Personnel with the U.S. Naval Observatory-Detachment Colorado and 2nd Space Operations Squadron move the rubidium fountain clock into its new home Tuesday at Schriever Air Force Base. The USNO monitors the GPS constellation and provides time offsets to the 2nd Space Operations Squadron for their daily navigation uploads to each individual GPS satellite. (U.S. Air Force photo/Christopher DeWitt).
The U.S. Naval Observatory’s Alternate Master Clock on Schriever Air Force Base received its second rubidium fountain clock February 4 to ensure it has the most precise time in the world.
Both the USNO’s Washington D.C.-based primary and its local Alternate Master Clock facility serve as the Department of Defense’s common time reference. Additionally, the USNO monitors the GPS constellation and provides time offsets to the 2nd Space Operations Squadron for its daily navigation uploads to each individual GPS satellite.
“With the new rubidium fountain clock, we are going from the time standard of 1 to 2 nanoseconds down to 300 picoseconds,” said Bill Bollwerk, Head of USNO Detachment Colorado.
One nanosecond is equivalent to one billionth of a second, while a picosecond is equal to one trillionth of a second. Though these small slices of time may not sound important, every nth of a second is significant, especially in GPS operations.
“A nanosecond matters because it is equivalent to a 1-foot of error for GPS,” Bollwerk said. “If the GPS satellite clocks were off by 3 nanoseconds, you have 1-meter of error introduced into GPS.”
Designed and produced by physicists at the USNO laboratory in Washington D.C., the powered rubidium fountain clock traveled by dedicated truck to Schriever. Once the fountain clock arrived at Colorado base, with the help of members of the 2nd Space Operations Squadron, the 50th Security Forces Squadron and 50th Civil Engineering Squadron, the USNO team moved it to a climate controlled chamber in the USNO’s laboratory via an airsled hover lifter.
“The 2 SOPS men and women are able to operate and provide accurate instantaneous reliable support to U.S. military forces around the world, thanks to our partnership with the U.S. Naval Observatory,” said Lt. Col. Thomas Ste. Marie, 2 SOPS commander. “We are happy to be able to work together to support their upgrade. Our relationship allows 2 SOPS to continually reach our goal of record breaking time-transfer performance and navigation accuracies.”
Although 2 SOPS was happy to support the move, it’s not as easy as one might think.
“The process of moving the rubidium fountain was very complicated,” said Ken Dreiling, USNO Detachment Colorado. “We had to ensure the fountain clock was not actually in contact with the floor or the walls as we moved it from the loading dock through the hallways and elevator into our facility.”
The careful transport of the fountain was essential to prevent damage that could affect the clock’s performance.
“The fountain clock collects billions of rubidium atoms, encased in a spherical vacuum chamber and laser-cooled to a millionth of a degree above absolute zero degrees Kelvin, approaching the coldest temperature anything can be,” Bollwerk said. “The reason we do that is because we want to observe and measure the atoms for long time in an environment that minimizes unwanted noise like the Doppler Shift.”
Though the Alternate Master Clock provides precise timing for several communication and space systems, Missile Defense Agency, DOD facilities and several civilian infrastructures around the world, the new system was installed primarily to support GPS operations.
“It is great to have the most precise time standard in the world but it is useless unless you can get it to the user, not everyone can come to the facility and set their watch,” said Bollwerk. “GPS is USNO’s primary means of providing global precise time to the warfighter. It is a great partnership between the Navy and the Air Force.”
Dreiling said the new fountain clock will help improve GPS operations.
“The new rubidium fountain clock is the next-generation new frequency standard,” Dreiling said. “This will boost the GPS’s timing by 10-fold.”
The VIC100 Series antenna by Panasonic is an active L1 GPS antenna designed for timing and synchronization. It offers immunity to noise and interference, and secure performance by attenuating noise and interference near the GPS L1 frequency through triple-filtering design.
The VIC100 Series is housed in a waterproof enclosure designed for excellent performance under severe environmental conditions. Its shape prevents accumulation of snow and ice, eliminating problems with bird perching and enhanced immunity to lightning surge.
Mapping turned up the heat in June, becoming a hot topic across the board. Apple ended negotiations to buy Waze, a provider of crowd sourced mapping and traffic, reportedly because the company did not want to relocate from Israel. Google quickly took Apple’s place as Waze’s buyer. With almost 50 million drivers using Waze, many via Apple Maps, Google would get another leg up in the race to own mapping. The connected car industry, gathered in Detroit last week, discussed the need for intelligibility in the market, particularly more organized categories of offerings. Also of interest this month is the backpack-mounted Google Trekker used to map the world where cars don’t go, as well as the LocationTech working group.
Surprisingly, reaction to Google’s sweeping design of new personalized maps, now in limited release, has been muted. The maps show landmarks, restaurants, and other details tailored to the user’s plans, habits, and interests that will become increasingly individualized with usage. One person’s map may include bars and public pools, another’s may include book stores and playgrounds. Google also introduced other map features like blending of Google’s place images, 360 degree views within retail shops, and 3-D satellite images of earth without a plug-in or download needed.
Connected Car Gathering. At Telematics Detroit last week, the connected car industry tried to reach much-needed clarity on the state of the connected car, with attention to standardization, consolidation, increased collaboration, and partnership. Many are trying to build a smartphone experience in the car but, “compared to a mobile phone, you’re always going to lose,” said Robert Acker of Harman. “The car is another device on the ‘Internet of Things,’ and we need to optimize that thing for consuming content while driving. Don’t make it a bigger smartphone device. That’s all Google or Apple can do. Rather, completely change the paradigm. Make it totally seamless; introduce gesture, head-up displays, steering controls. Make it truly revolutionized for the customer.”
Auto OEMs Are Changing Really. Smartphone-like capability in the vehicle is revitalizing the staid OEM industry and has encouraged car manufacturers to take more risk and speed up development time. It used to be de rigueur that a car maker would first pick a supplier like Denso to build a component, like a radio. “Now the automotive companies are first choosing a platform and layering on companies to build the solution. Tech companies are specialists,” says David Jumpa of Airbiquity. “We stand in the middle of the platform that makes it all work together.” Jumpa expects connected car services to get bundled and consumers to pay a subscription fee.
Freedom to Choose. To the unhappiness of wireless carriers, the automotive industry is planning on building cars with embedded subscriber identity module (SIM). Unlike current SIM cards that are carrier-specific, these are universal SIM cards that would enable customers to pick their vehicle’s wireless carrier and then change it at will. For OEMs, embedding SIM cards creates great efficiency. SIM cards can’t be easily replaced, as they must be soldered into vehicles because of vibration and shock. With OEMs shipping vehicles to multiple countries that have different carriers, a universal SIM card provides great flexibility and cost savings. Apple once tried to pursue an embedded SIM card and the carriers rose against it, but let’s see how the OEMs fare.
Mapping the World on Your Back. You’ve probably seen cars loaded with GPS and cameras for mapping streets. It is less likely you’ve seen mapping trikes, carts, or new photo-mapping backpack. Google uses the Trekker, a 42-pound backpack equipped with GPS and 15 cameras. Every 2.5 seconds it takes a picture as a person lugs it along trails, narrow streets, alleys, and mountains. Photos are stitched together to create panoramic images for StreetView.
Location Collaboration. A new initiative, LocationTech, has arrived on the location scene and is dedicated to individual and company collaboration on open-source software with an emphasis on location. The non-profit Eclipse Foundation, has created the working group LocationTech, led by Oracle, IBM, OpenGeo, and Actuate. LocationTech will allow companies to jointly develop and deploy components that bring location awareness to enterprise IT. “No single vendor can address the range of issues our LocationTech working group members are going to solve,” said Mike Milinkovich of Eclipse. “By creating a multi-vendor, open platform for location technologies, we intend to spur even broader adoption of location aware products, devices and services.” LocationTech might sound similar to the Open Geospatial Consortium (OGC) and the Open Source Geospatial Foundation (OSGeo); however, LocationTech offers full-service support and staffing for open-source location-aware technologies.
By bouncing eye-safe laser pulses off a mirror on a hillside, researchers at the National Institute of Standards and Technology (NIST) have transferred ultraprecise time signals through open air with unprecedented precision equivalent to the “ticking” of the world’s best next-generation atomic clocks.
Described in the April 28 issue of Nature Photonics, the demonstration shows how next-generation atomic clocks at different locations could be linked wirelessly to improve geodesy (altitude mapping), distribution of time and frequency information, satellite navigation, radar arrays and other applications. Clock signals of this type have previously been transferred by fiber-optic cable, but a wireless channel offers greater flexibility and the eventual possibility of transfer to and from satellites.
NIST researchers transferred ultraprecise time signals over the air between a laboratory on NIST’s campus in Boulder, Colorado, and nearby Kohler Mesa. Signals were sent in both directions, reflected off a mirror on the mesa, and returned to the lab, a total distrance of approximately two kilometers. The two-way technique overcomes timing distortions on the signals from turbulence in the atmosphere, and shows how next-generation atomic clocks at different locations could be linked wirelessly to improve distribution of time and frequency information and other applications.
The stability of the transferred infrared signal matched that of NIST’s best experimental atomic clock, which operates at optical frequencies. Infrared light is very close to the frequencies used by these clocks, and both are much higher than the microwave frequencies in conventional atomic clocks currently used as national time standards. Operating frequency is one of the most important factors in the precision of optical atomic clocks, which have the potential to provide a 100-fold improvement in the accuracy of future time standards. But the signals need to be distributed with minimal loss of precision and accuracy.
The signal transfer demonstration was performed outdoors over a two-way wireless link using two laser frequency combs. A frequency comb generates a steady stream of ultrashort optical pulses with a spacing that can be synchronized perfectly with the “ticks” of an optical atomic clock. (Click here for more on how frequency combs work.) In the experiment, the two combs were synchronized to the same stable optical cavity, which serves as a stand-in for an optical atomic clock. Each comb pulse was sent from one of two locations on NIST’s campus in Boulder, Colorado, reflected off a mirror on a mesa behind the campus, and returned to the other site, traveling a total distance of two kilometers.
Researchers measured travel times for pulses traveling in opposite directions between the two sites. The cumulative timing differences and frequency instabilities were infinitesimal, just one million-billionths of a second per hour, a performance level sufficient for transferring optical clock signals.
The transfer technique overcomes typical wireless signal problems such as turbulence in the atmosphere—the phenomenon that makes images shimmer when it’s very hot outside. Because turbulence affects both directions equally, it can be cancelled out. The transfer technique can also withstand signal losses due to temporary obstruction of the light path. The method should be able to operate at much longer distances, possibly even over future ground-to-satellite optical communication links as an added timing channel, researchers say.
The combs potentially could be made portable, and the low-power infrared light is safe for eyes. The research is funded in part by the Defense Advanced Research Projects Agency.
Downtown Seattle, a typical test-case environment.
Multi-Constellations Working in a Dense Urban Future
GNSS receivers in cell phones will soon support four or more satellite constellations and derive additional location measurements from other sources: cellular location, MEMS sensors, Wi-Fi, and others. The authors propose test standards covering these sources, meeting industry requirements for repeatable testing while considering the user experience.
By Peter Anderson, Esther Anyaegbu, and Richard Catmur
Cellular location test standards include well-defined and widely used standards for GPS-based systems in both the 3rd Generation Partnership Program cellular technologies of GSM/WCDMA/LTE, typically referenced as the 3GPP standards, and for CDMA technologies in the 3GPP2 standards. These standards provide a reference benchmark for location performance in the laboratory, when the unit under test is directly connected to the test system via a coax connection. In addition, standards are being rolled out, such as the CTIA — The Wireless Association total isotropic sensitivity (TIS) requirement, for over-the-air (OTA) testing and developed further with LTE A-GPS OTA using SUPL 2.0. These tests are typically performed in an anechoic chamber and allow the performance of the antenna to be included.
Recently developed standards such as the 3GPP Technical Specification (TS) 37.571-1 cover multi-constellation systems, typically GPS and GLONASS for a two-constellation system, or GPS, GLONASS and Galileo for a three-constellation system, with options for additionally supporting QZSS and space-based augmentation system (SBAS) satellites. During 2014, the standards will encompass additional constellations such as the BeiDou satellite system.
Figure 1A. GNSS systems available in the 2015-2020 timescale.Figure 1B. GNSS systems available in the 2015-2020 timescale.
Significant change is also happening with the additional technologies such as cellular location, Wi-Fi, and micro-electromechanical systems (MEMS) sensors providing location information. Hybrid solutions using all/any available location information from these multiple technologies present significant challenges to both the test environment and the related test standards.
The acceptance levels required for the platform integrators and their customers are becoming much more stringent, as the use cases of the location become more diverse. These present further challenges to the performance requirements for test standards for cellular location.
Measuring Performance
The rapid growth in the GNSS applications market has driven users to demand improvements in the performance and reliability of GNSS receivers. The test standards currently employed by cellular phone and network manufacturers to evaluate the performance of GNSS receivers are even more stringent than the regulatory mandates for positioning of emergency callers and other location-based services. Emergency-call positioning is an example of a service that must provide a position fix in both outdoor and indoor environments.
A user’s experience with a GNSS receiver begins when he switches on the device. The quality of his experience defines the basic performance criteria used to assess the performance of a GNSS receiver.
How long did it take to get a position fix?
How accurate is the position fix?
When the fix is lost, how long did it take the device to reacquire satellites and re-compute the fix?
These expectations define the performance of the GNSS receiver. Manufacturers use these performance metrics to compare the performance of different GNSS receivers.
The receiver’s time-to-first-fix (TTFF) depends on the initial conditions; that is, the type of acquisition aiding data (almanac data, ephemerides, knowledge of time and frequency, and so on) available to the receiver when it is switched on.
Users now expect location-based applications to work regardless of where they are and whether they are in a fixed location or on the move. They expect the same level of performance when they are indoors at home or at work, as outdoors in a rural or urban environment. This has led to an increased demand for accurate and reliable outdoor and indoor positioning.
Reacquisition time — how quickly a receiver recovers when the user goes through a pedestrian underpass or under a tunnel or a bridge, for instance — is not tested in any of the existing test standards discussed here.
The useable sensitivity of any GNSS receiver is key to its performance. It defines the availability of a GNSS positioning fix. The acquisition sensitivity defines the minimum received power level at which the receiver can acquire satellites and compute a position fix, while the tracking sensitivity of a receiver defines the minimum received power level at which a GNSS receiver is still able to track and maintain a position fix.
Different applications use different criteria to characterize the performance of a GNSS receiver. In an E911 scenario, for instance, position accuracy and response time are critical, whereas for navigation while driving, accuracy and tracking sensitivity are important. The test criteria employed by different manufacturers are intended to verify the suitability of a particular device for the required application.
The initial test conditions are defined by the manufacturers to ensure that the different devices are tested in the same way. These conditions describe how the test sessions are started, and what acquisition aiding data are available at the start of the test session.
The main divisions among performance tests are:
Laboratory-based tests, either conducted versus OTA RF testing, or simulated versus record-and-playback signal testing.
Real-world testing (field testing). This can be difficult because the test conditions are never the same. Fortunately, it is possible to record these scenarios using an RF data recorder. This allows the same real-world scenario (with the same test conditions) to be tested repeatedly in the lab.
Static scenario testing versus moving scenario testing.
Comparison tests — relative testing (comparing one receiver against another): for reported signal-to-noise ratio (SNR), reported accuracy, and repeatability tests.
Current GNSS Test Standards
Varying performance requirements test the TTFF, accuracy, multipath tolerance, acquisition, and tracking sensitivity of the GNSS receiver. The first three following are industry-defined test standards:
3GPP2 CDMA Performance Standards. The 3GPP2 CDMA test standards (C.S0036-A) are similar to the 3GPP test standards. The 3GPP2 is for CDMA cellular systems, which are synchronized to GPS time.
3GPP GNSS Performance Standards. The latest 3GPP TS 37.571-1 test standard describes the tests for the minimum performance requirements for GNSS receivers that support multi-constellations. It is slightly more stringent than the original 3GPP TS 34.171 test standard. In the 3GPP TS 37.571-1 coarse-time sensitivity test case, signals for only six satellites are generated, whereas in the TS 34.171 coarse-time sensitivity scenario, signals for eight satellites are generated.
Table 1 shows the power levels and satellite allocation for a multi-constellation 3GPP TS 37.571-1 coarse-time sensitivity test case. In this scenario, the pilot signal will always be GPS, if GPS is supported. The signal level of the pilot signal for GPS and GLONASS have been set as –142 dBm, while the non-pilot signal level for GPS and GLONASS have been set as –147 dBm.
Table 1. 3GPP TS 37.571-1 Satellite allocation.
For the 3GPP TS 37.571-1 fine-time assistance test case, six satellites are generated. For the dual-constellation fine-time test, the split is 3+3, and for a triple-constellation test case, the split is 2+2+2, as shown in Table 2.
OTA Requirements. Testing standards have been rolled out for OTA testing, where the testing is typically performed in an anechoic chamber, allowing antenna performance to be included, with tests for the receive sensitivity referenced to an isotropic antenna and over partial summations such as the upper hemisphere. They measure the TIS of the final receiver, and operator requirements typically require OTA acquisition sensitivity of –140 dBm and tracking sensitivity of –145 dBm or lower.
Other modified test standards used by manufacturers to assess the performance of the GNSS receiver include:
Nominal Accuracy Margin Test. This test is based on the 3GPP nominal accuracy test case. All signals are reduced in steps of 1 dB till the test fails to achieve a fix in 20 seconds.
Dynamic Range Margin Test. This test is based on the 3GPP dynamic range test case. All signals are reduced in steps of 1 dB till the test fails to achieve a fix in 20 seconds.
Sensitivity Coarse-Time Margin Test. This test is based on the 3GPP sensitivity coarse-time test case. Both the pilot and non-pilot signals are reduced in steps of 1dB till the test fails to achieve a fix in 20 seconds.
Pilot Sensitivity Coarse-Time Margin Test. This test is based on the 3GPP coarse-time sensitivity test case. The non-pilot signals are always kept at –152 dBm while the signal level of the pilot signal is reduced in steps of 1 dB till the test fails to achieve a fix in 20 seconds.
Non-Pilot Sensitivity Coarse-Time Margin Test. This test is based on the 3GPP coarse-time sensitivity test case. In this test, the pilot signal is always kept at –142 dBm while the signal levels of the other seven non-pilot signals are reduced in steps of 1 dB till the test fails to achieve a fix in 20 seconds.
These modified performance tests are used because they map directly to the end-user’s experience in the real world, measuring the position accuracy, response time, and sensitivity of the GNSS receiver.
Current Equipment. The equipment required for the current test standards are all GNSS multi-satellite simulator-based, either using a single constellation (for GPS), or a multi-constellation GNSS simulator as a component of a larger cellular test system.
Limitation of Current Standards
So far, tests for GNSS in cellular devices have been very much customer/manufacturer specific, starting with 3GPP-type tests, but adding to them. Each will have its own preferred type of tests, with different configurations and types of tests. They have included primarily GNSS simulator tests, either directly connected to the device under test or using radiated signals, together with some corner cases. With chips such as the ST-Ericsson CG1960 GNSS IC, this means that different tests need to be performed for each customer.
Typically the tests are focused on cold or hot TTFF type tests, or sensitivity type tests. Live signal tests have typically been used for drive tests, with a receiver being driven around an appropriate test route, normally in an urban environment. More recently RF replays have become much more widely used, but do require truth data to give validity. RF replay tests are typically used for specific difficult routes for urban drive tests or pedestrian tests.
The 3GPP types of test standards were developed to provide a simple set of repeatable tests. However, they are idealistic, and they do not relate closely to any real-world scenario, and the test connection is defined to be at the antenna port of the system. In reality, different manufacturers and network operator standards take these tests as a given, and define margins on the tests to allow for typical losses due to antennas and implementation on a platform. These margins might be as much as 8 or 10 dB. In addition, manufacturers and network operators define their own variants of the 3GPP tests to match typical real-world usage cases, such as deep indoor.
Challenges
Current location test specifications assume that the key input to the location calculation is always the GPS constellation. With the rise of additional constellations and alternative location sources, and the challenges of the urban environment, GPS will be one of many different inputs to the location position. The key for the future will be for standards focused on testing location performance, irrespective of which constellations are visible, and also being able to fully test the system performance. Tests will be suggested that allow the basic functionality of a system to be checked, but can be enhanced to stress-test the performance of a receiver. As future location systems will use all available inputs to produce a location, there will be challenges to the supporting test standards and test equipment to handle all of these in parallel.
The initial challenge for location test standards has been the use of GNSS constellations in addition to GPS. Current leading GNSS receivers in cellular devices make use of GPS, GLONASS, SBAS, and QZSS, and network-aiding information for A-GLONASS is being rolled out in the cellular networks. The 3GPP TS 37.571-1 specification has been derived from the original GPS-only specification TS 34.171, with the addition of GLONASS and Galileo constellation options. These allow single-, dual-, or triple-constellation tests to be performed. If there is GPS in the system, then GPS is viewed as the primary constellation, and tests like the sensitivity coarse-time assistance test would have a satellite from the GPS constellation with the highest signal level. The test standards also accommodate the use of some satellites from SBAS such as WAAS and QZSS. These tests require that the performance shall be met without the use of any data coming from sensors that can aid the positioning.
This is only the first stage in the rollout of new GNSS constellations, and in the near future, GNSS receivers in cellular phones will support four or more constellations, and possibly also on frequencies additional to the L1 band, covering some or all of: GPS, GLONASS, Galileo, BeiDou Phase 2, BeiDou Phase 3, QZSS, SBAS, and IRNSS.
Table 3. Suggested four-constellation mix (Pilot signal to rotate round constellations).
The challenge for the minimum-performance specifications is to accommodate these different constellations as they become fully available. For the new constellations, this will initially be purely simulator-based, but could be extended to use of live data for certain test cases as the constellations are built up. A further challenge for the test specifications is that some of the systems are regionally based, so a performance specification based on a global approach is not applicable.
Further, tests must be severe enough to stress the receiver. With multiple constellations, it can be simple to pass a test without using all available satellites or constellations.
Other Location Sources (Hybrid Solution). Within the cellular platform, location can be provided by a number of different technologies, either separately or compositely, to provide a location to the accuracy required by the user. Technologies currently available include:
Cellular network: cell ID and cell network triangulation
LTE Positioning Protocol
Fine time assistance (for aiding)
Wi-Fi network name (service set identifier, or SSID)
Wi-Fi ranging
MEMS sensors
Near-field communication
Bluetooth
Pseudolites, other beacons, coded LED lights, and so on.
Real-World Environments. Measuring performance in a real environment is becoming much more important, as the user experience becomes much more key. The product must not only pass particular specifications, but must also meet customer expectations. In the age of the blog, negative customer feedback can damage a product’s reputation. But with the various GNSS constellations and other sources of location information, performance testing is growing significantly in complexity, and test standards needed to cover this complexity will also become more complex. The simple user criteria could be stated as “I want the system to provide a rapid, accurate position wherever I am.” But how accurate?
The end-user of a location system does not use a GNSS simulator with clean signals, but a location device with live signals, often in difficult environments. This has been recognized by platform integrators, and live test routes for both urban drive and urban pedestrian routes are now required. The performance required of the receiver in these locations has also changed, from “just need to get a fix of limited accuracy” to getting accurate location information, both from a fix (even from a cold start in a built-up area), to continuous navigation (better than 30-meter accuracy 99 percent of the time) throughout a test run.
Typical environments for these test cases include locales in many major cities, such as the environment in the OPENING PHOTO of Seattle and one shown here of Seoul, Korea.
Seoul, Korea, a typical test-case environment.
Coexistence and Interference. Recent controversies have raised the profile of GNSS interference from other wireless technologies. However, within the cellular platform, significant coexistence and potential interference issues are already present. These can occur due to adjacent channel interference, or from harmonics of cellular frequencies on the platform, for example, the second harmonic of the uplink channel for LTE Band 13 overlays the BeiDou-2 frequency of 1561MHz, and the second harmonics of both Bands 13 and 14 create out-of-band emissions in the GPS band (Figures 2 and 3).
Figure 2. BeiDou and LTE bands 13/14.Figure 3. GPS and LTE bands 13/14.
Test Proliferation. The increase in the number of GNSS constellations together with the use of other location sources to provide a hybrid solution could increase the number of tests to be performed exponentially. When this is then combined with the need to test over a range of simulated and real-world locations, together with customer specific requirements, a set of tests could easily take weeks to run. It is therefore important to ensure that the cellular location test standards are carefully constructed to not significantly proliferate the number and time for tests to be performed.
Future Test Equipment
A new generation of test equipment is emerging to meet the new challenges and requirements of multi-constellation GNSS and hybrid location systems. These include:
GNSS Simulators. Simulators currently provide up to three GNSS constellations, together with augmentation systems. With the roll-out of BeiDou-2, four-constellation simulators will now be required. Currently all GNSS devices integrated in cellular platforms use the L1 band. This will also potentially change to multi-frequency use. The appropriate GNSS simulator will need to be included in the cellular test system.
New Hybrid Test Systems. As the need for testing hybrid positioning systems in cellular devices emerges, hybrid location test systems (HLTS) are becoming available that can simulate and test hybrids of A-GNSS, Wi-Fi, MEMS sensors, and cellular positioning technologies, all in one system.
Today, these test systems use separate simulators for the different individual technologies (like GNSS, Wi-Fi, and so on), but these are now being merged into multi-system simulators that combine a number of different technologies into one device (see Figure 4).
RF Replay. The use of RF replay units for replicating live trials is already widespread. This will extend with further constellations and further frequency bands.
The advantages of using RF recorded data include:
Gives real-world data, which if the location is chosen carefully will stress the device under test;
Allows use of recorded test data from several/many urban locations;
Good for drive and pedestrian test applications;
Will be integrated in the HLTS type of test system.
The disadvantages of using RF recorded data include:
Results not deterministic;
Taken at one point in time, do not allow for future development of satellite constellations;
Proprietary recording devices, difficult to define a standard;
Need to include an inertial measurement unit (IMU) to get accurate truth data.
The difficulties of using RF replays include:
Successfully integrating all the signal environment (cellular, Wi-Fi, MEMS, and so on);
Multiple runs required to give reliable data (for example, 13 runs at different times of day to give a range of satellite geometry and user speed, between rush hour and middle of night);
Multiple locations required to stress the system;
Test time can be up to a day of real-time testing to re-run tests on one location.
Proposal for Hybrid Positioning
Tests should include a mixture of simulator-based tests, RF-replay-based tests, and live tests. This would comprise the following suite:
GNSS Performance Tests. The 3GPP type of tests (TS 37.571-1) are a good starting point for a minimum performance test, but they rely on the person running the test to define the number of constellations. To automate this, there could be a single test at the start of each test sequence to identify which constellations are supported (one to four), and then the formal test run for that mix of constellations. The constellations supported should be reported as part of the test report.
An option should be provided to allow margin tests for specific tests to be run, and these should again be reported in a standard method in the test report, specifying how far the device under test exceeds the 3GPP test. The typical margins expected for a GPS-only test would be between 8 and 10 dB in the 2014 timeframe. For a multi-constellation test, it will depend on the specific constellations used, but could be between 5 and 8 dB margin.
Ideally, a multipath scenario should be created that more closely matches the environment seen in a real urban environment.
Hybrid Location Tests. The main purpose of the hybrid location test is to prove that the different components of a cellular platform providing location are all operating correctly. A basic test would provide a sequence where the different combinations providing location are tested for correct operation separately, and then together. This would not be envisaged as a complete stress test, but each technology should be running in a mode where a location solution is not simple.
A simple example sequence of tests would be:
GNSS performance test;
Cell ID static test;
Wi-Fi SSID static test
Cell ID and Wi-Fi SSID static test
Cell ID and GNSS static test (GNSS –142 dBm)
Wi-Fi SSID and GNSS static test (GNSS –142 dBm)
Cell ID, Wi-Fi SSID, and GNSS static test (GNSS –142 dBm)
Cell ID, Wi-Fi SSID, GNSS, and sensors moving test.
See how easily tests can proliferate!
A more stringent test could then be performed to stress-test the performance if required, and if required a playback test could be performed (see RF Replay test below).
The additional location sources can also aid in providing initial states and information for the position-determination system, in addition to the common assisted-GNSS information provided by the network. This will be particularly important in indoor and other environments where GNSS performance is compromised.
Further developments such as the LTE Positioning Protocol Extensions (LPPe) from the Open Mobile Alliance will also allow the sending of additional information to the device to improve the accuracy of the position. This additional information could include accurate time, altitude information, and other parameters. Future assistance standards should enhance the use of this information, and test standards should verify the correct use of this information.
RF Replay (or Playback) Tests. GNSS performance is statistical, and it is important to ensure that any tests have sufficient breadth and repetition to ensure statistical reliability. This applies to the more normal standard simulator tests, as well as to the uses of tests in the urban environment. For example, performance in the urban environment can vary significantly between two closely spaced runs, and can also be very dependent on the time of the day. A test done in the daytime may hit rush-hour traffic, whereas tests done at night will have relatively free flow, and hence faster average speeds. Additionally, the space-vehicle constellation geometry is constantly changing, which can enhance or degrade the GNSS performance. These factors need to be considered in generating any test routes.
For RF replay tests, a number of specific locations for urban driving and pedestrian routes should be specified. These locations should be based on network-operator test requirements, and include a mixture of suburban and deep urban environments (such as Tehran Street, Seoul). For each location, ten different data sets should be used, captured at different times, including peak rush hour at a specified hour. The data set should also include separate high-performance IMU data to provide truth data. To provide test consistency, a golden-standard data set should be used. But with different suppliers this would be difficult.
For pedestrian tests, a similar number of different routes should be defined, and data captured similarly. Ideally, all data useable for a hybrid solution should be captured, and available for replay. The test criteria analyzed for this could include: yield; horizontal position error, along-track error, across-track error, heading error, and speed error.
Interference Tests with Different Cellular Bands. It is important to have a standard test to demonstrate that the device under test does not have performance degradation due to interference from particular cellular subsystems interfering with the GNSS. For this test, the device should be tested in an OTA environment to ensure that all interference coupling mechanisms are present. Two tests should be performed: first, a tracking test. In this the A-GPS performance is tested by measuring the GNSS carrier-to-noise ratio for each GNSS band, while all the wireless channels on the platform are exercised sequentially. The test result would indicate the maximum number of dBs degradation that occurs.
Second, a cold-start test at –140 dBm should be performed separately while each wireless channel on the platform is exercised. Any extension in cold-start TTFF should be noted.
Conclusions
The challenges for cellular location test standards have increased significantly with the availability of new GNSS constellations, and the use of all available technologies within the cellular platform to provide the best appropriate location for the required use case. For test standards to be relevant, and also able to be run in an appropriate time, they must consider both the requirements to prove that the appropriate technology is operating correctly, and also bear a relationship to the final system performance required. This means, for example, that a multi-constellation GNSS receiver is really using all the constellations appropriately, and also that the end-user performance requirement is considered.
Existing cellular test standards are minimum performance requirements, but future standards should encapsulate the minimum performance requirements while also allowing standard extension to provide a consistent performance description.
Further to this, platform performance must be proved in all standing operating modes, which means, for example, that the cellular system be checked when operating in all supported bands.
Test equipment to support future cellular test standards is in development, but the significant challenges will be in providing equipment to fully support urban drive and pedestrian performance requirements.
In conclusion, the ability to appropriately test a hybrid location system, comprising multi-constellation GNSS and additional location technologies, presents almost as many challenges as generating the hybrid solution in the first place.
Acknowledgments
Many thanks to the GNSS team at ST-Ericsson, and at Spirent, and also to our customers for the challenges that they have presented as the required location performances have changed and increased.
Manufacturers
Figure 4 is taken from a Spirent Hybrid Location Test System (HLTS).
Peter Anderson received master’s degrees in electrical sciences from Cambridge University and in microelectronics from Durham University. Until recently, he was a GPS systems manager and the GNSS Fellow at ST-Ericsson; he is now a consultant with PZA Systems Ltd.
Esther Anyaegbu is a senior systems architect at ST-Ericsson. She earned her Ph.D. in data communications systems from the University of Leeds, where she focused on the processing of GNSS signals in the frequency domain.
Richard Catmur is head of standards development at Spirent Communications. He holds an M.A. in engineering science from Oxford University. He has served as rapporteur, editor, or major contributor to all 3GPP and OMA standards on the testing of positioning in wireless devices.
Qualcomm Technologies, Inc., has announced the industry’s first 4G LTE Advanced embedded data connectivity platform for mobile computing devices, including thin form factor laptops, tablets and convertibles. The technology, based on Qualcomm Technologies’ Gobi chipsets — the MDM9225 and MDM9625 — is the first embedded, mobile computing solution to support LTE carrier aggregation and LTE Category 4 with peak data rates of up to 150Mbps. The announcement came at the Mobile World Congress being held in Barcelona, Spain, this week.
The Gobi MDM9x25 embedded platform includes an embedded GPS receiver with GLONASS support for enhanced asset tracking, turn-by-turn navigation and other location-based services.
The introduction marks the arrival of Qualcomm Technologies’ third-generation 4G LTE embedded chip, extends Qualcomm Technologies’ modem technology leadership in mobile computing, and promises to deliver the fastest 3G and 4G LTE connections worldwide, while offering the broadest multi-region coverage via a single SKU solution, the company said. PC OEM customers can select from embedded module vendors that support a range of Gobi chipsets, from 3G solutions with speeds up to 42Mbps to cutting-edge 4G LTE Advanced. Coupled with pay-as-you-go, no contract data plans, these products enable thinner, lighter and better connected mobile computing devices running leading operating systems such as iOS, Android, Windows 8 and Windows RT, and support a variety of modules for thin form factors, including PCI Express Mini Card, PCI Express M.2, and Land Grid Array. Additionally, the Qualcomm RF360 Front End solution, providing expanded active band support integral to Qualcomm Technologies’ single SKU LTE World Mode solution will also be included.
“Our broad portfolio of Gobi chipsets — including 3G 42Mbps, 4G LTE and 4G LTE Advanced — features industry-leading LTE multiband support for seamless connections to the fastest networks worldwide,” said Cristiano Amon, executive vice president of Qualcomm Technologies and co-president of Qualcomm Mobile Computing. “This latest addition can be easily implemented across enterprise, SMB and consumer industries allowing end users to download and stream rich HD content, access enterprise applications, share large files quickly and connect virtually wherever they are in the world.”
Qualcomm Gobi MDM9x25 chipsets began sampling to module vendors last November and will enable commercial device launches in the second half of this calendar year.
Intuicom Inc. announced the MCR-900 to help increase productivity by extending the reach of wireless communications. Utilizing superior frequency hopping spread spectrum (FHSS) technology, the MCR-900 provides long-range data transport with continuous throughput of 115 Kbps. The MCR-900 offers the capability and flexibility to meet a variety of wireless networking challenges.
According to the announcement, designed for broad functionality, the MCR- 900 offers the flexibility to serve in simple point to point communication or in more complex multipoint and IP-bridged networks. For example, when combined with the Intuicom RTK Bridge™-C, the MCR-900 provides a way to extend the reach of RTK corrections far beyond normal cellular coverage. This application will enable precision applications such as farming, survey, and construction that use GPS/GNSS-based RTK corrections to work in areas where it would otherwise be impossible.
“The MCR-900 has been tested in the most extreme environments and continues to run day in and day out. It fits in perfectly with our overall Machine Control portfolio at Leica Geosystems and we’re proud to offer it to our customers,” said Nick Guadagnoli, OEM Business Development Manager for Leica Geosystems Machine Control Division.
Abaqus, Inc., developer of a device-neutral, cloud-based location and messaging platform, and 1Shop Wireless, national sales agent for T-Mobile USA, have teamed to provide the cloud-based myGeoTracking mobile workforce management service for T-Mobile customers.
The Abaqus myGeoTracking platform provides a cloud-hosted solution that combines network-derived and phone-based GPS location information with fine-grained privacy control options, location-enhanced SMS, and a powerful rules engine to let companies quickly and easily manage their field-personnel and assets, the companies said. It does not require special devices or smartphones, expensive data plans, cumbersome applications, or software.
Peter Giansante, director of Sales for 1Shop Wireless, called the service “an ideal device-neutral location-based-solution for companies that want to equip their employees with feature phones and smartphones, and want to avoid the deployment and training issues associated with mobile apps-based MRM services.”
“Abaqus’myGeoTracking bizTeam service is a great fit for any field-force oriented T-Mobile customer that needs to deploy a mobile management solution,” said Gillian Foley, vice president of One Shop Wireless. “T-Mobile is constantly seeking innovative ways to meet customer demands. myGeoTracking fits the bill with its cloud-based approach that provides a lot of flexibility and choice to companies seeking a better way to affordably manage their mobile workforce.”
Unlike smartphone-based applications that require special phones and client applications, the location-enhanced, SMS-based myGeoTracking mobile workforce management solution from Abaqus can locate any phone on a cellular network using either cell ID information, or a precise location using GPS inside the device. The platform provides fine-grained controls to the dispatcher or the employee in the field to pull or push location data using simple SMS commands.
The myGeoTracking platform has a rich web API which can be used to integrate with a range of back-office Enterprise systems, the companies said. The service provides SMS-based messaging for team job status reports, and has a powerful rules engine that can use geofencing, time, device identity, workgroup, and other terms to integrate into a company’s mobile workflow needs on a day-to-day basis.
The myGeoTracking bizTeam service is an MRM solution which is completely cloud-based and does not require special GPS devices, special phones or any new software.
The myGeoTracking bizTeam MRM service can use any standard feature phone to send location and event information, and complies with USPS location standards.
The myGeoTracking bizTeam MRM service provides fine-grained privacy controls, which can be managed by the end-user from their mobile phones or from the web site
Location-enhanced SMS (myGeoText) lets mobile workers actively send the date, time, and location stamped status message from the field and trigger additional dispatch functions.
The myGeoTracking bizTeam MRM service provides geofencing, geo-corridors, and a variety of other events-based rules to enable easy integration of real-world alerts into a company’s workflow
The myGeoTracking bizTeam service provides rich reporting tools which can be exported to a company’s backoffice systems
The myGeoTracking MRM bizTeam service provides an Enterprise SMS feature which lets customer’s enhance their dispatch and operations by messaging individuals, groups, or the whole company.
The myGeoTracking Platform integrates with a range of back-office programs through a rich web API.
“We’re thrilled to team up with 1Shop Wireless and T-Mobile to provide the myGeoTracking bizTeam service as a light MRM solution for small-to-medium business customers,” said Shailendra Jain, CEO/Founder or Abaqus, Inc. “We look forward to working with 1Shop Wireless and T-Mobile to grow the adoption of cloud-based location and messaging services with strong privacy controls, and help their customers achieve clear ROI and real bottom-line benefits from the service.”
Noting that the arena in which cellular emergency calling (E911) and location-based services (LBS) must be delivered has grown significantly more complex, Spirent Communications has expanded its LBS LTE test solution to support long-term evolution (LTE) positioning protocol (LPP) and observed time difference of arrival (OTDOA). The deployment of wireless LTE networks enables the introduction of new positioning technologies and protocols for communicating location information between the main network with exciting consequences for the mobile device entities, and new services they can enable.
“In 2911, we launched the secure user plane location (SUPL) 2.0 test capabilities on our LTE test solution for successful deployments of LTE SUPL positioning.” said Robb Van Brunt, vice president oaf Spirent’s Wireless business. “Our latest enhancements include extensive test coverage for SUPL 2.0/LPP and a fully automated system for testing user equipment performance for LPP and OTDOA. With our unique e understanding of the importance and complexity of these technologies, Spirent can assist customers in expediting the delivery of new location-based devices for the growing number of LTE networks.”
OTDOA is an LTE positioning technology that allows for an enhanced user experience of LBS indoors or in other areas where GNSS do not work well, and in doing so also serves assa key component for supporting E911. Based on timing measurements from multiple base stations signals, OTDOA can also be combined with GNSS measurements, creating a hybrid approach to deliver an even more significant LBS performance improvement.
LPP is the positioning protocol that enables OTDOA along with numerous other positioning technologies on the LTE network. LPP is a control plane protocol that can also be used for user plane positioning, in conjunction with SUPL 2.0.
Spirent’s expansion to its LTE LBS Test Solution supports LPP minimum performance testing, as well as a completely automated system for testing UE positioning performance, including OTDOA accuracy measurement analysis.
Federal Communications Commission chairman Kevin Martin plans to issue new rules for testing location-based E911 service, as well as a call for public comment on the technology itself. The FCC chair plans to rule soon that testing of location-based enhanced 911 wireless accuracy be conducted at local emergency call centers rather than at the state level. The Association of Public Safety Communications Officials International petitioned the FCC for such a move several years ago.
APCO will soon release its Project LOCATE (Locate Our Citizens in Times of Emergencies) report. The report studied the accuracy of location information that public safety answering points get from 911 calls made from wireless phones.
Martin plans to seek public comment on E911 technological advances and prospects for an across-the-board industry deployment of a hybrid approach to E911, which draws on both GPS technology in handsets as well as network triangulation techniques.
