Okay, 127ms for a 4000km Globalstar round trip and round the world. That's greased lightning. Beats ICO and the GEOs. People can't stand voice delay. The idea that they can't detect less than 200ms is like saying they can't tell when you water their beer, sell them low octane fuel and feed hens with mash instead of grass, grubs, insects and good old dirt. Eggs should have golden yolks and taste like food.
>200ms is supermarket quality. Mass produced, cheap, poor quality and not very satisfying. But might just be tempting for millions. But they might think they are like over-ripe bananas and leave them on the shelf. Supermarket shoppers won't accept just anything.
On ephemeris:
> Ephemeris is used to describe the location of a
> satellite. There are several formats but one of
> the most commonly used is called a NORAD two
> element set. It is used to allow you to calculate
Actually, it's called a "two-line element" set, or TLE. Maybe that was a typo, so I'll let it slide.
> where the satellite is at any given time by
> describing its orbit. In a typical stabilized
> satellite, the vehicle is compensating for drag
> and is either raising its altitude or remaining
> fairly constant. A vehicle that is descending in
> altitude raises questions because it appears to
> not be accounting for drag and thus no longer
> stabilized. One of the G* satellites appears to
> be decreasing in altitude and that is what
> prompted the question.
> CSM
This is wrong.
(1) Atmospheric drag at Globalstar altitude is almost completely negligible. In fact, while they take drag into account to compute a satellite's current orbit, they do NOT incorporate a drag model to compute the satellite's future orbits [for ground antenna pointing purposes]. Drag is not a big deal.
Ok, they do lose 100-300 meters of altitude each year to drag effects, but for the most part it affects all satellites equally. So the fact that all satellites may be 150 meters lower than they were last year is of no concern.
(2) They are probably in the process of moving satellites within their orbit planes. Using the face of a clock as an example, to move a satellite from the 3-o'clock position to the 5-o'clock position requires the satellite to drift forward in argument of latitude. To drift forward, they increase the orbital rate [termed the "mean motion", in revs/day] and thus lower the satellite's altitude. When the satellite reaches the 5-o'clock position, the s/c is then raised back to the altitude it started from.
To achieve relative drift rates of 0.5 - 2.0 degrees/day, they must lower [or raise, if you want to drift backwards] the altitude by 1-3 km.
(3) NORAD data is poor. This doesn't give most people a warm fuzzy, since they think of NORAD as their national defender against nuclear war and Martian invasion fleets. NORAD orbit data is usually only accurate to +/- several kilometres. A garden variety, off-the-self GPS receiver is accurate to 300 metres.
Processed, filtered, GPS orbit data computed by people who know is accurate to 20-30 meters, which is more than sufficient for pointing the ground antennae.
(4) Two-line element sets require the use of a standard NORAD-type orbit propagator, referred to as MSGP4. It, too, is poor quality. Developed from arcane mathematical models of the 1950s and 1960s, orbits that are predicted more than 3 or 4 days into the future by MSGP4 are worthless, except for hand-waving, qualitative analyses.
In summary, the satellite orbits are fine. All is well, and the satellites are exactly where they should be.
So, what else have you got?
Maurice |
