Tag: letters

Constellation Viability

We recently discovered that in a 5-day period three GPS satellites were taken off line indefinitely with no explanation as to possible return to service. This is significant in that the constellation that remains is “non-optimal” in terms of providing more than four satellites at all locations and so on. Specifically we have concerns about RNP Aviation operations that rely on excellent satellite availability to achieve the lower RNP, less than 0.2 capability for final approaches. This seriously impacts airline operations as our calculations indicate that RNP 0.18 operations have been reduced (worldwide) to an availability of about 86 percent. This is a very significant loss of availability. This potentially also impacts LPV and other FAA certified GPS approaches as well.

Do you have any information as to what has happened to these satellites and whether or not they may be returned to service? May the constellation be “rebalanced” — with satellites being moved to new orbits to make the remaining 26 satellites provide a more optimal configuration that would also substantially improve RNP and other aviation operations availability?

Any insights or references as to where I may go to learn more about this situation would be greatly appreciated.

—David Vacanti Aerospace Fellow,
RF Navigation Surveillance IPSC Lead, Honeywell

Editor’s reply: As of September 4, three on-orbit GPS satellites had been taken out of service. PRN 15 was taken off line on 21 August; PRN 3 off on 24 August, and PRN 29 off on 26 August. PRNs 15 and 3 were reportedly undergoing clock swaps. PRN 3 had a previous clock swap in June. See “Clipped Birds” in The System news coverage for constellation status as of press time, September 22.

NDGPS Budget

Eric Gakstatter’s on line article on NDGPS (Nationwide Differential GPS) is a very poor framing of the issues. I have two main problems with GPS World’s on-line and hardcopy reporting of this issue.

One, the print article relegates an issue that is one of the most important in more than five years for GPS to a “down-at-the-bottom-of-page-20-issue.”

Secondly, it allows a pro-WAAS policy wonk such as Mr. Gakstatter to report on it. Very disappointing. This issue should not be framed as an NDGPS vs. WAAS issue as Gakstatter continually tries to make it. This is not the reason that NDGPS funding is an issue. It is much more complicated than that. Additionally, Mr. Gakstatter undermines his own credibility by making light of the user community that requires NDGPS for real-time, 1–5 meter, differential corrections and cannot use WAAS reliably.

Both of these points illustrate the lack of understanding by him and GPS World of real-world problems that face the GPS user community and policy issues in Washington.

In an article titled “Managing the GPS Constellation for Today’s Needs” in this month’s GPS World publication (September 2006), Mr. Lavrakas states that “Accuracy is the thing, and real-time accuracy is the most important thing.” I wholeheartedly agree. The NDGPS funding issue concerns a fantastically reliable augmentation system that is both efficient and fiscally responsible. My hope is that some sanity will return to this debate and the right path will be taken for the betterment of the GPS user community and the U.S. taxpayer (“Let’s not forget whose money makes all of this possible,” A. Cameron, Editor, same issue of the magazine).

Please, GPS World is a great publication, let’s have some accurate and complete reporting rather than opinion being floated as fact.

—Tim Smith
GPS Program Coordinator, National Park Service

Eric Gakstatter replies: Clearly when valuable programs are threatened, emotions run high.

For the record, I did not claim there were no unique applications for NDGPS. I wrote that the primary threat to NDGPS is the lack of a “killer app” and that WAAS is a key technology that is putting pressure on NDGPS. I think it would be difficult to argue that it is not.

GPS World’s editor and I invite you to write a more substantive, detailed essay to make “The Case for NDGPS.” I think it would be in the best interest of the GPS user community to hear it.

Finally, I am neither pro-WAAS nor anti-NDGPS. I am pro-user community. I have used — and own — both WAAS and NDGPS equipment extensively in the field and am very familiar with the capabilities and limitations of both technologies.

 

Air Traffic

It was gratifying to read about the result of so many years of hard work to make the vision of GBAS a reality. As Honeywell’s former partner [Pelorus Navigation Systems] in the development of the current systems at SEA-TAC and Moses Lake [Bremen, Malaga, Spain and other locations] I was delighted to read the results of what I knew would be found. My congratulations to Tim and the others at Boeing that had the courage to support LAAS/GBAS/ GRAS from the beginning. It has been a road far too long but the proverbial ILS light at the end of the tunnel will soon be replaced with GBAS.

—Mike Beamish
President, Analysis Unlimited
Calgary, Alberta, Canada

 

I so much enjoyed the August news story “Next Generation Air Traffic Controlled by GPS.” As I wrote in my October 1991 GPS World article titled “GPS, Aviation, and Airport; The Integrated Solution,” this was the future. The 1991 article includes then-revolutionary concepts of seamless airport operations, Automatic Dependent Surveillance in the terminal area, computer automated safety logic, and precise digital maps used for navigation and air traffic control. ADS-B is an integral part of the 21st century aviation infrastructure modernization.

The recent Comair, Louisville, Kentucky, disaster could certainly have been prevented with a moving map display showing the right travel path or even a set of navigational waypoints used during the departure taxi and take off. As experienced in the FAA’s Safe Flight 21 and Capstone programs, this technology saves lives, reduces delays, hence lowering airline operating costs, and reduces fuel consumption, thereby reducing pollution, while providing superior cost-effective performance for the air traffic controller and pilot over legacy systems of yesterday.

There are still hurdles to fully realize the aviation infrastructure of the 21st century and an FAA request for ADS-B funding is certainly a major step. A change in FAA management or a change in the U.S. administration could impact or delay vital enhancements to the country’s aviation infrastructure.

—Bob Pilley

 

Race FX

Just wondered why your article “IndyCar Series Supplements Broadcast with GPS Data” concerns an open wheel racing series, but your photos are from the NASCAR Nextel Cup series. Probably both series use the same technology, but Dale Jarrett, Bobby Labonte and Dale Earnhardt, Jr., drive stockcars, not IndyCars.

—John Webster
Selma, Alabama

Editor’s reply: Good catch. Photos from the IndyCar Series weren’t available, but as you state, both series use the same technology, which was the focus of the article.

Flex Power

See “Potential Problems for Users of Modernized GPS Signals in Mixed-Mode Operations” by Eric Gakstatter in The System news section, page 15, June issue.

Dear Mr. Gakstatter,

The Navstar GPS Joint Program Office (JPO) appreciates receiving inquiries regarding GPS signals, because it often helps us improve the signal documentation. We are particularly glad to field questions from GPS World because of your ability to “spread the word.”

We have entered a new era. During the 27 years since the first GPS satellite was launched in 1978, there have been only three navigation signals. When the first Block IIR-M satellite was launched on 25 September 2005, the number of navigation signals doubled with the addition of L2C and M codes on L1 and L2. Block IIF satellites will add L5, and Block III satellites will provide L1C. From the original three signals the number will grow at least to eight, not including the fact that L1C, L2C, L5, and the M codes each have two separate components, a pilot carrier and data. Further complicating the mix is the need to multiplex more than the two original signals on L1 and L2 while maintaining a constant total amplitude but permitting power to be shifted from one signal to another, that is, flex power. Also, instead of only two fixed message structures, now there are many, and the new ones are more flexible than before.

All of these changes are designed to improve performance, and they will. However, it is inevitable that in the process some confusion will result. For example, IS-GPS-200D addresses the phase relationship between L2P(Y) and L2C in one of the notes in Table 3-III (and similarly in paragraph 3.3.1.5) by stating: “The two carrier components on L2 [L2P(Y) and L2C] may not have the phase quadrature relationship. They may be broadcast on the same phase.” It was expected that this would inform users of the two possible phase relationships. However, only when receivers actually observed a phase change during on-orbit testing of the first IIR-M did the full implication of these few words sink in.

The purpose of such phase flexibility is to optimize a satellite’s power efficiency throughout its life and for each of its signal configurations. Unlike the past where the C/A and P(Y) phase relationship was defined and fixed, the multitude of new signals and their components may take on different phase relationships from time to time and from satellite generation to generation. This flexibility is needed to achieve the best overall results from signal modernization, but it may impose new tasks on user equipment and on differential messaging services.

For example, the JPO was not aware that L2C user equipment would employ cross-mode phase measurements between legacy (code-aided cross correlation) phase measurements and direct L2C phase measurements. In a common mode system, a very rare phase shift should cause little or no problem. (One question to commercial users is whether the satellite should be taken out of service briefly during such a phase change.) If cross-mode measurements are used, then the phase relationships must be known. This can be done by messaging, such as having the information in an almanac. However, it probably is better and certainly faster for the user equipment to monitor these relationships.

The JPO has had a long-standing process for dealing with such issues. It is called the Interface Control Working Group (ICWG). Information about the ICWG, including how to join, is available at: http://gps.losangeles.af.mil/engineering/icwg/. The JPO has long wanted more commercial participation so these types of questions can be discussed and resolved and the specifications improved. We invite you and your readers to participate, especially during this time of rapid signal modernization. Thank you.Respectfully, best regards,

— Mark C. Crews

U.S. Air Force, Navstar GPS Chief Engineer