WhiteStar Corp. announced it’s added a new layer of oil and gas pipeline data to its Unlimited Basemap Access (UBA) product. The new WhiteStar Oil & Gas Pipeline Layer will be a nationwide, georeferenced shapefile showing the locations of all lateral and transmission pipelines in the United States.
The Company said existing subscribers to the WhiteStar UBA product will begin receiving segments of the oil and gas layer at no extra charge with their regular third-quarter UBA update in October. The first segment of the layer will include pipelines in Texas, Oklahoma and the Gulf of Mexico. The layer includes attribute information, such as owner and operator data, for each pipeline.
WhiteStar said they are creating the new UBA layer primarily from a U.S. Department of Energy (DOE) pipeline map that is available in PDF format on the DOE Energy Information Administration’s website (www.eia.doe.gov). A rich source of pipeline information, this map has frustrated hydrocarbon companies for years because it can be downloaded only in a non-GIS compatible PDF format.
“We’ve converted the PDF to a shapefile and georeferenced it to align with all of the other cultural-feature layers in the UBA product, which is fully GIS compatible,” said WhiteStar President and CEO Robert White. “This new layer allows UBA clients to easily integrate pipeline maps and attribute details into their digital mapping projects.”
According to the company, the UBA product is a seamless nationwide digital mosaic of basemap information layers from U.S. Census Bureau TIGER Files (with optional TeleAtlas upgrades). Designed for any geospatial mapping project that requires an accurate digital base map, UBA contains 42 layers of cultural features – such as political boundaries, roads, water bodies, and environmentally sensitive areas – that can be ‘cookie cut’ according to a user-selected area of interest and downloaded into most popular digital mapping package.
WhiteStar said they developed UBA with an interface that lets the user select layers with a few mouse clicks and then delineate the area of interest by choosing a specific county, outlining the project area onscreen or entering its latitude/longitude corner points. UBA users can then export the data into a variety of popular mapping formats, including ESRI, MapInfo, GeoGraphix, Petra, AutoCAD, SMT Kingdom and Golden software. In addition, the data can be projected in either NAD27 or NAD83 coordinate systems, including all related state planes and UTM zones.
“Our clients use UBA to populate their maps with cultural features for investor presentations, exploration & production logistics planning, infrastructure siting, and permit submissions,” said White. “The new pipeline layer will enable operators to quickly determine which lateral and transmission lines run near their leases.”
WhiteStar said they will roll out regional segments of the UBA Oil & Gas Pipeline Layer until the seamless nationwide data set is completed. Following delivery of the Texas, Oklahoma, and Gulf Coast segment, WhiteStar will deliver the region of Ohio, Pennsylvania, and West Virginia that is producing from the Marcellus Shale formation. UBA clients can expect that one to ship in early 2010.
Chronos Technology is introducing the academic and business research consortium working on the GAARDIAN Project at its booth (#728) at the ION GNSS conference taking place this week in Savannah, Georgia. Chronos is leading the consortium, which over the course of 2009 – 2011 will be researching the data-gathering necessary to develop a system for mission and safety critical applications that will certify the accuracy, reliability, integrity, and continuity of Positioning, Navigation and Timing (PNT) systems: GPS, the new enhanced LORAN (eLORAN), GALILEO and GLONASS.
GAARDIAN is the acronym for “GNSS Availability, Accuracy, Reliability anD Integrity Assessment for Timing and Navigation” and the Consortium includes University of Bath, General Lighthouse Authorities, BT, Ordnance Survey, National Physical Laboratory, and Imperial College London.
The project will create a mesh of remote PNT (Positioning, Navigation & Timing) interference detection & mitigation sensors (IDMs) which will be deployed in the vicinity of PNT dependent infrastructure & applications. These probes will monitor the integrity, reliability, continuity and accuracy of the locally received GPS (or other GNSS) and eLoran signals on a 24×7 basis and report back to a central server. The user will be alerted in real time to any anomalous behavior in either of the two PNT signals.
IDM sensors, which can be configured by the user to be personalized to a specific deployed location, permanently monitor the PNT signals and on detection of an anomaly warn of a potentially critical situation.
Users access the data over the internet from a secure server environment, enabling continuous monitoring from any internet enabled terminal – effectively providing access to detailed knowledge about local PNT health and pinpointing interference phenomena from anywhere in the world.
Likely phenomena or threats to PNT services which would cause an alarm include jamming, general interference, multipath from local reflections, space environment or weather related events and satellite or transmitter malfunction.
Traditionally it has been very difficult to analyze the specific nature of interference to a PNT signal, when monitoring one signal alone, e.g. GPS. By using the technically dissimilar eLoran signal and continually analyzing key data, the integrity, reliability, continuity and accuracy of either signal can be recorded with high confidence.
Likely applications will include homeland security, transport users such as harbors, airports, roads and railways, emergency services, military, utilities, scientific community, telecom infrastructure and any safety or mission critical application leveraging PNT signals.
By Marcel Baracchi-Frei,
Grégoire Waelchli, Cyril Botteron,
and Pierre-André Farine
The idea of a software receiver is to replace the data processing implemented in hardware with software and to sample the analog input signal as close as possible to the antenna. Thus, the hardware is reduced to the minimum — antenna and analog-to-digital converters (ADCs) — while all the signal processing is done in software. As current mobile devices (such as personal digital assistants and smartphones) include more and more computing power and system features, it becomes possible to integrate a complete GNSS receiver with very few external components.
One advantage of a software receiver clearly lies in the low-cost opportunity, as the system resources such as the calculation power and system memory can be shared. Another advantage resides in the flexibility for adapting to new signals and frequencies. Indeed, an update can easily be performed by changing some parameters and algorithms in software, while it would require a new redevelopment for a standard hardware receiver.
Updating capabilities may become even more important in the future, as the world of satellite navigation is in complete effervescence: Europe is developing its own solution, Galileo, foreseen to be operational in 2013; China has undertaken a fundamental redevelopment of its current Compass navigation system; Russia is investing huge sums of money in GLONASS to bring it back to full operation; and the U.S. GPS system will see some fundamental improvements during the next few years, with new frequencies and new modulation techniques. At the same time, augmentation systems (either space-based or land-based) will develop all over the world.
These future developments will increase the number of accessible satellites available to every user — with the advantage of better coverage and higher accuracy. However, to take full advantage of the new satellite constellations and signals, new GNSS receivers and algorithms must be developed.
Definition and Types
The definition of a software receiver (SR) always brings some confusion among researchers and engineers in the field of communications and GNSS. For example, a receiver containing multiple hardware parts which can be reconfigured by setting a software flag or hardware pins of a chipset are regarded by some communication engineers to be a SR. In this article, however, we will consider the widely accepted SR definition in the field of GNSS; that is, a receiver in which all the baseband signal processing is performed in software by a programmable microprocessor.
Nowadays, software receivers can be grouped in three main categories:
field programmable gate arrays (FPGAs), which are sometimes also referred to the domain of SR. These receivers can be reconfigured in the field by software.
post-processing receivers include, among others, countless software tools or lines of code for testing new algorithms and for analyzing the GNSS signal, for example, to investigate GPS satellite failure or to decrypt unpublished codes.
real-time-capable software receivers group that will be further considered here.
A modern GNSS receiver normally contains a RF front-end, a signal acquisition, a tracking, and a navigation block. A hardware-based receiver accomplishes the residual carrier removal, PRN code-despreading, and integration at the system sampling rate. Until the late 1990s, due to the limited processing power of microprocessors, these signal functions could only be practically implemented in hardware.
The GNSS SR boom really started with the development of real-time processing capability. This was first accomplished on a digital signal processor (DSP) and later on a commercial conventional personal computer (PC). Today, DSPs are increasingly replaced by specialized processors for embedded applications.
Challenges
Data rate. The ideal software receiver would place the ADC as close as possible to the antenna to reduce hardware parts to a minimum. In that sense, the most straightforward approach consists of digitizing the data directly at the antenna, without pre-filtering or pre-processing. But as the Nyquist theorem must be fulfilled (that is, sampling with at least twice the highest signal frequency), this translates into a data rate that is, for the time being, too high to be processed by a microcontroller.
Considering the GPS L1 signal and assuming 1 quantization bit per sample, this leads to the following values:
In order to reduce the data throughput, a solution such as a low intermediate frequency (IF) or a sub-sampling analog front-end must be chosen. In a low IF front-end, the incoming signal is down-converted to a lower intermediate frequency of several megahertz. This allows working with a sampling (and data) rate that can be more easily handled by a microcontroller. With the new BOC signal modulations (used for the Galileo E1 and the modernized GPS L1 signals) that have no energy at and near DC, a zero-IF or homodyne architecture is also possible without SNR degredation due to DC offset, flicker noise, or even-order distortions.
The sub-sampling technique exploits the fact that the effective signal bandwidth in a GNSS signal is much lower than the carrier frequency. Therefore, not the carrier frequency but the signal bandwidth must be respected by the Nyquist theorem (assuming appropriate band-pass filtering). In this case, the modulated signal is under-sampled to achieve frequency translation via intentional aliasing. Again, if the GPS L1 signal is taken as an example with assuming 1 quantization bit per sample, this leads to the following values:
However, as the sub-sampling approach is still difficult to implement due to current hardware and resources limitations, a more classical solution based on an analog IF down-conversion is often chosen. That means that the signal is first down-converted to an intermediate frequency and afterwards digitized.
Baseband Processing. Considering an IF-based architecture, the ADC provides a data stream (real or complex), which is first shifted into baseband by at least one complex mixer. The signal is then multiplied with several code replicas (generally early, prompt, and late) and finally accumulated. Figure 1 shows an example of a real data IF architecture.
FIGURE 1. Real IF architecture
In hardware receivers, the local code and carrier are generally generated in real-time by means of a numerically controlled oscillator (NCO) that performs the role of a digital waveform generator by incrementing an accumulator by a per-sample phase increment. The resulting value is then converted to the corresponding amplitude value to recreate the waveform at any desired phase offset. The frequency resolution is typically in the range of a few millihertz with a 32-bit accumulator, and a sampling frequency in the range of a few megahertz.
Assuming that a look-up table (LUT) address can be obtained with two logical operations (one shift and one mask), and the corresponding LUT value reads with 1 memory access — which is quite optimistic — the amount of operations needed to generate the complex waveforms per channel is given in Table 1.
Source: Marcel Baracchi-Frei, Grégoire Waelchli, Cyril Botteron, and Pierre-André Farine
The real-time carrier generation is computationally expensive and is consequently not suitable for a one-to-one software implementation. Earlier studies [Heckler, 2004] demonstrated that, assuming that an integer operation and a multiplication take one and 14 CPU cycles, respectively (for an Intel Pentium 4 processor), the baseband operations (without carrier and code generation or navigation solution) would require at least a 3 GHz Intel Pentium 4 processor with 100 percent CPU load. Therefore, under these conditions, real-time operations are not suitable for embedded processors. Therefore standard hardware receiver architectures cannot be translated directly into software, and consequently new strategies must be developed to lower the processing load.
Status
A major problem with the software architecture is the important computing resources required for baseband processing, especially for the accumulation process. As a straightforward transposition of traditional hardware-based architectures into software would lead to an amount of operations which is not suitable for today’s fastest computers, two main alternate strategies have been proposed in the literature: the first relies on single-instruction multiple-data (SIMD) operations, which provide the capability of processing vectors of data. Since they operate on multiple integer values at the same time, SIMD can produce significant gains in execution speed for repetitive tasks such as baseband processing. However, SIMD operations are tied to specific processors and therefore severely limit the portability of the code.
The second alternative consists in the bitwise parallel operations (sometimes also referred to as vector processing in the literature), which exploit the native bitwise representation of the signal. The data bits are stored in separate vectors, one sign and one or several magnitude vectors, on which bitwise parallel operations can be performed. The objective is to take advantage of the universality, high parallelism, and speed of the bitwise operations for which a single integer operation is translated into a few simple parallel logical relations. While SIMD operations use advanced and specific optimization schemes, the latter methodology exploits universal CPU instructions set. The drawback of the bitwise operations is the different representation of the values. To be able to perform integer operations, a time consuming conversion is needed.
Single-Instruction Multiple-Data
In 1995, Intel introduced the first instance of SIMD under the name of Multi Media Extension (MMX). The SIMD are mathematical instructions that operate on vectors of data and perform integer arithmetic on eight 8-bit, four 16-bit, or two 32-bit integers packed into a MMX register (see Figure 2).
FIGURE 2. Single-instruction single-data versus single-instruction multiple-data.
On average, the SIMD operations take more clock cycles to execute than a traditional x86 operation. Anyhow, since they operate on multiple integers at the same time, MMX code can produce significant gains in execution speed for appropriately structured algorithms. Later SIMD extensions (SSE, SSE2, and SSE3) added eight 128-bit registers to the x86 instruction set. Additionally, SSE operations include SIMD floating point operations, and expand the type of integer operations available to the programmer.
SIMD operations are well suited to parallelize the operations of the baseband processing (BBP) stage. In particular, they can be used to allow the PRN code mixing and the accumulation to be performed concurrently for all the code replicas. With the help of further optimizations such as instruction pipelining, more than 600 percent performance improvement with the SIMD operations compared to the standard integer operations can be observed [Heckler, 2006].For this reason, most of the software receivers with real-time processing capabilities use SIMD operations [Heckler; Pany 2003; Charkhandeh, 2006 ].
Bitwise Operations. Bitwise operation (or vector processing) was first introduced into the SR domain in 2002 [Ledvina]. The method exploits the bit representation of the incoming signal, where the data bits are stored in separate vectors on which bitwise parallel operations can be performed. Figure 3 shows a typical data storage scheme for vector processing.
Source: Marcel Baracchi-Frei, Grégoire Waelchli, Cyril Botteron, and Pierre-André Farine
The sign information is stored in the sign word while the remaining bit(s) representing the magnitude is (are) stored in the magn word(s). The objective is to take advantage of the high parallelism and speed of the bitwise operations for which a single integer addition or multiplication is translated into simple parallel logical operations. The carrier mixing stage is reduced to one or a few simple logical operations which can be performed concurrently on several bits. In the same way, the PRN code removal only affects the sign word.
In a U.S. patent by Ledvina and colleagues, the complete code and carrier removal process requires two operations for each code replica (early, prompt, and late). The complexity can be even further reduced by more than 30 percent by considering one single combination of early and late code replicas (typically early-minus-late). This way, the authors claim an improvement of a factor of 2 for the bitwise method compared to the standard integer operations.
The inherent drawback of this approach is the lack of flexibility: the complexity of the process becomes bit-depth dependent and the signal quantification cannot be easily changed (while performing BBP with integers allows the signal structure to change significantly without code modification).
To overcome this limitation, a combination of bitwise processing and distributed arithmetic can be used [described in Waelchli, 2009]. The power-consuming operations are performed with bitwise operations, and to be able to keep the flexibility of the calculations, standard integer operations are used after the code and carrier removal. The conversion between the two methods is performed with distributed arithmetic that offers an extremely efficient way to switch between the two representations.
Another important aspect in a software receiver is the code and carrier generation. As these tasks represent a huge processing load, new solutions must be developed in this domain.
Code Generation
The pseudorandom noise (PRN) codes transmitted by the satellites are deterministic sequences with noise-like properties that are typically generated with tapped linear feedback shift registers (for GPS L1 C/A) or saved in memory (for Galileo E1). But in order to save processing power, it is preferable for software applications to compute off-line the 32 codes and store them in memory.
One method stores the different PRN codes in their oversampled representation (the code are pre-generated) [Ledvina, 2002]. As the incoming signal code phase is random, the beginning of the first code chip is in general not aligned with the beginning of a word and may occur anywhere within it. To overcome this issue, either all the possible phases can be stored in memory, or the code can be shifted appropriately during the tracking. While the first approach increases the memory requirements, the second requires further data processing in function of the phase mismatch. Regarding the Doppler compensation, all the PRN codes in the table are assumed to have a zero Doppler shift. The code phase errors due to this hypothesis are eliminated by choosing a replica code from the table whose midpoint occurs at the desired midpoint time. The only other effect of the zero Doppler shift assumption is a small correlation power loss which is not more than 0.014 dB if the magnitude of the true Doppler shift is less than 10 kHz [Ledvina patent]. This approach is very popular in the SR domain and can be found in several solutions.
Carrier Generation
The generation of a local carrier frequency is necessary to perform the Doppler removal. The standard trigonometric functions or the Taylor decompositions for the sines and cosines computation are too heavy for a software implementation and are seldom considered.
However, several other techniques exist to reduce the computational load for the carrier generation: the values for the carrier can be pre-generated and then stored in lookup tables. As this would require several gigabytes of memory to store all the possible frequencies, the values are recorded on a coarse frequency grid with zero phases and at the RF front-end sampling frequency. The carrier will thus be available in a sampled version. The limited number of available carrier frequencies introduces a supplementary mismatch in the Doppler removal process. This error can be compensated with a simple phase rotation of the accumulation results. This method is very popular in the SR domain, and many solutions take advantage of it to avoid the power-hungry real-time carrier generation.
Based on the same principle as above, Normark (2004) proposed a method that pre-computes a set of carrier frequency candidates to be stored in memory. The grid spacing is selected so as to minimize the loss due to Doppler frequency offset. Furthermore, to provide phase alignement capabilities of the carriers, a set of initial phases is also provided for each possible Doppler frequency, as illustrated in Figure 4.
FIGURE 4. Set of carrier frequency candidates.
Contrarily to the Ledvina approach and thanks to the phase alignement capabilities, the number of sampling points must not obligatorily correspond to an entire acquisition period. Therefore, the length of the frequency candidate vectors can be chosen with respect to the available memory space and becomes quasi independent of the sampling frequency.
Another approach consists in removing concurrently the Doppler from all received satellite signals [Petovello, 2006]. The algorithm is implemented as a look-up table containing one single frequency, and the carrier removal is performed for all channels with the same frequency, but the frequency error results normally in an unacceptable loss. To overcome this problem, the integration interval is split into sub-intervals for which a partial accumulation is computed.
The result is rotated proportionally to the frequency mismatch in the same way as in the method described above. The algorithm can be applied recursively and with an appropriate selection of the sub-intervals, and the total attenuation factor can be limited to a reasonable value. The author claims an improvement of up to 30 percent compared to the standard look-up table method with respect to the total complexity for both Doppler removal and correlation stages. Regarding the computational complexity, the Doppler removal stage remains unchanged, with the difference that it is only performed once for all satellites. But the rotation needs to be done for each of the sub-intervals. However, this algorithm remains difficult to implement (number of samples varies in one or more full C/A code chip, and the data alignment is different than the sub-interval boundaries).
Available Receivers
Today, software receivers can be found at university and commercial levels. The development not only includes programming solution but also the realization of dedicated RF front-ends. As these RF front-ends are able to capture more and more frequencies with increasing bit-rates and band-widths, the PC-based software receivers require a comparably complex interface to transfer the digitized IF samples into the computer’s memory.
Two classes of PC-based GNSS SR front-end solutions can be found. The first one uses commercially available ADCs that are either connected directly to the PC (for example, via the PCI bus) or that are working as stand-alone devices. The ADC directly digitizes the received IF signal, which is taken from a pure analog front-end. This solution is often found at the university and research institute level, where a high amount of flexibility is required; for example, at the Department of Geomatics Engineering of the University of Calgary, Cornell University, and the University FAF Munich’s Institute of Geodesy and Navigation.
The second solution is based on front-ends that integrate an ADC plus a USB 2.0 interface. Currently, an impressive number of commercial and R&D front-ends are available for the GNSS market. NordNav (acquired by CSR) and Accord were among the first to provide USB-based solutions. Another interesting development comes from the University of Colorado, which in an OpenGPS forum published all details on the RF and USB sections. More companies announced and continue to announce front-ends that are not only capable of capturing a single frequency, but several different bands. To be able to deal with this increasing bandwidth, the USB port is very well suited for SR development, and its maximum theoretical transfer rate of 480 MBit/s allows realizing GPS/Galileo multi-frequency high bandwidth front-ends.
Embedded Market. As mentioned in the introduction, the embedded market will gain increasing importance during the next few years. A growing number of receivers are developed for this market, supporting different embedded platforms (for example, Intel XScale, ARM-based, and DSP-based). Several companies offer commercial software receivers for the embedded market, among others NordNav and SiRF (acquired by CSR), ALK Technologies Inc., and CellGuide.
Commercial PC-Based Receivers. The first commercial GPS/Galileo receiver for a PC platform was presented in 2001 by NordNav. This SR can be compared to a normal GPS receiver, although the CPU load of this solution is still quite impressive. Several other solutions have been presented more recently. One of the first (car) navigation solutions was presented by ALK Technologies under the name CoPilot. The CPU load was drastically reduced, and this solution works on a standard commercial personal computer. The client does not really see a difference compared to a solution that is based on a hardware receiver.
Research Activities. Use in teaching and training is one of the most valuable and obvious application for software GNSS receivers. Receivers, for which the source code is available, allow the observation and inspection of almost every signal data by the researcher.
Several textbooks have been published related to software GNSS receivers. The pioneer in this area is James Bao-yen Tsui, who in 2000 wrote the first book on software receivers, Fundamentals of Global Positioning System Receivers: A Software Approach (Wiley-Interscience, updated in 2004). Kai Borre and co-authors published in 2006 a book that comes with a complete (post-processing) software receiver written in Matlab: A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach (Birkhäuser Boston, 1st edition).
The European Union is financing development of receivers for Galileo. One project was the Galileo Receiver Analysis and Design Application (GRANADA) simulation tool. Running under Matlab, GRANADA is realized as a modular and configurable tool with a dual role: test-bench for integration and evaluation of receiver technologies, and SR as asset for GNSS application developers.
Other companies provide toolboxes (in Matlab or C) that allow testing of new algorithms in a working environment and inspecting almost all data signals; for example, Data Fusion Corporation and NavSys.
Outlook
Software receivers have found their place in the field of algorithm prototyping and testing. They also play a key role for certain special applications. What remains unclear today is if they will enter and drastically change the embedded market, or succeed as generic high-end receivers.
A software GNSS receiver offers advantages including design flexibility, faster adaptability, faster time-to-market, higher portability, and easy optimization at any algorithm stage. However, a major drawback persists in the slow throughput and the high CPU load.
Many different companies and universities have projects running that seek to optimize and develop new algorithms and methods for a software implementation. The developments not only consider the software levels, but also extend in the direction of using additional hardware that is already available on a standard PC; for example, using the high performance graphic processing unit (GPU) for calculating the local carrier [Petovello, 2008].
On the opposite end of the spectrum from the mass market, the following factors seem to ensure that, sooner or later, high-end software receivers will be available:
High bandwidth signals (GPS and Galileo) can already be transferred into the PC in real time and processed.
The processing power is increasing, allowing real-time processing with a limited amount of multi-correlators. The introduction of new multi-core processors will be advantageous for software receivers.
Post-processing is one of the most important benefits of a software receiver, as it enables a re-analysis of the signal several times with all possible processing options. Increasing hard disk capacity facilitates storage of several long data sequences.
Some signal-processing algorithms such as frequency-domain tracking or maximum-likelihood tracking are much easier to implement in software than in hardware, as they require complex operations at the signal level.
History
During the 1990s, a U.S. Department of Defense (DoD) project named Speakeasy was undertaken with the objective of showing and proving the concept of a programmable waveform, multiband, multimode radio [Lackey, 1995]. The Speakeasy project demonstrated the approach that underlies most software receivers: the analog to digital converter (ADC) is placed as near as possible to the antenna front-end, and all baseband functions that receive digitized intermediate frequency (IF) data input are processed in a programmable microprocessor using software techniques rather than hardware elements, such as correlators. The programmable implementation of all baseband functions offers a great flexibility that allows rapid changes and modifications. This property is an advantage in the fast-changing environment of GNSS receivers as new radio frequency (RF) bands, modulation types, bandwidths, and spreading/dispreading and baseband algorithms are regularly introduced.
In 1990, researchers at the NASA/Caltech Jet Propulsion Laboratory introduced a signal acquisition technique for code division multiple access (CDMA) systems that was based on the Fast Fourier Transform (FFT) [van Nee, 1991]. Since then, this method has been widely adopted in GNSS SR because of its simplicity and efficiency of processing load.
In 1996, researchers at Ohio University provided a direct digitization technique — called the bandpass sampling technique — that allowed the placing of ADCs closer to the RF portions of GNSS SRs. Until this time, the implemented SRs in university laboratories post-processed the data due to the lack of processing power mentioned earlier.
Finally, in 2001, researchers at Stanford University implemented a real-time processing-capable SR for the GPS L1 C/A signal [Akos, 2001].
However, the GNSS SR boom really started with the development of real-time processing capability. This was first accomplished on a digital signal processor (DSP) and later on a commercial conventional personal computer (PC). Today, the DSPs are increasingly replaced by specialized processors for embedded applications.
Marcel Baracchi-Frei received a physics-electronics degree from the University of Neuchâtel, Switzerland, and is working as a project leader and Ph.D. candidate in the Electronics and Signal Processing Laboratory at the Swiss Federal Institute of Technology (EPFL).
GRÉGOIRE WAELCHLI received his degree of physics-electronics from the University of Neuchâtel and is now at EPFL for a Ph.D. thesis in the field of GNSS software receivers.
CYRIL BOTTERON received a Ph.D. with specialization in wireless communications from the University of Calgary, Canada, and now leads the EPFL GNSS and UWB research subgroups.
PIERRE-ANDRÉ FARINE is professor and head of the Electronics and Signal Processing Laboratory at EPFL, and associate professor at the University of Neuchâtel.
Reaction to August’s editorial, “Fair Play to Those Who Dream,” concerning the as-yet unreleased Galileo signal specification, came swift — and mixed — from both sides of the Atlantic. Correspondents were unanimous, however, in wishing their names kept off the record.
From Europe: “I really think you go too far, too far to be honest.”
“I truly appreciate your open words. They come right out of my heart. I’m not sure, though, whether they are loud enough to make the European Space Agency and European Union wake up at last.”
From the United States: “. . . ranting sensationalism . . .”
“Nice job. I think you will get some kudos for this.”
Interference Counter-Effort Gets Cart Before Pony
Your May 2009 editorial bemoaned the fact that, despite rhetoric to the contrary, blessed little had been accomplished in our ability to identify and localize sources of GPS interference. Some of my colleagues might describe me as a bit obsessed with this issue, but I think it a healthy obsession.
The most recent attempt I’ve seen that addresses this issue is a program sponsored by the Department of Homeland Security (DHS). Based on a briefing of this effort that’s available on the Internet, I seriously doubt that this program, as presently structured, has much chance of yielding a useful product. Why? Because I believe the approach to be thoughtless rather than thoughtful. And I mean “thoughtless” in the literal, not pejorative, sense. Those involved in the program, as listed in the briefing, are among the brightest talents available to address this issue.
My past dissatisfaction with the way the government has treated this issue — at least from the early 1980s — stems from what I call its planning-to-plan mode. An agency would lay out an elaborate plan, replete with words like “coordinate” and “support,” but include few if any specific action items such as “Agency A should develop specific capability B.” So no one was on the line to deliver.
Preparatory to writing this letter, I thought I would do a few searches to see if there were any new initiatives underway. I was rather pleasantly surprised to see that DHS had produced, and briefed, a study titled “GPS Interference Detection and Mitigation.” That is, until I read it.
The DHS briefing is a 25-slide presentation. To its credit, it doesn’t plan-to-plan. The brief lays out a procedure to address the issue. But once we eliminate the boilerplate describing the non-specific tasking and coordination, we wind up with roughly seven slides devoted to a detailed description of the central data repository and the logging of data. Excluding a page that vaguely describes potential sources of data without describing how they could contribute to the tasking, there are zero pages devoted to the sensors! But these sensors are the heart of the system, and will dictate the types of data that the central facility must process.
The problem is compounded since, according to the briefing, the only aspect of the program assigned funding is the central data repository. So we’re putting the seed money into some generic repository? A classic case of buying the cart before the (as yet) undefined pony. What’s the problem? I think there are several.
First (and I’m guessing here), someone was given a mandate to do something. And with insufficient time to do the necessary thinking. Which primarily requires a few folks closing the door, putting their feet on the table, and actually thinking about the problem.
This, in turn, produced a description of a processing/repository center that had to be generic, since without defining the sensors one can’t define the data types, formats, and quantity that the sensor arrays would provide. So how does one spend the repository funding indicated by the DHS study?
And, it’s a hard problem. You can’t solve it by just brute force.
Let me end by suggesting how I would contrast a thoughtful approach from a thoughtless approach to the issue. The thoughtless approach would be to develop a one-size-fits-all solution, by which all data sources will filter through the distribution chain to the repository.
The thoughtful approach is for one of the guys/girls to eventually take his/her feet off the table, and suggest, “Hey, our major heartburn at this time is fratricide! Rather than spend a lot of time and money reporting this interference, why not adopt procedures for preventing it? And, given we provide a training program in place on potential problem sources and corrective procedures, we might spend a few dollars on the type of jam meter (see Phil Ward’s paper in Inside GNSS, Sept/Oct 2007) to show that violations will be monitored.”
Under this scenario, with a bit of luck, a potential San Diego (2006) or Rome AFB (1997–1998) event never gets to the reporting stage.
Is the above the ultimate solution? Of course not. Countering the intentional jamming of GPS will be the larger challenge. But a bit of forethought appears preferable to blindly funding the cart, before knowing how the pony behaves.
— Terry McGurn, Reston, Virginia
Military Handhelds
I just wanted to say how much I enjoyed Don Jewell’s column, “The Warfighter and Rorschach Shock.” It is a sad fact that the DoD has not woken up to the potential of a market-driven procurement process for products like handheld GPS. I’m of the opinion that if tamper-resistant SAASM OEM chips or boards were available to the Garmins of the industry, and the military user had his choice of units, we’d see a much superior offering to the warfighter at a lower net cost. Companies like Garmin are not GPS receiver builders, they buy their receivers from companies like SiRF and Broadcom. Their expertise is as systems integrators and they do a superb job of it. Obviously, the warfighter values this expertise and wants to use it — so why don’t we give them what they want?
— Logan Scott, Breckinridge, Colorado
Time for GPS 101
Ijust read Don Jewell’s article “Time for GPS 101.” I too am appalled by the ignorance of the public and, more importantly, of our political decision-makers, not just about GPS (critical enough in isolation) but about a myriad of national and global safety and security issues. As a 30-year member of the aerospace engineering community, I am fearful of the future of our country and society because of bad decisions made by ignorant decision-makers, sometimes supported by technical “experts” who provide bad information and advice, usually from their own ignorance but often cynically from a hidden agenda.
— Name Withheld
I truly enjoy Don Jewell’s editorials!
After reading the GPS 101 column, I wondered: Why is there nothing presenting this technology to children? (If there is, I have yet to find it.) Before their minds turn to mush, I believe if we start them with the basics, coupled with the present technology evolving as fast as it is, imagine what they could be doing with GPS when they graduate from high school! I understand that the teenagers have the iPhone with the hip applications and such, but wouldn’t it be neat to see a seven-year-old with a handheld scouring a park looking for his/her first geocache?
— John Pollard, Southeastern, Electric Cooperative, Sioux Falls, SD
I enjoyed your “Time for GPS 101” article. Your point was driven home last night as I watched the National Geographic Channel program, Known Universe: “The Fastest,” when they used GPS in their discussion of the relativistic effects of velocity. To my horror, they described how the GPS receiver sends signals to the satellites which are used to determine your time and space coordinates. The ignorance of the unwashed masses may be excused, but when a channel dedicated to science gets it so wrong, I really worry.
Apogee Mapping has released amLibrary, a spatial data bundle that includes the company’s four flagship products and is packaged exclusively for use by higher education institutions.
The bundle includes Apogee’s amElevation, amHillshade, amContour, and amWater. AmElevation is a national dataset of 1-arc second digital elevation data (DEM); amHillshade comprises nationwide 40, 200, and 1,000 foot contours in a smoothed vector format; amHillshade is a national set of tiled raster topography offered in both grayscale and full-color; and amWater is a premium vector hydrography dataset derived from information generated by the U.S. Environmental Protection Agency. AmLibrary also includes 50 data layers depicting environmental, climactic, and geologic data. Bundled together, this information provides GIS users with comprehensive and detailed terrain data that can be used as a basis upon which to conduct research and perform complex spatial analyses, according to Apogee Mapping.
AmLibrary is offered exclusively to colleges and universities in either MapInfo TAB format or ESRI Shapefile format. Full product documentation, layer, and metadata are provided with the product.
One of the satellites in the Chinese domestic satellite navigation system, Beidou, is no longer in geostationary orbit and appears to have been abandoned.
According to information from the U.S. Space Command, the orbit of Beidou 1D was raised by around 130 kilometers on February 18, 2009. This may have been an attempt to place the satellite in a graveyard or disposal orbit. Such a maneuver is carried out by spacecraft operators when a satellite reaches the end of its life due to a malfunction or some other reason. However, the recommended boost height for geostationary satellites is about 300 kilometers, where a satellite is above the zone used to reposition active geostationary satellites and also provides a buffer for natural orbit variations due to solar radiation pressure and other causes. Beidou 1D may not have had sufficient propellant to reach desired orbit height.
In its current orbit, Beidou 1D is drifting westward at a rate of about 4.5 degrees per day and has already completed one circuit of the Earth. On July 17, it was positioned just west of the Greenwich meridian.
China launched Beidou 1D in February 2007; according to the Xinhua news agency at the time, the satellite was to serve as a backup to the three satellites already in orbit, perhaps replacing the first Beidou satellite, Beidou 1A, when necessary. Subsequent reports did indicate that Beidou 1A appeared to have malfunctioned.
It is not known what kind of malfunction Beidou 1D suffered or whether its signals have been switched off. Accurate detailed information about the current status of the Beidou domestic system is difficult to obtain.
China has plans to improve its domestic navigation system and to develop a global system known as Beidou 2 or Compass. Its first medium Earth orbit satellite, Beidou M1, was launched on April 13, 2007, followed by a geostationary satellite, Beidou G2, on April 14, 2009.
— Richard Langley
Galileo, Too, Has Accounting Problems
The European Union’s Galileo program has been ill-prepared and badly managed, according to a report by the European Court of Auditors released on June 29. These defects have set back development by five years, it believes.
The report also criticizes the Union’s 27 individual member states for counterproductive promotion of their respective national aerospace industries. The auditors conclude that the original public-private partnership plan was “inadequately prepared and conceived” and “unrealistic.”
The European Commission (EC) “must considerably strengthen its management,” advice the EC has evidently taken to heart. For the last year, contract negotiations by the European Space Agency (ESA) have taken place under the watchful eye of an EC program manager.
Contracts. On June 15, ESA signed contracts for the procurement of so-called long-lead items required for the construction of the constellation with Astrium GmbH and OHB Systems, the latter a German company and the former a German-French partnership with British involvement. Both Astrium (€7 million) and OHB (€10 million) contracts relate to parts for equipment of the satellite platforms and navigation payloads. Award of the satellite contracts themselves is planned to take place by the end of 2009.
ESA and Arianespace contracted for launch of the first four operational Galileo satellites on two Soyuz launch vehicles from Europe’s Spaceport in French Guiana. The four IOV satellites will be placed in orbit by end of 2010.
Conversations at the Paris Air Show seemed to indicate that ESA and the EC may divide the satellite construction contract into two stages to permit a later modification of the design, and that they may also divide the first satellite contract between the two bidders, Astrium and OHB, as an insurance policy to reduce the possibility of further development delays, and as a boon to design flexibility.
The Astrium CEO sharply criticized this option, saying it would increase overall program cost. The OHB CEO seemed more sanguine, lauding ESA’s move as likely to maintain a competitive environment.
Yowza!!, an application designed for the latest GPS-enabled iPhone 3G and 3GS models and iPod Touch, brings relevant coupon offers to customers based on their location.
“Any time you insert a concept such as location into a marketing program, you end up with a far more compelling value proposition,” states Mike Wehrs, president of the Mobile Marketing Association.
Sales and discount offers via Yowza!! can be updated in real-time and targeted by region or store location. “The phone will deliver a list of stores within one mile that have offers on Yowza!!,” said August Trometer, co-founder of the recent startup. Users show the barcode and digital mobile coupon on their handset at checkout to redeem the discount on their purchase.
“We work directly with merchants; they provide us with their latitude and longitude, we get the GPS coordinates, do a database search with a proprietary algorithm,” said Trometer. “The phone constantly goes back and forth between our app, touching data from our database. When the person touches their location, it touches a new set of data in the database. The phone will work with them to keep delivering the closest store. There’s a lot of work on the database end of things.”
One drawback of the app is that it has to be turned on to work — it does not sit in the background, waiting to be activated by incoming offers. “Users have to give the application access to their GPS coordinates,” explained Trometer. “But the power of the device and all the applications it brings make it silly to turn off the location capability.”
Retailers that have signed with Yowza!! include Sears, McDonald’s, The Container Store, and more. Unlike traditional forms of couponing such as newspaper ads, Yowza!! offers can be updated in real time and targeted by region or store location.
Trometer expects to announce Yowza!! capability through other GPS-equipped phones: Blackberry Storm, Google’s Android-based phone, and the Palm Pre. “All three makers allow developer access to the GPS and this is very important, it’s crucial, obviously. They also have a high-res screen, which is a requirement for our scannable barcode that the user shows to the merchant.”
Referring to GPS handsets that lack a high-res screen, he claims “The other phone manufacturers really have an uphill battle right now.”
Whose GPS? The source of the GPS chip within Apple’s iPhone remains a mystery. “Even people who have done teardowns of the devices, the chips are completely blank,” says Trometer.
“There are so many possibilities, we’re just scratching the surface right now with what can be done,” Trometer said. “The mind reels with the things that can be done with that.”
>> SURVEY & CONSTRUCTION
Hemisphere, Juniper Jointly Offer DGPS Receiver for Demanding Environments
Juniper Systems and Hemisphere GPS offer the XF101 DGPS receiver for the Archer Field PC, designed to deliver sub-meter DGPS to location-based applications in demanding environments.
According to the companies, the Hemisphere GPS XF101 DGPS receiver provides: Crescent GPS technology for sub-meter accuracy; COAST technology to maintain accuracy during temporary loss of differential signal; optional external antenna for centimeter-level accuracy; low power consumption; modular connection for rapid field use; real-time or post-processed DGPS data collection; and multipath minimization.
The XF101 with the Archer is priced at less than $2,500. It fully supports mobile GIS applications such as ESRI ArcPad and OnPoz GNSS Driver.
>> AVIONICS
NovAtel Receiver for Next-Gen WAAS
NovAtel announced receipt of a contract from the U.S. Federal Aviation Administration (FAA) to develop the next generation Wide Area Augmentation System (WAAS) reference receiver, the GIII. Total contract value can go up to $9.7 million.
NovAtel has worked with the FAA WAAS program since 1995, providing and supporting two previous generations of reference receivers for the WAAS ground network. The technology refresh will add support for new L1C, L2C, and L5 signal capabilities, on a qualified RTCA DO-178B software and DO-254 hardware platform. The WAAS GIII receiver program is scheduled to be completed over the next three years, and will include growth provision for further signal capability such as Galileo. As many as 14 receivers will be produced in the GIII development and qualification program.
>> FLEET TRACKING
AT&T, Trimble Fleet Management
AT&T has broadened its fleet and mobile asset management portfolio with the latest version of Trimble’s GeoManager solution, which helps reduce fuel and maintenance costs by enabling operators to manage their vehicle assets more efficiently.
Trimble GeoManager enables transportation and field-service fleet operators to track their mobile workers and assets through software and GPS modems running on AT&T’s wireless network. GeoManager integrates GPS, wireless data communications, and a browser interface to help manage mobile workers, the mobile worker’s work, and the mobile worker’s assets.
AT&T and Trimble have jointly offered fleet-tracking solutions for several years. The GeoManager update features improved map and status, new landmark uploads, WLAN usage, schedule report enhancements, driver logs, and organizational hierarchy modifications.
>> TIMING
Timing Vulnerability Concern Grows
Industrial and enterprise users in telecommunications and utilities privately express concern over revelations from the April Government Accounting Office (GAO) report, “Global Positioning System: Significant Challenges in Sustaining and Upgrading Widely Used Capabilities.” The GPS signal is used for synchronizing almost all global computer networks belonging to the military, utilities, banks, telecomms, television companies, and many more.
Backup? What Backup? These same companies point to a continued lack of commitment on the part of the U.S. government to stable and reliable backup for GPS. As long ago as 2007, in comments before the Department of Transportation, wireless carrier Sprint Nextel stated: “Sprint Nextel Corporation respectfully requests that the U.S. government continue to operate and invest in the LORAN-C and eLORAN systems. Should the DOT and DHS decide to decommission the LORAN-C system, Sprint Nextel recommends that the agencies delay doing so until the eLORAN system is fully operational. Sprint Nextel and other communications providers use the frequency signals of the Global Positioning System, LORAN, and atomic clocks for multiple levels of redundancy and diversity in their networks. Therefore, Sprint Nextel urges the DOT and DHS to carefully weigh decisions which might impact LORAN’s availability to the nation’s voice and data communications networks.
“The loss of a primary reference source (PRS) can negatively impact a telecommunications network, and those impacts can vary from minor short-term noise impairments to long-term network-wide outages. Both traditional wireline services and newer wireless services require a precise frequency reference for basic service delivery . . . . The continental U.S. portion of the Sprint Nextel network requires a PRS at thousands of switch sites, interconnection sites
, and cell tower sites to ensure reliable service delivery.”
Deadlock on Capitol Hill. Competing resolutions to either discontinue or adequately fund LORAN and eLORAN continue fencing in Congressional subcommittees in both chambers. Nothing has changed since Sprint commented two years ago — aside from a potential rise in the susceptibility of GPS to jamming, unintentional interference, and decreased availability.
GAO REPORT, FIGURE 5. Probability of maintaining constellation of at least 18, 21, and 24 GPS satellites based on reliability data as of March 2009 and a two-year GPS III launch delay.
>> TIMING
Telecom Clock from EndRun
EndRun Technologies announced a Telecom Clock Option for its Meridian Precision GPS Timebase, which provides accurate and stable GPS-synchronized outputs for military communications, aerospace, broadcast, engineering and calibration laboratories, telecommunications, and more.
The option was designed as a plug-and-play module that can supply any combination of E1, T1, J1 and/or composite clock outputs. An alarm output is also available and single-satellite mode (SSM) is supported. The Telecom Clock Option can be installed in EndRun’s GPS or CDMA-based Meridian and Tycho product lines.
England’s CSR plc and U.S.-based SiRF Technology Holdings, Inc., have completed their merger, ending years of speculation over what may become of SiRF, a pioneering maker of GPS receivers that had become financially troubled during the current economic downturn.
“In bringing together the combined capabilities and broad range of CSR and SiRF technologies and platforms, we have created a new force in the industry and a world class organization with the commercial, technical and operational scale to build on CSR and SiRF’s existing customer relationships and deliver the next generation of connectivity and location enabled products,” said Joep van Beurden, CSR CEO. “Our strategic goal is to address the existing and emerging needs of our combined customer base for connectivity and location technologies. The potential applications and benefits to the end user of connectivity plus location are only just starting to open up, and these exciting new opportunities will be driven by our unique combination of leading location technologies and connectivity solutions.”
SiRF co-founder Kanwar Chadha echoed those sentiments. “CSR and SiRF have a shared vision of using innovation to bring the benefits of wireless connectivity and location to mainstream consumers and enterprises and to enable new and exciting user experiences,” said Chadha, now a CSR board member and chief marketing officer. “We believe that through this merger, our customers and consumers will derive benefits from a much stronger player whose focus is on delivering best in class connectivity and location platforms.”
For CSR’s customers, the merger with SiRF means CSR’s Connectivity Centre products are augmented by GPS technologies, including assisted GPS (A-GPS), dead reckoning, and location centric platforms, the companies said. Meanwhile, SiRF’s customers will see enhancements to SiRF’s location platforms with CSR’s Connectivity Centre capabilities.
The enlarged CSR group will have its global headquarters in Cambridge, United Kingdom, with SiRF’s headquarters remaining in San Jose, California, which will also serve as CSR’s U.S. headquarters. The combined CSR group is now among the top 10 fabless semiconductor companies, with a combined customer list including six of the top seven handset manufacturers, the top five personal navigation device makers, the top two automotive telematics suppliers, and other auto and consumer electronics providers, CSR said.
CSR plc of Cambridge, UK, and SiRF Technology Holdings Inc., of San Jose, California, on June 26 completed the merger between SiRF and a wholly owned subsidiary of CSR. The merger resulted in “creating a provider of connectivity and location platforms and a company with the scale, technology, and strategy to enable its customers to address the exciting and emerging opportunities in mobile markets,” according to a company statement.
The company said that customers of the enlarged CSR group will be able to deliver new user experiences of connectivity and location technologies in a diverse range of devices such as mobile phones, personal navigation devices, in-car navigation and telematics systems, laptop and netbook PCs, mobile internet devices, digital cameras, gaming machines, cellular accessories, and consumer electronic devices.
“In bringing together the combined capabilities and broad range of CSR and SiRF technologies and platforms, we have created a new force in the industry and a world-class organization with the commercial, technical and operational scale to build on CSR and SiRF’s existing customer relationships and deliver the next generation of connectivity and location enabled products,” said Joep van Beurden, CEO of CSR. “Our strategic goal is to address the existing and emerging needs of our combined customer base for connectivity and location technologies. The potential applications and benefits to the end user of connectivity plus location are only just starting to open up, and these exciting new opportunities will be driven by our unique combination of leading location technologies and connectivity solutions.”
“CSR and SiRF have a shared vision of using innovation to bring the benefits of wireless connectivity and location to mainstream consumers and enterprises and to enable new and exciting user experiences”, said Kanwar Chadha, co-founder of SiRF and newly appointed board member and chief marketing officer of CSR. “We believe that through this merger, our customers and consumers will derive benefits from a much stronger player whose focus is on delivering best in class connectivity and location platforms.”
“Technology innovation represents the foundation for both CSR’s and SiRF’s success in the market place,” said James Collier, co-founder, board member and Chief Technology Officer of CSR. “We look forward to combining the complementary expertise of our teams to take innovation to the next level in our multifunction radio and system platforms to address emerging customer and market needs.”
For CSR’s customers, the merger with SiRF means CSR’s Connectivity Centre products are augmented by GPS technologies that are well respected and enjoy widespread adoption, the company said, while SiRF brings to CSR a strong IP portfolio in GPS and assisted GPS (A-GPS), dead reckoning, and location centric platforms. The enlarged CSR group will have its global headquarters in Cambridge, UK, with SiRF’s headquarters in San Jose becoming CSR’s U.S. headquarters.
By Richard Langleuy, with an additional note by Oliver Montenbruck
The GPS Wing and its contractors have traced the cause of pseudorange errors on L1 and L2 broadcast by the newest GPS satellite, SVN-49, to the manner in which the L5 signal demonstration payload was added to the satellite. Signal leakage between the two input ports of the antenna coupler network for the satellite’s array of 12 helical antenna elements, reflected from the L5 filter and then transmitted, creates a second signal with a delay of approximately 30 nanoseconds, and the appearance of a multipath component.
While testing an adjustment to the signal-in-space to minimize the effect of the problem on receiver navigation solutions on Earth, the GPS Wing is interested in hearing from manufacturers and the user community concerning the different impacts of SVN-49 signals on the wide range products and applications in operation, before reaching a final decision on what to do with the satellite prior to setting it healthy.
The seventh modernized GPS Block IIR satellite was launched on March 24, 2009. Called SVN-49, its sequence number in the long line of GPS satellites, or PRN01, after its pseudorandom noise code identifier, this satellite is special. In addition to the equipment required to transmit the legacy GPS C/A-code and P(Y)-code signals and the new civil L2C-code and military M-code signals on the standard L1 (1575.42 MHz) and L2 (1227.6 MHz) frequencies, SVN-49 carries an L5 demonstration payload. L5 is the new civil signal to be transmitted on 1176.45 MHz by Block IIF and succeeding generations of GPS satellites.
The demo payload was included to claim the frequency, which was allocated by the International Telecommunication Union before the August 26, 2009, deadline. The deadline had been imposed seven years earlier when the GPS Joint Program Office (the forerunner of the GPS Wing) applied for the frequency. The Block IIF program schedule had slipped a bit and as a safeguard (and one which eventually saved the day), the demo payload was developed and assigned to SVN-49.
Shortly after the L1/L2 system on SVN-49 was activated on March 28, it became clear that the satellite had a small problem. The pseudorange data obtained by U.S. Air Force Space Command’s 2nd Space Operations Squadron (2 SOPS) monitor stations had larger than normal errors. Typically, the errors have a random characteristic, with a mean of zero and a peak-to-peak variation of two meters or so. But the SVN-49 ionosphere-corrected errors reached a level of about four meters and when they were plotted against the elevation angle of the satellite as viewed at each monitor station, a clear trend emerged (see Figure 1).
FIGURE 1. Ionospheric-refraction-corrected SVN4-9 pseudorange residuals from data collected at 2 SOPS monitor stations (courtesy GPS Wing).
Although larger than normal, the errors still fell within the accuracy tolerances specified for GPS signals. Nevertheless, the anomalous behavior of SVN-49’s signals was a cause of concern, and the GPS Wing and its contractors mounted efforts to find the cause.
Payload Source. They traced the source of the problem to the manner in which the L5 demo payload was added to the satellite. To understand the problem, we need to examine how the L1 and L2 signals are transmitted by a GPS satellite.
A primary and defining characteristic of GPS signals is that the received signal power should be approximately the same at any location on the Earth’s surface within view of the satellite. In other words, we should receive about the same signal power when a GPS satellite is overhead (and closer to us) as when it is low on the horizon (and further away). Any major variation in signal level seen by a receiver is typically due to the gain pattern of the receiver’s antenna.
To achieve a uniform power density at the Earth’s surface, a GPS satellite uses an array of 12 helical antenna elements, with an inner ring of four elements and an outer ring of eight, fed by an antenna coupler network (see Figure 2). The L1 and L2 signals are fed into the coupler through one of its two input ports: port J1. The inner ring of elements transmits most of the L1 and L2 power from J1 with a broad pattern, while the outer ring transmits a sharper pattern but with a weaker signal and a different phase. The net effect of this arrangement is to reduce the radiated power from the inner ring as seen at high elevation angles and boost it for lower elevation angles thereby achieving an almost uniform power density.
FIGURE 2. L-band antenna element locations (courtesy GPS Wing).
The antenna coupler’s other input port, J2, is used on SVN-49 to feed the L5 signal to the antenna array after first passing through a filter and a 162-inch (411-centimeter) cable. Most of the power from J2 goes to the outer ring, with less going to the inner ring — the inverse of the power distribution from J1. This is why initial reports of L5 signal acquisition noted its high directivity with much weaker signals at low elevation angles compared with the L1 and L2 signals. But this behavior was expected.
Not expected was the effect of the L5 filter and its associated cable run on the L1 and L2 signals. It turns out that some of the L1 and L2 signal from J1 exits the J2 port, is reflected from the L5 filter, and then is transmitted from the J2 port with a delay of approximately 30 nanoseconds. With hindsight, the J1 to J2 signal leakage and reflection from the L5 filter should have been prevented.
On the ground, a receiver sees both the direct signal and the weaker reflected signal, which looks like a multipath component. The GPS Wing and its contractors have attempted to model the effect of the reflected signal on GPS receiver measurements. According to their models, if the direct and reflected L1 signals are in phase at the zenith, then a standard code-correlating receiver will measure a C/A-code pseudorange that is 1.62 meters too long. The error becomes smaller as the elevation angle drops, due to the drop in power level of the reflected signal, reaching zero at an elevation angle of about 42 degrees, corresponding to a null in the antenna pattern and then rising slightly as the elevation angle drops to zero (see Figure 3).
FIGURE 3. Model of the differences between the SVN-49 L1 delayed (multipath) and direct signals (courtesy GPS Wing).
P(Y), L2, and L2C. The same error behavior is expected for L1 P(Y)-code pseudoranges. Maximum L2 P(Y)-code pseudorange errors are modeled to be zero if the direct and reflected L2 signals are in quadrature, or to have maximum values of about plus 0.95 meters if the direct and reflected signals have the same phase, and minus 1.1 meters if they have the opposite phase. Ground tests should confirm which of the three possibilities describes the actual signals. The L2C signal is expected to behave in a similar manner to the L2 P(Y) signal.
If ionosphere-free pseudoranges are computed from the L1 and L2 pseudoranges, the maximum errors are predicted to be 4.14, 2.66, and 5.84 meters for the quadrature, in-phase, and opposite-phase L2 direct and reflected signal possibilities.
The models also predict an effect on carrier-phase measurements, but these are very much smaller: a maximum error of 6.8 millimeters on L1 and 4.8 millimeters on L2.
It is not possible to fully fix the problem. The GPS Wing and its contractors are looking at ways to minimize the effect of the problem on receiver navigation solutions. One
experiment under assessment is to adjust the broadcast navigation message ephemeris of the satellite by placing the antenna phase center about 152 meters above the actual position of the satellite, while compensating with a satellite clock offset. Such navigation message adjustments reduce the peak-to-peak variation of the error by about a half; they do not eliminate it.
Status Quo? Another possibility is to broadcast the signal as is, without attempts to compensate for the error. It would then be up to the user to determine how best to use the signals. Initial indications show that certain receivers with advanced multipath mitigation correlators can essentially filter out much of the multipath component (see Narrow Correlators Screen Error section below). Receivers with standard correlators could use the SNV-49 signals but assign a higher uncertainty to the measurements when they are combined with those from other satellites.
The GPS Wing is interested in hearing from manufacturers and the user community concerning the impact of SVN-49 signals on products and applications before coming to a final decision on what to do with the satellite before setting it healthy, and a briefing and interview process has begun to obtain that information. The decision is expect by mid-September.
— Richard B. Langley, University of New Brunswick
Narrow Correlators Screen Error
The typical variation of SVN-49 multipath errors over time is illustrated in Figure 4 for semi-codeless P(Y)-code measurements on the L1 and L2 frequency from a commercial test receiver near Munich, Germany. SVN-49 was visible for roughly 6 hours at this site and reached a peak elevation angle of 80 degrees. The errors are most pronounced on L1 where they vary between –0.5 meters near the horizon and +1 meter near the center of the pass. L2, in contrast, is notably less affected. Here, multipath errors caused by signal reflections in the satellite are well below 0.5 meters in amplitude and cannot be clearly distinguished from local multipath at the receiver site.
FIGURE 4. Typical SNV-49 multipath errors for semi-codeless P(Y)-code tracking on L1 (top) and L2 (bottom) from a conventional correlator (using JAVAD GNSS Triumph receivers.)
While the example shown in Figure 4 is representative for many receivers currently tracking the new GPS satellite, a few receivers are able to filter out the satellite multipath component due to the use of special multipath-mitigation techniques. While implementation details are mostly proprietary, it is commonly known that strobe or double-delta correlators can effectively counteract short-range multipath when using an extremely narrow correlator spacing. The effectiveness of such techniques is shown in Figure 5 for C/A-code and L2C-code tracking by the same test receiver. Obviously, multipath errors are well below the thermal noise in this case and the measurement errors can hardly be distinguished from those of other GPS satellites.
FIGURE 5. SVN49 multipath errors for C/A-code (top) and L2C-code (bottom) tracking using special multipath-mitigation techniques with 20-nanosecond correlator spacing (using JAVAD GNSS Triumph receivers.)
From a practical point of view, users will probably have to decide on their own whether to employ receivers with advanced multipath-mitigation capabilities, whether to apply elevation-angle-dependent measurement corrections (primarily for L1 code measurements), or whether to simply accept the moderate degradation of the SVN-49 measurements. In view of the wide variety of receivers in use and considering their varied applications, a unique solution to the SVN-49 problem is probably not feasible, and care should be taken before applying a priori “corrections” that might cause more harm than good.
(Editor’s Note: The data used to track the anomalies of SVN-49 were gathered using JAVAD GNSS Triumph receivers.)
In the litigious society that we have become, it is not surprising to see GPS as a regular fixture in many civil and criminal proceedings in our nation’s courts. A new and growing outlet for the legal profession, it has also engaged many of the older GPS pioneers who, instead of just retiring, have found a relatively lucrative way to spend their free time. They now form the cadre of GPS expert witnesses, without whom many of the cases involving positioning could not be settled equitably.
These brave individuals must of necessity remain nameless, because all have signed non-disclosure orders regarding the details of any case they may be or have been working on. Even the public record of adjudicated cases affords but a small peek into the activities of these unheralded witnesses. Most civil cases are settled before trial, often with confidential terms, and many criminal cases plead out, so there is little to find in a search of public records for cases involving significant aspects of GPS.
Civil matters usually fall into one of the following categories:
misuse or misappropriation of intellectual property (IP), for example, patent infringement;
liability for accidents; or
product liability for latent defects.
Criminal matters involve some sort of tracking of suspects or felons, or use of GPS for evidence of an alleged perpetrator’s location at the time of the crime. The use of GPS in these instances comes smack up against the public’s right to privacy. In some states, many of these cases are thrown out for lack of warrants allowing use of GPS tracking, while in other states warrants are not required. In 2007, the 7th Circuit U.S. Court of Appeals held that no warrant was required, as did a court in Wisconsin. But the New York State Court of Appeals found the opposite on a 4–3 vote. It is likely that the U.S. Supreme Court will have to determine if such warrantless tracking of suspects violates the Fourth Amendment to the Constitution.
Patents. Most IP cases involve patent disputes wherein the patent in question in some way uses GPS or is itself a GPS component. An application relating to mapping in a car or the way differential GPS is performed provide examples of the former, while a method for improved receiver signal-processing would be of the latter type. These lawsuits are very contentious because experts from each side will disagree on what to others might seem to be obvious. These experts must opine on the meaning of the claims in the patent, the validity of the patent, and the likelihood that the device in question actually infringes on the patent. The cases are expensive to litigate and take a long time to come to an end. Many are settled just before going to trial.
During the pre-trial process, the expert witness must conduct research, provide reports, and testify in depositions. Early on, the expert will testify before a federal judge at proceeding called a Markman hearing, wherein each side presents his interpretation of the words in the patent claims that are in dispute. It is up to the judge to decide what the words mean. Lawyers refer to this as claim construction and how the claims are “construed.” If the case does go to trial, the experts testify in open court, usually before a jury.
Navy versus Air Force. A civil case well known to me involved whether or not GPS receivers would perform during and after the week-number rollover (WNRO) that occurred in the summer of 1999. This case came about as an adjunct to the hysteria involving Y2K. But it was a real concern to the tracking company and its customers, who had deployed thousands of GPS receivers, some in high-risk areas. They had valuable cargo and people at risk if their GPS failed.
The tracking company asked the receiver manufacturer if the units would operate through and after WNRO. The receiver company really didn’t know and delayed answering long enough that the exasperated tracking company commissioned a U.S. Navy test facility to experiment with a GPS simulator and the receivers in question to see what would happen. In the meantime, the receiver company told the tracking company that the Air Force expected everything to go ahead normally, that is the uploads performed at the Master Control Station in Colorado would continue on the same routine during WNRO as it had in the past, namely at least daily uploads. The Air Force would not guarantee that it would happen that way because its specification allowed for uploads plus or minus three days from the end of the week. As such, the receiver company told the tracking company it couldn’t guarantee the upload would be timely, but not to worry.
The tests by the Navy showed that if the uploads was early or late, there would be adverse consequences. One version of the receiver would stop operating for several days after the upload, and another version would stop operating and never recover. As a result of these tests, the tracking company purchased replacements and then sued the receiver company for the costs, claiming a latent defect in their products. The jury ruled for the tracking company and ordered the receiver company to pay for the replacement receivers.
Crash Course. Another case involved a fatal accident caused by the crash of an automobile company’s test van into an open-structured, desert racing car. The test van had GPS onboard as it was performing experiments. The data showed the speed and location of the van up to the time of the collision, and that was enough to cause a settlement.
GPS has figured in countless cases of property incursions where GPS survey data has been used to prove exactly where one property begins and another ends.
Probably the most celebrated and precedent-setting cases occurred in 2001, when a driver sued a rental-car company because it levied a $450 surcharge when a concealed GPS unit indicated he was speeding while driving the rental car. The judge threw out the case because the rental company failed to disclose that it had hidden GPS unit in the car, and that it had no right to collect a fine for speeding as only a government entity could do so.
Several ongoing cases involve patent disputes about GPS applications and receiver designs, but all are subject to non-disclosure restrictions.
Suspect Tracking. In the criminal arena, a large number of cases involve GPS use to track suspects. That sort of data was used to help convict Laci Peterson’s husband of murder in a recent and celebrated California trial. Today, courts all over America are pondering whether the covert use of GPS tracking is an invasion of privacy and should require a warrant before police can use it.
Authorities use GPS quite openly to keep track of felons, child molesters, parolees, indicted suspects out on bail, people sentenced to home restraint, and so on. Supposedly, in these cases the person has already broken the law so their rights are abrogated. Or, they may have signed an agreement giving consent to such tracking in exchange for their conditional release.
In one instance, a paroled sex offender in Florida was rearrested when the tracking company informed the sheriff that he was not where he was supposed to be. After an examination of the data and with help from Google maps, it was determined that if the tracking company’s data was correct, the parolee had to be traveling at 90 miles per hour across a field where there was no road. He was released forthwith.
Law enforcement routinely uses GPS to locate stolen cars equipped with services such as OnStar.
In Malibu, California, two fishermen were stopped by fish and game deputies and charged with illegal taking of lobsters. The officers had photos and onboard GPS fixes to present in court. Unfortunately for the district attorney,
the wily defense claimed that since magnetic north had moved more than 100 meters since the maps that Fish and Game relied on were made, the maps were not accurate, and therefore the GPS data was inaccurate. The jury did not seem interested in science, the law, or the facts, and it acquitted the lead defendant. His partner chose to plead to a lesser charge and was fined, while the boat owner went free.
Market Outlook. It is highly likely that litigation regarding IP will grow as more companies profit from GPS technology, in many instances not knowing that someone holds a patent on which they could possibly be infringing. Criminal proceedings will increase as well, now that GPS tracking is relatively inexpensive for law enforcement to deploy. Meanwhile legislatures and high courts ponder how to deal with potential violations of privacy and the need for warrants.
LEN JACOBSON is a consultant to the GPS industry and has participated as an expert witness in many cases involving GPS. He is the author of the book GNSS Markets and Applications, published in 2007.
In the April edition, an article titled “DAGR Extended” covered news from the Space & Missiles Center regarding the GPS Wing awarding a follow-on contract to Rockwell Collins to provide Defense Advanced GPS Receivers (DAGR).
At the end of the article appeared an “Unofficial Word,” which made derogatory and inaccurate remarks about the use of the DAGR.
We are disappointed that the staff of GPS World did not contact us for a response to the accusations made in the article. Had you contacted us, our response would have been the following:
The DAGR provides the only means for dismounted soldiers or special operators to obtain location information of sufficient accuracy, reliability, and integrity for targeting purposes. Our warfighters use the DAGR to call in close air support missions, which the DAGR delivers GPS-guided munitions with pinpoint accuracy through its Advanced Laser Range Finder and Fire Support functions. The DAGR also provides unique Gun Laying Azimuth Determination applications.
Use of a commercial GPS in these circumstances would entail significant risk that would be totally unacceptable. No other handheld GPS is authorized, nor should it be authorized, for use in military targeting operations.
In the combat theater, our soldiers and special operators are working in extremely difficult conditions — environmental conditions where the DAGR functions consistently and provides warfighters with the information they vitally need.
On April 30, we celebrated the delivery of the 300,000th DAGR, which is proven testimony to the utility and reliability of the product.
In the future, we’d appreciate an opportunity to respond firsthand.
— Robert Haag
Senior Director, Soldier Solutions
Rockwell Collins
Don Jewell, Military & Government Editor, replies:
I could not agree with you more. At the same time, I totally disagree with your comment that our remarks were “derogatory and inaccurate … about the use of the DAGR.”
The conclusions drawn in that “Unofficial Word” (not, by the way, written by me) came directly from several industry and government warfighter panels (many of them attended by Rockwell Collins), face-to-face interviews, letters, and a plethora of personal e-mails from warfighters over the last 24 months. The results were unanimous: the DAGR, according to our warfighters who have opted not to use it, is too big, too heavy, has limited battery life, a black-and-white screen, is basically obsolete, and has a very difficult, definitely not user-friendly interface. Our interviews and correspondence show that the DAGR, as a standalone device, has been replaced by various GPS handheld or wrist-mounted units, Garmin and Trimble primarily.
How can I then agree with your comments? Because your letter very carefully only defends the use of the DAGR as an embedded device. Indeed it has been our experience that the only warfighters that consistently give the DAGR high marks are the soldiers using the DAGR as an embedded device: those responsible for directing fire — bombs or artillery on target. In a recent interview session with more than 40 soldiers, only the soldier responsible for directing fire said that he used the DAGR in any capacity. He stated, “For directing fire I use my DAGR because it has the necessary interface for laser designators and communications to direct fire. Other than that, I depend on my Garmin, as does everyone else I know, for a personal GPS unit. The Captain uses the DAGR as an input to the Blue Force Tracking (BFT) system that stays in the Humvee or Stryker vehicle.”
As I, and many others, in many articles, have said all along, the DAGR as an embedded piece of equipment, with dual frequencies, encryption, and approved government interfaces, serves a necessary and critical function: supplying BFT information and directing fire. As you correctly point out, it is the only approved government PNT source for directing fire. That is a good thing; all information and interfaces needed for the direct-fire mission have been worked out and do not need to be duplicated. Warfighters directing fire use the DAGR because there is no alternative, but for every other purpose for which they rely upon handheld GPS equipment, the DAGR is found seriously wanting. Suffice it to say the design is more than 14 years old, and the unit was dated when first released.
I have spoken to several Rockwell Collins representatives about my concerns and those of the warfighters over the years, and usually they do not dispute the DAGRs’ shortcomings. However, recently I was shown a picture, by a senior Rockwell Collins representative, of a new Rockwell Collins government GPS unit that impressed me as much as a simple picture of a GPS unit can. I asked for more information and a unit to review and I am still waiting. My problem, and I say this in all sincerity, is not with Rockwell Collins, as I know you built the DAGR to outdated government specifications that were generated in the early 1990s; by Moore’s law that is more than seven generations old by today’s standards. My primary concern is the safety and welfare of our warfighters. I know you can do much better, but the antiquated and non-responsive government acquisition system has prevented you from making changes and updating the poorly designed user interface. Rockwell Collins makes tremendous radios and avionics, which I used successfully throughout my 30-year military career, except for PLGR and DAGR units, which I consistently found to be inferior.
I consider myself a sophisticated GPS user and have tested more than 80 individual GPS units from manufacturers around the globe, yet I find your equipment and interface totally confusing. So please help me. Send me the new proposed government equipment with the color screen, the new interface, and hopefully new capabilities, and I will gladly review it in the magazine.
Several of my articles have helped gain waivers from the U.S. government for official use of thousands of commercial and civilian GPS handheld units in theater, mostly military-hardened Trimble units. If you have a great new handheld unit, then please send me an example to review and I will do that. Maybe we can get official waivers to use it in theater. I sincerely hope that is the case.
The soldiers, sailors, and airmen of the U.S. military have voted by purchasing their own units or by obtaining waivers. Even the newest recruits, whose low salary qualifies them for state assistance and food stamps, spend their money on commercial GPS units. As a very distinguished friend and world-renowned GPS expert said recently in a public forum, “You may not know it, but there has been an unofficial competition among military users for GPS handheld units, and Garmin won.” You have delivered 300,000 DAGRs, but how many of those units are actually in use today as stand-alone devices?
GAO Report
In my opinion the “GAO Questions GPS Health” article in the June issue focuses too much on the IIF as the potential problem. The May 14–15 National Space-Based PNT Advisory Board meeting heard a presentation from the DoD on GPS issues and challenges. During the briefing, Brigadier General Hyten acknowleged (as asserted in the GAO report) that there are three somewhat equally scary risks: delay of IIF, delay of OCX contract award, and delay of GPS IIIA. In the GAO report, the real doomsday scenario (in the 2015–2017 time frame) was from a two-year slide on the GPS IIIA program. You should also be aware that the graphs in the GAO report don’t account for two mitigation tools the DoD has in reserve: retired satellite still in space that could be revived (there are three at the moment), and power management as a means to extend satellite life.
I’m less worried about the first graph in the report that shows a dip in the 2010 time frame than I am about the catastrophic dip in the second chart around the 2015 time frame. I think we have a good chance of having fired our silver bullets by that time and will be much more constrained with respect to available mitigations. It is good you are writing about this as it raises awareness of the issue which could aid in the development of a more robust risk mitigation plan before this becomes a crisis.
I have been somewhat troubled by the anti-IIF program bias in the overall dialog on the subject. I don’t have full visibility or historical knowledge of what all went wrong there; what I do know indicates there was plenty of culpability to go around between the contractor and the government. I am concerned that too much focus on publicly spanking IIF will detract from fixing the root causes of the dilemma we are in: the requirements development processes and acquisition programs applied to GPS are broken. That is exacerbated by a lack of stable policy with respect to the long-term strategy for GPS development and sustainment. There are definitely lessons to learn from the IIF experience. But the difficulties associated with that program should be seen for what they are: symptoms rather than the root cause.
