The elevation changes seen with civilian GPS are the result of two
fairly fundamental issues:
1. A GPS receiver doesn't really "know" where actual sea level is, it
uses a mathematical model. Since the earth is ellipsoidal-ish rather
than spherical on a large scale, and since there are smaller local
variations, the model is a trade-off between accuracy and complexity.
On State Route 37 just north of San Francisco, you are immediately
adjacent to San Pablo Bay and can see that you're about 10 ft above it.
My Garmin GPS pegs my elevation at 61 ft.
2. The second is "dilution of precision," or DOP. The relative
positions of the satellites in the sky change constantly, and finding
your coordinates is a process of finding the intersection point of lines
of position [LOP] from multiple satellites. When the satellites are
scattered all over the sky, many of the LOP's intersect at large angles
[90 deg is ideal] and the intersection is well defined. If the
satellites are more or less lined up in one area of the sky, the LOP's
tend to intersect at smaller angles and the exact intersection point is
less well defined. This is Horizontal DOP or HDOP.
Unfortunately, when it comes to elevation [VDOP], roughly half the
satellites are below the horizon at any given time, there are thus fewer
LOP's to work with, and the probability of finding large-angle LOP's is
inherently lower. Hence, VDOP is usually [almost always] greater than HDOP.
Thus endeth GPS 101.
I also think they are communicating using some service on the Iridium
constellation which is probably expensive, and the ship is only making
around 10 kts or so, so more frequent updates wouldn't indicate much.
Given the overall expense of this expedition however, communications
costs are probably below the round-off. [:-)
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
On 1/24/2018 5:52 AM, Bayard Coolidge, N1HO via Elecraft wrote:
The elevation discrepancies are normal - I see the same effect whendriving
around down here in South Florida, randomly dropping to 10-20feet below sea
level. The civilian GPS systems are not configured forextreme elevation
accuracy, particularly when close to sea level.
The main web site's position reporting may be outdated because, I think,that
they're relaying their position via satellite (Iridium, perhaps?) whichcan get
pretty expensive. Given the length of the journey, it's probablynot worthwhile
providing hour-by-hour position updates. They'll get therewhen they get there,
and they'll get on the air after they get things set up.
73,
Brandy, N1HO (EL96xh)
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