GPS Proximity to RF Downlink

[AnthonyRC]

I just completed a nice 1300ft flight with live video and a GPS data downlink. During the flight there appeared to be some moments in time where the GPS data was lost. After the flight I looked at the logged gps data and it appears that the number of satellites used for the fix never gets above 5, and drops down to 2, 1, or even zero quite often.

So, here's the question, do UAV designers need to worry about the proximity of the RF downlink antenna and the GPS antenna?. I'm worried that 600mW at 2.4GHz is going to cause problems for the sensitive GPS antenna.

[air]

That definitely doesnt sound too positive & wouldn't be great I'd imagine if you were relying on GPS for navigation.
I would have thought that with a good installation an airborne gps receiver should have no problem acquiring a large number of satellites & should stay locked on?

[PerfectStranger]

Ah, I love this stuff. They deal with the same satellite coverage we do. That being said, you will never receive 0 satellites due to bad constellation. More likely the antennae was obscured momentarily or interf. from your other electronics. I'm sure it a recreational grade GPS. The big boys do have an IMU (inertial measurement unit). GPS tells it where it's at and the IMU tells it exactly how it is moving in the alpha, phi, omega (ie, where it's going to be very soon!).

i work for an aerial photograhy/photogrammetry company. last year we bought a LiDAR (light detection and ranging) unit. for navigation, it operates just like a missile. has both survey-grade gps and the IMU unit. i process all the gps and imu data separately then integrate. it is routine to get sub 6 cm accuracy for a kinematic (moving) survey. measure 4-5cm with your fingers then wipe the smile off your face.

check out the noaa website. they put up an alert because of a huge solar storm. this type of thing will cause spotty coverage and ruin satellites.

did i say i love this stuff?

matt

[Medve]

would you want to put some sheilding between the GPS antenna and the Video TX antenna? Say some foil? and obviously move them as far appart as you can.

[Dave Jones]

AnthonyRC
What GPS receiver are you using?
I have had the same problum with some GPS receivers.
Dave Jones
AUAV.net

[AnthonyRC]

Thanks for all the comments on this one. I did some more experiments with the plane sat in the garden and it confirmed that the 2.4GHz transmitter is the issue.
The difference between the GPS antenna 10cm from the Tx, and 40cm from the Tx, is about a factor of two in the number of satellites acquired.

The GPS is a NAVI-S from falcom.de, based on the SiRF chipset. Works well generally, but looks like I need to keep the two apart in the future.

[sesat]

Dave, have you found some receivers being more risilient than others to this sort of thing?


Ram.

[Medve]

you may want to run a wire from the GPS and put a patch antenna as far from the 2.4Ghz TX as you can. Most GPS's have external antenna jacks.

[air]

mattalbr, Do you know if there are any high grade gps receivers available that would be compact enough for installation on an RC model?

[PerfectStranger]

mattalbr, Do you know if there are any high grade gps receivers available that would be compact enough for installation on an RC model?

Any GPS receivers that are specifically mapping or survey grade AND small enough to be carried around on our planes cost thousands of dollars. Many thousands. The software to post-process the data is costly as well. Unless you intend on making LOTS of cash with it or are independently wealthy, there's no way. If you are going to make LOTS of cash with it or if you are independently wealthy, drop me a line! I would love to help you out.

That being said, the recreational grade GPS receivers are not terrible. With a good constellation, continuous unobstructed observation, and min 6-7 satellites you can expect accuracies of 5-15 feet. It's the phase that it uses for timing that is the limitation on these receivers. I have a GarminIII+ and it's great for everything but making money.

I just found this page, might explain some stuff better than I.
http://www.digitalgrove.net/GPS.htm

Some quality GPS
http://www.trimble.com/
http://products.thalesnavigation.com/en/
http://www.applanix.com/
Kucera International (http://www.kucera-gis.com/) shameless plug!

[sesat]

Matt, do you have any comments on the Novatel OEM4-G2L receiver, as to whether it's worth its price?


Ram.

[PerfectStranger]

Matt, do you have any comments on the Novatel OEM4-G2L receiver, as to whether it's worth its price?


Ram.

ram

i looked it up, that g2l is a thing of beauty! so small yet so capable. i don't know how much it costs but if you can make money with it, and you need the accuracy, it's probably worth it. novatel makes good hardware. we have two of the dl4plus (http://www.novatel.com/Products/dl4plus.html), each mounted in a plane as the primary gps for both photo and lidar. just as important IMHO is thier customer support. it is excellent and we've needed them at wacky hours sometimes.

if you're looking at these, you know what you are looking for I assume. be sure that all your electronics can talk to each other. it sounds simple, and it is, but if not remembered can be costly. i know :) .

matt

[JettPilot]

Thanks for the heads up on the 2.4 transmitter interfereing with a GPS. I am building a UAV with a 5 watt video transmitter and would have run into the same problem :eek: . You guys have saved me lots of work :) I will now put the them on opposite wingtips, its much easier to change the design now than it would have been to find out after it was built :D .