GMV, one of the world’s leading companies in satellite navigation systems, announced the tracking of both data and pilot channels of Galileo first satellite signal with its own line of GNSS receiver products.
The first two Galileo satellites were launched from Kouru Spaceport in French Guiana on October 21st and are now in in-orbit test campaign. The Galileo PRN 11 started transmitting the first navigation signal last Saturday.
GMV has been involved in GNSS for the last 25 years and today GMV’s GNSS team includes more than 120 highly specialized engineers, some having more than 15 years experience in the GNSS field. GMV plays a critical role in the ongoing development of Europe’s GNSS strategy, being a key partner in the EGNOS and Galileo programmes.
GMV has developed its own GNSS software receiver products: SRX-10 on GPS, which has been optimized for the urban environment, NUSAR for GPS L1 and Galileo E1 and its own L1 front end. This experience has been applied, even previously to the development of the receivers, to many studies on receiver performances under very diverse signal conditions and designs, namely by processing the GIOVE satellites signal.
Supported by its line of GNSS receiver products, GMV now presents its results on the first Galileo signals on both data (E1-B) and pilot (E1-C) channels of the Galileo PRN 11 satellite.
This article gives a brief overview of the acquisition and tracking of Galileo IOV signals received from the GSAT0101 satellite on the morning of December 15. Researchers in the PLAN Group successfully recorded E1 and E5 data using a single dual-channel front-end and subsequently acquired and tracked E1 B/C, E5a and E5b signals using the PLAN Group GSNRx software GNSS receiver.
A little over seven weeks after launch, one of the two Galileo IOV satellites began to transmit on the E1 band. To the delight of eagerly waiting researchers worldwide, Galileo-PFM (GSAT0101) broke radio silence on December 10, 2011. Within hours the community was alive with reports of successful acquisition and tracking of the E1 B/C signals. Four days later the E5 signal was also activated. In the early hours of the morning of the 15th of December researchers gathered in the PLAN Group at the University of Calgary and observed the sky filled with broadcasting satellites from three GNSS. Using a dual channel front-end designed in-house, a Novatel GPS-703-GGG antenna and a laptop computer, IF data was collected to examine these new signals. This data was processed by GSNRx, a reconfigurable a multi-system, multi-frequency software receiver developed by the PLAN Group [1]. The equipment used to acquire and process the data is shown in Figure 1.
Figure 1 The equipment used to acquire and process the Galileo-PFM signals included an in-house dual frequency front-end, a 10 MHz OCXO, a Novatel GPS-703-GGG antenna and a standard laptop computer running the GSNRx software receiver.
At approximately 03:20 MST (UTC – 7:00) more than 20 GNSS satellites were visible from a rooftop mounted antenna. Having reconfigured the front-end to accommodate the E5 band, IF data was collected which included Galileo E1 B/C and E5 A/B, GIOVE-B E1 B/C and E5a, GPS L1 C/A and L5, and GLONASS L1 C/A. Following some last minute modifications to GSNRx to include the Galileo E5b signals, the samples were processed, simultaneously tracking GPS and Galileo on both the L1/E1 and L5/E5 frequencies and GLONASS on L1. A screenshot of the receiver in operation is shown in Figure 2.
Figure 2 Screenshot of GSNRx while processing the Galileo PFM signals
The versatility of GSNRx had been exploited in the past when new signals were brought online. In particular, the modular design adapted for PLAN’s software receiver had been utilized to quickly add new signals and new signal processing techniques. Once again this flexibility was drawn upon to facilitate the last-minute addition of the E5b I/Q signals (that very night) and to enable the stand-alone tracking of each signal component. By the same means, of course, this structure could be easily manipulated to enable composite tracking of data/pilot signal pairs or even facilitate vector tracking of all signals in view.
A subset of the raw correlator values for the E1 B, E1 C, E5a I and E5a Q signals are shown in Figure 3, (note that the E1 C values have been offset by -2.0×105 for clarity). A data-rate of 250 symbol/s is clearly visible on the E1 B and E5b signals while a 50 symbol/s stream can be observed on the E5a I signal. The 25 chip secondary code is also evident on E1 C at a rate of 250 chip/s.
Figure 3 Raw Correlator Values for the E1 B/C, E5aI/Q and E5bI/Q signals. The bit periods can be clearly seen on E1B, E5aI and E5bI. The secondary code can be observed on E1C while the pilot signal can be seen on singals E5aQ and E5bQ.
All six components of the Galileo-PFM signals shown above (transmitted on PRN 11) were tracked independently and their signal modulations were found to agree with the Galileo Open Service ICD [2]. A trace of the measured carrier-to-noise floor ratios for the Galileo signals is shown in Figure 4. As indicated by the ICD, the E5b signals were observed at 2 dB lower power than the E1 B and C signals. The E5a signals, however, were expected to be received at the same power as E5b and yet were observed at approximately 4 dB lower power. This is believed to be a combination of the antenna and IF filtering within the front-end as the E5a center frequency is located relatively near the pass-band edge of both. This front-end was initially designed for 40 MHz bandwidth, but used in this experiment at 50 MHz, as will be discussed later.
Figure 4 Measured C/N0 for Galileo-PFM Signals
The software receiver was once again reconfigured, this time to produce signal correlator values spaced along a delay of approximately 700 m and 70 m for the E1 A/B and E5 A/B signals, respectively, such that the cross-correlation of the received and local-replica PRN sequences could be examined. The signals were tracked for 10 seconds and the 1 ms correlator values averaged, to produce estimates of the code cross-correlation function. The characteristic ripple of the CBOC modulation on E1 B/C can be seen in Figure 5 (left), particularly on the right-most ascending feature of the envelope. Likewise, the alt-BOC cross-correlation of E5a Q in Figure 5 (right) is as expected. It is noted that the E5a I signal has suffered some distortion due to the filtering effects mentioned above.
Figure 5 Measured cross-correlation functions for the Galileo PFM E1 B and C signals (left) and E5a I and E5b I signals (right).
The PLAN group’s front-end is a highly flexible GNSS signal capture tool ideally suited for use with the GSNRx software receiver. The front-end, photographed in Figure 6, allows software reconfiguration of oscillator source (onboard, or external), antenna bias voltage, sampling rate, and IF bandwidth in addition to other low level control options making it highly adaptable. Furthermore, the center frequency, and filter bandwidth of each of the two hardware channels is independently configurable between 1150 – 2000 MHz, and between 4—40 MHz bandwidth (single sided) respectively.
Figure 6: PLAN group two-channel reconfigurable front-end with main system blocks labeled. The external clock and GNSS antenna SMA connectors are along the right edge, while the data interface is via mini-USB on the opposite side of the front-end.
Typically the front-end is configured to collect dual bands of 40 MHz two-sided bandwidth in order to cover the L1 and L2 transmission bands of both GPS and GLONASS as is shown in the right and central blocks within Figure 7. To allow the capture of E5a/E5b, the front-end configuration software was used to move the center frequency of channel B from 1237 MHz to 1192 MHz, the bandwidth of channel B from 33 MHz to 50 MHz, and to increase the sampling rate of both channels from 40 to 50 Ms/s.
Figure 7: Front-end channel A and channel B typically configured to capture GPS and GLONASS L1+L2, but reconfigured here to allow capture of Galileo IOV E5a+E5b signal in lieu of L2 band.
While each of the E5a and E5b signals have main lobe widths of 20.46 MHz (two sided), the composite E5 signal covers 50 MHz of spectrum, overlaying both the current GPS L5 signal at 1176, and the future GLONASS L3 signal near 1207 MHz. In order to demonstrate the capabilities of the GSNRx software receiver as an L5/E5 + L1/E1 system, it was desirable to capture the new IOV signals in their entirety.
The Galileo PFM satellite was observed from the Calgary Laboratory on the E1 link since the 12th of December at approximately 08:00 hrs and on the E5 link since the 14th of December at approximately 18:00 hrs. The last successful acquisition of the satellite on either E1 or E5 was at 03:20 hrs on the 15th of December and indicated a Doppler of approximately +2.3 kHz at E1. This figure is compatible with a reported elevation of approximately 40 degrees and rising, as reported by a number of software packages operating on a TLE [3]. Researchers recorded IF data once again at 03:55 on the 15th of December but failed to acquire any of the Galileo-PFM signals, suggesting the satellite may temporarily have ceased transmission.
References
Petovello, M. G., and C. O’Driscoll, G. Lachapelle, D. Borio and H. Murtaza (2008), “Architecture and Benefits of an Advanced GNSS Software Receiver,” Journal of Global Positioning Systems, vol. 7, no. 2, pp. 156-168.
Galileo Project Office. Galileo OS SIS ICD. http://ec.europa.eu/…/galileo/files/galileo-os-sis-icd-issue1-revision1_en [Accessed: 15 December 2011].
NORAD Two-Line Element Sets. http://celestrak.com/NORAD/elements/, [Accessed: 15 December 2011].
ITT Exelis and Chronos Technology Ltd. have agreed to jointly pursue and develop product offerings for the GNSS interference, detection and mitigation (IDM) market.
Satellite-based positioning, navigation and timing (PNT) systems are vulnerable to many factors, such as signals jamming, resulting in potentially devastating system failures. The collaboration between ITT Exelis and Chronos Technology will allow both companies to respond to the IDM market by offering a set of complementary products and solutions.
“The IDM threat is real and the risks are increasing,” said Charles Curry, founder and managing director, Chronos Technology Ltd. “ITT Exelis has recognized the technological innovation driven by the GAARDIAN research project into GPS jamming and interference detection, and will bring cutting-edge innovations to enhance the GAARDIAN platform.”
GAARDIAN has largely concluded its three-year run to deliver prototype sensors and probes to detect interference and give alarms, as well as detailed analyses of the GNSS environment.
The British, European, U.S., and global economies are vulnerable, by their dependence on GPS/GNSS, to interruption of the energy supply, breakdown of communications, transport, and financial services, and potential loss of life — all with no operational monitoring, detection, recourse, or back-up, prior to GAARDIAN and SENTINEL.
The follow-on SENTINEL is mid-way through its two-year life to take the next requisite steps:
Actually locating the interference;
categorizing it;
determining its extent;
giving a determination of trust in GNSS,
and addressing spoofing.
The project has a large user base in law enforcement and government.
For more than 37 years, ITT Exelis payloads and payload components have been on board every GPS satellite and have accumulated in excess of 500 years of on-orbit life without a single mission-related failure due to ITT Exelis equipment.
ITT Exelis Geospatial Systems, headquartered in Rochester, N.Y., is a global supplier of innovative night vision, remote sensing and navigation solutions that provide sight and situational awareness at the space, airborne, ground and soldier levels. Key applications include image intensification and thermal imaging; advanced power supplies; multi-spectral image systems; weather and climate monitoring; space science; intelligence, surveillance and reconnaissance; GPS-based positioning, navigation and timing systems; and image exploitation software.
Chronos Technology Limited is a world leader in timing synchronization solutions and GNSS jamming and interference detection, and is currently the lead for the UK Government sponsored SENTINEL research program, which followed on from the GAARDIAN GNSS interference detection project to research the location of GNSS jammers. Established in 1986, Chronos is a leading provider of technical solutions including time and timing for wireline and wireless telecom operators; highly versatile telecoms sync testing and monitoring systems and quality of service applications. Chronos also supplies GNSS (GPS) products from receivers for all application types including covert tracking, avionics and embedded systems, to test equipment (simulators) and GNSS infrastructure (antennas, splitters, repeaters) for the distribution of GNSS RF signals into sensitive environments. Chronos has developed a range of bespoke GPS timing products for time and frequency synchronization in power and communication systems.
The Galileo PFM IOV satellite (GSAT0101) began transmitting E5 signals early on December 14. It had already started airing E1 signals on December 10. Several COoperative Network for GIOVE Observation (CONGO) stations, including one at the University of New Brunswick, are now tracking both the E1 and E5 signals.
Meanwhile, the European Space Agency (ESA) has released a statement on the start of IOV satellite transmissions, titled "Galileo in tune: first navigation signal transmitted to Earth":
"Europe’s Galileo system has passed its latest milestone, transmitting its very first test navigation signal back to Earth.
"The first two Galileo satellites were launched into orbit on 21 October. Since then their systems have been activated and the satellites placed into their final orbits, positioned so that their navigation antennas are aligned with the world they are designed to serve.
"Last weekend marked the first orbital transmission from one of these navigation antennas. The stage was set, the singer in place and an audience – in the shape of engineers on the ground – was waiting eagerly.
"The question was would the singer make music, and if so, would it be in tune?
"The turn of Galileo’s main ‘L-band’ (1200-1600 MHz) antenna came on the early morning of Saturday 10 December. A test signal was transmitted by the first Galileo satellite in the ‘E1’ band, which will be used for Galileo’s Open Service once the system begins operating in 2014.
"To prepare for the test, the payload power amplifiers were switched on and ‘outgassed’ – warmed up to release vapours that might otherwise interfere with operations – before transmission began.
"The signal power and shape was well within specifications. The shape is especially important because its modulation is carefully designed to enable interoperability with the ‘L1’ band of US GPS navigation satellites: Galileo and GPS can indeed work together as planned.
"The test campaign is concentrating on the first satellite for the reminder of the year, with the focus moving to the second Galileo satellite from the start of 2012. The plan is to complete In-Orbit Testing by next spring.
"The next pair of Galileo In-Orbit Validation satellites will also be launched next year, to form the operational nucleus of the full Galileo constellation. Meanwhile the next batch of Galileo satellites are currently being manufactured for launch in 2014."
On Saturday, December 10, at about 06:00 UTC, one of the two Galileo In-Orbit Validation (IOV) satellites launched on October 21 started transmitting navigation signals on the L1/E1 frequency using the E11 ranging code.
According to prediction visibilities based on NORAD/JSpOC tracking information, the transmitting satellite is PFM, the ProtoFlight Model (GSAT0101). The FM2 (Flight Model 2) satellite (GSAT0102) has not yet started transmitting navigation signals.
Stations of the COoperative Network for GIOVE Observation (CONGO) were among the first to track the satellite. Results have also been reported by Thales Avionics, JAVAD GNSS, Politecnico di Torino's NavSAS group, and Thales Alenia Space.
The following figure shows C/N0 values in dB-Hz of PFM 1-Hz data collected at the University of New Brunswick CONGO station on December 10. Time axis runs for 24 hours starting at 01:00 UTC. Receiver is a Javad Delta-G2T.
"An important point is that we tracked it with our units that are already in the market," said Javad Ashjaee, CEO. "This is not a lab tests. Our customers can track it too."
Here are the company's tracking results of PRN-11 for now, plots of pseudorange (in chips), doppler (in Hz), and SNR (relative number):
JAVAD GNSS expects to publish additional results soon.
Following the recent launch of two Galileo in-orbit validation satellites, Thales Avionics of Valence, France, has successfully acquired and tracked the new L1 Open Service signal transmitted by one of the space vehicles (PRN 11) on Monday, December 12, at 13:30 (GMT). Thales Avionics has developed a Galileo receiver capable of processing the Open Service, Commercial Service, and Safety of Life service of the Galileo constellation.
Figure 1 shows a screenshot of the receiver interface program highlighting the L1 signal energy (top right) and the pilot secondary code (bottom).
Figure 1: Real-time measurements.
The satellite Doppler and C/N0 values have been recorded and are provided below.
The raw navigation message has been decoded. It contains INAV type 0 and INAV dummy data as shown in the next figure. These messages enable Galileo system time transfer.
The signal modulation and characteristics show no discrepancy relative to the Galileo Open Service ICD released last year.
The fact that only L1 frequency is broadcast for the moment prevents providsion of further results based on dual-frequency measurements.
Thales has developed a coherent processing of the Galileo E5 AltBOC(15,10) signal compatible with hardware architecture designed for independent processing of both E5a and E5b. This processing is fully compatible with the mismatch between the two RF channels on E5a and E5b, thanks to real-time calibration based on satellite signals. This processing only requires software implementation, without additional recurrent costs. The technique is relevant for future receivers operating in the E5 band, in order to significantly enhance the accuracy, with respect to thermal noise and multi-path, and to improve the cycle slip probability.
Thales Avionics, involved for many years in GNSS receivers design and production, has developed a Galileo receiver capable of processing the Open Service, Commercial Service, and Safety of Life service of the Galileo constellation. This high-end receiver includes patented state of the art algorithms capable of processing up to four different frequencies.
The Beidou-2/Compass IGSO-5 (fifth inclined geosynchonous orbit) satellite was launched on December 1 from Xichang, China. Exact launch time was 21:07:04.189 UTC. The third stage of the CZ-3A rocket with the satellite attached achieved a geosynchronous transfer orbit and the satellite subsequently separated according to NORAD/JSpOC. As of December 7, the satellite is still in geosynchronous transfer orbit (GTO), orbiting the Earth about twice a day with a highly eliptic orbit. To get to geosynchronous orbit, the satellite's apogee kick motor will have to be fired. The satellite is not drifting to its intended orbit, for example, like a GLONASS satellite might.
According to an announcement on the official government Beidou/Compass website, this launch completes the construction of the basic regional navigation system for service to China and will be operational by the end of the year. However, completion of the Phase II development, to provide service to the Asia/Pacific region, will require further satellite launches in 2012. Phase III global coverage, with a 30-satellite system, will be achieved by 2020 according to the website.
The GNSS community outside China still awaits a Compass interface control document (ICD), which has been promised by the end of 2012.
By Chris Rizos, Co-chair, Steering Committee of Multi-GNSS Asia
A dramatic increase over the next five years to roughly 100 GNSS satellites in the skies over Asia and Oceania makes that region the fastest growing area in GNSS. The Multi-GNSS Asia (MGA) initiative, a cooperative international demonstration campaign, seeks to take full advantage of this scientific and technical windfall, gaining early experience with the new signals and services of multi-constellation GNSS.
The MGA organization is sponsored by Japan’s Aerospace Exploration Agency (JAXA), and seeks to promote the region as the “showcase of the new GNSS era” through this demonstration campaign. See an animation of the burgeoning satellite availability over the coming decade. The MGA demonstration campaign consists of a series of activities from 2010 to 2015.
Infrastructure Deployment. JAXA is currently deploying a multi-GNSS monitoring (MGM) network, consisting of continuously operating reference stations equipped with multi-GNSS receivers, that will support the production of precise orbit and satellite clock offset information for the multiple constellations. The MGM-net will be deployed in two stages. The first 20 receivers supplied by JAXA will go to hosting countries and organizations in the Asia-Oceania region by early 2012, with an additional 40 available for deployment globally in 2013. The MGM-net is a component of the tracking network of the International GNSS Service (IGS) global multi-GNSS experiment (M-GEX, igs.org). Both MGM-net and M-GEX will include data and analysis centers and faciltate the sharing of information and resources among participating organizations. The multi-GNSS tracking data will be available to everyone in the form of RINEX files.
Projects. Joint experiments involving new or extended multi-GNSS applications, such as disaster management, intelligent transportation systems, precise positioning, and location-based services will be promoted among GNSS providers, receiver manufacturers, local service providers, government organizations, and universities in the Asia-Oceania region. Some of these will take advantage of the special characteristics of the first QZSS satellite, Michibiki. Several project proposals have been submitted over the last year; one of particular interest is a call by JAXA for a “Multi-GNSS Joint Experiment.” Such experiments could include using the broadcast augmentation message known as the L-band Experimental (LEX) signal, modulated on the L6/E6 frequency at 1278.75MHz, to support precise positioning. China has recently proposed a “BeiDou Application Demonstration & Experience Campaign” (BADEC) as an MGA project activity.
Regional Workshops. An important MGA activity is the organization of an annual workshop to report on joint experiments and results and to promote new joint projects. The First Asia Oceania Regional Workshop on GNSS (AORWG), held in January 2010 in Bangkok, Thailand, drew 195 participants from 18 countries.
The second AORWG took place that November in Melbourne, Australia, and drew 101 participants from 11 countries.
The most recent AORWG was held November 1–3 this year on Jeju Island, South Korea, attracting 86 participants. Five demonstration projects were proposed by researchers from Japan, South Korea, Taiwan, Australia, and Malaysia, and were all endorsed by the MGA Steering Committee.
They are:
Evaluation of Multi-GNSS for Precision Agriculture in Korea; Chungnam National University, Korea
Sustainable Resource Utilization by Precision Farming of Oil Palm Plantation; RTK-Auto Guided Oil Palm Planter; On-the-Go Soil ECa Mapping; University Putra Malaysia , Malaysia
Automated rice transplanter guided by using Multi-GNSS including QZSS; Agricultural Research Center, National Agriculture Research Organization , JAPAN
Joint QZSS/GPS positioning using L1/L5 band signals; National Cheng Kung University, Taiwan
Multi-GNSS Experiment; Royal Melbourne Institute of Technology University, Australia.
The status of the MGM-net deployment and the results of the demonstration projects will be presented at the fourth AORWG scheduled for November 2012 in Kuala Lumpur, Malaysia.
China’s BADEC. At the third AORGW, Dr. X. R. Dong, an expert from the International Cooperation Research Center of China Satellite Navigation Office (CSNO), introduced plans for several long-term project activities under the banner of the BeiDou/GNSS Application Demonstration & Experience Campaign (BADEC). This is another important proposal from China, following the International GNSS Monitoring and Assessment Service (iGMAS) that has drawn attention and support from GNSS providers, users, and international organizations.
A subgroup dealing with iGMAS is approved and setup by ICG-6; the sub-group is co-chaired by Dr. X. R. Dong, IGS and Satoshi Kogure from Japan. Besides continuing to advocate for iGMAS, the goals of BADEC include seeking to make the Asia-Oceania region a showcase of the new GNSS era, and including BeiDou-specific goals such as “welcome the introduction and utilization of BeiDou services,” “let users experience the Multi-GNSS including BeiDou,” and “encourage GNSS provider and users to carry out experiment and demonstration jointly.”
Both IGMAS and BADEC will contribute to promote the GNSS open-service performance, compatibility, and interoperability, to be implemented through extensive international cooperation, especially with the IGS’s M-GEX and MGM-net.
Chris Rizos is professor and head of the School of Surveying & Spatial Information Systems, University of New South Wales, Sydney, Australia. He is president of the International Association of Geodesy, serving from now into 2015.
Disaster-preparedness plans recognize the individual’s role in his or her own survival. When storms approach, have water, food, and basic survival gear on hand. It takes time for help to arrive.
The civil GPS industry faces an oncoming storm of interference, and the receiver is the first line of defense. As we integrate GPS into all facets of our lives and infrastructure, we become more subject to disruptions, both unintentional and intentional. Newark International Airport now sees several jamming events per day. In Taiwan, one airport experiences an average of 117 events per day!
How can civil PNT infrastructure be made more resilient?
Faced with jamming, spoofing, and cyber attacks, receivers must take basic precautionary measures. They must recognize jamming and spoofing attacks to avoid generating hazardously misleading outputs. Situational awareness is key. Accurate and specific alarms must be generated so users can take action and authorities can be notified. Regular threat-signature updates can improve situational awareness, much like antivirus updates on a computer. Fire alarms don’t put out fires but they do save lives and improve response time.
Twenty years ago, computers rarely had firewall or antivirus protection. As GPS becomes more deeply integrated into communications-enabled systems, its utility increases exponentially but so does its vulnerability to cyber attack. When you update your GPS software or your maps, how do you know they have not been compromised? How do you know your receiver is authentic?
Figure 1. There are demonstrated, well known attacks that can cause receivers to output misleading information without warning. Many of these attacks can be detected using simple methods. Some receivers incorporate detection and countermeasures techniques. Many don’t. Receiver certification provides GPS buyers with a starting point for selecting GPS receivers. Certified receivers can accurately report on interference so it can be located and stopped.
The U.S. Navy recently discovered counterfeit routers in several of their installations. Well-developed computer security methods such as the Trusted Platform Module found in more than 300 million computers can help secure GPS receivers without impeding innovation.
The government can also play a role in improving receivers by providing an authenticatable civil signal structure. Well-documented Public Key Infrastructure methods such as digital signing and occasional, short-spread spectrum security-code bursts can be added to the new L1C signal. Receivers voluntarily using these signal features can establish signal provenance with extremely high confidence.
The public, unclassified keys needed to process these features could be sold and used as a revenue source for the GPS system. Receivers that choose not to use these features can ignore them without adverse impact other than weaker security. The large numbers of in-theater military users who rely on civil signals would also stand to benefit.
Finally, I would note that situationally aware receivers can provide specific and detailed reports about what they see. Interference-monitoring systems such as Patriot Watch will need detailed reports to sort and associate the multitude of reports they receive into a coherent picture of what is actually happening. To provide adequate geographic coverage, interference monitoring systems will need to accept reports from diverse receiver types on an opportunistic basis. In short, they will have to rely on crowdsourcing as a major operational input.
As Brad Parkinson noted during my presentation of this material to the November 9 meeting of the National PNT Executive Committee Advisory Board (“Receiver Certification: Making the GNSS Environment Hostile to Jammers and Spoofers,” at www.pnt.gov/advisory/2011/11/), in the early days of electricity, a lot of houses burned down because of electrical problems. Underwriters Laboratories helped immensely by testing electrical equipment to make sure it was reasonably safe, and consumers looked for the UL label. A voluntary, basic receiver certification process similar to Underwriters Laboratories should be pursued to provide the user community with a basis for selecting receivers.
Logan Scott has more than 32 years of military and civil GPS systems engineering experience. At Texas Instruments, he pioneered approaches for building high-performance, jamming-resistant digital receivers. While at Omnipoint, a cellular carrier, he developed cross-system interference mitigation strategies. He holds 33 U.S. patents.
By Saeed Daneshmand, Ali Jafarnia-Jahromi, Ali Broumandan, and Gérard Lachapelle
Most anti-spoofing techniques are computationally complicated or limited to a specific spoofing scenario. A new approach uses a two-antenna array to steer a null toward the direction of the spoofing signals, taking advantage of the spatial filtering and the periodicity of the authentic and spoofing signals. It requires neither antenna-array calibration nor a spoofing detection block, and can be employed as an inline anti-spoofing module at the input of conventional GPS receivers.
GNSS signals are highly vulnerable to in-band interference such as jamming and spoofing. Spoofing is an intentional interfering signal that aims to coerce GNSS receivers into generating false position/navigation solutions. A spoofing attack is, potentially, significantly more hazardous than jamming since the target receiver is not aware of this threat. In recent years, implementation of software receiver-based spoofers has become feasible due to rapid advances with software-defined radio (SDR) technology. Therefore, spoofing countermeasures have attracted significant interest in the GNSS community.
Most of the recently proposed anti-spoofing techniques focus on spoofing detection rather than on spoofing mitigation. Furthermore, most of these techniques are either restricted to specific spoofing scenarios or impose high computational complexity on receiver operation.
Due to the logistical limitations, spoofing transmitters often transmit several pseudorandom noise codes (PRNs) from the same antenna, while the authentic PRNs are transmitted from different satellites from different directions. This scenario is shown in Figure 1. In addition, to provide an effective spoofing attack, the individual spoofing PRNs should be as powerful as their authentic peers. Therefore, overall spatial energy of the spoofing signals, which is coming from one direction, is higher than other incident signals. Based on this common feature of the spoofing signals, we propose an effective null-steering approach to set up a countermeasure against spoofing attacks. This method employs a low-complexity processing technique to simultaneously de-spread the different incident signals and extract their spatial energy. Afterwards, a null is steered toward the direction where signals with the highest amount of energy impinge on the double-antenna array. One of the benefits of this method is that it does not require array calibration or the knowledge of the array configuration, which are the main limitations of antenna-array processing techniques.
Processing Method
The block diagram of the proposed method is shown in Figure 2. Without loss of generality, assume that s(t) is the received spoofing signal at the first antenna.
Figure 2. Operational block diagram of proposed technique.
The impinging signal at the second antenna can be modeled by , where θs and μ signify the spatial phase and gain difference between the two channels, respectively. As mentioned before, the spoofer transmits several PRNs from the same direction while the authentic signals are transmitted from different directions. Therefore, θs is the same for all the spoofing signals. However, the incident authentic signals impose different spatial phase differences. In other words, the dominant spatial energy is coming from the spoofing direction. Thus, by multiplying the conjugate of the first channel signals to that of the second channel and then applying a summation over N samples, θs can be estimated as (1)
where r1 and r2 are the complex baseband models of the received signals at the first and the second channels respectively, and Ts is the sampling duration. In (1), θs can be estimated considering the fact that the authentic terms are summed up non-constructively while the spoofing terms are combined constructively, and all other crosscorrelation and noise terms are significantly reduced after filtering. For estimating μ, the signal of each channel is multiplied by its conjugate in the next epoch to prevent noise amplification. It can easily be shown that μ can be estimated as (2)
where T is the pseudorandom code period. Having and a proper gain can be applied to the second channel in order to mitigate the spoofing signals by adding them destructively as (3)
Analyses and Simulation Results
We have carried out simulations for the case of 10 authentic and 10 spoofing GPS signals being transmitted at the same time. The authentic sources were randomly distributed over different azimuth and elevation angles, while all spoofing signals were transmitted from the same direction at azimuth and elevation of 45 degrees. A random code delay and Doppler frequency shift were assigned to each PRN. The average power of the authentic and the spoofing PRNs were –158.5 dBW and –156.5 dBW, respectively.
Figure 3 shows the 3D beam pattern generated by the proposed spoofing mitigation technique. The green lines show the authentic signals coming from different directions, and the red line represents the spoofing signals. As shown, the beam pattern’s null is steered toward the spoofing direction.
Figure 3. Null steering toward the spoofer signals.
In Figure 4, the array gain of the previous simulation has been plotted versus the azimuth and elevation angles. Note that the double-antenna anti-spoofing technique significantly attenuates the spoofer signals. This attenuation is about 11 dB in this case. Hence, after mitigation, the average injected spoofing power is reduced to –167.5 dBW for each PRN. As shown in Figure 4, the double-antenna process has an inherent array gain that can amplify the authentic signals. However, due to the presence of the cone of ambiguity in the two-antenna array beam pattern, the power of some authentic satellites that are located in the attenuation cone might be also reduced.
Figure 4. Array gain with respect to azimuth and elevation.
Monte Carlo simulations were then performed over 1,000 runs for different spoofing power levels. The transmitted direction, the code delay, and the Doppler frequency shift of the spoofing and authentic signals were changed during each run of the simulation. Figure 5 shows the average signal to interference-plus-noise ratio (SINR) of the authentic and the spoofing signals as a function of the average input spoofing power for both the single antenna and the proposed double antenna processes. A typical detection SINR threshold corresponding to PFA=10-3 also has been shown in this figure.
Figure 5. Authentic and spoofed SINR variations as a function of average spoofing power.
In the case of the single antenna receiver, the SINR of the authentic signals decreases as the input spoofing power increases. This is because of the receiver noise-floor increase due to the cross-correlation terms caused by the higher power spoofing signals. However, the average SINR of the spoofing signals increases as the power of the spoofing PRNs increase.
For example, when the average input spoofing power is –150 dBW, the authentic SINR for the single-antenna process is under the detection threshold, while the SINR of the spoofing signal is above this threshold. However, by considering the proposed beamforming method, as the spoofing power increases, the SINR of the authentic signal almost remains constant, while the spoofing SINR is always far below the detection threshold.
Hence, the proposed null-steering method not only attenuates the spoofing signals but also significantly reduces the spoofing cross-correlation terms that increase the receiver noise floor. Early real-data processing verifies the theoretical findings and shows that the proposed method indeed is applicable to real-world spoofing scenarios.
Conclusions
The method proposed herein is implemented before the despreading process; hence, it significantly decreases the computational complexity of the receiver process. Furthermore, the method does not require array calibration, which is the common burden with array-processing techniques.
These features make it suitable for real-time applications and, thus, it can be either employed as a pre-processing unit for conventional GPS receivers or easily integrated into next-generation GPS receivers. Considering the initial experimental results, the required antenna spacing for a proper anti-spoofing scenario is about a half carrier wavelength. Hence, the proposed anti-spoofing method can be integrated into handheld devices.
The proposed technique can also be easily extended to other GNSS signal structures. Further analyses and tests in different real-world scenarios are ongoing to further assess the effectiveness of the method.
Saeed Daneshmand is a Ph.D. student in the Position, Location, and Navigation (PLAN) group in the Department of Geomatics Engineering at the University of Calgary. His research focuses on GNSS interference and multipath mitigation using array processing.
Ali Jafarnia-Jahromi is a Ph.D. student in the PLAN group at the University of Calgary. His research focuses on GNSS spoofing detection and mitigation techniques.
Ali Broumandan received his Ph.D. degree from Department of Geomatics Engineering, University of Calgary, Canada. He is a senior research associate/post-doctoral fellow in the PLAN group at the University.
Gérard Lachapelle holds a Canada Research Chair in wireless location In the Department of Geomatics Engineering at the University of Calgary in Alberta, Canada, and is a member of GPS World’s Editorial Advisory Board.
Thanks to Richard Langley for the constellation update in November GPS World, from ION-GNSS. I’m a GPS constellation junkie, and if there was a history of each GPS space vehicle on orbit, I’d read them all. I love hearing the operational tidbits, about a IIF having problems with its cesium clock, or a reaction wheel failing, or how many spare SVs are on hand, and if SVs are slated to be disposed of, and so on. I’ve never been able to find a good centralized source of that type of information, as it seems to be something that just kind of leaks out into the industry press, from uncited sources. I’d been waiting for an update to The Almanac but it’s a moving target, so I understand why you don’t rush to update it every time a new SV is launched, or an SV’s clock changes. Especially with the increase in GNSS launches.
So thanks for those new updates, and passing them along as they happen.
A second thing, just kind of my musing of the state of the GNSS constellations, and how the U.S. GPS system is so much different than the others: The cost of reliability.
With continued launches by Russia, the GLONASS system has, for all practical purposes, reached a fully operational status with 27 satellites set healthy, being commissioned or in flight tests. They are definitely putting far more SVs into orbit faster than the GPS program ever has. Over the years, they’ve put up so many satellites that they have three times as many disposed satellites (90) as they have operational (27) satellites.
Compass has launched 13 satellites; at least eight are known to be usable.
In the GPS constellation, there are still more SVs active on orbit than have been disposed of, in the entire history of the GPS program. Think about that for a minute.
30 active satellites on orbit, and in the entire 40-year history of the program, only 29 have been disposed of. This is a testament to both the forward-thinking design of the GPS system by its many architects, contractors, and builders of the SVs and their payloads. And of course the Air Force that manages the constellation. The GPS system sets the standard for all other GNSS systems. It is not only the most accurate and dependable GNSS system in the world, it is also the most obsolete, in terms of age of spacecraft on orbit.
The user segment enjoys reliability, at the expense of new features. Because the Block II and II-A satellites exceeded their design life, and now the last of the II-R satellites are reaching their design life, we don’t have all of the signals we could have from a more modernized constellation. Non-professionals like myself don’t have an operational L2C, for ionospheric correction in consumer-level devices. (Waiting on OCX.) We don’t have operational L5 (Waiting on OCX, again.) And what about all of those inter-satellite links for ranging that the IIR, IIR-M, and the IIF (and IIA as well?) satellites have? Are those waiting on OCX too?
Originally, the IIF satellites were supposed to number 51. Then it was reduced to 33. Then 15. Now 12. 12 isn’t even enough to replace the entire remaining IIA fleet, while maintaining the current level of active SVs. Of course, it doesn’t make any sense to launch lots of IIF birds when GPS III is out there on the horizon, only three short years — we hope — away.
If the II, IIA, IIR, and IIR-M GPS spacecraft would have had lifetimes similar to GLONASS satellites, the whole constellation would have either fallen into disrepair, or, more likely, been upgraded to IIF satellites a decade ago. And we’d have all of the modern signals that we could ever hope to need. Civilians have the same signals that we’ve had since the beginning of the GPS program. We could have had new signals years ago. but the old birds keep on flying, and so far, we only have two IIF satellites in orbit.
— Jerry Pasker
Monticello, Iowa
Occupy GPS
It occurred to me recently that maybe all these people all over the country are protesting the fact that 1 percent of the world’s GNSS receivers control 99 percent of the attention.
While 99 percent of receivers actually outperform that select 1 percent in most metrics — time to fix, accuracy in cities, power consumption, sensitivity, dynamic range, jam immunity, and so on — because they live and work in cell-phones and tablets, they are poorly compensated and don’t always get the respect of their better-dressed cousins.
