Home › Laptop GPS Hardware › The GPS constellation
We all take our personal navigators for granted treating the GPS constellation as just another utility. Here's artwork depicting the satellites that currently serve us everyday:
http://www.gps.gov/multimedia/images/
According to the Air Force operational data of 9 Nov 09, the constellation consists of :
Twelve Block IIA -- three unusable
Twelve Block IIR
Eight Block IIRM -- one unusable
Although that's thirty two satellites, historically thirty one is the greatest number ever set usable. That seems to be a 'de facto' limit for now.
Block IIA satellites are operating long past their predicted on orbit life; satellite number 23 (transmitting PRN32) being launched way back during November of 1990. Most of those satellites are on a 'watch list' meaning the are just a component away from end of navigation mission.
Twenty one Block IIR satellites were built. One was lost when its Delta II launch vehicle exploded seconds after lift off.
Eight of these were converted to IIRM versions which, in addition to the signal we all use, can transmit 'Modernized' military and civilian signals. Unfortunately, one IIRM satellite exhibits a signal anomaly so there's been reluctance to set it usable.
Block IIF satellites are about five years late. The first one has finally been scheduled for launch during May-June of 2010. Twelve have be ordered for constellation sustainment until Block III satellites are delivered around the middle of the next decade.
The constellation requires twenty four satellites for promised world wide coverage. We know things work a lot better when we have at least thirty working.
http://www.gps.gov/multimedia/images/
According to the Air Force operational data of 9 Nov 09, the constellation consists of :
Twelve Block IIA -- three unusable
Twelve Block IIR
Eight Block IIRM -- one unusable
Although that's thirty two satellites, historically thirty one is the greatest number ever set usable. That seems to be a 'de facto' limit for now.
Block IIA satellites are operating long past their predicted on orbit life; satellite number 23 (transmitting PRN32) being launched way back during November of 1990. Most of those satellites are on a 'watch list' meaning the are just a component away from end of navigation mission.
Twenty one Block IIR satellites were built. One was lost when its Delta II launch vehicle exploded seconds after lift off.
Eight of these were converted to IIRM versions which, in addition to the signal we all use, can transmit 'Modernized' military and civilian signals. Unfortunately, one IIRM satellite exhibits a signal anomaly so there's been reluctance to set it usable.
Block IIF satellites are about five years late. The first one has finally been scheduled for launch during May-June of 2010. Twelve have be ordered for constellation sustainment until Block III satellites are delivered around the middle of the next decade.
The constellation requires twenty four satellites for promised world wide coverage. We know things work a lot better when we have at least thirty working.
Thanks for that satellite backgrounder, MrUmbra. Most interesting.
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If you take the global view, the situation is not as dangerous as it seems.
The Russian GLONASS system already has 18 operational satellites in position, and needs 6 more to reach worldwide coverage. Three satellites per launch, so only two launches away from completion.
The European Galileo system is scheduled to be completed in 2013. Looking farther out, the Chinese Compass system is targeted for 2020. (Although the program could conceivably end once it has reached full coverage for China. This is less of a risk with the Russian and European systems, due to their geographical location.)
The Russian GLONASS system already has 18 operational satellites in position, and needs 6 more to reach worldwide coverage. Three satellites per launch, so only two launches away from completion.
The European Galileo system is scheduled to be completed in 2013. Looking farther out, the Chinese Compass system is targeted for 2020. (Although the program could conceivably end once it has reached full coverage for China. This is less of a risk with the Russian and European systems, due to their geographical location.)
According to the Air Force operational data of 1 Dec 09, the constellation now consists of:
Twelve Block IIA -- one unusable
Twelve Block IIR
Eight Block IIRM -- one unusable
PRN01, the IIRM satellite with a signal anomaly is still unhealthy. This satellite operates within the Air Force published specification but its basic error is 'out of family' with respect to the seven other IIRM space vehicles. This is what happened:
Upcoming IIF GPS satellites will add a new 'safety of life' frequency designated as L5. But, GPS faced a deadline to transmit an L5 signal from medium earth orbit as imposed by the international body that coordinates frequencies . With the slippage in IIF delivery, that deadline would have been missed allowing an L5 assignment to the first user that transmitted there. Consequently the Air Force commissioned Lockheed Martin to add an L5 'demonstration' to a remaining IIRM satellite. When connected to the antenna array, the L5 transmitter distorted the L1 signal beam we all use. Unfortunately, the problem wasn't detected during ground testing.
PRN24 is a IIA satellite removed from service for undisclosed reasons.
--- CHAS
Twelve Block IIA -- one unusable
Twelve Block IIR
Eight Block IIRM -- one unusable
PRN01, the IIRM satellite with a signal anomaly is still unhealthy. This satellite operates within the Air Force published specification but its basic error is 'out of family' with respect to the seven other IIRM space vehicles. This is what happened:
Upcoming IIF GPS satellites will add a new 'safety of life' frequency designated as L5. But, GPS faced a deadline to transmit an L5 signal from medium earth orbit as imposed by the international body that coordinates frequencies . With the slippage in IIF delivery, that deadline would have been missed allowing an L5 assignment to the first user that transmitted there. Consequently the Air Force commissioned Lockheed Martin to add an L5 'demonstration' to a remaining IIRM satellite. When connected to the antenna array, the L5 transmitter distorted the L1 signal beam we all use. Unfortunately, the problem wasn't detected during ground testing.
PRN24 is a IIA satellite removed from service for undisclosed reasons.
--- CHAS
According to the Air Force operational data of 10 Jan 10, the constellation now consists of:
Twelve Block IIA -- one unusable
Twelve Block IIR
Eight Block IIRM -- one unusable
That's still 30 usable satellites but there have been recent constellation changes. PRN25 was decommissioned and PRN24 was returned to service. These are older Block IIA satellites.
Of significant note, field commanders in Afghanistan have been reporting GPS outages in the steep sided mountain valleys. The US Air Force plans to improve that situation during the next 24 months by reorganizing the constellation. Currently, those usable 30 satellites are organized to include backups at the principal 24 orbital slots. So, in essence, the 30 satellite constellation provides performance that's only marginally better than a baseline 24 satellite system.
After the satellites are redistributed, we civil users should also see improvements in our mountainous or urban environments.
--- CHAS
Twelve Block IIA -- one unusable
Twelve Block IIR
Eight Block IIRM -- one unusable
That's still 30 usable satellites but there have been recent constellation changes. PRN25 was decommissioned and PRN24 was returned to service. These are older Block IIA satellites.
Of significant note, field commanders in Afghanistan have been reporting GPS outages in the steep sided mountain valleys. The US Air Force plans to improve that situation during the next 24 months by reorganizing the constellation. Currently, those usable 30 satellites are organized to include backups at the principal 24 orbital slots. So, in essence, the 30 satellite constellation provides performance that's only marginally better than a baseline 24 satellite system.
After the satellites are redistributed, we civil users should also see improvements in our mountainous or urban environments.
--- CHAS
That's something to look forward to!
Last spring, the US General Accounting Office issued findings implying the GPS constellation will weaken as Block IIA satellites, that are all operating well past their projected design expectations, are removed from service with no replacements forthcoming.
Significantly, a GPS Block IIF satellite has finally arrived arrived at the Cape Canaveral for launch this May. Then, eleven more IIF satellites are to be delivered. Hopefully, these new satellites will sustain GPS until Block III satellites arrive in five or six years.
--- CHAS
Significantly, a GPS Block IIF satellite has finally arrived arrived at the Cape Canaveral for launch this May. Then, eleven more IIF satellites are to be delivered. Hopefully, these new satellites will sustain GPS until Block III satellites arrive in five or six years.
--- CHAS
While GPS IIF-1 is being prepared for its 20 May launch, the rocket assigned to perform the duty has been moved to the launch pad. This is a Delta IV Evolved Expendable Launch Vehicle. Pictured is the booster core. Two solid fuel rocket motors will be attached for this launch.
The Delta II rocket, that has launched previous Block II series GPS satellites, is being phased out of service.
--- CHAS
The Delta II rocket, that has launched previous Block II series GPS satellites, is being phased out of service.
--- CHAS
It's time for a GPS constellation update.
Following several launch 'scrubs', GPS IIF-1 was successfully launched on 28 May 2010. It was set usable on PRN25 during 27 August 2010 bringing the constellation back to 31 usable satellites.
--- CHAS
Following several launch 'scrubs', GPS IIF-1 was successfully launched on 28 May 2010. It was set usable on PRN25 during 27 August 2010 bringing the constellation back to 31 usable satellites.
--- CHAS
There's a new navigation satellite that will eventually 'sort of' become part of the GPS constellation, it's called MICHIBIKI. According to the Japan Aerospace Exploitation Agency the satellite was placed into a Quasi-Zenith orbit on September 27. It was launched from Japan's Tanegashima Space Center on September 11, 2010.
JAXA | Topics
The orbit is designed to favor overhead visibility within Japan for eight hours per day. Its signal coverage augments the existing GPS which, evidently, doesn't provide adequate coverage there in the cities and mountains.
MICHIBIKI's signal structures are identical with those of GPS. User equipment is simplified by allowing receiver 'front ends' to employ tracking and demodulation techniques identical with current GPS practice.
Three satellites will eventually provide continuous service over Japan.
--- CHAS
JAXA | Topics
The orbit is designed to favor overhead visibility within Japan for eight hours per day. Its signal coverage augments the existing GPS which, evidently, doesn't provide adequate coverage there in the cities and mountains.
MICHIBIKI's signal structures are identical with those of GPS. User equipment is simplified by allowing receiver 'front ends' to employ tracking and demodulation techniques identical with current GPS practice.
Three satellites will eventually provide continuous service over Japan.
--- CHAS
A GPS satellite has recently celebrated 20 years on orbit. It's a block IIA that was launched way back in 1990 on 26 November. It transmitted PRN23.
There's an interesting history. A mechanism that points the solar panels towards the sun prematurely failed forcing the system operators to manually operate the satellite for fourteen years. It was finally removed from service on Feb 2004.
Fortunately, the array drive mechanism regained functionality prompting a series tests beginning early 2007 with the satellite set to transmit PRN32. The Air Force had been reluctant to utilize PRN32 .. evidently, receivers not programmed for PRN32 were in service. The satellite was again set usable on PRN32 during 26 FEB 2008 (NANU 2008024). It continues to perform well.
Eleven Block IIA satellites from the nineties continue to operate. All but one are on the so called 'watch list' being one critical component failure away from losing the navigation mission.
--- CHAS
There's an interesting history. A mechanism that points the solar panels towards the sun prematurely failed forcing the system operators to manually operate the satellite for fourteen years. It was finally removed from service on Feb 2004.
Fortunately, the array drive mechanism regained functionality prompting a series tests beginning early 2007 with the satellite set to transmit PRN32. The Air Force had been reluctant to utilize PRN32 .. evidently, receivers not programmed for PRN32 were in service. The satellite was again set usable on PRN32 during 26 FEB 2008 (NANU 2008024). It continues to perform well.
Eleven Block IIA satellites from the nineties continue to operate. All but one are on the so called 'watch list' being one critical component failure away from losing the navigation mission.
--- CHAS
It's quite impressive that a piece of technology that was new 20 years ago, in 1990, can still perform well today, in 2010!
Chas, thanks for keeping us up to date on these things. It's always a pleasure to read.
Chas, thanks for keeping us up to date on these things. It's always a pleasure to read.
MrUmbra, appreciate the updates. Nicely done.
Ashtech just released a FREE online program to predict satellite position, count and PDOP, among other things. Covers GPS, GLONASS and SBAS. Thought this might be of interest to others as well. Find it at Web Mission Planning - Automatic Web-Accessible GNSS Survey Planning Tool
Ashtech just released a FREE online program to predict satellite position, count and PDOP, among other things. Covers GPS, GLONASS and SBAS. Thought this might be of interest to others as well. Find it at Web Mission Planning - Automatic Web-Accessible GNSS Survey Planning Tool
The satellite sending PRN01 has been decommissioned from active service. Although it transmitted navigation signals, we never were able to use it. Here's a tale of woe:
Back during 2009, a newly designed satellite was scheduled to introduce the new 'Safety of Life' L5 signal. Unfortunately, delivery of GPS IIF-1 was hopelessly delayed pushing the US Air Force against an internationally imposed deadline to transmit the L5 signal from medium earth orbit. Solving that problem, Lockheed Martin equiped a remaining modernized Block IIR satellite with a 'demonstration' transmitter. History records 24 March 2009 as the successful launch day for GPS IIR (M7) with its L5 signal.
When the satellite was activated, monitor stations worldwide immediately detected a strange anonymity .. a ranging error that varies with observed elevation. Analysts quickly determined the demonstration transmitter is not properly isolated from the master navigation antenna. That causes associated electrical problems which distort the transmitted beam in a manner that resembles multipath conditions when perceived on the ground.
Attempting to save the satellite, the Air Force petitioned the GPS community for technical solutions. Many ideas were presented but none was a panacea solving everyone's requirements. The satellite was never set as healthy.
--- CHAS
Back during 2009, a newly designed satellite was scheduled to introduce the new 'Safety of Life' L5 signal. Unfortunately, delivery of GPS IIF-1 was hopelessly delayed pushing the US Air Force against an internationally imposed deadline to transmit the L5 signal from medium earth orbit. Solving that problem, Lockheed Martin equiped a remaining modernized Block IIR satellite with a 'demonstration' transmitter. History records 24 March 2009 as the successful launch day for GPS IIR (M7) with its L5 signal.
When the satellite was activated, monitor stations worldwide immediately detected a strange anonymity .. a ranging error that varies with observed elevation. Analysts quickly determined the demonstration transmitter is not properly isolated from the master navigation antenna. That causes associated electrical problems which distort the transmitted beam in a manner that resembles multipath conditions when perceived on the ground.
Attempting to save the satellite, the Air Force petitioned the GPS community for technical solutions. Many ideas were presented but none was a panacea solving everyone's requirements. The satellite was never set as healthy.
PRN01 Goes Off the Air
--- CHAS
This is GPS IIF-2 being prepared for its April delivery to Cape Canaveral. Block IIF GPS satellites are built by Boeing at the El Segundo satellite manufacturing facility.
It's launch atop a Delta IV rocket is tentatively scheduled for the morning of 14 July 2011. If all goes well, the satellite will be ready for use after a thirty day on orbit engineering evaluation.
The US Air Force is to buy 12 Block IIF satellites for replacement of 90s vintage spacecraft that remarkably continue to function well past their design requirements,
--- CHAS
It's launch atop a Delta IV rocket is tentatively scheduled for the morning of 14 July 2011. If all goes well, the satellite will be ready for use after a thirty day on orbit engineering evaluation.
The US Air Force is to buy 12 Block IIF satellites for replacement of 90s vintage spacecraft that remarkably continue to function well past their design requirements,
--- CHAS