyor units are like aircraft units in that they won't assume they are on a uniform sphere because that would be too inaccurate for survey work.

The other difficulty investingating this effect is being able to determine where, when, and magnitude of ionospheric disturbances are wrt to the GPS satellites when you take a measurement.
-Craig
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Postby chocolatethunder » Mon Dec 10, 2007 10:14 pm

I have already written my twelve page research paper on GPS receivers and geomagnetic storms, and in my project I am attempting to determined the effects that these storms will have on the accuracy of gps receivers. My initial thought would be to obtain a receiver that can be placed in a set location and I would take daily readings and compare it to the storm activity posted on the NOAA website or to more detailed data from an expert at NOAA. I have tried looking for gps receivers to fit my purpose but have come across a few problems such as will the gps give altitude readings, can it generate readings on demand, and most importantly the cost factor. Since I do not own a GPS receiver myself, I was wondering if anybody could please supply me with information about where to look for a gps receiver for this project
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Postby Craig_Bridge » Tue Dec 11, 2007 8:29 am

I have already written my twelve page research paper on GPS receivers and geomagnetic storms,

That is nice, but I fear you have done a lot of work without dealing with:
1) Coming up with a hypothesis
2) Doing an experimental design to test the hypothesis
3) Investigating what measurement and or equipment needs are required to perform the experiment

The heart of any scientific investigation is figuring out how to apply the scientific method to evaluating a question.

Aircraft GPS units that are designed to work in bad weather for approaches have gyros for dead reckoning to deal with signal drop outs.

Car navigation units that are design to work in "concrete cayons" like midtown Manhattan also have gyros for dead reckoning and have provisions for external anteneas so that the shielding effects of the car do not reduce the signal strength and they are typically 12 to 14 channel units with some of the newer sensitive reciever circuitry.

My initial thought would be to obtain a receiver that can be placed in a set location and I would take daily readings and compare it to the storm activity posted on the NOAA website or to more detailed data from an expert at NOAA.


GPS satellites are not in geosynchronous orbit so their trajectories are constantly changing wrt a stationary receiver. This means that the GPS unit is designed to integrate the information obtained from all satellites being recieved over time to come up with a positional reading. The longer a GPS unit stays in the same position, the more accurate the reading becomes because it has more information from more trajectories which means it has more intersections of planes. You are going to have to have a GPS unit that you can turn off completely to reset its knowledge of where it is and all past satellite data.

Finding a way to do your experiment inexpensively is a major challenge.

Obtaining point in time NOAA information that can be used to correlate to a GPS location at the point when you power it on is an extremely expensive equipment intensive problem. Even commercial airliners don't have this kind of access. Small planes that are equiped to fly under 12,000 feet in bad weather carry their own weather radar for a variety of reasons. The FAA radars are not designed for weather. The dopler radars that are excellent weather tools do not publish enough of their data in real time. Many local TV stations have their own dopler weather radar and are actually better data sources; however, correlating any of this data to your GPS location and current satellite trajectories is well beyond what you can probably do.

You probably need to come up with a hypothesis that can be tested with equipement that you can afford.
-Craig
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gps receivers

Postby peteryoung » Tue Dec 11, 2007 10:16 pm

I have used a digital data recorder from Eagle Tree Systems with a GPS
receiver add-on. The advantage of this particular system is small size,
several hours of data recording/storage (exact max duration depends on acquisition rate), and direct playback into a PC or laptop. The measured GPS data also has enough granularity to provide very fine graduations of measured latitude and longitude data (digital displays usually don't provide much granularity).

The problem is that the Eagle Tree system, while offering many attractive
features that would be ideal for your project, isn't inexpensive; go to
http://www.eagletreesystems.com and price the totals for the flight data recorder and gps module. Options: find a radio control modeler who has a system that he's willing to loan out; or see if your high school would be willing to purchase this system for future academic use.

Another issue you have to deal with, discussed by a previous Expert, is how to nonambiguously characterize the ionospheric disturbance environment that may or may not be influencing the GPS receiver's
precision or accuracy. As I've found out with my own measurements,
a set of measurements for a fixed receiver shows quite interesting
variations in position - well within the spec bounds for a non-WAAS corrected receiver but certainly not a static unchanging quantity. The data sample times were short enough (5-15 minutes) that ionospherics were probably not the cause, either, for these "normal" variations.

Peter Young
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Postby Ray Trent » Wed Dec 12, 2007 4:02 pm

It occurs to me that you might have better luck getting this data from a place where it's already collected, the WAAS system. This is a set of ground stations that record this ionospheric disturbance and send correction factors up to the GPS satellites for rebroadcast to GPS receivers, which use the data to improve their accuracy.

There's a lot of WAAS data available at http://www.nstb.tc.faa.gov/RT_VerticalP ... nLevel.htm, and pages that are linked from there. Also, there's a really interesting paper I found with Google that you might find very interesting for your project: http://www.ima.umn.edu/~toews/research/waasfilter.pdf.

In general, Google is your friend :-).

If you really want to record this data yourself, most of the commercially available GPS receivers are excellent. Make sure you get one that supports NMEA output. This is a widely used serial port communication standard implemented by most GPS receivers that are intended for hiking or boating (as opposed to car trips... those tend to be pretty dumbed down)). If your PC doesn't have a serial port, you might also need a USB-Serial converter, but those are cheap.

There's a pretty good description of what you'd need to do to hook it up to a PC here: http://www.windmill.co.uk/gps.html, but that's just an example... I'm sure you can find better info if you search for it, possibly even in the GPS's documentation.

Personally, I've done a lot of geocaching, which involves using a GPS in a sort of "treasure hunting" game (see http://www.geocaching.com0, and I've found that Magellan receivers are somewhat more accurate and pull in a signal more strongly under trees than Garmins, but the differences are minor, and the Garmins tend to be cheaper.

Ironically, because it will be less accurate, you'll want to either buy a GPS that does *not* have WAAS capability, or make sure that the one you buy can *disable* WAAS, because it will throw off your measurements, making them almost useless. The whole point of WAAS is to correct for the exact ionospheric disturbances that you're trying to measure. I would suggest looking on the Garmin and Magellan websites for documentation of their various models.

My preference would be for one that has WAAS, but can disable it, because that way after you're done with your experiment you can turn it back on and have a much more accurate receiver.
../ray\..
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Postby Craig_Bridge » Wed Dec 12, 2007 4:54 pm

The MRCS and WAAS comparison that Ray posted a link to http://www.ima.umn.edu/~toews/research/waasfilter.pdf (had a trailing period that should not have been there) deals with factors involved in calibrating the distance information derived by the difference in the two frequencies received from a single satellite. What it doesn't deal with is knowing the satellite angle for each satellite being received by a GPS unit or how many are being received.

In short, there are a LOT of sources for errors and each one has a different statistical error space.
-Craig
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