OMEGA/VLF is a long-range, random-route navigation system based on Very Low Frequency (VLF) transmissions from a small number (8 stations originally, now???) of very powerful land based transmitters, each capable of providing signal coverage at distances as great as 11,000 NMs. It is an enormously complex system in nearly every sense and is somewhat vunerable to atmospheric distortions. I consists of computers and atomic clocks and combines principles of dead reckoning with ground based electronics. It is a very very technical system and therefore most average operators are incapable of using it properly leaving room for gross navigational errors.
The system consists of two separate systems, OMEGA & VLF. Each exists for entirely different reasons. OMEGA for navigation and VLF for communication. (VLF is a very high powered communication system operated by and for the US Navy primarily for direct comm with submarines anywhere around the world with a range of 6000 to 11,000 NMs. VLF signals are an excellent source of navigation, since they are very stable and predictable with worldwide coverage.)
OMEGA is in simple terms ground baesd GPS, consisting of 8 stations located around the world. The signals travel along the earth's surface all the way around the earth, in fact the Omega recieves two sets of signals from each station, one traveling the long way around the world which if not disregarded by the OMEGA reciever will cause one of the many propogation anomolies called "wrong way propogation".
Each of the eight OMEGA stations transmits a series of three freqs. These are always broadcast in the same order, with the same interval(0.2 secs) between each freq, but with a different interval after the last freq and the begining of the new cycle. The complete cycle for each station, including the unique interval between stop and start, lasts exactly 10 seconds and is repeated continiously. The OMEGA reciever can identify each station by it's trasmission pattern, however each station alse transmits a fourth freq for more positive and rapid ident.
OMEGA's advantage over IRS (INS) is that via the ground station signals it regularly computes and updates it's present position, unlike the pure INS/IRS system which is a self contained setup of gyros & accelerometers and no matter how accurate it will drift over time and without updates it will eventually provide erroneous data. These updates for the INS/IRS are provided via GPS, DME, VOR, etc... normally a combination of the above making it full proof. But in the old days (pre-GPS) long-range overwater oceanic flights with no VOR, no DMEs around had to either rely on INS drift accuraccy (very complex system as well but much easier to use than maintain) or OMEGA/VLF with it's regular auto updates providing more accuracy. Omega/VLF was subject to many differrent propogation anomolies which required very high operation proficiency & system operation knowledge to recognize and compensate for.
OMEGA/VLF onboard systems were much cheaper and lighter than comparable INS systems and were much less prone to mechanical breakdowns.
I'm now dizzy having just remembered all that. I'm surprized anyone still asks about this ancient system at interviews, it's not even in use anymore as far as I know.
Last edited by tomcat69; 27th Jul 2005 at 11:38.