The origins of satellite navigation date back to the late 1950s, when the U.S. Navy began to develop the Navy Navigation Satellite System (better known as TRANSIT), which became known as the world’s first global navigation satellite system (GNSS). TRANSIT, and its successors—the U.S. Global Positioning System (GPS) and the Russian GLONASS system—was originally designed to serve military needs, but later evolved to serve civilian purposes as well. This sparked the formation of a rapidly developing GNSS industry and has made possible the development of a variety of different GNSS applications. Specifically, car and personal navigation devices as well as location-based services in the form of various smartphone apps have made GNSS a part of our everyday lives.
The Evolution Of GNSS
Before discussing GNSS simulators, it’s important to gain a basic understanding about the different GNSS types available today and how they work. GNSS help to pinpoint a geographic location of a user’s receiver anywhere in the world by using a constellation of orbiting satellites in conjunction with a network of ground stations. For example, the most widely used GNSS—GPS—relies on a combination of 24 nominal satellites in approximately 12-hour orbits.
Each satellite transmits coded signals at exact intervals, and the receiver converts that information into position, velocity, and time estimates. These estimates are then used to calculate the position of the transmitting satellite, as well as the distance between it and the receiver, based on the transmission time delay. Finally, the receiver accurately determines the user’s position after coordinating signal data from four or more satellites.
There are currently two global systems fully operational today: GPS and GLONASS, with the most well-known GNSS being the United States’ GPS. GPS was initially developed for military purposes, and selective availability was applied to the signal, which introduced errors to the civilian signal. However, in the year 2000, the U.S. government set the selective availability of GPS signals to zero, thereby eliminating the deliberate errors that were previously introduced into civilian receivers. This action paved the way for widespread consumer adoption.
The other active GNSS today is Russia’s GLONASS, which first launched in 1982, but fell into a state of disrepair until it recently achieved global coverage and full operational capability at the end of 2011.
Following the success of GPS and GLONASS, other countries are currently developing navigation systems and have already launched satellites in space for construction and in-orbit validation. The European Union (EU) is creating a civilian-operated GNSS called Galileo, which will be interoperable with GPS and GLONASS. Additionally, China is designing a global system known as BeiDou, and India and Japan also have regional satellite navigation systems in the works, respectively known as the Indian Regional Navigational Satellite System (IRNSS) and Quasi-Zenith Satellite System (QZSS).
Typical GNSS Applications
In addition to the personal navigation use that most of us associate satellite navigation systems with, there are many other useful applications1 for GNSS technology, including:
• Aviation: Air flight navigation requires a high level of accuracy for the en route navigation, approach, and landing.
• Automotive: Customized in-vehicle navigation systems help users by providing reliable driving directions. New modernized systems include safety enhancements that improve vehicle handling characteristics.
• Weak signal navigation: Certain applications, such as indoor environments where the signal quality is poor, require an enhanced GNSS rather than standalone system.
• Space: GNSS are primarily used in low Earth satellites, but are increasingly being used in space vehicles operating at higher altitudes.
• Agriculture: GNSS field measurements combined with geographic information system tools provide accurate regional maps for resource monitoring and management.
• Geodesy and surveying: The precise positioning information afforded by today’s sophisticated GNSS enable us to monitor the movements of the Earth’s crustal plates or ice shelves.
• Scientific: GNSS can be used for remote sensing of the environment and space weather studies.
The Benefits Of GNSS Simulators
An RF-based GNSS signal simulator is an excellent way to validate the performance of GNSS receivers and systems for research and development, manufacturing and system integration testing. GNSS simulators are used in approximately all applications, from aviation to civilian and military use, and offer an advantage over live GNSS signals by enabling complete control over the signals and conditions. This allows users to more accurately test GNSS systems before they are used in a real-world setting.
A simulator produces the identical signals that are transmitted from GNSS satellites under a controlled setting so users can regulate certain parameters such as the date, time, and location; vehicle motion; environmental conditions; and signal errors and inaccuracies. Therefore, using an RF-based GNSS signal simulator is the preferred method for testing satellite navigation receivers during R&D, design, manufacturing, certification, and maintenance stages because it offers a more reliable approach than using a live satellite.
Types Of Simulators
In order to understand why it is useful to perform GNSS tests with a simulator, it is important to comprehend the testing needs of different user groups and the simulation requirements that can be derived from various test applications. There are a number of simulator devices, and each is ideally suited to a particular application or user group based on its design.
There are two types of full-scale RF signal simulators: single-channel and multichannel2. A single-channel system can simulate a signal from one satellite and usually has the capability to control a signal’s Doppler profile. This type of simulator is suited for production and R&D testing.
On the other hand, multichannel simulators perform simulation of multiple satellite signals and are commonly used for R&D, design, manufacturing, and post-launch tests. One key benefit of a multichannel RF simulator is that it provides repeatability of the signal generation. Not only does it simulate multiple satellite channels, but also complete constellations at runtime. RF simulators are also capable of simulating single or multiple frequencies, which is advantageous for users, as they can work with a number of frequencies from a sole platform.
While analog simulators were the original type of system developed, today’s modernized digital simulators offer many benefits, including the flexibility to reprogram the simulator on the fly. An analog simulator usually requires a separate frequency generator for each satellite, whereas a digital one may only need one synthesizer for each frequency. This means that digital systems are able to eliminate interchannel biases that are more prevalent in analog simulators. Analog systems also introduce higher phase noise compared with digital versions, making digital systems the ideal solution for today’s GNSS simulator needs.
In general, an arbitrary band limited RF signal can be generated by an I/Q modulator3. This analog component has three main inputs: I, Q, and LO. It receives two independent baseband signals at the I and Q ports, commonly referred to as the in-phase and quadrature components, respectively. This is also the origin of the term I/Q modulation. The LO input of the modulator is usually connected to a frequency synthesizer generating the RF carrier wave.
Mathematically, I/Q modulation works by using the carrier wave twice, once directly and once with a phase shift of 90°. The two orthogonal carrier signals with the frequency fc are individually multiplied with either the I or Q input signal. The output is generated by adding both products.
Consequently, I and Q form a complex baseband signal with real and imaginary parts. The main advantage is that the baseband signal is already the combination of all simulated satellite vehicles at this frequency. In this case, interchannel bias is eliminated completely.
An RF recorder/replay system is yet another type of signal simulator that collects data which is then processed by a software receiver. A recorder simulator is perfect for aviation applications, where it is necessary to test the integration of GNSS receivers with inertial navigation systems. In this situation, a RF recorder system can be used to generate the information from one flight that would otherwise require hundreds of flights. Replay devices enable a user to play back a recorded signal and reverse the operations it went through. This is helpful in situations where the digitized intermediate frequency (DIF) signal is insufficient and needs to be bypassed.
A final type of simulator is a pseudolite, which is a single-channel simulator that provides in-the-field simulation of future satellite systems. This type of system is ideal for testing the signals of upcoming GNSS such as Galileo in Europe or BeiDou in China; however, a pseudolite simulator is limiting in that it can only be used for R&D purposes and in restricted areas.
Modern Simulation Capabilities
Modern GNSS simulators are designed to meet the needs of any application, ranging from research and development of GNSS safety and professional applications, to system integration and production testing of mass market applications including automotive SatNav, mobile phone apps, chip-sets, and handheld personal navigation devices. As discussed earlier in this article, they support a variety of channels and frequencies and, while most standard GNSS services can only simulate one simultaneous transmit antenna, some of today’s more sophisticated systems provide additional flexibility by giving users the capability to simulate two, three, or four multi-GNSS sources from a single unit. Additional advancements include:
• Smooth Doppler shift simulation for precise signal simulation even when simulating high-dynamic trajectories.
• GNSS correction data generation. Correction data, usually in the RTCM format, minimizes the effects of atmospheric and satellite errors on position determination.
• CAN-Bus interface for automotive applications, which emulates a wheel’s sensor data in real time according to the motion behavior of the vehicle.
• Antenna patterns to simulate the effects of different receiver antennas
• Multipath models, such as point reflector, statistical channel model, and import of multipath data by file help users more precisely simulate environmental effects.
• A remote control interface that includes commands to control and steer the simulator without using the GUI. This means that the GNSS simulators can be integrated with an existing automatic test system, thus providing users with the capability to build commands into existing test systems.
One practical example of a modern GNSS simulator is shown in the picture on the next page. For this experiment, a WORK Microwave NavX®-NCS Professional GNSS simulator was used and a u-blox AEK-4H served as the receiver. The generated RF signal is directly fed into the receiver input. A static position is simulated, and the Google Earth visualisation displays its position, which is directly located at the foot of the Statue of Liberty. The receiver on the right shows the correct position and simulated satellites in the sky plot.
Of course, it would also be possible to simulate movement, such as driving from New York to Philadelphia or flying from Paris to Amsterdam. The receiver perceives no difference in the RF signal than it would from a signal picked up from an antenna. It is also possible to add specific errors to the signal for receiver tests, which cannot be achieved using the real signal in space.
More Streamlined Systems Yet To Come
These are just some of the innovative features that modern GNSS simulators offer while aiding in the successful research and development of global satellite navigation systems. As additional GNSS signals become available and augmentation systems drive forward new growth, more enhancements will be made to GNSS.
Technological developments in the near future promise to greatly improve the functionality of GNSS. For example, the United States is currently working on the third generation of GPS, and Russia continues to innovate GLONASS.
The accuracy and reliability of GNSS will also be improved by third-party augmentation systems, such as WAAS and EGNOS. As these augmentation systems are optimized, users will experience an even more streamlined satellite navigation system.
Editor’s note: The capabilities discussed in this article are based on the WORK Microwave and IFEN NavX-NCS GNSS RF signal simulator (pictured below), a full-featured, multifrequency, multisystem, and multichannel signal generator. NavX-NCS provides support for all of today’s SatNav systems, including GPS, Galileo, GLONASS, QZSS and BeDou, simultaneously, and from a single platform. It is ideally suited for R&D of GNSS safety and professional applications, as well as system integration and production testing of mass market applications, including automotive SatNav, mobile phone apps, chip sets, and handheld personal navigation devices. NavX-NCS GNSS simulators feature as many as nine L-band frequenceis and 108 channels, offering users more than twice the number of channels than standard simulators. Advanced features include Doppler shift simuation, multiple RF outputs, RTCM correction data, and more. For further information, contact sales@work-microwave.de.
About the authors
Markus Bochenko is an R&D Engineer and has worked with Work Microwave GmbH in Holzkirchen, Germany since 2009. He is responsible for development and the hardware implemention of complex algorithms as well as FPGA and Embedded System Design.
Andreas Blumenschein is a R&D Enginner who has worked with Work Microwave GmbH since 2008 and is responsible for FPGA design and microcontroller programming as well as the coordination of hardware production.
Dr.Guenter Heinrichs is a Communications Engineering Expert who has been the Head of Customer Applications with IFEN GmbH in Poing, Germany, since 2002. He is responsible for global product strategy of the Multi-GNSS Navigation RF Constellation Simulator (NavX-NCS) and is also accountable for all customer specific application developments that relate to the NavX-NCS.