record and playback: applications and testing examples
DESCRIPTION
Radio-frequency Record and Playback Systems (RPS) are crucial tools in GNSS (Global Navigation Satellite Systems) receiver and chipset testing. Learn what Spirent’s industry-leading GSS6400 RPS can do. Discover: - The quality benefits of testing with recorded, repeatable RF signals - The time and cost benefits of minimising testing in the field - How our RPS can help with hardware and software testing, and algorithm studiesTRANSCRIPT
GSS6400 RPSApplications and Testing Examples
This document…
… describes the applications, uses and results that can be obtained when using the GSS6400 signal Record and Playback System for the development and test of GLONASS and GNSS receivers.
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The GSS6400 Record and Playback System reduces your testing, trials and travel costs substantially by allowing you to record real life GLONASS or GPS signals (including the real world fades, multipath and interference) onto the internal hard drive of the unit. Once you have this data you can then repeatedly play it back into your system under test in the comfort of your own lab, without the need to return to the field, saving you time and money.
Figure 1Spirent GSS6400
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Typical Applications include
• Software and Hardware Testing• Repeatability Tests• Manufacturing Test• Performance analysis• System trials• A-GPS trials• Ionospheric and interference monitoring • Algorithm studies• Position• Multipath• Sensitivity
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Example uses for the GSS6400
Chipset and product design Tuning of software and algorithms to provide optimum performance in real world situations occupies a lot of development time. In particular environments the traditional approach has been to position development resources on site and conduct real time trials whilst developing improvements and solutions to problems. This has the added difficulty of not recreating the exact conditions on subsequent tests, and required engineers to be away from mainstream development for extended periods of time.
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With the RPS system the GNSS signal environment can be recorded once, maybe at different times of day and then used back at “base” for subsequent development and testing. If local personnel are available for recording, then travel for development engineers is eliminated entirely.
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Prove new algorithms, especially for new high sensitivity receivers
GNSS receivers begin their testing and performance proving with simulator signals and long test campaigns. Moving to the “real world” as experienced by end customers often requires the algorithms to be field tested, using repeated runs in known environments such as city areas and locations with high interference or obscuration. By holding a library of pre-recorded signals from such areas, much of this work can be achieved without leaving the bench, thus cutting both costs and development time. Changes to algorithms are easily proven using the same signals over and over, without the need for more field testing.
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Production testing
Production testing usually involves the replay of the same signal scenario for each test run, or using off air signals for testing. The GSS6400 provides a cost effective method of providing repeatable GNSS signals under the control of a test PC, or from its front panel. Once the test signal has been recorded (from a simulator or from live sky), consistent replay can then be obtained, freeing up the simulator for other work and preventing live sky anomalies interfering with testing.
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Reducing Field Trial Costs and Position Repeatability
A Spirent GSS6700 simulator was used to generate a scenario in HONG KONG, using both GLONASS and GPS. The direct output from the simulator was fed into a GPS/GLONASS receiver and the positions determined and output into Google Earth. The same scenario was then recorded onto the GSS6400 and then subsequently played back.
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The following picture shows the tracks of the positions as calculated directly from the simulator signal, and those played back through the GSS6400. The green dots are direct from the simulator and the red after playback.
Figure 2Overlay of calculated positions direct from a simulator, and played back from the GSS6400
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Figure 3Positions from simulator, and replayed (zoomed in)
If we zoom in further we can see the results between the outputs direct from the simulator and those recorded and played back on the GSS6400 are within a metre.
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Figure 4Multiple replays from the GSS6400
This is the same test again but with 3 playbacks from the GSS6400, compared with the direct simulator output (green). This shows that the signals played back from the GSS6400 can be used to produce both ACCURATE and REPEATABLE positions from the stored GLONASS/GPS signals.
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The GSS6400 has several features to make the field recording simple:
• Single box with no need to attach a PC or external drive• 500GB internal hard drive for 30 hours of recording• No unnecessary wires• An internal battery that lasts for 1 hour while recording• Simple one touch recording (a long press on the front panel Enter
button), see below• LCD and front panel controls• Works with 12V Car power adapter, and no need to worry if this
power source inadvertently fails as the internal battery will seamlessly take over
• Carry case and handle for walking trials
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Algorithm Tuning - Position Repeatability - Batch Testing - Lab use
When GNSS tests are undertaken in dense urban canyons there are fewer observations because of the obscuration of the satellites. This leads to the solution only just having enough satellites to be able to make a position determination, and the accuracy is degraded. Typically in dense urban areas to obtain a position fix at all you have to rely on multipathed signals reaching the antenna via several bounces off buildings. With only a few satellites in view it is possible that the solution appears tens of metres off the true route.
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The GNSS receiver also has to work very hard to re-acquire the satellites as it moves in and out of areas of satellite visibility. The software will be searching in the time and frequency domains in order to re-acquire a lost satellite, or find new ones, and will be increasing its integration steps to allow it to acquire satellites at very low signal levels. System designers and software engineers therefore spend a lot of time tuning the filters and rerunning test after test.
Once recorded, the GSS6400 replays exactly the same signal each time. GNSS system manufacturers can therefore use the GSS6400 to replay, time and time again, the RF signals stored during the initial trial. It allows the developers to improve their tracking and navigation software so that the effects of multipath, interference and fading signals are minimised.
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For example filters may have to be tuned and acquisition strategies determined to get the best possible fix while the unit under test is being driven round a roundabout, while at the same time losing lock on the satellites and re-acquiring the signals as it goes under the fly-overs. So instead of having to do the test in the field on many an occasion, the GSS6400 can record then data once and then it can be played back multiple times.
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In the following picture the vehicle drove round the roundabout two times (the white lines) but then the same signals were recorded and subsequently played back from the GSS6400 a total of six times.
Figure 6Algorithm tuning
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For use in the Lab the GSS6400 has some beneficial features:
• Can be controlled by the front panel OR you can attach a Screen and Keyboard and use the inbuilt Webserver, which gives you full control over the system:
Figure 7Webserver Page 18SPIRENT eBook Page 18
• The signals can also be attenuated to see the effect that has on algorithms under test, by up to 31dB in 1 dB steps.
• Data can be archived onto external SATA drives using the fast data transfer functions of SATA (150MB/s).
• Data files recoded on one system can be played back on another unit.
The GSS6400 also allows you to automate your test process as:
• Scripts can be written to automatically play back given files in a given order
• Be controlled locally or remotely via the web to maximise the use of the system
• You can VNC into the system for additional flexibility
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How GSS6400 works:
The GSS6400 RPS employs a low noise amplifier for optimum overall system noise performance prior to down-converting to IF for digitising. An AGC circuit caters for a wide range of active or passive antenna gains. Standard antennas with gains of up to 27dB and 3.3 V nominal DC power can be used directly.
The digitised IF is stored on an internal 500Mb hard drive giving up to 30hrs of record time. Along with the data file containing the digitised signals, an accompanying information file is automatically generated containing details such as record time and date. Further data can be added to this file for record keeping purposes.
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Data files can be stored using two bit (sign and magnitude) or one bit (sign only) quantisation, depending upon the requirements. On playback the IF signal is recreated and then up-converted to RF at the GPS L1 frequency of 1575.42MHz, and the GLONASS centre frequency of 1602 MHz, using a built in oven stabilised local oscillator (OCXO) for minimum phase noise.
A common 26.0MHz TCXO is used as the clock for GPS/GLONASS devices to keep I/F samples in phase with each other. The I/F is sampled at 13MHz for GPS with a filter bandwidth of 2.5MHz, and at 26MHz for GLONASS with a filter bandwidth of 8MHz.
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Carrier to Noise performance
The GSS6400 faithfully records and plays back the RF signals observed during a trial to within a few dBs of the value observed directly in the field. Typically for 2 bit GPS or GLONASS recording the difference is just 1dB, while at 1 bit it is 3dB. The recording includes all the real-life fades, multipath and interference as seen in the real-world.
As we have seen if you compare the position output of the GPS/GLONASS receiver used in real time and the output from the GSS6400 with the same receiver the positions will match almost exactly.
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We can do a similar analysis by looking at the Carrier to Noise ratios of a recorded, and then played back set of data. In this test we are recording 2 bit GPS signals over a period of 15 minutes.
If we calculate the AVERAGE C/No from the Real time and the Played back over the 15 minute period we have:
In this 15 minute test the average played back Carrier to Noise Ratio is 0.65 db below that of the real time signal.
Figure 7Carrier to Noise comparison
Real Time Average C/No (dB) Played Back Average C/No (dB)
44.91 44.26
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CONCLUSION
The GSS6400 record and playback system can greatly reduce the cost of field trials, and the time taken to develop new algorithms, by bringing the complexities of ‘real-world’ GNSS signals back into the office.
The GSS6400 is highly portable and self contained, allowing easy recording in the field. When used in the office however it is transformed into an highly efficient piece of laboratory equipment allowing GNSS signals to be replayed with accuracy and repeatability. It is very flexible, allowing automated operation for testing, and can be controlled remotely.
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If you found this eBook of interest we think you’ll also be interested in downloading “An introduction to GNSS Record & Playback System Pt 2”
We are continually adding new content to our website on a regular basis. Bookmark this link: www.spirent.com/positioning
Visit the Spirent GNSS Blog, there are currently more than 90 posts with 2 to 3 new posts added per week. Catch up on what’s new: www.spirent.com/Blog/Positioning
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