Testing with SDR-based GPS/GNSS simulators

Article By : Mohammad Usman

Simulating complex interference environments in the lab offers a range of clear benefits over live range tests, and makes situations that would be virtually impossible to test on a live range feasible through a simulated scenario.

Editor’s Note: This is the third article in a three-part series about software defined radios in GPS/GNSS simulators. Click to read Part 1 and Part 2.

Software Defined Radios (SDRs) are a valuable resource for test & measurement (T&M) of RF device function and capability development. These radio systems are particularly enticing due to their ability to be upgraded via software and/or field-programmable gate array (FPGA) IP cores, and thereby providing T&M engineers with extended flexibility, capabilities, and cost-efficiency.

It is imperative to do extensive testing of GNSS-based products during the design and development phase. It can be beneficial to include an SDR-based GNSS simulator in the testing regime. One of the advantages is that the necessary test routines in the lab can be automated, tests can be programmed to cater for necessary power thresholds in order to protect RF components from damage, and tests can be planned in a particular order and sequence to ensure reliability and consistency and to reduce the problems caused by human error.

The test equipment can be integrated into lab servers and accessed remotely through IP connections by test engineers in case they are not in the same physical area as the equipment. The SDR can be pre-programmed to transmit signals at specific time intervals during field operations and testing. This function can be extremely beneficial in remote and hard-to-access areas such as mobile phone towers and geographically challenging locations  where routine access is not possible. Since an SDR is a single piece of equipment it is an optimal solution for size, weight, and power (SWaP) and would be beneficial in areas where these measures are constrained.

SDRs are not limited to just one transmission scheme or waveform, and can be reconfigured to add new waveforms or to operate as a different type of radio altogether. SDRs can store a larger amount of data than traditional stand-alone equipment and have very high performance network interfaces to transfer huge amounts of data to a host system. An ever-increasing number of technologies are using unprecedented levels of bandwidth. To meet the dynamic capture and processing requirements of spectrum monitoring, SDRs and external data processing systems have become the de-facto standard.

Multiple-input multiple-output (MIMO) wireless technologies have the ability to significantly increase raw data throughput in spectrally limited environments, while at the same time providing immunity to the multipath effects common in urban settings. The availability of multiple configurable radio channels allow for the SDR to receive and transmit for MIMO operation, and thus operate and process information on various frequencies and channels.

SDRs have the ability to reconfigure the system to accommodate changing testing conditions and scenarios, in addition to processing data, and acquiring and recording various tests and measurement results. It is easier to upgrade the software in the system to the latest protocols and algorithms, and can be attached to various T&M interfaces.

The design and assembly of necessary hardware is usually associated with high overhead costs, on the other hand, offering software updates are much more cost effective and save valuable after sale expenditure. Using an SDR does not lock a platform into a specific set of communications and functions, as they can be easily upgraded even by a remote connection, thus saving on potential upgrade and logistic costs. SDRs provides an efficient and comparatively inexpensive solution to the problem of building multi-mode, multi-band, multi-functional wireless devices that can be enhanced using software upgrades. As such, SDRs can really be considered an enabling technology that is applicable across a wide range of areas within the wireless industry.

GNSS/GPS simulation examples

The low cost and high performance of GNSS receiver chipsets has resulted in many original equipment manufacturers (OEMs) including a GNSS receiver in their products. The selection and integration of suitable receivers requires a lot of effort and engineering development. In order to test a GNSS receiver, it needs to be subjected to similar conditions as that of an actual field environment. This requires a simulator to closely synthesize the signal emanating from the GNSS satellite i.e. satellite orbits and coordinate system, ephemeris data generation, code generation and signal modulation as well as simulate noise, attenuation, atmospheric effects and multipath etc. A GNSS SDR based simulator allows for repeatable testing of devices with GNSS receivers in simulated environments. The following are some of the basic tests that are performed to test a GNSS receiver.

Time to First Fix

Time to First Fix (TTFF) is the amount of time, normally measured in seconds, that the receiver requires to calculate the position, navigation and timing (PNT) solution, also known as position fix or a fix. These are known as cold, warm, and hot starts. When a cold start is performed, the receiver memory is cleared, and the receiver must perform calculations without using any saved data, which includes information about the satellite constellation (almanac), and precise orbit and clock data from each satellite (ephemeris). The TTFF is longest with a cold reset as the receiver must systematically search for all possible satellites. After acquiring a satellite signal, the receiver can begin to obtain approximate information on all the other satellites, called the almanac. This almanac is transmitted repeatedly over 12.5 minutes. Almanac data can be received from any of the GPS satellites and is considered valid for up to 180 days.

A warm reset or hot start TTFF is often performed by powering down and then powering a device back on. A warm reset keeps the almanac information in memory and deletes the ephemeris and the date, time, and location information. The time required of a receiver in this state to calculate a position fix may also be termed time to subsequent fix (TTSF). This test is important for applications where the user must get the position or time information as quickly as possible. For example, it is important for GNSS receivers used in automobiles for navigation because the user needs to rely on directions, which cannot be provided until a fix is obtained. The values of this parameter for modern GNSS receiver like SEPTENTRIO (model AsteRx-m2a UAS) are as follows; Warm Start: <20s, Cold Start: <45s and Re-Acquisition (average): <1.2s.

GNSS Sensitivity Test

GNSS sensitivity is defined as the lowest signal level at which a GNSS receiver is able to track and achieve a position fix. Sensitivity is measured in dBm and is one of the most important parameters of a GNSS receiver. The main areas of test are acquisition sensitivity, tracking sensitivity and the navigation sensitivity.

Typically, the RF power level received by an antenna on the ground will be between -125 dBm and -150 dBm depending on environmental factors and time of day. To generate very low RF power levels, it is required to use external passive attenuators to reduce the power output.

Acquisition Sensitivity

Acquisition sensitivity represents the minimum power level at which a GNSS receiver can achieve a position fix. This is the minimum level to successfully perform TTFF under cold start. Acquisition sensitivity is usually around -140 to -150 dBm.

The acquisition algorithms perform a two-dimensional search for the signal, the first dimension is time and the code phase is the time alignment of the pseudorandom noise (PRN) code. The second dimension is frequency, since the frequency of the signal from a specific satellite can differ from its nominal value as the line-of-sight velocity of the satellite causes a Doppler shift resulting in a higher or lower frequency. The acquisition plot of a GPS PRN in Matlab is depicted in following figure. It can be seen that RPN code peak visible in back ground noise. In case of low signal power or in the presence of noise or interference, the receiver is not able to acquire the GPS signal.

GPS Signal acquisition plot for PRN 31.
Figure 1: GPS Signal acquisition plot for PRN 31. (Source: Per Vices)

Tracking Sensitivity

Tracking sensitivity is the minimum power level at which a receiver can receive and maintain a connection with a satellite or constellation of satellites. In terms of TTFF states, this would be the warm/hot start state. Since the acquisition process has acquired the satellite, the receiver has information about the satellites in view. The tracking sensitivity is usually lower than the acquisition sensitivity (-150 to 165 dBm).

Navigation Sensitivity

Navigation sensitivity is the minimum power level at which a receiver can maintain a connection with the GNSS Satellites to provide an accurate location during navigation.

Interference Testing

There are some shortcomings with GPS in particular (and other GNSS systems), mainly due to the weak received signal strength. For example, the GPS L1 signal strength is of the order of -160 dB (lower than the noise floor) and the signal can be easily blocked by buildings and other objects, including foliage. Furthermore, the GPS signals can be easily jammed or degraded by unintentional RF interference, jammers, and spoofers. Interference testing is a type of meta-test, in that some of the above tests such as sensitivity or TTFF are done with the simulation of an interfering signal. Some other GNSS receiver tests include location accuracy, multipath testing and antenna testing etc. All of the tests can be conducted with the help of an SDR-based GPS/GNSS simulator.

GPS Simulation/testing for Military Test Ranges

For years, governments and OEMs had to rely on ‘JamFests’ or exercises, often conducted on remote test ranges to test GPS receivers or other GNSS-reliant equipment against real-world interference. These field tests presented a range of problems from the logistics (moving equipment and trained engineers to an often remote location) to issuing warning notices in the local area. However, due to the advances in GNSS simulators using SDRs, these test can be conveniently performed in lab environments with an embedded, software interference simulator or an external hardware interference alongside a simulator. Simulating complex interference environments in the lab offers a range of clear benefits over live range tests, and makes situations that would be virtually impossible to test on a live range feasible through a simulated scenario. It is now possible to test scenarios that include natural atmospheric effects, dynamic and static sources of interference, spoofing and multipath errors, among others.

GPS Field Testing

Although an SDR based GNSS simulator will facilitate development of GNSS products during the design and development phase, for some high grade and military GNSS equipment, it is necessary to test the end-product in actual field environment, especially during final qualification process. It will ensure GPS receiver performance while testing in tactical, live environments, and help characterize the impact of actual interference.

One such facility, the U.S. Army Electronic Proving Ground (EPG) executes developmental testing in direct support of Department of Defense (DOD) and various industry partners. The GPS Test Facility performs a variety of specialized GPS tests using EPG outdoor test ranges and the GPS Instrumentation Suite (GPSIS), along with specialized data collection, reduction and analysis tools. The GPSIS consists of three GPS simulators that provide full control of the signal structure to replicate the live GPS constellation, allowing repeatable laboratory testing under controlled conditions. The Fort Huachuca’s Buffalo Soldier Electronic Testing Range allows personnel to conduct testing in a variety of real-world environments including desert, canyon and mountainous terrain with or without foliage. With over 25 years of GPS test experience, EPG is well known and respected in the test community. Sites such as these can (and some are) make use of SDR based simulators to help further the flexibility and performance of traditional GPS/GNSS simulators.

There are a number of different elements that need to be tested for a variety of GPS/GNSS receivers which require complex simulators. These simulators can benefit significantly from the incorporation of SDRs to provide flexibility associated with tuning range/frequency, number of channels, changing of RF parameters, and the ability to simulate different environments easily. These SDR based systems can allow for more in-lab testing as well as an upgrade path for new measures to be tested and deployed to GPS field testing facilities.

 

This article was originally published on Embedded.

Dr Mohammad Usman did his BE (Electrical Engineering) from University of Engineering and Technology Lahore, Pakistan and MSc and PhD from the University of Manchester, UK. He has more than twenty eight (28) years of experience in the private industry and public sector research organization of Pakistan in the field of electronics design, software development, communications, project management and aerospace product development. His research interests include GNSS (Global Navigation Satellite System) based remote sensing, GPS based bi-static SAR (Synthetic Aperture Radar) and GNSS/GPS signal processing.

 

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