RhettTAFE

Hi everyone,

I have been given an assignment for my GPS Theory class and I am struggling mightily to find any information at all on the subject. I came across this forum and thought I would sign up and see if anyone here can point me in the right direction.

My assignment is about the satellites in the GPS Constellation..
Where are the spare satellites??? As you guys probably know there are 35 satellites up there for GPS, 30 are being used today.
What are the other 5 doing? Orbiting at a higher or lower altitude ready to be moved into place? Sitting around doing nothing up in space? or following the same orbital plane as the other satellites but switched off?

Any info or relevant websites would be greatly appreciated..

Thanks in advance.

zee

I read something on the status within the last month. I'm still working on my morning second cup. Might be something on the enclosed, if I can paste it right.
GPS Almanac Information - USCG Navigation Center

HumVee

The satellites are replaced one at a time, on a regular basis. They require reaction mass to keep them in their orbits, and it runs out. There's quite a cost involved in giving us our free GPS coordinates...I'm sure us tax payers are paying for it - but it's a worthwhile service!

RhettTAFE

I understand this HumVee but my problem is trying to find out what the 5 satellites from the GPS constellation which are not in use are doing?
Does any one know the name of these satellites which are not sending signals are called?
Searching doesn't come up with any thing because i am using words like "Unused satellites" or "Spare Satellites"

And Zee thanks for the site, looks like it might help if i can get my head around it..

"You may very well need to change the health settings when you use Yuma or SEM almanac data. While the orbital description data is generally usable for months, the satellite health may change at any time. For information on satellite health at times other than the almanac "Time Of Applicability", see the NANUs or status messages. An almanac entry of 000 indicates the satellite is healthy."
^ Is a copy from that site..
Does this mean that a healthy satellite is set to unhealthy when its not being used? and can therefore be swapped in for a healthy satellite at any stage?

Im getting rather confused now

HumVee

This website discusses exactly what I think you're interested in:

The GPS Satellite Constellation

It states that there are 4 standby satellites that are in different orbits than the operational satellites. It's a quick and fascinating read, and even has a chart showing which satellites wouldn't be visible at certain times from a certain location and time period (Johor Bahru, Malaysia; January 1998). There's no mention on how the chart was compiled. Unfortunately, the web page is dated 1999, which makes it a decade old - but the basic information still looks useful to me.

This site is a decade more recent (2009), and states that there's only 3 spares - so maybe one of the spares had to go active to replace a failed satellite. And this site has a very alarming message that the entire GPS satellite system is in jeopardy, becase the US Air Force hasn't been able to keep up with scheduled replacement satellite launches! So maybe there have been multiple failed satellites, and the USAF has been sending up replacement birds...but not quite as fast as they are beginning to fail. If a number of GPS satellites fail in short order, and there's not enough standby satellites to activate and fill the gap, then that would definitely cause "holes" where fewer satellites would be available to be acquired. I'm sure that the article is exaggerating the level of alarm, but the information is certainly worthy of concern since so many depend on this system!

GNSS

The "concept" of spares as per the original spec is no longer valid as satellites in orbit regardless of being operational or not are running on borrowed time and part of the design life span.

There are 32 "possible" operational slots, 1 is reserved for on ground pre launch testing, 1 other is used for "other things" for "2SOP stuff"

There are several satellites piggy backing others in a couple of slots and these non operational satellites have been retained in orbit on a just in case basis. Looks something like as in the following link


http://gge.unb.ca/Resources/GPSConstellationPlot.pdf

RhettTAFE

Thanks a lot guys, that was exactly what i was looking for..
Me and my classmates thank you

HumVee

Great info, GNSS! Thanks.