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Highest Altitude GPS?

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Bruce

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I'd like to eventually try for a Tripoli altitude record and was wondering what GPS might be the best for extremely high flights?

I realize that a lot of research and hard work goes into every record attempt. Even if I never do make it above 150,000 feet, I'd still like to have a GPS that's capable of it.

I read of the COCOM Limits,

https://en.wikipedia.org/wiki/Coordinating_Committee_for_Multilateral_Export_Controls

"a limit placed on GPS tracking devices that disables tracking when the device calculates that it is moving faster than 1,000 knots (1,900 km/h; 1,200 mph) at an altitude higher than 18,000 m (59,000 ft).[3] This was intended to prevent the use of GPS in intercontinental ballistic missile-like applications."

How is this dealt with for high altitude rocket flights?

Ideally I'd like to have a GPS that saves the flight data onboard as well as sends it back to the ground in real time. Just to cover all the bases...

It seems like a fast rocket would have the best chance of a straight vertical flight. I would think that the fins stabilizing effect would diminish as the rocket ascends since the air is getting thinner as you go up. So a fast rocket with its greater momentum would hopefully stay on course better than a slower rocket.

But normal GPSes don't work if you go too fast. What's the solution?
 

Bruce

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Yes, it looks like the Multitronix Mx150 Kate 2.0,

https://www.multitronix.com/kate-2-transmitter.html

and the BigRedBee High Altitude 70cm 100mw GPS/APRS Transmitter,

https://shop.bigredbee.com/collecti...high-altitude-70cm-100mw-gps-aprs-transmitter

are 2 of the best solutions.

Aren't both of them restricted to speeds below 1,150 MPH (1000knots)?

It would seem likely that a high altitude rocket would exceed this speed. Is that part of the flight just not recorded and the GPS resumes operation when speed drops below 1,150 MPH?
 

VernK

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Aren't both of them restricted to speeds below 1,150 MPH (1000knots)?
It would seem likely that a high altitude rocket would exceed this speed. Is that part of the flight just not recorded and the GPS resumes operation when speed drops below 1,150 MPH?
Yes, Kate 2 is restricted to only report GPS fixes at less than 1000 knots. Above 1000 knots, Kate 2 will revert to using accelerometer data in order to fill in for the missing GPS data so that it can still report altitude and velocity on the way up. This makes sure you still know what is going on. The instant the velocity drops below 1000 knots, the GPS in Kate 2 will immediately resume reporting valid fixes because satellite lock is maintained even above 1000 knots. It is just the reporting of the fixes that is interrupted. This means it will resume reporting long before you reach apogee and want to record the maximum altitude. There is no limit on the maximum altitude for Kate 2. Kate 2 records all the data onboard the rocket as well as sending it to the receiver on the ground to be recorded there too.
 
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Walter Longburn

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Bruce

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I couldn't find any numbers on the Featherweight web site that indicated what the limits of their altimeter's velocity, altitude and acceleration were.

Does anyone know?
 

Bruce

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Yes, Kate 2 is restricted to only report GPS fixes at less than 1000 knots. Above 1000 knots, Kate 2 will revert to using accelerometer data in order to fill in for the missing GPS data so that it can still report altitude and velocity on the way up. This makes sure you still know what is going on. The instant the velocity drops below 1000 knots, the GPS in Kate 2 will immediately resume reporting valid fixes because satellite lock is maintained even above 1000 knots. It is just the reporting of the fixes that is interrupted. This means it will resume reporting long before you reach apogee and want to record the maximum altitude. There is no limit on the maximum altitude for Kate 2. Kate 2 records all the data onboard the rocket as well as sending it to the receiver on the ground to be recorded there too.
Thanks for the good info Verne.

Are there limits on acceleration for Kate 2? I understand that some GPSs only work below 4Gs.

Also, what about spin? If the rocket is spin stabilized, is there a limit as to how fast it can spin before the GPS stops working?
 

Ethan

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I couldn't find any numbers on the Featherweight web site that indicated what the limits of their altimeter's velocity, altitude and acceleration were.

Does anyone know?
It uses a U-blox Module, so it is restricted to 1000 knots and has a maximum altitude of 50km.
 

jderimig

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I'd like to eventually try for a Tripoli altitude record and was wondering what GPS might be the best for extremely high flights?
If an official TRA record is important then you may want to check what electronics are approved by the TRA records committee.
 

VernK

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Thanks for the good info Verne.
Are there limits on acceleration for Kate 2? I understand that some GPSs only work below 4Gs.
Also, what about spin? If the rocket is spin stabilized, is there a limit as to how fast it can spin before the GPS stops working?
There are no guarantees but experience has shown that Kate 2 will maintain lock about 90% of the time if the acceleration is 10 G's or less. It will loose lock about 90% of the time if the acceleration is 20 G's or more. If it loses lock, it will regain lock within just a couple of seconds once the acceleration subsides. In other words, right after motor burnout. I have never seen a GPS problem related to spin during any flights. Although admittedly, most of those flights were probably not spinning very fast. I have ground tested it to 2000 deg/s spin rate and the GPS was operating just fine.
 
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Adrian A

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It says 300,000 ft. on the website.
300,000 feet is the radio range of the Featherweight GPS tracker with its standard antennas. With a directional ground antenna the range can be higher. If your rocket goes above 50 km AGL (164,000 feet) or 500 meters/second (~Mach 1.45) the u-Blox GPS receiver will withhold its data until your unit is below that speed and altitude. In the meantime you will still have communication and data on the battery voltage, communication signal strength, etc.
 

ksaves2

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For a DIY person the Ublox chipset is either/or. Yeah it locks out at a 1000 knots but as soon as the speed drops, it starts reporting altitude again. I think 50km/~164,000 feet is probably the limit for commercial off the shelf units. Some GPS units once the altitude limit was reached they stopped sending altitude. I believe the available trackers for rocketry use GPS units that can read to the higher altitude as long as the velocity is below the limit. Yeah, like John mentions above, go to the TRA site and they have listed GPS trackers acceptable for altitude records.
I remember reading an article about this something like 10 years ago where hobbyist high altitude balloon fliers had problems with altitude lockout with certain GPS chipsets. Of course a gas balloon doesn’t travel at rocket speeds so those folks posted a list of acceptable chipsets. The deal here is they’d fly digital cameras and APRS radios so they could track and find the balloon and camera after the balloon burst.
Then there was an British Ham who built an ultra tiny APRS tracker that had a small lithium battery, charge controller and small solar panels. Small party balloon flew around the world 5 times and eventually went down off the coast of Iceland. Kurt Savegnago
 

Joshua Smith

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Yeah, Featherweight uses LoRA for transmitting data back to the receiver, low data rate, low power, long range. The big restriction will still be the GPS side. The site says it uses u‑blox M8 for simultaneous reception of GPS and GLONASS. I don't know if there are similar restrictions when using GLONASS? I tried looking at the data sheet for what I think is the M8, and operational envelope was 50,000m, 500m/s, and 4g. The 4g seems like it would be an issue so I'm wondering if I'm looking at the right chip or not, there are a number of variants. I'm going to see what chip info I can get off of mine and I'll post if I learn anything

update: it's this https://www.u-blox.com/en/product/sam-m8q-module which has the same operational limits as I mentioned above
 
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AllDigital

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I use a u-blox M8 in my package and it cuts out regardless of using GPS, GLONASS, or BeiDou. So, I don't think their ruleset is specific to a network.

I would be very interested to know what chipset the BigRedBee is using and/or if it was commercially modified for them. If I ever fly that high I would likely use the BRB, but in our DIY telemetry it would be nice to be using the gold standard for high altitude rockets.
 

jderimig

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GPS antennas are very important. Good ones cost more than the chipset.
 

cerving

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GPS antennas are very important. Good ones cost more than the chipset.
Yup. If cost is more important than performance, the passive patch antennas (like the integrated one on the uBlox M8Q and the Lantronix A2235A that we use) are OK... but don't expect them to keep a lock at mach speeds, or if your rocket starts spinning or otherwise gets wonky. If performance is #1, you can't beat an amplified quadrafilar antenna... but it's way more expensive than the GPS chipset. Somewhat cheaper would be a passive quadrifilar antenna... you will get better coverage than a patch antenna, but with lower gain it's not going to keep a lock quite as well.
 

AllDigital

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If performance is #1, you can't beat an amplified quadrafilar antenna...
If price is not the primary concern, do you have a recommendation for an amplified quadrafilar antenna that doesn't weigh a pound or look like it should be on top of my truck? I can find plenty of small quadrafilar antennas, but not amplified, like this one:
Are the amplifiers separate or usually integrated?
Thank you, Mike
 

cerving

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Try this one... it appears to be readily available.


The Sarentel SL-1206 was the gold standard for compact amplified helical antennas, they were bought by Maruwa. As far as I can tell, the Maruwa p/n is now MHC-1575G. You'll have to contact them to see who their distributors are now, AFAIK none of the major US distributors (DigiKey, Mouser, Farnell/Newark) carry them.
 

Adrian A

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Yup. If cost is more important than performance, the passive patch antennas (like the integrated one on the uBlox M8Q and the Lantronix A2235A that we use) are OK... but don't expect them to keep a lock at mach speeds, or if your rocket starts spinning or otherwise gets wonky. If performance is #1, you can't beat an amplified quadrafilar antenna... but it's way more expensive than the GPS chipset. Somewhat cheaper would be a passive quadrifilar antenna... you will get better coverage than a patch antenna, but with lower gain it's not going to keep a lock quite as well.
I also used to think that antenna orientation and pattern would be very important for good in-flight GPS performance. Then I learned that if a receiver has enough signal strength from a satellite to download the ephemeris (orbit information), it only needs about 1/10 the signal strength to continue to track them, and with a low gain patch antenna like the M8Q the tracker orientation has less effect than that. This means that it can keep using the signal even when the antenna is pointed away from the satellite or straight at the ground. You can see this for yourself in the live data that the Featherweight GPS receiver puts out. This makes the rocket roll angle or spin rate a complete non-issue for the performance. The receiver can just keep listening continuously however the rocket is pointed.
 

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VernK

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I also used to think that antenna orientation and pattern would be very important for good in-flight GPS performance. Then I learned that if a receiver has enough signal strength from a satellite to download the ephemeris (orbit information), it only needs about 1/10 the signal strength to continue to track them, and with a low gain patch antenna like the M8Q the tracker orientation has less effect than that. This means that it can keep using the signal even when the antenna is pointed away from the satellite or straight at the ground. You can see this for yourself in the live data that the Featherweight GPS receiver puts out. This makes the rocket roll angle or spin rate a complete non-issue for the performance. The receiver can just keep listening continuously however the rocket is pointed.
I will second what Adrian stated. That has been my experience too.

I would also add that it is really important not to contaminate the GPS signal with interference (EMI) generated by your own electronics. A good design needs to keep the GPS module and especially the GPS patch antenna well away or shielded from sources of RF energy such as the onboard microprocessor, SPI data bus, USB port, switching voltage regulators, etc. It is also important to keep the downlink signal from the onboard transmitter from blasting the GPS antenna preamp and overloading it. Selecting a GPS patch antenna with a built-in SAW filter before the LNA will help a lot. Some designs even have a second SAW filter after the first LNA. Once you have assembled your full system, it should be able to lock onto 4 satellites in about 45-60 sec from a cold start. If it is taking longer than that then something is not right. In other words, there might be some interference going on. The GPS should proceed to lock-up all the rest of the visible satellites in a matter of another 3-4 minutes or so at the most. Again, if it is taking longer than that something is not quite right. If it is a simple reset where the GPS already has recent ephemeris information from before the reset then it should only take a couple seconds to lock.
 

AllDigital

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"OR 4 interest-free payments of$40.63"
to rich for me..
Ha. That RTK CORS antenna on NewEgg is made by Beitian in China. I just bought one off of AliExpress (Alibaba) for $25.31 (including US shipping). @cerving I also dropped a note to Maruwa about the 1575G. We'll see what they say. Here is the alibaba link:

 

UhClem

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Once you have assembled your full system, it should be able to lock onto 4 satellites in about 45-60 sec from a cold start. If it is taking longer than that then something is not right. In other words, there might be some interference going on. The GPS should proceed to lock-up all the rest of the visible satellites in a matter of another 3-4 minutes or so at the most.
It takes about 30 seconds to receive the ephemeris data. For modern all in view receivers that is the limiting step. 30 seconds or so after power up and you should get data.

When I bought a Big Red Bee I looked at the heat shrink meant to keep the battery in place but also covering the patch antenna and realized that would cause trouble. Without the heat shrink it had a position 31 seconds after a cold start. It took 45 to 50 seconds with the heat shrink. The heat shrink changed the center frequency of the patch enough to degrade performance. The heat shrink wasn't all bad news since the RF inputs on GPS engines can be extremely static sensitive.

Another thing to worry about with GPS is that the national ranges will occasionally test weapon systems in a GPS jamming environment. The jammers of course cover a lot more space than that. A NOTAM is typically issued when this happens.
 

ksaves2

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Try this one... it appears to be readily available.


The Sarentel SL-1206 was the gold standard for compact amplified helical antennas, they were bought by Maruwa. As far as I can tell, the Maruwa p/n is now MHC-1575G. You'll have to contact them to see who their distributors are now, AFAIK none of the major US distributors (DigiKey, Mouser, Farnell/Newark) carry them.
Yeah, I had one off those I was experimenting with in the past. Cost me $73.00 back then. Amplified quadrifiler antenna. The thing would give a decent fix from my basement! Was going to make a tracker except I dropped the danged thing and busted it! Found out they were out of stock at the GPS chip dealer I was working with. Should have ordered 2 or 3 when I could. The performance was fantastic.
I played around with SDR radios for awhile but I eventually gave up. The prospect of a $26.00 GPS tracker was very enticing and would be disposable. Core sample? No problem just toss it out! The stumbling block was I couldn’t get the SDR unit to program my Ham callsign in some of the NMEA strings it was transmitting to stay legal. I expect the inability to program was a result it was a cheap radio. Spend bigger bucks and I’d probably be able to program away....... But then that defeats the purpose of a super cheap GPS tracker. I gave up on it. Kurt
 

Bruce

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I played around with SDR radios for awhile but I eventually gave up. The prospect of a $26.00 GPS tracker was very enticing and would be disposable. Core sample? No problem just toss it out! The stumbling block was I couldn’t get the SDR unit to program my Ham callsign in some of the NMEA strings it was transmitting to stay legal. I expect the inability to program was a result it was a cheap radio. Spend bigger bucks and I’d probably be able to program away....


Sounds interesting!

Might you be willing to share the model name of the $26 SDR radio transmitter that you used?
 

Bruce

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For a DIY person the Ublox chipset is either/or. Yeah it locks out at a 1000 knots but as soon as the speed drops, it starts reporting altitude again.

So as I understand it, if you are building your own altimeter setup, if EITHER the max speed or max altitude conditions are exceeded, then the GPS will stop reporting coordinates.

But if you are a manufacturer, you are allowed to exceed ONE of the max speed or max altitude conditions and the GPS will still continue to report coordinates?

Why the difference? And how do manufacturers do it?
 

Adrian A

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So as I understand it, if you are building your own altimeter setup, if EITHER the max speed or max altitude conditions are exceeded, then the GPS will stop reporting coordinates.

But if you are a manufacturer, you are allowed to exceed ONE of the max speed or max altitude conditions and the GPS will still continue to report coordinates?

Why the difference? And how do manufacturers do it?
I'm skeptical that this is true, but it if it is, I'll be kicking myself for not asking them for this the last time I bought 500 units. There are some GPS module manufacturers that welcome customer-requested mods to firmware, but I would be very surprised if u-Blox is one of them.
 
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