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 emergence of an ever-growing number of GNSS applications has driven the need to use GNSS simulators to ensure time- and cost-efficient development cycles. This article provides an overview of how GNSS applications can efficiently be tested, the type of simulators and simulation methods that are used, and which simulation capabilities a modern simulator should include.
Typical GNSS Applications
In addition to the personal navigation use that most of us associate satellite navigation systems with, there are many other useful applications 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.
- Marine: GNSS are standard on all boats today.
- 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 exact same signals that are transmitted from GNSS satellites under a controlled setting so that 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 multichannel. 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.
In general, an arbitrary band limited RF signal can be generated by an I/Q modulator. (See Figure 1.) 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.