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Sedalia Dave

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Posts posted by Sedalia Dave

  1. Hundreds of years ago, people developed ingenious methods to secure their letters from prying eyes – and they did it with only paper, adhesive and folds.


    Late at night on 8 February 1587, an imprisoned Mary Queen of Scots composed her last ever letter to her brother-in-law. "Tonight, after dinner, I have been advised of my sentence: I am to be executed like a criminal at eight in the morning," she wrote. "The Catholic faith and the assertion of my God-given right to the English crown are the two issues on which I am condemned." With a sad acceptance of her fate, she asked him to take care of her affairs and pay her servants, wishing him "good health and a long and happy life".

    After Mary had finished writing, she then began to fold up the letter to secure its contents. She didn't want her captors snooping – and particularly not her cousin Queen Elizabeth I. However, envelopes were not used in the 1500s – not least because paper was expensive – and there was no trustworthy postal service at the time.

    Instead, Mary cut a thin strip from the paper margin, before folding up her message into a small rectangle. After poking the knife through the rectangle to make a hole, she then fed the strip through, looping it and tightening it a few times, creating a "spiral lock". No wax or adhesive was required, but crucially, if someone tried to sneak a look, they would have to rip through the strip, so her brother-in-law would know the message had been intercepted.



    Letter Locking


    Letterlocking videos

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  2. Like the gun forums, the Amateur radio forums are filled with people that have no clue, yet they want to be the center of attention so that parrot information without a true understanding of what they are talking about. 


    I learned more about HF communication and antennas by reading books written in the late 40s and through out the 50s than I ever have reading current articles. 


    Amateur radio magazines suffer the same issues as the gun magazines. Product reviews as mostly fluff and gloss over or fail to point out the short comings of the equipment they review.

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  3. 21 minutes ago, Henry T Harrison said:

    It took me less than thirty seconds to google the answer to your daily cry for attention 


    Your 30 second google search wouldn't have really answered his question. and in actuality you would have been wrong.

     However those of us that are curious and want a detailed answer would have discovered that Yes it would have been possible to have access to a man portable satellite based navigation system within the time frame of the story.


    This is the Saloon and in getting a detailed answer I wouldn't have discovered that a satellite based navigation was first operational in 1964.


    Personally I find Alpo's questions beneficial as they often lead to tangents and those of us that have the inclination can broaden our horizons by following the thread as it meanders along. 


    If you don't like his questions why do you read his posts?

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  4. MX 4102 Transit Satellite Navigator


    MX 4102



    When Sputnik was put into orbit in 1957 some scientists at the Johns Hopkins Applied Physics Lab came up with the idea of using the doppler shift measured on the ground to solve for the 3 dimensional position of the receiver.  This required a satellite that transmits it's orbital elements.  There are a number of papers on the JHAPL Legacy of Transit web page.  The Navy Navigation Satellite System, as it was officially known, was designed mainly to allow the inertial navigation system on Polaris nuclear subs to be updated.

    The very first Transit receiver took up about a dozen six foot high rack cabinets.  The receiver for the Polaris sub was in two cabinets designed to fit through the hatch as so were narrower than a normal relay rack.  The transistor had just come out and integrated circuits were non existent when the Transit satellite system went into operation.  By the time Magnavox built the MX 4102 integrated circuits were common, hence it's small size and much lower price than the very expensive early Transit receivers.

    The MX 4102 was designed by Magnavox, Torrence, CA (now Leica Geosystems Group) for use on pleasure boats and since there can be a number of hours between Transit fixes it includes dead reckoning to keep the position updated.  This requires external compass and speed inputs from any number of the common types of sensors used abroad ships and the appropriate interface options for the MX 4102.

    The MX 4102 uses a oven controlled oscillator (OCO) marked:
    Electronic Research Company
    Model EROS 800-MA-97
    Mfg. p/n 626358-1 B
    Freq 5.00 MHz
    s/n 11703
    Date 89-17

    This is oscillator has a short term stability of 1E-10, equivalent to rack mounted lab type standard oscillators.  A high stability oscillator is needed to measure the doppler curve and to set the receiver tuning offset to account for the doppler shift of the carrier on the low orbiting Transit satellites.



    The Navigator's Manual and the Installation and Service Manual are dated June 1989.



    The unit operates from 10 to 30 Volt Dc power with positive going through the fuse.  the current is just over 1 amp at a cold start and decreases to about 0.8 amps after the oscillator oven has warmed up.



    By setting the approximate position, date and time by pressing INIT repeatedly the receiver can start searching for signals.  When the signal strength at 400 Mhz gets above some threshold the receiver starts looking for the digital modulation.  Now when there is no active satellite or I don't have a high enough antenna gain the receiver beeps and displays no fix (NFX).  Apparently this receiver does not use the signal at 150 Mhz, only the 400 Mhz signal.  This means that the corrections for ionosphere refraction are either approximated or not used.

    The front panel has a Vacuum Fluorescent Display (VFD) with buttons below and to the right so the VFD can act as a menu system.  In addition there are a couple of membrane keyboards, one with specific labels related to functions and the other with digits for numerical input.



    On 1 Jan. 1997 all the Transit satellites were turned over to the Navy Ionospheric Monitoring System (NIMS) and no longer use the Transit data modulation.  Instead they now use a different modulation that's a classified NIMS format.  These satellites are in a fairly high orbit and will stay there for a very long time, i.e. they will not decay and fall to Earth like a low orbiting satellite.  But they are all now very old and so will cease to operate electrically when failures take them off the air.  
    The primary navigation frequencies are 149.985 and 399.970 and telemetry on 136.650 MHz.

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  5. Portable units existed at that time frame. However, it was not called GPS it was known as "Transit".

    Transit was the precursor to GPS as we know it today. By the mid 80s single channel units were being produced that could have been transported over land if you brought along a big enough battery to power it. By today's standard accuracy would have been poor with a single fix only being accurate to about 400 meters. By averaging several fixes sub-meter accuracy was possible.


    Below is a chart showing the relative accuracy of navigation systems.


    Accuracy of Navigation Systems.svg  


    Transit (satellite)


    The below is a PDF copy of an article printed in the Johns Hopkins APL Technical Digest from 1984


    The Navy Navagation Satellite System Transit.pdf



    Picture of a low cost receiver available in the mid 80s


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  6. For a voice only rig the Alinco DR-735T is hard to beat. Dual VFO's, intuitive to operate, and easy to program.


    The Icom 2730 is also a nice voice only rig but the cooling fan is annoying and Icom nickle and dimes you to death because no accessories come with the radio.


    Consensus on Yaesu rigs is that while very high quality they are not very intuitive to program and use.


    A great source of equipment reviews from other HAMs is www.eham.net


    If your interested in D-Star / DMR capable rigs I'll need to talk to a couple of guys at our Friday Lunch get together.


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