In message <970221202026_1812486234@emout14.mail.aol.com>,
Gramin@aol.com writes
>In a message dated 22/02/97 00:04:38, you write:
>
>> they would still get cooked if they flew too close to a transmitter
>
>Ah, but if we weren't occupying their airspace there would be no high power
>radars trying to fry them.
>
>While on the subject of high power, I have now quite a few recorded gps
>tracks from trips up and down the country showing gaps which can be
>associated with ground transmitter sites. It occurs to me that if all
>fliers sent in their tracks to a central agency, they could be overlaid to
>show the regions of doubtful coverage (but I'm not volunteering !).
This is a snip of something I wrote on the subject about four years ago
which is still largely relevant today.
startclip
Users of GPS systems operating on the "Civil" L2 C/A downlink frequency
of 1575.42 Mhz should be aware that operation close by or near to a
number of high power transmitter sites will experience irrational
behaviour with their equipment. Very often this will take the form of a
sudden degradation of accuracy or the dreaded "poor GPS coverage"
message.
The reason is that UHF TV stations radiate energy at between 470 and 854
Mhz at power levels up to 1000kW ERP. Their siting on high ground means
we might be flying level with the antenna and therefore in the area of
most intense energy. An inevitable consequence of radio frequency
generation and amplification is the production of harmonic energy which
is normally suppressed by filtration yet can still be of a high enough
level to disturb GPS receivers. The highly sensitive front end of
satellite receivers will be overwhelmed by the offending RF energy
swamping the ephemeris data and timing information necessary for
position and velocity calculations.
UHF transmitters, because of their wide bandwidth, operating around
channels 60 (783-789Mhz) and 27 (519-525Mhz) will be particularly
susceptible to harmonic and intermodulation product interference
especially if the site is the main serving station for the area. These
would include Dover, Heathfield, Midhurst, Oxford, Beacon Hill, Huntshaw
Cross, Mendip, Blaenplwyf, Waltham, Sandy Heath, Tacolneston, Carmel,
Llandonna, Winter Hill, Pontop Pike, Selkirk, Angus and Limavady.
A vast number of relay transmitters also radiate on these channels but
at much reduced power levels (1-500W ERP) though even these may cause
temporary degradation of accuracy.
Quite apart from the obvious physical danger of transiting close to a
high mast a number of the sites are close to regulated airspace where
navigation errors may inadvertently place you in conflict. Notable
amongst these are Mendip, Oxford, Midhurst and Winter Hill where VFR
routing outside of controlled airspace is bound to pass closer to a site
than is desirable. From experience passing closer than three miles will
in most cases cause a significant loss of reliability. Much also depends
on the passing level as the radiation patterns for TV masts are very
flat so clearing by two or three thousand feet above may cause little
problem. On some earlier models of the Garmin 100 model the software was
unable to recover from signal interference causing the whole system to
crash often losing valuable routing and waypoint data.
There are also a number of radar installations around the country where
similar effects can be observed. Oakhanger, Portsdown, Staxton Wold,
Fylingdales, Croughton, Goonhilly, Defford and Malvern in particular
could upset most units.
endclip
The list was also produced before the advent of Channel 5 so the
interfering sites may well have grown in number. Garmin have since
produced updated software for the 100 which overcomes the problems.
More recently I have been involved with installation of GPS into UK
military jets where problems were also found with interference from
Transponder/IFF systems though the jets did have dual antennas and it
was largely found to be the proximity of the top surface antenna.
Something that is less likely to occur on civil layouts.
The other known problem in aircraft is the 12th and 13th harmonic from
airband transmissions typically channels 121.050-121.350 and 131.000-
131.425 on VHF, 262.375-262.750, 314.775-315.075 and 393.750-394.025 on
military UHF.
GPS and aircraft frequencies can be unhappy bedfellows.
--
Graham E Laucht
|