Eggfinder Problem

[Zbench]

Also keep in mind that the accuracy is dependent on the number of satellites -and the position of the satellites relative to your ground position. It would be interesting to see if you both had the same set of satellites through the test process.

Is the GPS receiver for the mini different than the standard sized GPS receiver?

Randy,

You've been saying that the distribution in the sky affects accuracy, but it just doesn't hold water with respect to on the ground testing. To wit, there is a free software program called Visual GPS View. You start the program, plug in your eggtimer serial cable. Go to start GPS, look for the serial port that is active, set to 9600 baud. Next, connect the black pin to ground on your TX/Mini and the white pin to the TX pin. Then, plug in the battery to the mini or TX.

What you will get is a real time read out of the satellites, their position in sky, relative strength and the PDOP, HDOP and VDOP values. I did it a bit ago for my TX and came up with the following screen shots. I took one each time I picked up a new satellite. I let it run for about 30 minutes, but never got better than 7 satellites:





In looking at the above plots, you'll notice a few things:

Satellites are not all in one quadrant, but rather randomly distributed. Anytime I've done this, which is typically when I build a new device, I get the same random distribution

You'll notice that the satellites on the extreme edges of the horizon while seen, typically don't establish a lock. I assume that is because the signal is weak and satellites on the horizon have to cut through the maximum amount of atmosphere

You'll also notice how the HDOP gets better with each additional satellite

To Mike's earlier point, if you run this experiment where you are, wherever that is, I'm certain you will see something similar. There are currently 31 active GPS satellites, and the system is designed for accuracy considering smart bombs use the GPS network to guide their payload to target.

Fun fact, USA-132 is the longest serving satellite in GPS history, still operational after being launched on 23 July 1997 aboard a Delta II rocket. Still getting it done over 25 years later.

One more analytical tool you can use to understand your 500 foot variance.

 

[rjenkins133]

Randy,

You've been saying that the distribution in the sky affects accuracy, but it just doesn't hold water with respect to on the ground testing. To wit, there is a free software program called Visual GPS View. You start the program, plug in your eggtimer serial cable. Go to start GPS, look for the serial port that is active, set to 9600 baud. Next, connect the black pin to ground on your TX/Mini and the white pin to the TX pin. Then, plug in the battery to the mini or TX.

What you will get is a real time read out of the satellites, their position in sky, relative strength and the PDOP, HDOP and VDOP values. I did it a bit ago for my TX and came up with the following screen shots. I took one each time I picked up a new satellite. I let it run for about 30 minutes, but never got better than 7 satellites:

View attachment 538303
View attachment 538304
View attachment 538305

In looking at the above plots, you'll notice a few things:

Satellites are not all in one quadrant, but rather randomly distributed. Anytime I've done this, which is typically when I build a new device, I get the same random distribution

You'll notice that the satellites on the extreme edges of the horizon while seen, typically don't establish a lock. I assume that is because the signal is weak and satellites on the horizon have to cut through the maximum amount of atmosphere

You'll also notice how the HDOP gets better with each additional satellite

To Mike's earlier point, if you run this experiment where you are, wherever that is, I'm certain you will see something similar. There are currently 31 active GPS satellites, and the system is designed for accuracy considering smart bombs use the GPS network to guide their payload to target.

Fun fact, USA-132 is the longest serving satellite in GPS history, still operational after being launched on 23 July 1997 aboard a Delta II rocket. Still getting it done over 25 years later.

One more analytical tool you can use to understand your 500 foot variance.

my comments about accuracy and satellite position are based on this article: Dilution of Precision

Randy

 

[Tractionengines]

WOW...that goes back a while 1999. I will read that in detail later. (Only glanced thru it. )

FYI... That is about the time I got my first handheld GPS unit. It's also BEFORE they turned OFF the forced inaccuracies in consumer GPS units. ( That was 8pm on May 1st 2000. When typical error went from greater than 100m to less than 10m. I remember testing how close it returned me to my saved waypoints, before and after the change. )

 

[Zbench]

WOW...that goes back a while 1999. I will read that in detail later. (Only glanced thru it. )

FYI... That is about the time I got my first handheld GPS unit. It's also BEFORE they turned OFF the forced inaccuracies in consumer GPS units. ( That was 8pm on May 1st 2000. When typical error went from greater than 100m to less than 10m. I remember testing how close it returned me to my saved waypoints, before and after the change. )

I came to the same conclusion. I don't take much from that 22+ year old article. Other than Selective Availability being turned off in 2000 (thanks Bill), the other big thing I noted that impacts the accuracy figures reported is the author talks about 24 GPS satellites in orbit. That has increased to 31, with 32 being the max allowed. He further spends a lot of time talking about how position accuracy changes going from 4 to 5 satellites. I think the assertions about satellite location in the sky is all true, but when you increase the number of transmitters by 30%, you have a much larger surface area and as a result, much smaller HDOP values. You can see that above just by looking at the numbers posted and without any calculation.

As you can see from the data above, you'd have to be pretty unlucky to only see 5 satellites. At the launch site in Amherst, I typically see 10 or more (* on the GPS receiver). In my experiment last night, I did flip on the GPS receiver to see how it correlated with what I was seeing on the computer screen. The figures were always exactly in sync. When they changed on the GPS software, they similarly were updated on the hand help receiver.

 

[user 30299]

Randy,

You've been saying that the distribution in the sky affects accuracy, but it just doesn't hold water with respect to on the ground testing. To wit, there is a free software program called Visual GPS View. You start the program, plug in your eggtimer serial cable. Go to start GPS, look for the serial port that is active, set to 9600 baud. Next, connect the black pin to ground on your TX/Mini and the white pin to the TX pin. Then, plug in the battery to the mini or TX.

What you will get is a real time read out of the satellites, their position in sky, relative strength and the PDOP, HDOP and VDOP values. I did it a bit ago for my TX and came up with the following screen shots. I took one each time I picked up a new satellite. I let it run for about 30 minutes, but never got better than 7 satellites:

View attachment 538303
View attachment 538304
View attachment 538305

In looking at the above plots, you'll notice a few things:

Satellites are not all in one quadrant, but rather randomly distributed. Anytime I've done this, which is typically when I build a new device, I get the same random distribution

You'll notice that the satellites on the extreme edges of the horizon while seen, typically don't establish a lock. I assume that is because the signal is weak and satellites on the horizon have to cut through the maximum amount of atmosphere

You'll also notice how the HDOP gets better with each additional satellite

To Mike's earlier point, if you run this experiment where you are, wherever that is, I'm certain you will see something similar. There are currently 31 active GPS satellites, and the system is designed for accuracy considering smart bombs use the GPS network to guide their payload to target.

Fun fact, USA-132 is the longest serving satellite in GPS history, still operational after being launched on 23 July 1997 aboard a Delta II rocket. Still getting it done over 25 years later.

One more analytical tool you can use to understand your 500 foot variance.

I use the VisualGPS program quite a bit to test & check my trackers. Very helpful.

 

[user 30299]

Is the 500 ft. variance a one-off incident?

 

[cerving]

A GPS fix can be obtained with as few as 3 satellites, but it depends on where they are. If they were all at 45 degrees elevation and 120 degrees apart you'd get a pretty decent fix. That's not the reality, however, so 5 or more satellites is what you're looking for. In a populated area there are going to be buildings and trees at the lower elevations, making it harder to get a fix. Out in the open, that won't be a problem... at the launch sites that I attend (deserts and furrowed farm fields) I typically see at least 7 satellites, usually more.

Note that the number of satellites in the LCD display is the number being used to generate the fix, not necessarily the number that are being tracked. Up to 22 satellites can be used in the fix, out of 66 that it can track... if it sees them first and knows where they are. From a power-on cold start, you're not going to get 22 satellites... that takes time, we're talking hours and possibly days. People often ask me how their phone can be more "accurate" than their tracker, it's because your phone is always on and over time learns where the satellites are. If you kept your tracker powered on for a long time, it would be equally accurate. GPS modules have a backup power function that can be used to save the location data while powering down the rest of the module (RF, interface, etc.) to save power, but given the relatively infrequent usage of rocketry trackers that's not really a feature that can realistically be implemented to improve resolution, at least in most cases.

 

[Tractionengines]

@cerving Thanks for the information. Both @Zbench and I have shown that within 1 minute the fix is within a 30 feet and buy 30 minutes it's within 10 feet for all of our Eggtimer GPS modules. (Even in a crowded industrial or residential setting.)

That is plenty fast, and accurate enough for rocketry use as far as I am concerned. Using more battery to get to <10ft faster is not worth the shortened battery life, or, more weight of a bigger battery. I have a audible beepers that even in mature corn, I can hear 10 feet away.

 

[Zbench]

Cris,

Seems like the only feasible cause of a 500 foot error is a faulty GPS chip. Is that likely? I would think it either works or it doesn’t. Interested in your opinion of root cause.

 

[waltr]

I have had some interesting results with a mini-GPS. I have an SD logger in the LCD receiver that saves every NMEA sting.

Just after power on of mini-GPS I have had it report locations hundreds of feet away but typically gets within a few dozen within a few minutes. Did have it report I was in Spain from half a minute then jumper back across the pond to Pa.

If I power on the mini-GPS 10 minutes before a flight then have gotten very good results.

As has been mentioned a few times:
the 500 foot error the OP saw maybe have been due to GPS antenna poited to the ground and was using fewer satalites.

 

[cerving]

As I've mentioned before, that patch antennas that are commonly used in rocketry don't really get a very good view of the sky. They're pointing the wrong direction most of the time... instead of pointing "up", they're pointing "sideways", so at best you get 1/2 of what you should be getting in a full-sky view. If your rocket lands antenna-down, they're not getting anything at all... hopefully your last received location was pretty close. The best antenna would be a helical antenna along the longitudinal axis of the rocket, but even then you'd probably be missing the "top" 30 degrees or so if you had an aluminum tip on the nose cone, or aluminum bulkplates.