Eggfinder Problem

[rjenkins133]

I'm looking for guidance for a problem I encountered during yesterdays NOTRA launch. I am relatively new to GPS tracking. Hopefully someone on this forum has more experience and can provide guidance.

I have an eggfinder rx with bluetooth, and the eggfinder TX mounted in a nosecone with no metal anywhere around it. I use the rocket locator app on my phone.

The rocket launched fine and landed in a medium tall soy bean field. no problem, my phone showed the location of the rocket out in the middle of the bean field - perhaps 1500 feet away.

I walked towards it and eventually walked up to the position of the red locator on the Rocket Locator app screen - but - no rocket anywhere around (the altimeter was beeping and I would have heard it if it was nearby). When we did find the rocket it was about 300 - 400 feet north of the locator marked position. I kept checking the RX led on the receiver and I was receiving an active signal from the transmitter - but the location was just wrong.

I have two hypothesis:

1. the phone GPS location was wrong - the app shows the blue tolerance circle around the current phone position. sometimes it would grow very large but it was very small when standing right where the rocket position was marked.

2. the rocket GPS location was wrong - the app does not show any uncertainty area around the marked position of the rocket.

Are there any other possible issues that would cause this behavior?

What is the tolerance area for the GPS module in the Eggfinder TX? Is this data available?

Thanks,
Randy

 

[Scott_650]

Randy - I’ve had a tough time at the NOTRA field getting a precise lock on the location with Google Maps and the Flight Sketch wind drift app on my phone so it may not be your Eggfinder but something about the conditions at the field that creates a problem for a GPS. More than once I’ve had to manually flag the location - the default “bulllseye” will show up as far east as the intersection of 113/Vermilion Rd. It might just be my crummy old generic Android phone but something to consider. Next launch maybe we can call a quick huddle of GPS locator users and see if anyone else is experiencing anomalies.

John Bryan

 

[waltr]

A few things I can think of:
1- Cell phones don't always use GPS but triangulate from Cell Towers. Lack of multiple Cell towers might cause an issue.
2- I have found that the Eggfinder GPS unit needs a good 10 minute or more to get an accurate location.
I log the NMEA data to SD card (in LCD RX) then bring into Google Earth. First 5 minutes the location can be very far away but gets better. Once showed my location as in Spain.
3- If you don't zoom in on the phone's map then it looks like you at on top of the rocket. Zooming in shows you are not.
4- Did you carry the Eggfinder receiver with you to ensure getting a Location while the rocket is on the ground?
If not then possible got last good signal when rocket still above the ground then chute drifted a bit.
5- If not enough GPS Sats received then accuracy suffers. Lots of different reasons.

My L1 cert flight had the rocket land 1600 feet away. I could see the general direction but not where it landed. Took my iPhone and LDC RX. When close entered the GPS coordinates into phone. Walked until looked like I was at the rocket then zoomed in the map. Walked to within 20 feet of rocket before seeing it in the tall grass.

 

[rjenkins133]

Hi Waltr - Thanks for all of the comments...

1- Cell phones don't always use GPS but triangulate from Cell Towers. Lack of multiple Cell towers might cause an issue.
The launch field is adjacent to the Ohio turnpike - I think the cell coverage is good - but I could be wrong. I have purchased the LCD receiver and GPS module from Eggtimer because I don't trust the phone (for multiple reasons).

2- I have found that the Eggfinder GPS unit needs a good 10 minute or more to get an accurate location.
I log the NMEA data to SD card (in LCD RX) then bring into Google Earth. First 5 minutes the location can be very far away but gets better. Once showed my location as in Spain.
Yes. I agree this is true right after turning it on - as it acquires more satellites the accuracy improves. But this had been turned on for more than 10 minutes. Perhaps I will log the data as well for debugging purposes at least - once I get my LCD RX built.

3- If you don't zoom in on the phone's map then it looks like you at on top of the rocket. Zooming in shows you are not.
I did zoom in on the phone - but I don't know the resolution when fully zoomed in. I need to research this further.

4- Did you carry the Eggfinder receiver with you to ensure getting a Location while the rocket is on the ground?
If not then possible got last good signal when rocket still above the ground then chute drifted a bit.
Yes I did have the receiver with me as I walked in the field. I was also checking the RX LED and I was receiving transmissions from the rocket mounted transmitter as I was walking around.

5- If not enough GPS Sats received then accuracy suffers. Lots of different reasons.

I agree - and there doesn't seem to be a method in the rocket locater app to determine the accuracy of the received rocket position.


Randy

 

[waltr]

I don't disagree on any of your comments since this is another 'Magic black box' so common today.

Logging the data stream could be helpful since there are multiple NMEA sequences sent.
Most apps only use part of the data in the $GPGGA but this does have #satellites used which the Eggfinder LDC Rx displays. Other NMEA lines ($GPGSA) have DOP (Diliution of Position) info.
The $GPGSV has info about the Satellite in view.

The Eggfinder's GPS module does send $GPGGA, $GPGSA, $GPRMC, $GPGSV sequences.
In my logged data I do see dropped data, partial sequences and time gaps. It is up to the App receiving the NMEA data to sort and present this info and reject anything that is bad.

 

[rjenkins133]

OK - it looks like the rocket locator app has the ability to log NMEA data to a file. I will enable this for the next launch.

I see DOP is a qualitative value and not a quantitative value - Looks like a value of 5 or less should be acceptable. Do you agree? Is there an accepted method for calculating a tolerance area around the indicated position?

Thanks for all of the guidance.

Randy

 

[waltr]

Logging the data would be good.

Did a bit of reading on the DOP values and less than 5 seems good.
I did not find any good explanation of calculating tolerance or even how these values are obtained.

I did write a couple of Python scripts to extract info from the NMEA data. One just does Lat, Long & Alt from $GPGGA. The other does the DOP values from $GPGSA. The DOPs can be quite high after powering on the Eggfiner TX. Have had a few flights where these values got pretty high (>40) during the flight then settled down after chute deployed. Other flights than stay <5.

The best results have been when I power on the TX and wait much more than 10 minutes. I also leave the LCD RX off until ready to put rocket onto pad. At the last launch (bit windy and the corn is over 6') I used the mini TX on three flights and left it powered between flights (it is in a coupler tube that gets tie wrapped onto the shock cord). Data on all three flights was very good, even when plotted in Google Earth.
Of course since I log from the LCD RX I didn't see the initial DOP values from the GPS TX.

Oh yea, if you save the log file then you can import it to Google Earth and see a 3D path of the flight on a 3D satellite view. Pretty neat.

 

[waltr]

Found this PP that is pretty good explaining GPS.
https://web.archive.org/web/20141214015213/http://www.montana.edu/gps/slides/2GPSAccuracy.pdf

And realized your comment:

1- Cell phones don't always use GPS but triangulate from Cell Towers. Lack of multiple Cell towers might cause an issue.
The launch field is adjacent to the Ohio turnpike - I think the cell coverage is good - but I could be wrong.

It is not cell coverage- which only needs one cell tower to make a call but multiple towers to triangulate your position. Min of three towers needed within range of your phone.

Did the map on your phone show you at the correct location? Turn on Sat View and check against a know landmark, corner of a field, road, etc. Might be interesting to do at your next launch.

 

[cerving]

If you have the LCD receiver then you're using GPS on both sides, so it should be very accurate. We don't currently report the LCD-GPS's coordinates, only that it has a fix; that may be something that we do in a future firmware build, or at least the time since last fix and HDOP so you know how good your ground fix is.

 

[waltr]

Reporting LCD RX coordinates has been a wish list item. With the TX powered and on the same table as the RX then the coordinates should be the same. Might be another check to know both are giving good results.

I know the LCD-GPS RX works but I have trouble using it to walk to rocket. One issue I have is not being able to walk fast enough for it to give a direction (30 feet in 10 seconds). Tall grass and weeds or tall corn makes going slow.
I have been entering the coordinates into my iPhone map then use the map. This works very well.

 

[cerving]

Reporting LCD RX coordinates has been a wish list item. With the TX powered and on the same table as the RX then the coordinates should be the same. Might be another check to know both are giving good results.

I know the LCD-GPS RX works but I have trouble using it to walk to rocket. One issue I have is not being able to walk fast enough for it to give a direction (30 feet in 10 seconds). Tall grass and weeds or tall corn makes going slow.
I have been entering the coordinates into my iPhone map then use the map. This works very well.

Chances are that if you didn't move 30' in 10 seconds then you're still pretty close. Maybe I'll make it force a new GPS fix if it hasn't updated it in 20 seconds.

 

[rharshberger]

Chances are that if you didn't move 30' in 10 seconds then you're still pretty close. Maybe I'll make it force a new GPS fix if it hasn't updated it in 20 seconds.

Corn fields are an example of less than 30' in 10 seconds...and zero visibility, forced updates every 20 seconds might help, I chased all around my rockets 7.5" diameter nosecone with 60" chute for about 15 minutes before getting visual (next row over) on the chute, I knew I was close (easily withing 50-60') but the slow updating kind of ran me in circles.

 

[cerving]

Duly noted.

 

[Tractionengines]

I'm looking for guidance for a problem I encountered during yesterdays NOTRA launch. I am relatively new to GPS tracking. Hopefully someone on this forum has more experience and can provide guidance.

Next launch maybe we can call a quick huddle of GPS locator users and see if anyone else is experiencing anomalies.

My data was a little odd at the NOTRA Launch July 2nd. That was first time using TX, and LCD. The 2nd flight seemed better.

If I make the next launch, I am up for a get together of GPS users. We can see how close positions match. And check for channel usage. As we have more people with 900mhz systems, good to check for interference.

After that launch, I gave the nosecone with tracker to my 2 youngest and had them play "hide and seek" in the few acres around the house. (It let me get a feel for "close" versus "very close".)

Faster updates for the last bit while your wondering around the "area" would help. For now I keep a 100+db beeper attached to shockcord. The GPS gets you close, then listen for the beeper to hone in the last few feet.

Mike

 

[rjenkins133]

It is not cell coverage- which only needs one cell tower to make a call but multiple towers to triangulate your position. Min of three towers needed within range of your phone.

Got it - per Cellmapper.com - looks like there are two verizon cell towers within about a 3 mile radius of the launch field. These closest are along the turnpike which runs east-west, one tower to the west and one to the east. The closest tower to the north or south is quite a bit further away - maybe 5 miles. So maybe this is a clue - perhaps the phone could more readily determine the east-west position but could not determine a precise north south position due to the cell tower arrangement. (but I don't know how the triangulation algorithm works).

Here is an interesting quote from a white paper on cell phone triangulation for the purposes of locating a 911 caller:

By using cell tower triangulation (3 towers), it is possible to determine a phone location to within an area of about ¾ square mile. In densely populated urban areas, the cell towers are close together, and a much closer estimation of phone location can be made than in a rural area, where the towers are far apart.

When I was checking the my location on the displayed map it appeared to show me in the correct location east to west - that is how the landmarks are arranged where we launch on this field- north-south lines between the fields on the maps that show the previous crop locations from the last time the satellite image was updated.

However the problem appears to be that the north-south location was not correct - the rocket was actually located a few hundred feet north of the displayed location on the phone. But there are no good landmarks in this area to visually fix the north south location and compare to the phones display.

Bottom line for me - I'm ditching the phone and switching to the Eggfinder LCD Rx.

Randy

 

[Tractionengines]

You can have both. The LCD can have the Bluetooth transmitter too. That way you can log to Rocket locator app in phone, and use LCD to get data real time.

 

[waltr]

Randy
Sounds like your cell phone is using Cell tower triangulation more than a GPS signal.
Nice research on checking this.

Yes, The LDC RX can have both a GPS and Bluetooth to phone so can use both.

 

[sl98]

The LDC RX can have both a GPS and Bluetooth to phone so can use both.

You can also have both Bluetooth and a data logger in the LCDrx in addition to the GPS. All you need to do is make a simple pigtail to connect the BT and logger to your LCD

 

[waltr]

Yes, both Logger and Bluetooth will work. The LDC board is simply an Async serial output. Both the logger and Bluetooth are simple serial inputs.

I do have a logger connected, No Bluetooth since I have an iPhone.
Thread here:
https://www.rocketryforum.com/threads/eggfinder-lcd-rx-logger.167896/

 

[Charles_McG]

Corn fields are an example of less than 30' in 10 seconds...and zero visibility, forced updates every 20 seconds might help, I chased all around my rockets 7.5" diameter nosecone with 60" chute for about 15 minutes before getting visual (next row over) on the chute, I knew I was close (easily withing 50-60') but the slow updating kind of ran me in circles.

I had a similar experience in tall reeds at the edge of a swamp. But I was carrying a directional mic, so when the the Eggfinder RX wasn’t useful (because it couldn’t tell my direction anymore) I was within audio range of the Proton beeper.

 

[waltr]

I too have heard the Quark or Quantum beeping for the last 30 feet to find the rocket.

 

[Zbench]

We were on vacation so I wasn’t at the NOTRA launch this past weekend. But hope to be at the next one. I have many TX and minis and they have always worked very well for me. I use an lcd receiver and prefer to just transcribe the gps location in GPS tracks on my iPhone.

Here is a screen shot of a cornfield retrieval. While it wasn’t in the corn very far, I couldn’t see it from the outside. I’ve found the transmitted coordinates to be very accurate.

We may need to start using a log board to ensure no one is using the same frequency.
Getting to be quite a few of us using Eggfinder products.

 

[ksaves2]

Folks, Give it up as a GPS tracker, no matter what one uses, NMEA streaming or APRS Ham band will not be able to get second to second accurate positions with a rocket under boost. APRS is once every 5 seconds and NMEA (Eggtimer-like stuff) is once every one or two seconds. Only issue is under boost, the positions are kind of dorky no matter what protocol is used. Forget about it.
The thing with GPS tracking of any kind is blow your main chute as high up as you can tolerate for the field you are flying at. I try to do a 1000 foot main deployment. At altitude, one has good reception of positions as the slow descent allows to get positions and the tracker to transmit them reliably. Can see the trend of the descent if wind is involved so if the rocket isn't there at the last known position, one knows which direction to proceed to find the rocket. Has never failed for me.
Remember, blow the main as high as one can tolerate. I understand some launch sites one might have to blow at 600 to 800 feet but do the best and highest as you can.
Another thing is get a multi element 900Mhz Yagi as when you get closer to the ground footprint of the rocket, the Yagi will increase the ground foot print. I've tested this compared with a simple straight dipole and Yagi. Took the Yagi off and put on the dipole and signal was gone. Put the Yagi back on and the signal came in again. They say that a Yagi is not so good to track a rocket in flight on 900Mhz but I've seen folks do it and did pretty well.
I use a 70cm/400Mhz or 2 meter/144Mhz Yagi antenna with an electronic attenuator for Ham GPS/APRS trackers and can track throughout a flight pretty well. Works great with a Beeline GPS and there are a pile of 2 meter GPS trackers out there that are readily adaptable to large rockets. Kurt KC9LDH

 

[Donnie]

Just discussed this with a friend who witnessed a similar issue at Argonia over Labor Day. Theory is that the tracker landed with the GPS ANTENNA (*not* the transmitting antenna that sends the location to the rx) face-down. Doesn't matter how good the signal is from the transmitter to the receiver, garbage in garbage out.

GPS antenna needs at least "some" view of the sky to get a great fix.

YMMV.

 

[ksaves2]

Just discussed this with a friend who witnessed a similar issue at Argonia over Labor Day. Theory is that the tracker landed with the GPS ANTENNA (*not* the transmitting antenna that sends the location to the rx) face-down. Doesn't matter how good the signal is from the transmitter to the receiver, garbage in garbage out.

GPS antenna needs at least "some" view of the sky to get a great fix.

YMMV.

Did they find the rocket anyways? I thought of this scenario in the distant past and felt it was technically possible.
It's the last known position that is important. If able to plot the incoming positions on a live map it really makes the recovery job easier as one can see what the drift trend is. Proceed in that direction and will find the rocket.
I use a 900Mhz Yagi on recovery after the rocket is down as it does increase the reception ground footprint of the downed rocket. They were cheap and easy to carry when I bought mine several years ago.
A GPS module face down in the dirt can lose a position lock as surmised above. A handheld Yagi even without an attenuator might still be able to get one an RDF directional fix as long as an RF carrier is transmitted. If the GPS lock is broken AND the tracker stops transmitting RF even if it's garbage, the above technique won't work.
That's where a live mapping program comes in to help as it shows the drift trend of the rocket as it is getting lower. Just keep going in that direction and make sure you put a noisemaker on the harness. When getting close in, the ears are very good for terminal tracking. Get rocket back fast and fly the next one! Kurt

 

[Hobie1dog]

great thread, I'm still trying to figure out which way to go with GPS

 

[cerving]

GPS antenna positioning in a rocket is a bit of a problem... ideally you want to get a "top view" of the sky the entire time, but there's no way to keep it "up" in the vehicle throughout the flight. A patch antenna facing sideways is a bit of a compromise... it the nose is pointing "up" (boost) or "down" (descent) then you'll get a 1/2 view of the sky, and a 1/2 view of the ground which isn't going to help you very much. When you land, you might get a full view of the sky... or if it ends up facing down you might get nothing. That last-received packet is all-important... the rocket is going to be very close to it.

 

[BenCook]

Hey Cris I have a quick question along these lines. I have a mini and I launched to about 6800 feet and my LCD with GPS was beeping with pings to whole time the main was set to deploy at about 700 feet. we lost sight of the rocket but we heard events coming from my 70cm telemetry module with voice on a separate LCD. I received beeps from my LCD connected to the Mini. Until Both units stopped receiving signal. I went to the location the tracking LCD showed me following the arrows. When I got to the location The LCD lead me too there was no rocket and no pings.I wandered around for about 40 minutes with no luck. I then tried going a different direction from where I thought I should go and got a ping on the LCD. That had the proper location of the rocket and I walked right too it but it was about 2000 feet away from the original location that the LCD pointed me too. After I found my rocket I understood why I wasn’t getting an data. It was in a gully surrounded by rock. It almost seemed like the GPS lost its fix and then didn’t regain it again until my rocket was already on the ground? Is that possible? Any pointers to ensure I get a location that is closer than 2000 feet away when I start my search?

 

[ksaves2]

Hey Cris I have a quick question along these lines. I have a mini and I launched to about 6800 feet and my LCD with GPS was beeping with pings to whole time the main was set to deploy at about 700 feet. we lost sight of the rocket but we heard events coming from my 70cm telemetry module with voice on a separate LCD. I received beeps from my LCD connected to the Mini. Until Both units stopped receiving signal. I went to the location the tracking LCD showed me following the arrows. When I got to the location The LCD lead me too there was no rocket and no pings.I wandered around for about 40 minutes with no luck. I then tried going a different direction from where I thought I should go and got a ping on the LCD. That had the proper location of the rocket and I walked right too it but it was about 2000 feet away from the original location that the LCD pointed me too. After I found my rocket I understood why I wasn’t getting an data. It was in a gully surrounded by rock. It almost seemed like the GPS lost its fix and then didn’t regain it again until my rocket was already on the ground? Is that possible? Any pointers to ensure I get a location that is closer than 2000 feet away when I start my search?

Ahhhh, you need a noisemaker on your harness. If one's rocket is in a "hole" or cornfield furrow, the ground foot print of the radio is going to be "chit". The Ham radio stuff will be a "little bit" better but likely not that much though 2 meters APRS would be dynamite. Want to learn more about propagation? Read the books and get a Ham radio license Technician rating or above. Best thing I ever did, THEN, I went into GPS tracking.
If your rocket is 2000 feet away and in a hole, that last known position is the key to start the search pattern. In a hole an Eggfinder's ground foot print is going to be "chit" and I suspect Chris will agree with that.
I use a 900 Mhz Yagi for ground recovery only and it does increase the ground footprint. As long as one knows which direction to proceed, they'll be fine no matter what antenna they use. A totally sight unseen flight, that last known position no matter what radio band one uses is most important.
If the GPS antenna on the tracker is unfortunately pointing into the ground, GPS lock can be lost. as outlined in the OP. If the transmitter stops transmitting, one is toast. Unless they got the last lock on the last known position. If a photomap program can be used for tracking, one can proceed in the direction of the last known fix and believe me, you'll find your rocket. I've never lost one with a tracker 900Mhz, 400Mhz, APRS or otherwise. I've seen a rocket (not mine) with an RDF tracker have a full recovery deployment failure, auger in and the tracker survived. Had a weak signal above the hole and they couldn't dig down deep enough to get the rocket back! Rocket got into clay and that was it. I think the flier got the butt end of the rocket back but that was it. Tracker was beeping up to the end when they gave up on the dig.
It was way down there.
RDF is the only option with very small rockets but GPS is the way to go with larger projects.
Man, I saw RDF work really well in the olden days when GPS trackers, weren't available of any type or too pricey.
Shoot, I have Yagi antennas with electronic attenuators so I can go back there when I want.
Man, it was cool to see someone using a Rocket Finder or Walston device and trail them out into the field and actually find a totally "sight unseen" flight. Only problem back then those cost, big $$$$$$. APRS came out shortly thereafter and I did the "Ham radio" thing too save some $$$$. Then the 900Mhz NMEA unlicensed stuff came out and I went there as I liked building/soldering the trackers from parts. I had the stuff and magnifying lamp to do it. Only dorked one kit mind you and I built a pile of them and have no problem flying them. Did some 400Mhz kits too as I had a Ham callsign and the makers would them sell to me. Found a way to port the 900Mhz stuff to Ham radio mapping programs and I was ecstatic. To know my rocket landed "over the barn and out of the stream" was priceless. Could also navigate away from obstacles and take a "roundabout" recovery path as there were too many streams to ford and ditches to go through for a straight line approach. Take a road to get around all that stuff and walk right to the downed rocket on a flat part of the field! Pick it up and fly the next one! Kurt Savegnago

 

[rjenkins133]

At Airfest, I used the eggfinder tracker with my android phone using the rocket locator app, and as with my OP above - the tracker showed me the location on the phone and I walked to the location and there was no rocket at that location. So then I started walking around the location all the while checking the tracker location and after 15 -20 minutes of searching, the tracker suddenly showed a different location for the rocket - in one case about 150 feet from the initial location and in the other case about 500 or more from the original location. The entire time I was receiving a signal from the GPS transmitter. I turned off the phone network so that the phone had to use GPS and could not triangulate cell towers.

If the GPS receiver has lost lock, does the transmitter continue to transmit the last lock location? When outside, it normally doesn't take 15 minutes to establish lock - so why would it take so long after a launch?

Plus for each launch I could clearly see the descent pattern for a rocket under parachute - so it had a position lock during the descent - could it lose lock on impact and then why would it show a position that is 150 feet or more from the last lock position? and the at the wrong location that wasn't even along the descent path!

I have a log file from the rocket locator software, but I cannot see how to decode the contents - the log file is not ascii text and so I cannot determine if it contains NMEA data. - it is a .APK file - does anyone know how to look at the log file for this program?