What is skywave contamination ?

[flame_bringer]

Recently iv been reading about LORAN C navigation system and i came accross the term skywave contamination several times and i dont know what is it refering to , i tried to look in google for a definition but i didnt come up with a sufficient or a discernible definition .
so can anybody explain sky wave contamination ?
Thanks in advance

[Dan Winterland]

LORAN is a hperbolicbolic navigation system which relies on receiving transmissions and determining position through analysis of the TD (Time Difference) of the received signals. It transmits signals in the Low Frequency range and the receiver is designed to compute position using the ground wave signal - in otherword the signal which follows the curvature of the earth.

However, as with all transmissions in this band, there is also a skywave which reflects off the ionosphere. If the receiver picks up the skywave, the computed position will be in error because it had taken longer to reach it and the TD will be in error. Skywaves can be used for fixing by applying a correction, however, the ionosphere is multi layered and for an accurate fix the height of the ionosphere has to be taken into account. This is relatively easy because the different layers have different strengths at various times of the day.

But it does cause a problem, because often, the attenuation of sky waves is less than ground waves and the sky wave will be the stronger signal and the operator may not be aware he is reciving the sky wave, although some advanced LORAN sets have a sky wave warning system. The problem is resolved by pulsing the signal.

IIRC, the orignal system, LORAN A used skywaves.

[flame_bringer]

Dan Winterland
Thank you for replying
I see what u mean
So you're saying that, it is the error that occurs due to the diffrence in Arrival time at the reciever between the ground wave and the sky wave.
However another question arrises ,
How does pulsing the sky wave fix this error ?

[Dan Winterland]

1. Yes.

2. No idea. I think it's the signal which is pulsed. Not the separate waves as they are both a product of the same transmission. How pulsing stops the confusion, I've no idea. I'm a pilot and that goes too deep for someone who doesn't even have a physics O level!

[tob2]

If a continuous wave is used you have no way of knowing how it arrived to you, by pulsing the signal the ground wave will arrive first followed by the sky wave which can then be ignored

[flame_bringer]

tob2
thanks for replying
hows that preventing skywave contamination ?
please be more specific bro

[CONSPICUITY]

Years ago I studied Loran C for my radio licence.The position of the aircraft was determined by hyperbolic navigation by measuring the time difference of signals received from widely seperated stations. A master station transmitted a pulsed signal which when received triggered the slave stations to also transmit a pulsed signal the slave transmitted after a time delay to ensure that the master signal arrived first at the receiver these were displayed on a crt in such a manner that the time difference of there arrival can be accurately measured by strobe positioning.The use of skywaves could extend the day time range of 600 miles to 1200 miles at night by the reception of skywaves which were waves bounced of the E and F layers of the atmosphere care was required if skywaves were being received as a single pulse could appear as a series of multiple pulses on the indicator If taking a reading it was essential that only the 2 ground waves are used or only the first skywave pulses are used otherwise errors occurred

[flame_bringer]

Thanks everyone for the replies