GPS – Engineering Radio

Does this thing work?

I found this obviously used GPS antenna in a storage room at one of our client’s transmitter sites.

How often have you asked that same question about some older piece of equipment lying around? There is a trend among engineers to hold off on getting rid of old equipment because someday, perhaps, it can be used again. Often, these treasures so lovingly stored away for many years or decades do not work when that day finally comes along, leading to disappointment and despair.

This GPS antenna falls into that category.

What to do, what to do…

Fortunately, there is an easy way to test this antenna and do other things with GPS. I had one GT-U7 GPS receiver module left over from a previous project.

Couple that with an FT232RL FTDI USB to serial converter and a bit of software from u-blox. The GPS receiver is a clone of a u-blox M6 GNSS chip, meaning the u-center software will work with it. That is a free Windows software application. The u-center software is great because you can access all of the options on the GPS receiver chip. Since this is to be used for testing, I enabled the LNB voltage sensing and protection features in the antenna configuration menu. Thus, the software will notify if there is a short circuit or open circuit in the GPS antenna under test.

I had this nice Hammond 1590WB diecast enclosure left over from a previous project. It’s probably a bit of overkill, any small enclosure would work, but why buy something new?

Here is a complete list of parts:

GT-U7 GPS Module Satellite Navigation Positioning GPS Receiver, u-blox NEO M6 clone, Amazon B07P8YMVNT, $12.99
FT232RL Mini USB to TTL Serial Converter Adapter Module 3.3V/5.5V, Amazon B00IJXZQ7C, $6.49
IPEX to SMA jumper, RG-178, 4 inch, Amazon B0B9RYL56H, $8.78
Hammond Manufacturing 1590WB, Amazon B005T59VNS, $9.63
Mini USB 2.0 Cable, USB A to B Cable, 3 feet, Amazon B00006B6PH, $3.95

The cross-connect between the two modules is fairly straightforward:

GT-U7 pin	use	FT232RL pin	use
2	TXD	2	RXD
3	RXD	3	TXD
4	GND	5	GND
5	VCC	4	VCC

Pin out

This particular FTDI module has a jumper to set the VCC to 3.3 or 5 volts DC. I left it on 5VDC to run the GPS receiver and provide LNB power to the connected GPS antenna.

I used a piece of packing foam tacked into the inside of the enclosure with gorilla glue, then tacked the FTDI module to the foam with gorilla glue. The GPS module is tucked under the header for the FTDI chip.

The software is pretty easy to use. The most difficult thing is figuring out which com port and baud rate to use. To find the com port, open the Windows device manager then plug the FTDI module in. The new serial device should register automatically. Click on the new device to get the com port information. I find the GT-U7 modules are most often set to 9600 from the factory, but it could be anything. I suggest trying different baud rates until you start seeing data.

Putting all of those things together, we get this miniature USB power GPS receiver with software that can show how well a GPS antenna is working and whether or not the location has good (or good enough) reception. One could also check the coax going to a GPS antenna and make sure that it is working right and not too long. Or check and see if a line amplifier is working, etc.

The answer to the above question is, yes!

u-center software screenshot, GPS antenna under test

The used GPS antenna is picking up plenty of signals from a less-than-optimum position. I’d say this is a keeper.

Device under test; GPS antenna on window sill

A little bit of orange paint, also left over, and a few labels and it looks like a professional unit. Not bad for some leftover parts I had lying around.

GPS Jamming devices: FCC says No!

The FCC has become concerned about Jamming devices for Cellphones, GPS and WiFi. So much so, they have released Enforcement Advisory No. 2012-02, which specifies fines in excess of $100,000 per incident.

The advisory states:

In recent days, there have been various press reports about commuters using cell phone jammers to create a “quiet zone” on buses or trains. We caution consumers that it is against the law to use a cell or GPS jammer or any other type of device that blocks, jams or interferes with authorized communications, as well as to import, advertise, sell, or ship such a device. The FCC Enforcement Bureau has a zero tolerance policy in this area and will take aggressive action against violators.

I have ~~two~~ three thoughts:

GPS jammers are increasingly being employed by those who are concerned about their privacy. That set of people can range from truckers who don’t what their bosses to know every aspect of their journey, citizens concerned about GPS tracking devices on their vehicles, or ordinary people who don’t want the phone company tracking their every move via GPS-enabled cellphones.
If only the FCC were as diligent and judicious in pursuit of other interference issues in the radio frequency spectrum. A few immediately spring to mind; IBOC to analog adjacent channels, broadband over power line, electrical noise on the medium wave band, illegal 2-way radios on RPU frequencies, etc. Of course, there is no money in those issues.
Wasn’t the FCC about to allow LightSquared to install high-powered 4G data transmitters all over the place, thus jamming GPS anyway? I know they nixed that plan after the general uproar over the loss of GPS by things like aircraft in flight, etc.

It remains to be seen how, exactly the FCC is going to find things like this:

Hypothetically speaking, in a mobile operating environment while traveling down the interstate at 70 MPH with thousands of other vehicles, it would be the proverbial search for a needle in a haystack. This would be especially true for a GPS-only jamming device, which would require a very small amount of power to jam the weak satellite signals. It presents an interesting problem for the cash-strapped enforcement bureau.

Not all jamming devices are this small, however. After doing a Google search for GPS jamming devices I notice that some of them are great big honking things, with heatsinks and fans, capable of generating large signals on every cellphone, WiFi, 3/4G, and GPS frequency. Those larger jamming devices would be very easy to locate and disable.

Perhaps if the technology wasn’t so pervasive and readily abused by certain corporate and government entities, the desire to jam it wouldn’t exist.

Which do you prefer, GPS or 4G data?

In some locations, it is apparently going to be an either-or situation if this is to be believed:

Representatives of the GPS industry presented to members of the Federal Communications Commission clear, strong laboratory evidence of interference with the GPS signal by a proposed new broadcaster on January 19 of this year. The teleconference and subsequent written results of the testing apparently did not dissuade FCC International Bureau Chief Mindel De La Torre from authorizing Lightsquared to proceed with ancillary terrestrial component operations, installing up to 40,000 high-power transmitters close to the GPS frequency, across the United States.

Sound vaguely familiar? Seems that LightSquared took a page from the iBiquity play book when it comes to dealing with the FCC. The article goes on to say:

On January 26, the FCC waived its own rules (emphasis mine) and granted permission for the potential interferer to broadcast in the L Band 1 (1525 MHz—1559 MHz) from powerful land-based transmitters.

A little research on the LightSquared website shows they are rolling out an extensive L band 4G data network, not exactly what I would call broadcasting, at least not yet anyway. GPS system inhabits 1559 – 1610 MHz, centered around 1575.42 Mhz in the L-band. The signals coming from GPS satellites are very, very low, with the open sky signal around -130 to -135 dBm. Indoor signals can be as low as -150 dBm. Further, GPS receivers currently in the field were not built to operate in environments with high levels of RF energy on nearby frequencies.

So, who uses GPS? Just about everybody, including the military, the aviation industry, broadcasters, and the general public. Think about all the confused drivers who can no longer find their way to the grocery store without Tom-Tom. By far, the biggest impact is likely to be the entire cell phone network, which depends on GPS for its multiplex timing. It seems very likely that LightSquared network will be installed on existing broadcast and cell towers, right on top of the current cellular tenants. Even if they work around this by providing better GPS receivers with high pass filters, many existing consumer and aviation GPS receivers will be useless.

The potential interference is charted here:

Interference to consumer grade GPS receivers

For FAA-certified GPS receivers, the data is worse:

Interference to FAA certified GPS receivers

The FAA-certified aviation receivers are more sensitive, therefore, more likely to be impacted.

It makes me wonder, what is going on in Washington?

UPDATE: March 3, 2011, KNX, Los Angles runs with the story: Planned 4G service could cause widespread GPS jamming