Note: The 121.5 MHz frequency continues to be used, but only for short-range earth-based searches as
many emergency beacon transmitters emit a much weaker, continuous-duty
signal on this frequency to aid search parties as they close in.
Asked if I was interested in looking for the source of this signal, I
agreed and John passed on a frequency - 406.2 MHz, some
coordinates, and the number of a contact at the FCC. Calling that number and talking to the contact I quickly
determined that the coordinates given were not, in fact in decimal
degrees, but rather in degrees, minutes, and seconds - and these
coordinates, 40 degrees, 48 minutes North by 111 degrees, 49 minutes
West - reportedly had a "circle of probability" of 1.1 km in terms of
accuracy, placing the source of the signal 1.9 miles (about 3km)
northeast of the "U" on the mountain north of Salt Lake, about halfway between Twin Peaks
and Black Mountain.
It was reported that there had been several "hits" on this signal going
back several days and since this territory was somewhat
remote and not likely to have any power nearby so it was presumed that
whatever it was that was causing the problem was likely to be battery-powered
and would not persist for much longer.
Being that this frequency (406.2 MHz) is well-below the 70cm amateur
band, Yagi antennas cut for that band were not likely to be useful in
their directional characteristics. Fortunately, Glen, WA7X, had
previously acquired a circularly-polarized GOES downlink antenna
surplus for only a few dollars with the intent of modifying it for 70cm
use - but had not yet done so. Since this antenna was
designed to work at 402 MHz, it was still usable at 406 MHz and an additional feature
was that the elements of this antenna could easily be removed, making
it practical to carry while hiking.
That evening, after work I went over to Gordon's house. He
reported being able to hear a signal on his scanner near the frequency
of interest and had already driven around a bit to see if he could make
sense out of what was being heard as well as to scout out the
trail head for the most likely access. A point of confusion was
that even though this signal went away when he removed the antenna, he
couldn't hear it on any other receiver - and neither could I - so it
seemed likely that it may be emanating from the radio itself or could
possibly be an image response of some kind.
 |
Sunset over Ensign Peak, as viewed from
the ridge just below Twin Peaks.
Click on image for a larger
version. |
We piled into Gordon's Jeep and drove up through some fairly fancy
neighborhoods until we arrived at the trailhead to Ensign Peak. At this point
Gordon was wondering where his scanner had gone and a fairly thorough
search of his Jeep
(assuming that such was possible) did not yield
it. Fearing the worst, Gordon drove back home while there was
still some sunlight to look for it while I proceeded up the trail,
talking
occasionally with Glen who also relayed my transmissions when
Gordon as I didn't have a simplex path to him from my location due to geography. Once he arrived home
he searched some more for the scanner and carefully retraced some of
his steps to see if he could find it - as well as keeping a careful eye
along the road: No luck. As best as he could determine he'd
set the scanner on the roof of the Jeep and taken off, probably leaving
the scanner on the road somewhere...
By the time Gordon got back to the trailhead I'd arrived at a ridge to
the west of Twin Peaks where I could see into City Creek Canyon.
Up there, for the first time, I noticed a fairly weak signal on 406.28
MHz that sounded either like a repeater blowing squelch noise or, more
likely, a data transmission of some sort. Getting out the Yagi,
the signal bearing seemed to indicate that it was coming from the
direction of Ensign Peak, so I decided
to break out another receiver, a Kenwood TM-733A to get a second
opinion and to help eliminate the possibility that it was an image or
spur of some kind. Confirming that my FT-530 HT was, in fact,
really
hearing a signal, I decided to do a lateral traverse of this ridge to
double-check the bearing. The destination was a minor peak about a
half mile to the west and it was hoped that by taking several bearings
along the way, I could determine if the bearing that I'd taken was, in
fact, likely to be a "true" bearing and not merely a reflection.
While making the traverse I was in occasional contact with Gordon, who was well
underway
(sans scanner, unfortunately) and was keeping him apprised as
to what I had found. Upon arrival at the minor peak I checked
the signal strength and bearing once again and it was, in fact, still
coming from the direction of Ensign Peak and of about the same strength
- a reading entirely contrary to what the satellite coordinates had
seemed to indicate. Armed with this information, I headed back to
the saddle.
By the time I got to the saddle Gordon had been there for a few minutes and it
was starting to get quite dark with the city lights having turned on a
short time before. By now I was wishing that I'd done a
bit more research as to the passband of the SARSAT system and had a few
specific other specific technical details as to how it worked:
For example, I was wondering if a signal at 406.28 MHz would
be bothersome to a system that received at 406.20 MHz? I also
wished that I'd queried the contact at the FCC a bit more about any relevant details of
this signal that he might have such as whether or not that it was
modulated - and if so, what was the nature of this modulation?
 |
The block diagram of a typical SARSAT
transponder. This is some of the information that I'd wished that I had
with me on the first trip to look for the signal. This diagram
includes the older 121.5 and 243.0 MHz subsystems which are no longer
used onboard the spacecraft.
Click on image for a larger version. |
Being that this saddle was still a few kilometers west of the
coordinates we decided to follow the roads and trails to the east
towards Black Mountain while keeping an eye on the
clouds, virga, and lightning that we were seeing to the south and
west. Following the trail we wound around to the back side of
Twin Peaks and before too long, we split from the old jeep road and
began to follow a foot/bicycle trail to the east, lighting our way via
flashlight. At some distance past Twin Peaks we started to break
out from behind the peaks at about the same time that the wind, rain,
and lighting started to pick up. After a few lighting strikes
that appeared to be much less than a mile away, we beat a hasty retreat
to the only thing resembling shelter that we could find - a shallow
ravine along the trail near some scrub oak with a few 10's of
feet of ridge above us. There, we hurriedly donned our ponchos
and
waited out the now-heavy rain, fierce wind and
lightning. Fortunately, this storm wasn't too long-lived and it
moved off toward the east and after it had passed we stuffed our rain-soaked
ponchos into our packs and, defying common sense, continued on
our way in the direction of Black Mountain.
By this time, I was
really starting to wonder about some of the
technical details of the SARSAT system so I called Glen on the cell
phone and requested that he see what he could find on the web.
Before long we found ourselves in a clear area on a ridge top a
little more than a kilometer from the coordinates that I'd been given
so we decided to see what we could hear. At about this
time Glen called back on the radio with the results of his quick
research on the web. He reported that the SARSAT satellite
operated on a center frequency of 406.025 MHz and
NOT
406.2 MHz. He also reported that in this frequency range there
operated "Personal Locator Beacons" (PLB's) that could be activated by
someone in distress. These PLBs operate with several watts of
power and transmit a short data burst that contains not only a
unique serial number with which a person, boat, or vehicle could be
associated, but a GPS-derived location as well.
Armed with this new information we started to look around in the
406.025 MHz range as well and sure enough, we heard a few bursts of
what sounded like data transmissions. Another interesting signal
source seemed to appear at about 406.18 MHz - a bit off the SARSAT's
center frequency - but since we weren't sure of the bandwidth of the
SARSAT system, we couldn't readily discount it, either. This
latter signal proved to be problematic, however: Although it
could be heard on both the TM-733A and the FT-530, it seemed to resist
attempts at determining a bearing. The real clincher came about
when I wielded the step attenuator and noted that adding just 3dB of
attenuation seemed to cause the signal to go completely away: It
seemed to us that this indicated that this signal was likely an
intermod product, but we were still somewhat puzzled by the fact that
it was a clean, unmodulated carrier and we were wondering what signal
sources would be likely to cause that?
At this location we had quite a good view of the Salt Lake Valley and
over toward the Oquirrhs. Not too long after arriving we were
looking across the valley and our eyes caught a tremendously bright
flash of light on the ground near the north-center of the valley.
What was striking about this flash was that not only did it seem to
last the better part of a second, but it clearly had some obvious color
to it and after disappearing for a few seconds, it appeared again,
lasting for about the same amount of time. Not having any clue
what it was, we made several guesses, including the possibility that it
was a
malfunctioning power transformer.
 |
This was just some of the electrical
activity witnessed occurring over the Salt Lake valley that evening.
Click
on image for a larger version. |
Another thing that we could see from this spot was another thunderstorm moving in from the West but since this one was quite a ways to the south and since the skies above us
were relatively clear, we weren't too worried. As we sat there, I
took advantage of the viewpoint and snapped several dozen short time
exposures, hoping that I would be able to catch a half-decent lightning
strike on at least one of them: The results speak for themselves -
see the picture above.
After a few minutes of listening, looking and taking pictures we
decided that because it was approaching midnight and that the storm
seemed to be inclined to approach us after all, it was time to go. At
this point, when putting gear back into my pack, that I'd realized that
I couldn't find my transit compass: I'd tried to find it a little
bit
earlier when we were trying to take bearings on the signals that we
had
heard, but it now seemed not to be buried in my pack after all.
All that I could figure was that in the haste to don the poncho in the
rain I must have either dropped it or inadvertently taken it
off, so on the way back we stopped at the place where
we'd taken cover from the rain
(after several back-and forth attempts
to try to positively identify the exact location) and tried to find the
compass by flashlight - but to no avail: It seemed that the radio
gods weren't satisfied with just Gordon's scanner, but they wanted my
compass as a tribute as well!
The trip back down to the vehicle was mostly uneventful: The only
pause was a brief stop to attempt to take more lightning pictures -
this time of a storm that was starting to appear over mid-southern
Oquirrhs. At about the time we got the the vehicle, it was sprinkling
very lightly and the wind started to pick up, but by the time we got
back to Gordon's house the wind was gusting fairly strongly and the
lightning was much more intense and and accompanied by a hard rain. We decided that we'd timed our retreat about right!
The next day I toyed with the idea of going back up to listen for the
signal and to look for the compass but my plans were changed by the
appearance of a strong thunderstorm that dropped marble-sized hail at
my house. Apparently Gordon toyed briefly with the idea as well,
but he, too, was dissuaded by mother nature.
The next day
(Sunday) was very nice, but I didn't feel inclined to tromp around in
the 90 degree weather and I figured that the batteries running whatever
it was that was generated the nuisance signal probably weren't likely
to last 5 days, anyway...
 |
| A topographical map showing the hiking
route (in blue) along with the precise satellite fix and the "circle of
probability".Click on image for a larger version. |
More reports of signals:
On Monday (8/14) I received another email from the FCC that not only reported
that the signal was still there, but it had satellite "hits" going all
the way back to the 3rd of August - 11 days before.
An interesting piece of information was that this signal usually seemed
to appear in the late afternoon or early evenings - a
possible clue of its origin. Also included
in the email was a new position fix: 40 deg, 48' 00" North and
111 deg, 54' 36" West. This new fix was about 5 miles
west
of the original fix, indicating that it had either moved, or that the
quality of the earlier fix wasn't quite as good as was believed. By this
time we'd also done enough research to know that the
proper
frequency to listen on or near was, in fact, 406.050 MHz, plus or minus
40-50 kHz.
As it turns out, the modern SARSATs have two separate receive systems
onboard for 406 MHz: One of these simply receives and does a
store-and-forward of the data packets from the PLB's. The other
part of the system is a passband-limited, frequency conversion receiver that translates
the spectrum centered on 406.050 MHz with a passband of about +- 40
kHz, converts it down to 170 kHz and then phase-modulates it (along
with the converted baseband signals of the other frequencies) on a
1544.5 MHz carrier. In this way the effects of the Doppler shift
are almost exclusively confined to the receive system onboard the
satellite itself owing to the fact that the satellite downlink was not
a direct frequency translation.
Note: Another mode of
operation for the SARSAT is with the transponder centered on 406.025
MHz with a passband of +-10 kHz: We do not know for certain what
mode the SARSAT transponder(s) were in when the signal was received.
With this new information Gordon and I decided that we might, in fact,
be able to find this thing. To be sure, the new coordinates were
much "friendlier" than the others as they put the signal source just
off the east side
of Victory Road - just above its junction with Beck Street - all in places where we were likely to able to drive.
When trying to locate a signal such as this it is helpful to have an
all-mode receiver capable of operating in the frequency range.
While one would normally listen for a signal in FM, being able to
listen for the same signal on a receiver with a BFO
(e.g. an
SSB-capable receiver) is extremely helpful for several reasons:
It is possible to detect far weaker signals using SSB than with FM
alone, it is easier to determine the precise carrier frequency of the
signal being sought when using an SSB receiver, and it is fairly easy
to analyze the "signature" of the signal to determine something about
its origin. This last point can be particularly important as not
only can one often identify one specific transmitter in the presence of
others, but its observed frequency stability may provide a clue as to
whether the signal being heard is one that is stable and on-frequency
or more likely to be a free-running spurious emission of a malfunctioning device.
 |
The blue cross at the center of this map shows the location of the "new"
SARSAT fix obtained on 8/14.
The ACTUAL location of the transmitter turned out to be near the lower-left corner of this
map, at approximately the location of the first letter of the word "LINE".
Click on image for a larger
version. |
While the most likely candidate for all-mode use was my Yaesu FT-817, it
does not cover below 420 MHz.
What to do?
An answer
occurred to me: Throw together a quick-and-dirty frequency
converter.
Fortunately, John, K7JL, had onhand several UHF bandpass cavities
and he was able to tune one of these well out of its original frequency
range all the way down to 406 MHz so that it could function as a
bandpass filter. The next step was to provide a local oscillator
reference and this was provided by my Schlumberger 4031 service
monitor
- a device that can produce an SSB-quality carrier
and run from
12 volts. As it turns out, I had some diode ring mixers handy,
but they were rated only to 250 MHz. A quick test revealed that
they did, in fact, function reasonably well at 406 MHz, albeit with a
few extra dB of insertion loss. The final component needed was a
good preamplifier, but I was able to quickly retune a hombrew 70cm
GaAsFET that I had onhand to operate at 406 MHz. Cabling
everything
together I set the service monitor to generate a 280 MHz signal for
the
local oscillator, tuned the FT-817 to 126 MHz and, using another
service monitor, I checked the sensitivity of this lashup and found
that, on FM its 12dB SINAD sensitivity was about -115 dBm - more than
good enough for our purposes!
After heaving all of this equipment into the car
(along with a pair of
100 amp-hour batteries to run the service monitor, which draws 8-10
amps) I set off and picked up Gordon from his house. From there
we drove through downtown Salt Lake and past the state Capitol
building and as we climbed the hill we could hear some activity in
the 406.025 MHz area that sounded like data transmissions. We had
heard what we thought had been similar-sounding signals on the previous
Friday when we were
overlooking the valley and had presumed that they might, in fact, be
some Personal Locator Beacons transmitting. The fact that there
was an outdoor exhibitors' show in town seemed to make it even more
likely to us that one or more of these transmitters was
active.
Driving past the Capitol building the signal that we'd been hearing
wavered a bit, but became fairly consistent as we went down the Victory
road, and
it sounded very much like what we'd heard on Friday. Near the
bottom of the hill - just above the junction with Beck Street - I
pulled off the road into a gravel parking lot. A quick check
revealed that we were just a few 10's of meters south of the precise
coordinates relayed to us based on the most recent satellite
data. Getting out the Yagi, we tuned around and listened.
After a few minutes of thinking about what we were hearing, it occurred
to us that we were hearing only one transmission at this
location. While it sounded just like data bursts, we also noted
that these transmissions were not of consistent length and it sounded
as though they were being truncated randomly. Switching to SSB,
we also noted that the signal that we were hearing was fairly unstable,
having several kHz of observable "chirp" when it keyed up. From
this evidence, we concluded that this was most likely our
suspect: We surmised that it
could still be a PLB, but
one that was malfunctioning but this seemed to be unlikely as such a
device would likely be operating from a battery that should have long
been exhausted. The other obvious possibility was that it could
be an existing, malfunctioning transmitter throwing out spurious
transmissions.
Wielding the Yagi we determined that its bearing was west-southwest
from our current position so we set out to find another vantage point
from which we could hear this signal. Wanting to guard against
the likely possibility of being misled by a reflection Gordon
suggested somewhere above the capitol so we drove in that direction to
find a good overlook. Driving behind the capitol we happened to
drop behind the ridge, effectively shielding us from anything to the
west and we noticed that the signal that we were listening for
disappeared, indicating that it was not likely to be either to the
south or east of us. After a few more minutes of driving we
found a nice observation point at the end of the road, just below
Ensign Peak, and parked.
Judging by the sizable crowd up there
we concluded that this was a popular spot as it commanded a
spectacular view to the south and west and once we started waving the Yagi around we not only got some stares, but also an indication that the signal was quite strong, yielding a bearing
that was almost due west. As we were getting ready to depart
several people asked what we were doing and
(nearly correctly) had
surmised that we were tracking animals.
"Close!" we told them as
we explained what it was that we were
really doing.
Interestingly, we again noticed a very bright flash of light on the
ground in a midvalley location - just like the one that we'd seen on
Friday: We still don't know what it was!
Armed with the new bearing we wound our way down from the foothills
and headed west on 6th North. As we dropped, the signal became
quite weak for a while and it wasn't until we got to about 20th West
that it started to pick up again. Soon, we turned south, finding
ourselves passing the Executive Airport terminal and the local FAA
offices and
we parked for a moment to find another bearing: From here, the
signal appeared to be coming from the northwest - but it wasn't
particularly strong.
Needing another clear location, we decided to head
toward the International Center on the west side of the International
Airport in the hopes that we'd be able to get a counterbearing.
En route, we lost the signal, but we assumed that the path to
the source had been obstructed but once we got to the International
Center we still could not hear it. What we
could
hear, however, seemed to a
very occasional data transmission on
the frequency
(perhaps once per minute) that sounded different from the one that we
were seeking. A quick listen on SSB revealed that, unlike our
suspect, its carrier frequency was quite stable.
At this point we decided to go back to where he'd heard the signal
before - in the general area of the Executive terminal. Returning
and parking
in an empty lot we listened for a few minutes and didn't hear
anything other than the "occasional" transmission that we'd noted
previously. I then surmised that being a spur, it may have
drifted off-frequency a bit and eventually found it about 30 kHz higher
than it had been when we'd first heard it earlier in the evening.
I then decided that since it was likely a spur, it would seem
reasonable that the frequency on which the malfunctioning transmitter
operated was likely to be nearby, so I decided to start tuning around on the TM-733 in my Jeep while monitoring and tracking the unstable suspected signal on the FT-817.
After a few minutes of tuning I found a signal on 410.075 that sounded
very much like our suspect and after comparing its appearance and
modulation with that of the spur, they appeared to match exactly.
For
this signal the beam heading was due south from our location, but
there was a problem: Because that the antenna was designed for
operation on 402 MHz, we weren't sure if it would function properly
and yield a usable bearing that far off frequency. Fortunately,
we had a nearby-frequency signal with
which we could calibrate our antenna: A backhaul feed of the main
National
Weather Service transmitter on 410.575 MHz. The problem was that
we didn't know where this transmitter was located, so I called John, K7JL. He wasn't sure of its exactly location, either, so he
looked it up in the
telephone book and we determined that it was just a few blocks south of
us - a location that correlated with our bearing. At this point
it also occurred to us that the bearing of our suspect signal was about
the same as the NWS signal.
Getting back in the car we drove a few blocks south and stopped in a
parking lot adjacent to the NWS office and observed that the 410.075
MHz signal was extremely strong with a bearing pointing into a side
yard that, in the dark, appeared to have a number of antenna-like
structures. To be certain, we drove around to the south side of
the building and again, we resolved a bearing in their side yard.
At this latter location, I looked at this signal - and its spur - on
the service monitor's spectrum analyzer and determined that the 406 MHz
component was 38 dB below that of the 410.075 MHz one: If the
transmitter was running at just a couple of watts, this would imply
that the power in the spur was a few milliwatts - more than enough to
be heard around town and even by an orbiting spacecraft!
John, who was still on the phone, happened to know
the
technician at the NWS responsible for their transmitters and left a
message indicating that there may be a problem requiring his
attention. At this point we were satisfied that we'd uncovered
the source of the signal and went home. After all of this we
were somewhat surprised that our snooping
around government buildings, after dark, wielding odd-looking gear
hadn't
seemed to have attracted any attention!
The next morning the technician from the NWS returned the call and John explained
what had been happening and what had been found: The tech immediately
arranged to have
the transmitters looked at and, as a future preventative measure,
installed bandpass cavities. As it
turned out, this transmitter was part of the ASOS - the Automated
Surface Observing System - which is used
for gathering close-in meteorological data at airports.
Interestingly, the FCC's NOAA
contact pointed out that, several years ago, they had a
spate of similar interference issues at airports caused by these same-model
transmitters in which a weak 400 MHz-area spur had caused some
interference with some
operations. He also then mentioned that this was the first time that
a "non-airport" transmitter had caused some problems, but
this NOAA facility, which was adjacent to the
airport, had escaped prior scrutiny since its transmitter wasn't actually
on airport property like the other errant transmitters.
The irony did not escape us that this signal was generated from an NOAA
facility and that it was, in fact, NOAA that had reported this signal to
the FCC in the first place!
Interestingly, the latter satellite fix
(the one indicating a site
near Beck Street) was about 5km northeast of the actual location.
This error is understandable when one considers the nature of the
signal: It was likely fairly weak on the satellite. It also
was quite unstable in frequency, a factor that would tend to increase
the uncertainty when trying to make a Doppler fix on the
location. Furthermore, being that the satellites used to
determine these sorts of location fixes are in very high inclination
(being nearly in polar
orbit) they would have their best accuracy when determining
latitude: The accurate determination of longitude is a bit more
problematic as its uncertainty relies on observations over multiple
passes and/or through different satellites, hence the fairly close
agreement on latitude, but the much larger longitude error.
(Note:
This system is "reverse" of the way the old TRANSIT (link)
satellite system operated.)
The
observation that
this signal seemed to be only
present in evening hours also made sense: Only when the outside
temperature was in the mid 90's Fahrenheit did it seem to drift into
the SARSAT
passband. This temperature peak would, in fact, occur in the late
afternoon and as it cooled off, it would drift back up in
frequency, out of the transponder's passband - possibly disappearing
altogether from the transmitter's spectra.
A few days after the transmitter's servicing another email
arrived reporting that since the work had been done, no further SARSAT
hits had been noted. An email from the FCC indicated that they were pleased
that they did not have to make a special trip to Salt Lake just to find
this signal and offered to take us to the Red Iguana next time they
were in town.
End
