The Hunt for RFI

January 1, 2003 By: Wilbur R. Vincent, Richard W. Adler, Paul McGill, James R. Clynch, George Badger, Andrew A. Parker GPS World

Unjamming a Coast Harbor

In April, 2001, the captain of the research vessel PT SUR, based in Moss Landing, California, made a radio telephone call from at-sea to one of the authors, stating that signal reception of GPS in the whole of Moss Landing Harbor was jammed. He was advised to contact the U.S. Coast Guard (USCG) and the Federal Communication Commission (FCC). When the problem persisted for another month, we launched an effort at the local level to determine the cause of the jamming.

A view from the location of an unintentional GPS jammer across Moss Landing Harbor to the Monterey Bay Aquarium Research Institute. A GPS receiver with its antenna on the other side of the roof was continuously jammed for months.

Moss Landing is a moderate-sized harbor about 100 kilometers south of San Francisco, in the middle of Monterey Bay. It has a mixed fleet of working fishing boats, pleasure craft, and three large research vessels used by the local scientific community.

The Naval Postgraduate School (NPS), with a large program in science and engineering, is located at the south end of Monterey Bay. The Monterey Bay Aquarium Research Institute (MBARI) has its headquarters in Moss Landing and two major research vessels berthed there. This organization supports the Monterey Bay Aquarium and also has a large engineering program, especially in underwater remotely operated vehicles.

Locations of the RFI emitter and MBARI; power plant upper right

MBARI has used GPS for precision location of their vessels since the early 1990's, before the U.S. Coast Guard set up their system of DGPS stations along the coast. MBARI, with assistance from NPS, set up a differential station at their location at Moss Landing, using a UHF data link to send the corrections to their vessels.

After the April jamming report, NPS set up a monitor of the MBARI DGPS corrections to log the number of satellites being tracked. This clearly showed that the station was being heavily jammed. Reports of other GPS users in Moss Landing confirmed that it was a jamming issue and not a faulty receiver.

Click to enlarge image.

The jamming had impacted MBARI in several ways, including causing it to loose its GPS-based high-accuracy time reference. It would have caused difficulty at the narrow harbor entrance in fog. In at least two cases it caused small-boat owners to buy new GPS receivers, only to find they still could not get GPS in and around Moss Landing. One of the major ships in the harbor paid for a technician and new equipment to fix the problem, but finally had to turn off GPS in the harbor area, give the alarm that GPS was off line, and use radar only for harbor entrances in bad weather.

The GPS signal that feeds the MBARI reference station was also distributed to several laboratories and offices in the MBARI headquarters building, through a series of splitters and inline amplifiers. In an office with one of these drops, we set up a high-quality spectrum analyzer to examine the energy in a wide band about the GPS L1 frequency. Because there were several long cables and amplifiers between the antenna and the spectrum analyzer, the signals were not calibrated at the time they were taken. Later the system was calibrated. Figure 1 shows an example of the data recorded with a clear peak from the radio frequency interference (RFI) source many dB above the level of the GPS signals.

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Identifying Source-1

We began our search for the source of the jamming radiation in early May, 2001, spending several days looking for it. Two factors complicated the effort: the large number of metal objects that reflected the energy, and the shifting of the frequency of the emitter.

George Badger fabricated a 17-element antenna with about a 30-degree beamwidth and used this with a portable communications receiver, a general purpose radio that fit in a shirt pocket. The initial search drove along the roads in the area and stopped at widely spaced locations to record the peaks of the RFI signal. We found multiple peaks in all locations, coming from the many reflecting structures in the area, including the largest conventional power plant in California.

From its normal location inside the paint locker (see arrow), the antenna jammed all of Moss Landing Harbor and an area at least 1 kilometer out to sea.

Figure 2 shows the locations where bearings were taken as green circles, and the bearings in blue. The red circle shows the actual location of the emitter. Without the red dot, it is hard to define where the most likely position is. After ruling out the power plant, we decided to look where there were no building or other reflectors.

Closing In. The team put the spectrum analyzer on a cart along with the small radio, and took them to the dock area. Even then it was confusing. Only by turning off shore power to individual boats could we determine the actual emitter location. The signal stopped and started again as we turned power to the vessel emitting the RFI signal off and on. The photograph, taken by a "kite camera" at about 200 meters, shows the locations of the RFI emitter, MBARI, and the power plant.

Source-1 with cover open, showing the small preamplifier that jammed GPS.

We contacted the boat owner and gained access, quickly determining that the emitter was a commercially available VHF/UHF television antenna with built-in preamplifier. The antenna was powered by an AC/DC adapter plugged into boat AC power. The preamplifier was thus powered all the time, even when the TV was not on. In fact, the TV was seldom on, and most of the time the TV antenna was in a paint locker inside the locked boat. From this interior location, its emissions jammed all of Moss Landing Harbor and an area at least 1 kilometer out to sea.

The day after we located the jamming antenna, we purchased it from the owner, took it to NPS for study, and informed the Federal Communications (FCC) San Francisco field office. We also distributed a memorandum describing the facts of the case to the U.S. Coast Guard and the GPS Joint Program Office (JPO).

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Characteristics of Source-1

At the Naval Postgraduate School, we studied the antenna under controlled conditions and found it to have an internal preamplifier that exhibited unintended oscillations. The unit was normally powered from an inexpensive 12-volt AC/DC converter. In the tests it was powered from both this unit and a battery.

We studied the characteristics of the emission using another spectrum analyzer with its output sent to a waterfall display.

The unit proved extremely sensitive to the physical and electrical environment. We knew this from our search procedure, when modulation on the signal was recognized by its distinctive sound as a boat bilge pump. In an ad hoc experiment, we noted that the frequency varied over 3 MHz when one of us slowly moved his hand about 20 centimeters when it was 3 meters from the antenna. This is shown on the left in Figure 3. When the hand was held still, the frequency was much more stable, as seen by the section at the top of the traces.