[Elecraft] April 6th GPS Reset

[Fred Jensen]

The week number register is in the hardware aboard the GPS satellites, 
although receivers usually read and store it too.  Each rollover is a 
"GPS epoch," epoch being a term used by space travelers, astronomers, 
and other folk dabbling in space-ish things to confuse the rest of us.  
The first epoch ended in Aug 1999 ... 1024 weeks is 19.<mumble> years.  
We are approaching the end of epoch 2.  In 1999, a few of the early GPS 
receivers hit the WNRO [week number roll over] and thought it had become 
1980 again.  Using a GPS receiver to discipline the K3 involves the 
clock signal and unless you are using your very very old receiver to 
tell you what the current date is, the WNRO will have no effect.

Early in the first GPS epoch, it was discovered that the "civilian" 
signal was yielding somewhat better accuracy than the US Dept. of 
Defense had planned, and they implemented SA or Selective Availability 
to perturb the satellite clocks making it appear that you were moving 
around randomly in a 100-200 m area when you were in fact stationary.  
This would affect the clock signal at the receivers and thus the 
"discipline" applied to the K3.

It wasn't very long before a number of entities, including manufacturers 
and the US Coast Guard ... found ways to get around SA.  One is 
Differential GPS which places stations at very carefully surveyed 
locations, monitors their GPS reported positions, and transmits the 
errors.  The DGPS receiver uses the errors to correct its own GPS 
position.  If you have the new synthesizer for your K3 [or a K3s], you 
can hear and decode the DGPS signals, they're 100 bps MSK in the 284-458 
KHz range and have a distinctive signature on a P3.  The US DoD gave up 
on SA long ago but the DGPS stations remain, or at least did the last 
time I checked.

One GPS conspiracy theory posited that there was a "back door" into the 
satellites put there by another country [which we'll leave nameless] 
which, when activated, would be used to cause all the aircraft in the 
air to converge on one point and create a great conflagration.  
Conspiracy theories have a half-life and fortunately this one was fairly 
short, similar to the half-life of disco and leisure suits. [:-)

73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 4/3/2019 11:24 AM, na5n at zianet.com wrote:
> Fred Jensen writes:
>
>> Eventually, the 10-bit register holding the week number will overflow 
>> and reset to zero. That's all that's happening, it happens every 
>> 1,024 weeks, the first was in Aug 1999.
>
> The GPS reset has been the source of hysteria in various circles, 
> including exhaustive discussions on the various private pilot and 
> drone forums, since most aircraft and drones use GPS for real time 
> positioning and navigation while in flight.
>
> In short, when this happened in 1999, changes were made to lengthen 
> the roll over period to beyond our lifetime.  The April 6 rollover 
> only effects older GPS devices with the 10-bit registers.  Basically, 
> every GPS embedded device made in the last 10-15 years or so will not 
> be affected - only the first generation devices.
>
> There are absolutely no warnings or cautions on the websites for 
> Garmin, Magellan, or Trimble showing it is a non-issue for their GPS 
> devices - not even mentioned.  The FAA has a list of the very few GPS 
> devices used in older private aircraft that could be effected, only if 
> in flight when the rollover occurs.  With 10,000+ people in the air at 
> any moment, there would be a huge air of caution by the FAA and 
> airlines if the rollover had any chance of effecting the GPS devices 
> now in use.  In this case, "Silence is golden."
>
> 73, Paul NA5N
>