GPS CNAV Debate, and GNSS Interoperability Moves Forward

March 27, 2014  - By
Image: GPS World

The opening plenary session of the Munich Satellite Navigation Summit is convening as this column goes to the electronic press for distribution. Coverage of these top-level system briefings before a select international GNSS audience in Munich will appear in two e-newsletters next week, The European GNSS and Earth Observation Report (EAGER), and in a shortened form via the Navigate! Weekly.

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Until then, here’s an update on the CNAV debate in the United States and wider system-operator background from two recent meetings.

CNAV So Far.  In the closing hours of 2013, a departing U.S. Cabinet under-secretary for Transportation dropped a verbal bomb on the Pentagon, in the form of a communiqué expressing concern about reliability of the new civil navigation message (CNAV) signal scheduled to emanate in April from select GPS satellites on orbit. Subsequent explosions were detected in halls from Washington to Colorado and Los Angeles.

The Department of Transportation issued a call for back-up in the form of public comment via the Federal Register. That comment period closes on April 4.

Meanwhile, one semi-public organization communicated to its members that it finds nothing disturbing about the plan, set to take effect sometime in the coming month.

IGS Steps Forth on CNAV.  The International GNSS Service, a voluntary federation of more than 200 worldwide agencies that pool resources and permanent GPS and GLONASS station data to generate precise GPS and GLONASS products, issued a statement to its members and participating institutions in March. “We are confident that the IGS network is not at risk due to this change, and it is a welcomed step towards GPS modernization.”

The communiqué from the Infrastructure Committee went on to say that “This event is considered innocuous to the stability of the receiver network since during a limited GPS CNAV test campaign in June 2013 the IGS network was not affected, only a very specific receiver problem was detected by the IGS Multi-GNSS Experiment, which was informed to the GPS ground segment and addressed.

“Most modern receivers can track L2C and L5 and the CNAV messages, but the decoded messages should not be used by the receivers. The traditional L1 NAV messages (LNAV) will continue to be transmitted as usual and thus the receiver navigation files, birds, etc., will continue unaffected. Older receivers will be completely unaffected as they do not track L2C or L5.

“In any case IGS Station Operators and Station Network Managers are advised to keep an eye on receivers and on their data outputs during the start of the CNAV activation. Just in case something strange is observed please stop data submission and notify the IGS (Network Coordinator, Infrastructure Committee) so that we may investigate the issues quickly. In case of doubt with your own equipment please contact the receiver manufacturer and inform the IGS.”

PNT Advisory Board Airing. Prior to the appearance of the CNAV letter from the departing deputy secretary, the U.S. PNT Advisory Board heard a report in early December from Air Force Space Command on said implementation plan for the GPS CNAV message on L2C and L5. The minutes of that meeting were recently released.

The minutes relay the gist of General Whelan’s CNAV remarks as: “CNAV has been under discussion for a considerable time. Currently, L2C and L5 signals are being transmitted, but without a navigation message. AFSPC is working hard to activate these messages as soon as possible. One of the reasons for the delay is that additional time was needed to complete testing prior to activation. Testing began in late summer 2013 and, based on initial test results, a way ahead has been plotted. . . . Current plans are to begin initial broadcasting in the spring of 2014. CNAV uploads will occur twice weekly. The signal will meet GPS Standard Positioning System (SPS) standards, but may not achieve current accuracy levels until full implementation in late 2014.

“CNAV live-sky testing occurred in June [2013] and was conducted in cooperation with civil, industry, and international partners. The two-week test series included independent assessment and verification. The tests identified four errors that required action. The first, which was addressed in real time, related to implementation of the test series. The second required improvement to the tools suite, which should be totally integrated into the ground segment by December 2014. The third and fourth errors required patches to satellite software. All four issues are now regarded as closed.”

A subsequent presentation to the PNT Advisory Board from a Department of Transportation spokesperson did not directly mention CNAV, according to the meeting minutes, but did include this reminder on civil signal monitoring:

“DOT is responsible for performance monitoring of GPS civil signals. The International Committee on GNSS’s (ICG’s) transparency principle states that ‘Every GNSS provider should publish documentation that describes the signal and system information, the policies of provision, and the minimum levels of performance offered for its open service.’ Currently, this is only done on GPS L1 C/A signals. Performance standards for L2C and L5 have not yet been established. The crucial function of signal/service monitoring is to verify that commitments to GNSS performance are being met. Additionally, monitoring improves the situational awareness for GNSS operators, and provides assurance that any civil service failure is detected and resolved promptly.”

Other Global Developments. The International Committee on GNSS (ICG) held a meeting of its Working Group A on Compatibility and Interoperability, in November 2013 in Dubai, United Arab Emirates. A brief summary of those proceedings is now available.

The notes evidence steady, deliberate organizational and international progress on collaboration between system providers of GNSS signals.

Among new presentations to the body came several from Russia. Viktor Kashenko, Russian Federation, presented on the “Prospects for Status and Development of GLONASS System Space Complex,” an update on the GLONASS space segment noting that there is a full constellation of GLONASS-M satellites. CDMA signals at L1 and L2 are expected to be available beginning around 2016 or 2017.

Grigory Stupak, Russian Federation, followed with a presentation titled “SDCM Present Status and Future GLONASS Signals Development.” There are currently 22 SDCM ground stations around the world with a goal of creating seamless coverage throughout Russia with LPV-200 capability. The U.S. asked a question about whether SDCM provides corrections for other constellations in addition to GLONASS. The Russian Federation explained that SDCM currently augments both GLONASS and GPS, but additional constellations could be added in the future.

Oleg Denissenko, Russian Federation, discussed the goals of the GNSS Monitoring and Assessment System being developed in Russia and identified a list of parameters to be monitored by the international systems.

Xurong Dong from China gave the status of the International GNSS Monitoring & Assessment Service for OS (iGMAS). Initial operational capability (IOC) is expected in June 2014.  Ten tracking stations have been installed so far, and 25 additional stations are expected to be added in the future. A signal quality monitoring station has also been established in China and a new 40-meter antenna is expected to be installed in 2014.

Jeffrey Auerbach from the U.S. State Department presented on outcomes of the second Interference Detection and Mitigation (IDM) April 2013 workshop. The European Union noted that they are conducting a survey of professional users in Europe about privacy concerns, and perceptions and understandings of interference and jamming.

Stanislav Kizima, Russian Federation, provided an overview of the International IDM system concept and recommended the creation of an IDM system database server to be used for monitoring GNSS facilities. He suggested identifying formalized data exchange formats for IDM. A question was asked about whether something like this already exists in Russia. Kizima responded that Russia does have an active system  for  monitoring interference, but not  specifically for GNSS. There are some issues with the existing system because GNSS is not listed as source of interference and the technical facilities are not able to analyze parameters specific to GNSS. Hence the need for development of specific GNSS monitoring facilities. Tom Stansell from the U.S. responded that cell phones could be enabled to become individual detectors of GNSS interference, and the interference source location could be determined this way. This technique is known as crowdsourcing. Kizima noted that cell phones give information on signal power, but not measurement equipment.

China continued the session on spectrum protection with a presentation by Weimin Zhen on a proposal to develop a template for GNSS interference detection and reporting. He suggested that a generic template specific to reporting GNSS interference be developed.

Upcoming principal WG-A related meetings:

  • WG-A Inter-session Meeting, Geneva, Switzerland, possible dates July 16-18, 2014)
  • ICG-9, Prague, November 10-14, 2014.

About the Author: Alan Cameron

Alan Cameron is the former editor-at-large of GPS World magazine.