Author Topic: RS232 jamming (Read 548 times)

Wolfala · « **on:** October 04, 2007, 08:04:26 PM »

Food for thought for you guys. Lets say we have a geographic area you are traveling over. There are 2 GPS's which RS232 outputs going to a MFD. Whenever you go over that region, the GPS's keep their location without issue, but the MFD loses its mind saying it is not getting good position data from the GPS's, but the BIT info and signal info from the GPS's is unhindered and it gives good position data - and you know it is good position data.

And these problems only occur in a specific geographic region. My question is this, IS it possible to introduce transient voltages into a RS232 cable externally to prevent data from moving 4 feet from 1 box to another?

Wolf

Russian · « **Reply #1 on:** October 04, 2007, 08:11:32 PM »

Slightly OT, Why do you need two GPS receivers?

Wolfala · « **Reply #2 on:** October 04, 2007, 08:18:05 PM »

Aircraft have 2 GPS's for redundancy.

Russian · « **Reply #3 on:** October 04, 2007, 08:27:44 PM »

if I approached this problem as a system-admin, I would update firmware and disable one receiver to see if problem continues. LOL

eagl · « **Reply #4 on:** October 04, 2007, 08:29:52 PM »

Are you *sure* the gps receivers are getting good signals in that area?

The MFD might be smart enough to realize that the reported position doesn't make sense (like you travel for 5 minutes and jump 200 miles) so it reports bad data.

Are you by any chance having this problem in an area where GPS selective availability might be turned on and cranked up?

Meatwad · « **Reply #5 on:** October 04, 2007, 08:31:21 PM »

Around here there is a big natural gas pipeline and an oil pipeline. When you pass over these in spray and spreader trucks, it temporally screws up the GPS and the guidance lightbars in the trucks.

Why would a pipeline cause that

Wolfala · « **Reply #6 on:** October 04, 2007, 08:38:07 PM »

Quote

Originally posted by eagl
Are you *sure* the gps receivers are getting good signals in that area?

The MFD might be smart enough to realize that the reported position doesn't make sense (like you travel for 5 minutes and jump 200 miles) so it reports bad data.

Are you by any chance having this problem in an area where GPS selective availability might be turned on and cranked up?

There were no FDC notams saying reduced GPS accuracy was kicked in. The signals page was opened the entire time, and it happened to 2 separate aircraft with identical avionics configurations in the same geographic region.

The GARMIN stack didn't give any integrity warnings and the EPE was steady at 1.9 meters. Also the problem was reported from 300 feet all the way up to 12,000 feet. Been reported by multiple pilots in separate aircraft.

eagl · « **Reply #7 on:** October 04, 2007, 09:55:45 PM »

Selective Availability isn't supposed to give any warning. The GPS signal is simply skewed to make gps devices in that area inaccurate. Some devices may recognize the increased error, some will not.

Or it could be a software bug in the display unit. Or maybe there are unusual radio frequency reflections in that area, or some sort of transmitter that interferes with the GPS signal enough to spoof the GPS devices.

Or someone is running an illegal GPS jammer. I read somewhere that the Russians (and probably French and Israelis) have some nifty GPS jammers for sale. They're obviously a bit more complex than your average cellphone jammer, but they shouldn't be too difficult to make since the civilian GPS technical specifications are pretty much open information.

Heck, even a malfunctioning differential GPS transmitter might cause a problem like that.

http://en.wikipedia.org/wiki/Differential_GPS

Wolfala · « **Reply #8 on:** October 05, 2007, 04:24:05 PM »

OK, so it could be something as simple as a guy with VHF/UHF antenna with a pre-amplifier like we had out here at Moss landing a few years ago blasting away and no one knowing about it.

Hornet33 · « **Reply #9 on:** October 05, 2007, 06:46:45 PM »

OK I worked on the NDGPS project in the Coast Guard for around 5 years so I know a little something about the topic. I have in the past gotten phone calls from some of our ships reporting stuff like this from time to time concerning GPS input into the ships nav system. The GPS receivers still showed good data but the nav system would drop the fix.

What we found out was from time to time was that a number of the satalites would converge into a tightly packed group and then diverge again during the course of their orbits. With some of the older GPS receivers that only had a 6 channel receiver they would loose fix because there wasn't enough angle offset for the unit to compute a fix with the strongest satalite signals since they were directly overhead. With the newer 12 channel receivers this doesn't seem to cause a problem anymore because they are still able to generate a fix with lower signals from satalites lower on the visible horizon.

With me so far? OK now with many of the nav systems that are on the market, they normally are configured to accpet all of the NMEA from the GPS receivers that are connected to them, BUT and this is important, the internal nav software is programed to ONLY use the BEST 6 signals received from the GPS units. The nav systems determine this by the H-DOP and P-DOP reading sent to them from the GPS receivers. They do this to speed up the processing time for generating a fix and 99.9% of the time it works just fine.

So if you are in an area that is experiancing a satalite convergance, a 12 channel GPS receiver will still function but a nav system set up to only use the best 6 will loose fix. Now you should be able to go into the NAV system set up menus and configure it to use 12 channels or lower the signal ratio to pick up on lower to the horizon visiable satalites.

Now all that being said, is what you experianced a one time thing and your wondering if you have a problem in your system, or does it do it EVERY time you fly over a certain area?

I love working on problems like this but at the same time it also helps to be able to see the equipment and the actual problem in person for further trouble shooting.

I tried to find the link I used to use at work for reporting GPS outages and looking at the satalite tracks but it seems that is not a public domain site. Military only it seems and I no longer have access to it for the moment. My new job I'm getting ready to start I'll be able to look at it again.

At any rate if it's a problem that happens in the same location all the time you should file a report to the FFA. They might have more information.

eagl · « **Reply #10 on:** October 05, 2007, 07:06:13 PM »

Quote

Originally posted by Hornet33
At any rate if it's a problem that happens in the same location all the time you should file a report to the FFA. They might have more information.

Although the local Future Farmers of America will no doubt be fascinated by the problem, the FAA might be interested in the report too.

Hornet33 · « **Reply #11 on:** October 05, 2007, 08:01:22 PM »

Quote

Originally posted by eagl
Although the local Future Farmers of America will no doubt be fascinated by the problem, the FAA might be interested in the report too.

Yeah yeah yeah....fat fingers but you know what I ment.

Wolfala · « **Reply #12 on:** October 06, 2007, 03:26:05 AM »

Hornet,

Good stuff. I'm gonna copy and paste the things pilots have seen - keeping in mind they have different aircraft but identical avionics.

1. We have had a number of MFD glitches where it loses nav data, sometime permanently. We originally thought this might be a connection problem or something. However, my flight instructor flew into our area (baltimore-washington) and experienced something similar in his SR22. We have noticed that it seems all these glitches happen near (10-30nm) P40 (Camp David).

I am wondering if anyone else has experienced this; the theory is that there is some type of electronic jamming operative in that area that can interfere with our MFD or other avionics. If other SR22 drivers have experienced this I would consider it possible. If not it probably is just a far-fetched theory. So: has anyone else experienced something like this in the P40 area?"

2. Derek:

It is not just around P-40. The entire area north of Baltimore seems to have this happen all the way from the Camp David area to 20 miles west of Philadelphia. It has been happening for a few years now with no explanation. It can be intermittent but on days that it happens it keeps happening on and off during the time spent in that airspace."

3. Alexander:
As I said, this is an issue really not unique to Camp David. My plane is hangared only 14 miles form Camp David and we see no interference anywhere around Frederick. It is a phenomenon north of Baltimore. All of the anti aircraft batteries are around Washington, DC and there seems to be no interference down in that direction.
This is mostly an issue "nav source not valid" appearing on the MFD. The Garmin signals continue to be strong and there is no evidence that the GPS itself is functioning abnormally.
So go figure. "

4. My experiences are the same as Brian's. The MFD keeps trying to refresh itself to no avail yet the Garmin's do not indicate INTEG problems. This happens in the same areas time after time. I believe jamming is occurring; there are several sensitive military installations in these areas."

5. When it has happened to me the altitude has not really an issue. It has happened at 300 feet on up to over 12,000 feet. The Geography is the only consistent variable in the equation. I have really not seen this to any great degree anywhere else but has been reported by multiple pilots in that area. "

6. It has happened to me between 3000 to 6000. I can count on it happening halfway between SCAPE and KCXY. It is also very common east of EMI approaching KMTN.

7. This happened to me several times when I tried to squeeze between P-40 and the ADIZ, and it happens entering Philly from the West pretty often- between Lancaster and MXE. The 430s are still navigating, but it was disconcerting at first. The effect is short lived, and intermittent.
I'm not sure why, but I never noticed it West bound.

Hornet33 · « **Reply #13 on:** October 06, 2007, 10:36:55 AM »

OK based on some of those reports from other pilots you can rule out active jamming, since the MFD's are the only items affected and the actual GPS receivers are still functioning. If there was active jamming of the GPS signals you would loose everything.

Now based on the areas up around Baltimore there are many things that could cause this type of problem. That area has the most flight traffic in the US. You have several ATC operations going on up there, military installations, NSA, and who knows what else. So there is ALOT of EMI floating around, (Electro Magnetic Interferance). Radio traffic, satalite uplinks, cell towers, radar systems. You name it. There is alot of that stuff up in that area and it will cause problems if you systems aren't designed to handle it.

With the MFD's being the only thing affected the first thing I would check is the type of cable being used to connect the GPS receivers to the MFD's. I'd be willing to bet it's a single braided shield on that cable and it's not properly bonded to either of the units it connects too. A standard commercial RS232 cable works fine for most applications but in high EMI situations it will pick up alot of interferance and cause problems.

I would recomend at least a double shielded, double braided outer shield, foil inner shield cable for high EMI situations. The more shields the better. It cost more but it's also more reliable. MILspec cable is the best and use solder type pins on the connectors vs crimp on pins. You also want to use non ferrous metal connector shells instead of plastic. Pull the outer shield out through the back of the connector shell so you have enough to go from the connector port to the ground lug on the back of the unit. Use a crimp on ring terminal end on the end of the shield, and solder it as well, then secure that to the ground lug on each unit.

We had to replace all the inter connect cables on the Cutter James Rankin in Baltimore for the same kind of problem. Their nav systems would constantly loose fix if they were anywhere near Baltimore. The GPS receivers worked fine but everything else would go to crap on them. We went up there and conducted an EMI study and found all sorts of stuff. They wouldn't even use one of their radios because every time they keyed it up the nav system would drop the nav fix.

Once we replaced all the commercial cables with heavy shielded cables we built on site the problems went away.

Wolfala · « **Reply #14 on:** October 06, 2007, 11:40:49 PM »

I've had more then a few inquiries into the cable construction after you mentioned it. Do you have a vendor that builds these cables to MILSPEC that you can point me towards? Preferably someone who works with the DOD and doesn't **** around.