The System: New Kid on the Block: IIF Readied

March 1, 2010  - By 0 Comments

New Kid on the Block: IIF Readied

The first Block IIF satellite destined for orbit arrived at the Navstar Processing Facility at Cape Canaveral, Florida, aboard an Air Force C-17 cargo aircraft on February 12. It is now undergoing preparations for its launch this spring on a Delta IV rocket. Block IIF will enhance GPS performance by reportedly providing twice the navigational accuracy of heritage satellites, more robust signals for commercial aviation and search-and-rescue, and greater resistance to jamming in hostile environments.

New L5 Signal. The new IIFs will broadcast the operational civil L5 signal, whose spectrum allocation was secured by broadcast of the signal by a IIR(M) satellite last year. L5, at 1176.45 MHz, lies in the Aeronautical Radionavigation Services band and can be used for safety-of-life aviation. It will be compatible with Galileo, GLONASS, and QZSS, with the goal to be interoperable as well. L5 will transmit at a higher power than current civil GPS signals, with wider bandwidth, and lower frequency that may enhance indoor reception.

More L2C Beacons. The IIF generation will also add to the number of satellites on orbit that broadcast the L2C signal at 1227.6 MHz, bringing it closer to full operational capability. L2C enables the development of lower-cost, dual-frequency civil GPS receivers for correction of ionospheric time-delay errors. Once the control segment modernization is complete, enhancements such as dataless and pilot channels for improved performance and an improved navigation message with more precise clock and ephemeris information will be available. L2C will also be interoperable with the Quasi-Zenith Satellite System (QZSS) under development by Japan.

Long Life. Built by Boeing, the IIF has a longer design life of 12 years, faster processors, and more memory. It will be followed by 11 other IIFs before modernization shifts into a higher gear with the GPS III generation.

It takes four hefty guys to wheel the new satellite along the tarmac, but it will only take one Delta IV rocket to lift it 20,171 kilometers into space on May 13.

It takes four hefty guys to wheel the new satellite along the tarmac, but it will only take one Delta IV rocket to lift it 20,171 kilometers into space on May 13.

Some Receivers Run Afoul of GPS Ground Control Software Update

On January 11, 2010, when the GPS Wing and the 2nd Space Operations Squadron (2SOPS) loaded the updated AEP 5.5C software to the ground control segment, a problem surfaced with a specific subset of GPS selective availability anti-spoofing module (SAASM) receivers.

The GPS Wing did not revert to the previous AEP 5.4 because of the upcoming IIF-SV1 launch. The scheduled sequential AEP 5.5C and AEP 5.5D updates are required before the ground control segment can adequately manage the more advanced capabilities of the IIF satellites.

One purpose of the 5.5C AEP update is to enable SAASM functionality in coded receivers. The software for this functionality has been resident in various certified SAASM receivers for some time, but was never implemented in the ground control segment. The update alleviates that problem for the majority of SAASM receivers, but for one manufacturer it has caused problems. The updated software sends a specific code to SAASM receivers that enables them to authenticate the message and ensure that the code is correct, and is being sent from the GPS and not some other source. For most receivers this worked without a hitch, but for one manufacturer, a software (SW) bug or glitch occurred that must be corrected before the receiver can authenticate. This fix is in progress and will most likely be implemented as a software or firmware update to the receivers.

Timing. Another problem with a different set of receivers manifested itself exactly two weeks after the AEP 5.5C update occurred. Those that have researched this problem in some depth feel that the problem is totally unrelated to the AEP update and would have occurred regardless.
This is also considered to be a receiver software bug for the manufacturer, and that process is ongoing.

ICD. Prior to activating the software update, the GPS Wing issued an updateable ICD or Interface Control Document that all receiver manufacturers use as a voluntary guide to determine compliance. Strict compliance by the manufacturer with the receiver interface control document (ICD) may have prevented the first issue, but the second may be a serendipitous event of the type that occurs from time to time no matter what precautions are taken.

The GPS Wing has issued two Notice Advisory to NAVSTAR Users (NANUs) for civilian and commercial GPS users and for military users, asking for user comments.

Letter to the Editor. Meanwhile, a reader wrote in: “I have issues with misleading e-mails containing inaccurate titles of articles posted on the site. There have been multiple cases recently claiming AEP software (SW) upgrades caused problems with receivers. In fact, and as proven by the vendors involved and others analyzing the problems, the AEP SW did not cause any of the observed conditions. ICD noncompliance of SAASM user equipment (UE) caused the problems, and the AEP SW upgrade allowed DoD, FAA, and vendors to finally discover the noncompliance issues and begin the process to resolve them. The community should view the 5.5 SW upgrade for what it is: a valuable new capability implemented correctly, which helped us all understand some unexpected shortcomings in UE.”

The editor concurs, and apologizes for misleading article titles. However, hard information was scant — in fact, completely unavailable — at the time.

GLONASS Gets Regional; Beidou Moves; Galileo Inks

The three new GLONASS-M satellites launched on December 14 have been set operational: GLONASS 730 in orbital slot 1 was set healthy on January 30, joining 734 and 733, which were set healthy earlier in the month. This brings to 18 the number of satellites currently in service, although GLONASS 722 continues to provide a healthy signal only on its L1 frequency. At present, the constellation only suffices to provide a 24-hour regional signal over Russian territory, although satellites can and frequently are pulled in by global high-precision users to complete an RTK solution, along with GPS.

Two satellites are in maintenance mode and set unhealthy, and two others, launched in 2003 and 2005, respectively, are in the process of being decommissioned.

The next GLONASS launch, of the GLONASS Block 40 satellites originally set to rocket up last September but returned to the Reshetnev factory with problems in the signal generator, is scheduled for March 2. Three more will rise in August, and a November 10 booster will put two GLONASS-M satellites and the first GLONASS-K satellite into orbit.

Beidou. According to tracking data from the United States Strategic Command, Beidou’s G1 satellite has drifted from its original location of 160°E and is currently at about 147°E longitude, that is, no longer in geostationary lock. Perhaps it is moving to another assigned Beidou slot, to back up or replace one of the other satellites in the constellation, but this can be no more than speculation. Hard data on the Beidou/Compass system is extremely difficult to come by. The new Chinese government Beidou/Compass website does not provide up-to-date information on the status of the constellation — something we take for granted with GPS, GLONASS, and Galileo.

Galileo. The European Space Agency signed contracts for Galileo’s full operational capability phase on January 26: with OHB for the manufacture of 14 satellites, delivery of the first in July 2012, followed by two satellites every three months; for launch services with Arianespace; and for system support with Thales Alenia Space.

 

24+3 FAQ

Eric_Gakstatter_125Survey editor Eric Gakstatter posed these questions to the GPS Wing; their answers follow.

Will the satellites (SVN24, SVN26) remain healthy during their repositioning journey?

Yes. The satellites will be set unhealthy for the initial Delta-V, but will return to healthy status approximately 24 hours after initiation of the Delta-V. Initial Delta-V for SVN24 was accomplished on 13 Jan 10 and returned healthy on 14 Jan 10. SVN24 will take up to a year to reach its final destination. Initial Delta-V for SVN49 was accomplished on 21 Jan 10 and will arrive at its expanded position in Jun 10. Initial Delta-V for SVN26 will begin early Feb 10.

Why the two-year timeframe to realize the benefits when all repositioning will be complete in 12 months?

The two-year timeframe is a conservative estimate which takes into account potential operational necessities which could extend the time required for completion. We must take a disciplined approach to cover possible failures and ensure continuity of coverage during the transition. We will be adding GPS IIF vehicles to the constellation and older vehicles may fail during the transition timeframe. As vehicles are added and removed, the current plan is subject to change in order to provide the best service to all civil and military users. Some of these decisions could require additional time to complete the expanded constellation. However, benefits will likely be realized well in advance of 24 months.

What is the reasoning behind using SVN49 as a key component of the 24+3 configuration since it won’t benefit a significant portion of the civilian user community, namely aviation and marine navigation as well as other SBAS (WAAS) and DGPS users? In my understanding, the FAA’s and the Coast Guard’s user bases are primarily single-frequency pseudo-range, users who won’t be able to use SVN49.

SVN49 was selected because it is a brand new satellite with four good clocks. Although issues with SVN49’s navigation signals may make it unusable for all civil use, it could still put out a valid set of signals for military use. The Air Force team is continuing to work “open book” with civil and industry GPS experts to determine the possible outcome of SVN49. Although SVN49 is not currently healthy, GPSW and 50th SW are actively working a mitigation that may allow setting the vehicle healthy in the future. As a mitigation in case we are unable to set SVN49 healthy, SVN30 will be rephased to the same slot following a successful launch and on-orbit checkout of IIF-1. We expect to have either SVN30 or SVN49 healthy and broadcasting from the expanded slot within a 24-month timeframe. At this time, no decisions have been made and no options have been ruled out regarding SVN49.

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Alan Cameron

About the Author:

Alan Cameron is editor-in-chief and publisher of GPS World magazine, where he has worked since 2000. He also writes the monthly GNSS System Design e-mail newsletter and the Wide Awake blog.

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