[time-nuts] Lady Heather and Lucent RFTGm-II-XO / RFTGm-II-Rb

Tom Van Baak tvb at LeapSecond.com
Sun May 28 15:29:28 EDT 2017


> One annoying thing about the RFTGm's is that they don't report satellite positions (just signal levels)...
> so no nice antenna survey maps are possible.

Well, yes and no. It is true that signal levels can only be *measured*. And you've got that. No problem.

Now, realize that satellite positions are only ever *calculated* by a GPS receiver, not actually measured. So it's quite easy to generate satellite maps with or without a working GPS receiver. I mean, each and every GPS SV is-where-it-is-right-now regardless if you exist or not, if you've got a receiver or not, if your receiver outputs positions or not. Make sense?

So all you need is:
- a copy of a recent constellation almanac or ephemeris (on the 'net, or from quality GPS receivers, especially in binary mode),
- the approximate UTC date/time,
- your approximate location,
- a handful of wonderful orbital mechanics equations, which you can look up in any GPS textbook or online tutorial.

If you want to see an example of this, fire up Trimble Planning.exe, which is part of the free TBolt s/w suite (along with TBoltMon.exe, etc.). Again, remember that the whole point of GPS is that the precise location of each SV must be knowable by the CPU; not measured with a telescope or directional antenna or something. So it's quite easy to create maps for any and all known satellites once you look-up the orbit parameters. There are apps / programs / web sites that do this. NASA used to have the wonderful JTrack3D. Instead check out http://www.heavens-above.com/ for info.

For extra credit... The joke is that LH has a feature which "hides" the user's lat/lon. Privacy? Nope, LH still reports precise UTC date, time, PRN, Az, El, and Doppler! So it's not rocket science (well, it is in a way) to solve that nice set of precise and overdetermined numbers on the screen to obtain a good guess at the redacted position. Oops.


----- Original Message ----- 
From: "Mark Sims" <holrum at hotmail.com>
To: <time-nuts at febo.com>
Sent: Saturday, May 27, 2017 11:15 PM
Subject: [time-nuts] Lady Heather and Lucent RFTGm-II-XO / RFTGm-II-Rb

>I have Lady Heather working fairly well with the RFTGm's.    I used a serial port monitor program to capture the traffic in and out of the serial port and used the Lucent control program to set and read various parameters.  By analyzing the captured traffic and comparing the results to what the Lucent program was reporting / sending I worked out the protocol and message formats.
> The one message that I have problems with is the one that reports the EFC DAC voltage and temperature.  The message appears to be reporting the DAC value and temperature as a 16 bit integer.   Scaling that to actual values could be a problem.  The DAC is not that big a deal... I scale it to a 0-100% value... no real need to be concerned with the actual voltage.  The temperature value will require a lot of work.  It has an 8 bit granularity and seldom changes more than one step.
> One annoying thing about the RFTGm's is that they don't report satellite positions (just signal levels)... so no nice antenna survey maps are possible.

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