On Sun, 22 Jul 2007 17:18:56 -0000, Tina
wrote:

Does anyone know if it's legal to interfere with nav sat reception? It
would be interesting to know, for example. if there were known outages
when the president was at his father's estate in Maine.



On Jul 20, 10:29 pm, "LWG" wrote:
I had an interesting experience Thursday. I often fly from Baltimore to
Cumberland for business. This past Thursday, I decided to drive. I took my
Garmin Nuvi GPS along for the ride. In the vicinity of Hagerstown
(Maryland) the GPS went tango uniform. The screen worked, but the unit
indicated that satellite reception was lost. A few minutes later, the GPS
came back on, but then quickly died. On the way back to Baltimore late
Thursday morning, the unit remained nonfunctional. The satellite reception
page showed absolutely no signal from any bird. I tried wiggling the little
antenna panel, thinking that perhaps the antenna failed. I have a spare
antenna from my 295 which I thought I could use to test the receiver
function. I tried recycling the GPS, but nothing worked. The unit went
through its startup procedure, inquiring about relocation since last use,
etc. Even when reception is poor, the satellite page always shows some
level of signal unless the unit is indoors. There was nothing.

I left the unit on at the satellite page, primarily because I was too lazy
to reach up and turn it off. As I was coming down the ridge towards
Frederick (east), the unit lit up, and worked perfectly since, up through
today.

For those of you not familiar with this area of the country, P-40 or Camp
David is a little north of the route I was driving, just to the east of
Hagerstown. I received an email from AOPA that P-40 was supersized the
following day, Friday, indicating presidential or VIP presence. I have seen
notams about NAS Pax River spoofing/degrading/screwing with the GPS signal
in their vicinity, but I haven't seen anything about a remote interference
with the GPS signal (but since I drove, I didn't really check recently,
either).

So, for those of you (like me) who have become dependent upon GPS, you may
want to think about whether the government has a reason to block the signal
in the vicinity of your flight. If so, you may wish to make sure those VOR
frequencies are handy. The disappearance and reappearance of the signal was
so dramatic that my only conclusion is that the signal was blocked locally.

Where I fly in Southern California, it is not uncommon for certain
agencies in restricted areas to degrade or completely turn off GPS
signals. However, these are always preceded by NOTAM and the area of
non-operation are strictly defined. Lately, these have been cone
shaped outages originating from a point on the ground and gradually
increasing in diameter as altitude increases. The area involved
seems to be adjustable, but does not change once it has been defined
in the NOTAM. If one flies to the area you will get an immediate loss
of signal exactly where they define it, and the signal will return
when you depart the exact area defined. These guys are good.

My son was involved in some flight tests that were conducted over the
Pacific off the Southern California coast. They would get
notification that GPS was not reliable beyond a certain longitude and
watching the GPS count down while flying westerly, the signal dropped
at exactly the longitude they said it would.

So, yes, the government can and does interfere with satellite
navigation. However in our case it was always stated in advance for a
fixed amount of time, and they never have shut down the system over a
congested flight area. As far as we are concerned, it is a non-event.

Ron