-------- In message <55a53a67.7010...@rubidium.dyndns.org>, Magnus Danielson writes:
>> If the goal is a reliable backup for GPS, there are smarter ways to >> use the 100kHz band than Loran-C pulses, and there really isn't much >> reason to stay compatible with Loran-C receivers. > >True. I would look at PRN-codes if I where to do such a system today. >What may be an issue is the amount of sidebands allowed, as it would put >limits on the chipping-rate of PRN-codes or for that matter other forms >of modulations. I would probably stay with the pulses, they have some very desirable properties in terms of transmitter and antenna design and bandwidth. But I would get rid of the current spread-spectrum design and do something like this: We pick a basic period as a prime number of microseconds, for instance 262139µs (just below 2^18) and we define an epoch for this. This means that all transmitters are autonomous based on a local TAI reference. Each transmitter emits an individual PRN-spaced pattern of 32 pulses in the basic period. The exact pulse patterns for a transmitter will be picked based on vectors to neighboring transmitters. The pulse polarity is "plus, minus, data" where every 3rd pulse is used to implement a serial data-channel which communicates chain-configuration data, TAI/UTC info with some bits left over for civil defence warnings. Using one global "GRI" means that there no longer any "chains". This eliminates a host of failure-modes on the transmitter side and the receivers will automatically be "all-in-view". With all transmitters autonomous and independent, RAIM is possible. The basic period is relatively long to attenuate any CW interference for time/frequency purposes but the higher pulse-per-period count compensates this for location purposes. Making the pulse-pattern per transmitter and PRN-like eliminates all the "shadow" effects ("baseline extension" etc) and makes for quick (re)acquisition based on pattern-matching. The PRN pattern will also dramatically attenuate the "loran-lines" which polluted nearby VLF and LF bands. The +/-/data pulse polarity makes it possible to detect the start of the period by tracking where in the potential basic period pulses do not change polarity: 3 doesn't divide 32, so there is a + - + - sequence from all transmitters around the start of the period. But then again, I have spent far more of my life on Loran-C than can ever be justified :-) -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.