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GPS 2F-1 being completed at Boeing  (Source: Boeing)

GPS 2F satellite and Delta II rocket  (Source: United Launch Alliance)
Dozen new satellites will revitalize constellation

 

The Global Positioning System (GPS) has become an important part of global commerce, a far cry from its roots as a purely military system designed to aid in guiding Inter-Continental Ballistic Missiles. Everything from oil tankers to running shoes now incorporate GPS technology, thanks to shrinking designs and mass production.

Although the first Block I satellite was launched in 1978, the GPS network didn't achieve initial operational capability until December 1993. The GPS network requires a minimum of 21 satellites in medium Earth orbit to ensure global coverage, and a full network of 32 satellites is preferred for optimal performance and reliability.

There are currently thirty GPS satellites in orbit, but eleven of those are Block IIA satellites which were launched between 1990 and 1997. Those satellites were designed for a 7.5 year operational span, and the oldest four satellites in the constellation were launched in 1992. They have now been working for ten more years than planned; over twice the expected lifetime.

Those eleven satellites could fail at anytime, so the need for the newest Block 2F satellites has never been more apparent. However, the first GPS 2F launch had been delayed five times due to various problems. It finally launched at 2300 EDT and reached  orbit in 3.5 hours.

Boeing is building the twelve Block 2F GPS satellites for the US Air Force, which will continue to manage the constellation. New features include a doubling in the predicted signal accuracy; new L5 signals for civil and commercial aviation; a new "M-code" and variable power levels for the military that boosts performance and provides better resistance to jamming in hostile environments; a 12-15 year design life; and reduced operating costs. A new reprogrammable processor is also being used that can receive software uploads for improved system operation.

When the GPS system was first designed, a feature called "Selective Availability" was added that would create errors in the civilian signal, thus degrading performance. President Clinton ordered Selective Availability turned off in 1996, and the designs for Block 2F satellites were the first to not include SA hardware.

"The 12 GPS 2F satellites will provide improved signals that will enhance the precise global positioning, navigation and timing services supporting both the warfighter and the growing civilian needs of our global economy," said Col. Dave Madden, commander of the Global Positioning Systems Wing at the Space and Missile Systems Center.

GPS 2F-1 is currently taking over the Plane B, Slot 2 position of the constellation, replacing GPS 2A-27 launched in September 1996. It is a primary position in the network, which is divided into six orbital groupings with several satellites flying in each.

GPS 2A-27 will move into another slot in order to shore up weaker, but less critical parts of the network. Plane B, Slot 2 was originally supposed to be occupied by GPS 2R-20 in March 2009, but problems developed which precluded its usage.

Previous GPS launches over the past 20 years relied on Delta 2 rockets for delivery into orbit, but the $121 million Block 2F satellites use the larger and more capable Delta 4 and Atlas 5 rockets that can propel it directly into its orbit. Previous launches with Delta 2 rockets needed booster motors and several days to reach the desired position.

GPS 2F-1 is the 61st GPS satellite to launch, and the 50th GPS launch on a Delta rocket. Ground control will undergo a series of testing procedures over the next 90 to 120 days, after which GPS 2F-1 will enter service. The next satellite, GPS 2F-2, is currently scheduled for launch in November.

Three new Russian GLONASS satellites will attempt to launch in August in a bid to restore global coverage to their competing system, after being delayed from March. The first satellites in Europe's Galileo system are not expected to launch until 2014, but their signals could theoretically be used with GPS signals for even greater resolution with the properly designed hardware.

 



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RE: Error in article
By rcc on 5/28/2010 11:46:26 AM , Rating: 2
There are a few errors in there. As originally designed, the military signal from the GPS satellites actually ran at 10x the civilian signal, hence the 10:1 accuracy advantage.


RE: Error in article
By Jansen (blog) on 5/28/2010 12:04:21 PM , Rating: 1
I'm sorry, but you're incorrect. Select Availability was the only way to reduce accuracy, there were no seperate military and civilian signals until recently.


RE: Error in article
By dardas on 5/28/2010 12:54:02 PM , Rating: 2
AFAIK, GPS sats broadcasted several different frequencies. civilians and military.
SA was introduced only to the civilian freq, so the military applications could continue unimpeded just by the use of a different radio.

so you can say the military signal was "more accurate", but not due to the inherent nature of the frequency, but due to SA used on the civilian one.


RE: Error in article
By Solandri on 5/28/2010 1:25:23 PM , Rating: 2
Civilian and military GPS receivers used the same signals. Selective availability was a pseudorandom sequence of numbers added to the timing signals thus increasing the error radius of anyone using the raw signals. Military receivers were coded with the key to the pseudorandom sequence. This allowed the GPS unit to generate the same sequence and subtract it from the timing signal, thus obtaining the correct timing and hence the correct position. (You also needed a daily key which was distributed by sneakernet, so as to render captured military GPS units no better than commercial units.)

When SA was turned off, all they did was set the value of the pseudorandom sequence to zero.


RE: Error in article
By rcc on 5/28/2010 1:48:09 PM , Rating: 2
bull crunchies.

The system as originally build had clear distinctions between civilian and military usage and signals.

While I realize we look down on Wikipedia as a source, you might review http://en.wikipedia.org/wiki/Global_Positioning_Sy...


RE: Error in article
By Jansen (blog) on 5/28/2010 2:34:36 PM , Rating: 6
If you mean the P-code, that was only used for anti-spoofing purposes. It is only broadcast once a week and is virtually useless for standard applications.

Civilian and military GPS units use the Coarse/Acquisition code, which Selective Availability degraded by introducing pseudo-random errors which could be corrected by military GPS receivers that had the appropriate code key.

The L2C signal which I think people are confused by was only introduced in 2005 with the first Block 2R-M satellites.


RE: Error in article
By zbacanek on 5/28/2010 3:13:16 PM , Rating: 4
A couple of clarifications ... P-Code is broadcast continuously BUT the sequence is so long that it takes 1 week to repeat. C/A-Code is a short sequence and it repeats every 1 milliseconds.

P-Code is useful in addition to anti-spoofing... once you calculate the atmospheric distortion using C/A-Code transmitted in both the L1 and L2 frequency bands, you can use the P-Code which has 10x the bandwidth of C-Code to improve your position. There are methods to utilize P-Code without having to decrypt the encrypted P-Code and still get most of the benefits.

As I recall, Selective Availability was implemented by transmitting a time-varying clock correction in the navigation signal transmitted along with C-Code which was "slightly" wrong. Every satellite transmits its own clock corrections uploaded from ground stations to compensate for atomic clock drift relative to the ground station atomic-clocks. AFAIK, SA is not implemented as you describe.


RE: Error in article
By demiller9 on 5/28/2010 3:33:28 PM , Rating: 1
No, there have been separate military and civilian signals from the start. The civilian signal was called CA (coarse acquisition) and sent on the L1 frequency, the military signal is called the P code (Precise) and used L1 and L2. The CA signal has a bit rate of 1.023 million chips and the P code is 10.23 million chips.

The higher chip rate allowed higher accuracy to the military. Selective Availability reduced the civilian accuracy by intentionally misstating the satellite position and by introducing jitter to the timing signal (which modified the pseudorange value).


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