Wednesday, June 30, 2010

GPS Interference

A week ago I received a Yaesu VX-8GR VHF/UHF APRS hand held transceiver with GPS. The transceiver performs as expected except in one extremely annoying respect - its GPS takes a very long time to get an initial fix on its position after switching on the radio. It cannot get a fix from inside the shack at all. By contrast, my HTC smartphone will get a fix in a couple of minutes whilst sitting in its charger cradle on the shack desk. Or at least, it did.

This morning I noticed on aprs.fi that the last reported position of my smartphone, G4ILO-10, was somewhere in Somerset. I started the APRSISCE application with the intent of "bringing it back home" by sending a position report with the correct location. But after ten minutes the phone had not managed to get a fix.

I switched off all my radio equipment in case one of them was an interference source, and rebooted the phone, and eventually after several more minutes it obtained an accurate fix. I am beginning to suspect that something is interfering with GPS reception in the area of my house.

If you Google "GPS interference" you will find links to numerous articles and research papers raising concerns about what is apparently an increasingly common problem. One article states that a directional television receiving antenna widely available in the consumer market contains an amplifier which can emit spurious radiation in the GPS L1 frequency band with sufficient power to interfere with GPS reception at distances of 200 meters or more. Other potential interference sources include spurious outputs from TV transmitters.

Another website states that "We are seeing increasing evidence of GPS interference and also apparent erratic behaviour (e.g. mis-reported location)" and provides a form for reporting cases of interference. This page provides links to two reports on the issue which unfortunately require registration in order to access them.

Have other amateur GPS users experienced difficulty in receiving the satellites or an increase in inaccurate position reports? For many GPS applications the effects of this could have rather greater impact than a radio ham's inability to report his position to aprs.fi. Perhaps the US administration was rather hasty in its decision to decommission Loran.

6 comments:

K9CHP said...

My experience with the GPS on top of my VX-8R has always been good, Fast acquisition and solid hold of the satellites. Comparable with the 60CSx and/or the Oregon, not as good but not far behind.

Interference? So far, I've been lucky but I know of quite portable GPS jamming devices being imported from China and used to block GPS signals from phones etc. Some of the shadier members of society need to use those in their illeicit businesses. Maybe you got one in your neighborhood. Strictly illegal here in the US, but has that ever stopped criminals?

Amir K9CHP

Chris said...

One of the reasons why your HTC gets a fix so fast might be that it probably uses A-GPS, or "Assisted GPS":
http://en.wikipedia.org/wiki/Assisted_GPS
Basically, your smartphone knows how to fetch the GPS almanac / orbital data from a server on the Internet, so it can do a cold startup much faster.


-chris N2YYZ

Julian said...

Yes, the HTC does that. According to the manual, the VX-8 does as well. I had a VX-8R and that could get a fix inside the shack. So I don't know if the VX-8GR is poorer or whether some interference has occurred in the meantime.

Theodore said...

My memory may have failed me, but was it in the movie "Blade Runner" where people were getting sick from E.M. waves overload?
The way our society is going it won't be too long before we are living in a pea soup of E.M. radiation.
Putting new sources of R.F. on every bit of spectrum, many of them on 24/7 is an interesting experiment in assisted evolution.
I am sure it will take a while yet to get to the tipping point, but with mobile phones zapping our young constantly,
perhaps we will get a new form of life caused by subtle genetic damage, maybe we can call him radioman. Oh! thats us.

noone said...

EMC regulations are not enforced, the classic example is PLT. As a result we are sinking into a sea of noise.

Petr said...

I have the same experience with my VX-8GR. After power-up it needs at least 5 minutes to get a GPS fix. I think that there is no hot-start or it doesn't have a backup battery and processing the cold-start every time.