I have been flying a lassen iq gps in a few rockets this spring.
The last rocket just went 32,363ft at 1,300+mph
http://www.youtube.com/watch?v=I701JpONHmk
My problem is with the altitude and velocity the lassen does not catch up until the rocket is already comming down.
This has also held true for lower and slower flights to 500ft.
What is out there that would work better? I understand that nothing may not be perfect but maybe something better?
You did your L3 on an N2000? Phew. Congratulations.
I may be terribly mistaken about this, but I think that the only velocity data in the GPS strings is speed over ground, which means that you’re not going to get a reading of the rocket’s vertical velocity under power. Do you know something I don’t?
I’ve been told by other rocketeers that you will always lose GPS lock on the way up. Have you asked this question over on The Rocketry Forum? I’d think you’d get more people with relevant experience there.
How are you running the GPS? Is it going into some kind of packet APRS tracker and then radio-ing AX.25 packets down to your ground station? Or have you rigged something of your own?
Wow! Cant help you out with your problem, but the sound that thing makes on lift-off is incredible! Nice job.
I may be terribly mistaken about this, but I think that the only velocity data in the GPS strings is speed over ground, which means that you’re not going to get a reading of the rocket’s vertical velocity under power. Do you know something I don’t?
The Lassen IQ outputs both vertical and horizontal velocity.
I’ve been told by other rocketeers that you will always lose GPS lock on the way up. Have you asked this question over on The Rocketry Forum? I’d think you’d get more people with relevant experience there.
Correct to overcome this altitude is determined from accel and baro. However the rule for altitude records (flawed as it maybe) states that flights over 30,000 ft altitude needs to be determined by gps.
How are you running the GPS? Is it going into some kind of packet APRS tracker and then radio-ing AX.25 packets down to your ground station? Or have you rigged something of your own?
Lassen IQ and accel read by micro saves accel data to eeprom and sends data to ground with Xtend radio.
Ah, interesting. I did not know that.
And you have a homebuilt system? Impressive as well. I suspect you might be pretty close to the forefront of this stuff.
I’m so glad to see some folks putting decent electronics of their own design into their rockets. I find that much more interesting than all the “bigger, higher, faster” stuff.
I am working on a similar type of system incorporating GPS and radio telemetry back to the ground using XBee or ax.25 APRS. You can check out what I’ve got so far here: [http://www.digitalmisery.com. What are the issues with the GPS and maintaining lock on the way up? I will be experimenting with a 5Hz GPS that I thought would be good for this sort of payload but if GPS can’t keep lock during the ascent, is there really any point of trying to log/transmit high-rate GPS data? I can understand that it becomes useful at apogee to determine max altitude, but it sounds like a barometer and an accelerometer are more widely used methods of determining altitude. I am just barely getting interested in rocketry so I won’t have to worry about altitudes >1000’ for some time, but I just wanted to know what to look forward to and to plan my electronics accordingly. What sort of barometer/accelerometer combination is typically used? Is it a digital barometer?](http://www.digitalmisery.com)
The easiest way to send altitude data is to use a Perfectflite MAWD altimeter, turn on the telemetry function (in software), and connect the TX and ground wires from the data output connector to an XBee radio’s Din and ground pins. As soon as the altimeter goes into flight mode (at 160 feet) it will begin transmitting the altitude to the XBee, which will in turn transmit them to your ground station. The format is 7 bytes: 5 digits of altitude (with leading 0s), CR, LF. The MAWD figures altitude from a barometric sensor - with this setup you’re just basically letting the MAWD do the work for you, and the cost is that you’re going to have to trust the MAWD’s data.
I’ve flown this setup 11 times with good data throughout the flight each time. Now obviously if you’re going for altitudes/records that require GPS altitude, this is meaningless for your purposes. I wasn’t aware before this thread that anyone really cared about GPS altitude, but then, I fly only about 1000-4000 feet up, and I’ve almost always got a MAWD on board anyway.
The easiest way to send altitude data is to use a Perfectflite MAWD altimeter
This truly is the easiest. I have done it before and it works good.
What sort of barometer/accelerometer combination is typically used? Is it a digital barometer?
Currently use MPX4115 (free samples) for baro but looking into something with temp compensation. For accel I have used the ADXL78 and MMA2301. I find the ADXL to be most reliable.
I have attached a pic of the unit. The gps and radio are on the backside of the board. For rocket flights I use a yagi antenna.
mike whorley:
Currently use MPX4115 (free samples) for baro but looking into something with temp compensation.
The MPX4115 is temperature compensated.
When you say that the GPS doesn’t “Catch up” just what do you mean. Is there an obvious error, no lock on satellites or something else? If it is maintaining a lock, the internal position/altitude filters might be the problem. According to the Lassen IQ data sheet, those can be disabled.
mike whorley:
For accel I have used the ADXL78 and MMA2301. I find the ADXL to be most reliable.
I have attached a pic of the unit. The gps and radio are on the backside of the board. For rocket flights I use a yagi antenna.
Beautiful. You don’t have any problems with GPS lock when it’s mounted on the board with the other parts?
I have been working on a GPS board, and while it works fine on a protoboard, when on the pc board it takes forever to get a lock, and loses it very easily. I’ve taken the GPS a few inches away (plugging wires into the pc board) without resolving the problem. I’m sure it has to do with my lack of understanding of how RF stuff needs to be laid out and connected, or possibly my lack of knowledge of how to put together power supplies for GPS.
When you say you find the ADXL to be most reliable, I take it that means you had some reliability problems with the MMA unit? I’ve been working on telemetry boards designed to use MMA accelerometers (lower G versions than yours, though - the MMA3201 +/-40G version: I’m flying I, J, and K motors). Should I be concerned?
The MPX4115 is temperature compensated
Correct but I would like digital w/ temp comp
The MPX4115 is temperature compensated.
When you say that the GPS doesn’t “Catch up” just what do you mean. Is there an obvious error, no lock on satellites or something else? If it is maintaining a lock, the internal position/altitude filters might be the problem. According to the Lassen IQ data sheet, those can be disabled.
1 of 2 modes
Same sentence string is repeated
Altitude reading does not change
The first few flights where made with the filters disabled and I had the same issues.
Beautiful. You don’t have any problems with GPS lock when it’s mounted on the board with the other parts?
I have been working on a GPS board, and while it works fine on a protoboard, when on the pc board it takes forever to get a lock, and loses it very easily. I’ve taken the GPS a few inches away (plugging wires into the pc board) without resolving the problem.
I did have a problem just like this. I was using the ultra compact antenna for the Lassen gps. I wanted to mount this right on the pcb (on specified ground plane). In this config I t took forever to get lock and once lock was aquired it was lost with little movement. To resolve the issue I switched to the magnet mount antenna.
When you say you find the ADXL to be most reliable, I take it that means you had some reliability problems with the MMA unit? I’ve been working on telemetry boards designed to use MMA accelerometers (lower G versions than yours, though - the MMA3201 +/-40G version: I’m flying I, J, and K motors). Should I be concerned?
n
I don’t have too many flights with the mma2301 (replacement for the 2300) but one of my first projects was comparing the 2 accelrometers. I seem to get more noise(?) wth the mma. I have now switched the 2300 to a 2301 but have yet to fly it.
mike whorley:
I did have a problem just like this. I was using the ultra compact antenna for the Lassen gps. I wanted to mount this right on the pcb (on specified ground plane). In this config I t took forever to get lock and once lock was aquired it was lost with little movement. To resolve the issue I switched to the magnet mount antenna.
Ah, so it’s not just me. That does sound exactly like the issue I’m having. With essentially the same board but a DB9 connector going to a “hockey puck” receiver, it works just fine, including in flight, and including picking up a lock again quickly after losing it during boost.
Maybe I’ll abandon the plan to have the GPS directly on the PC board and stick with the magnet mount version.
Could it be that the 900 Mhz transmitter is interfering w/ the ability of the GPS module to retain lock? I had this problem when I played around with a similar configuration. Even with my current setup, I see a drop on received signal strength every time my 70cm transmitter sends a packet.
Greg Clark K7RKT
It’s possible. BTW, I’m currently using the 2.4 GHz XBees, not the 900 MHz ones. I also don’t have any processor in the loop, so the XBee is in transmit anytime it receives data from the GPS (that is, once per second), so it’s transmitting quite a bit.
It does work properly though on a protoboard, with the GPS and the XBee separated by little more than an inch. Maybe the orientations of the two devices makes a difference?
It looks like most GPS antennas are looking for a 65mmx65mm or bigger ground plane to be mounted to in order to have widest coverage. See this data sheet for patterns with 70mm and 43mm ground planes.
http://php2.twinner.com.tw/files/onshin … 06-NEW.pdf
Also GPS antennas usually have a LNA with a very narrow bandwidth, so the 2.4 Ghz signal from your Zigbee module should have little affect, unless it is too close the the GPS module itself.
Here is some data from a flight to show the slow responce of the gps to altitude. This was not a high and fast flight either.
Here is the data from the flight to 32K.
Looking at the gps file shows the problem I had. The gps output the same sentence for around 14 seconds. I assume the gps would catch up again like it did on previous flights but it has never output the same sentence multiple times.
I beleive the loss of contact to ground station to be from the airframe. The airframe is carbon fiber with a fiberglass nosecone. The gps is located in the nosecone due to the inability for rf to go through carbon.
The current theory is the higher the rocket goes the farther away the ground station needs to be for the airframe not to get in the way.
According to Wikipedia: GPS,mike whorley:
the rule for altitude records (flawed as it maybe) states that flights over 30,000 ft altitude needs to be determined by gps.
“All GPS receivers capable of functioning above 18 kilometers (11 mi) altitude and 515 metres per second (1,001 kn) [3] are classified as munitions (weapons) … These limits attempt to prevent use of a receiver in a ballistic missile. … This rule applies even to otherwise purely civilian units … Disabling operation above these limits exempts the receiver from classification as a munition. Vendor interpretations differ. … This has caused problems with some amateur radio balloon launches which regularly reach 30 kilometers (19 mi).”
Is there any way to tell if your GPS is deliberately choosing not to work in an attempt to avoid the munitions classification, and is perhaps being excessively conservative?
Yes this is an issue but as I understand it most gps units OR velocity and altitude (meaning if one OR the other is over limits condition is true). The Lassen IQ uses AND ( altitude AND velocity need to be over limits for condition to be true)