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GPS/GNSS Recording and Playback Overview and Setup

In the field, capture up to 80 MHz of multi-channel bandwidth and return to your lab with a rich library of GPS and GLONASS signals and impairments to accelerate RF product designs and research. WBT offers these GPS & GNSS recording and play back features:
  • Record and play back real-world RF signals, up to 80 MHz wide
  • Capture actual RF spectrum like FM, DAB, GPS, GLONASS, and cellular communication protocols
  • Visualize and record weak GPS signals
  • Advanced signal analysis with the Spectrum, Waterfall Histogram, Signal Analysis Toolkit
  • Log GPS NMEA data at the same time as recording RF
WBT can record and play back live any of the L-Band GNSS GPS signals and up to 80 MHz of the associated environment. The WBT records not only the GNSS signals and any SBAS when present, but also the background noise or interference, making the replayed signals truly representative of the real-world conditions. WBT Capture and playback covers Galileo, Compass, QZSS, GLONASS, GPS, SBAS, and CNSS Bei Dou signals in the field, and playing them back in the lab with optimal fidelity and performance.

Overview

Record an L1 C/A GPS signal over the air onto a WBT, and playback later into a GPS receiver. The picture shown above is from a Linx Technologies RXM-GPS-FM development board, and represents any GPS Receiver that can take an RF input with an isolated GPS input.

GPS Analysis Tool


Other GNSS systems would use a very similar procedure to the one outlined using GNSS Frequencies, see Differences with GNSS from GPS below for more details

Setup

The following items are needed:

  • WBT-2xx
  • An active GPS antenna (such as a “puck”) with > 25 dB internal gain, powered by 3V DC, preferably with an SMA connector. The antenna that comes with your WBT kit
  • GPS Antenna Puck

    Note: For other GNSS types, a different antenna should be selected for GNSS Record and GNSS Playback
  • A GPS receiver with external antenna input that is to receive the playback of the recorded GPS. The antenna input should completely override any other RF source the GPS would use (e.g., internal antenna).
  • A short SMA Cable (optionally with adapters, if needed) to connect the WBT to the GPS receiver for playback.
  • An optional second GPS puck to record NMEA GPS strings in the data file while recording.
GPS Record Note
We used a Linx Technologies RXM-GPS-FM development board for our testing, as well as QRC’s Surveyor-500 doing TDOA position estimation to confirm the accuracy of GPS Playback.

GPS Record

  1. From the Session GUI set Tuner A or B as follows;
  • Set the Center Frequency 1575.42 MHz.
  • Set the Span 3.125 MHz or greater.
GPS Record System Diagram


GNSS Recording and GNSS Playback Addtional Note
For other GNSS signals, including P-type GPS a larger number may be required. The WBT is capable of recording 80 MHz losslessly and can handle all GNSS Record and GNSS Playback needs. For questions on your specific needs, please contact us at QRC and we cna help you find the right settings for your specific need
  1. Attach the GPS puck antenna to the Special Use Connector.
  2. Optionally attach the second GPS puck antenna to the GPS Connector.
  3. Be sure the GPS antenna(s) have a reasonable view of the sky. For indoors playback when there’s a GPS repeater in use, see below in section 5.4.
  4. Record at least 5 minutes of data to allow the receiver to “ warm up ” later when playing back.
  5. Connect the TX/RX port of the tuner used to collect the data, to the GPS receiver with an
  6. Be sure the GPS receiver is OFF (cold).
  7. Start playing back the GPS data file.
  8. Turn on the GPS receiver from a cold start (the almanac and clock must be cleared).
GPS Record System

GPS Playback

GPS Playback System Diagram

  1. Connect the TX/RX port of the tuner used to collect the data, to the GPS receiver with an
GPS Playback Diagram

  1. Be sure the GPS receiver is OFF (cold).
  2. Start playing back the GPS data file.
  3. Turn on the GPS receiver from a cold start (the almanac and clock must be cleared).
GPS Measurement Software

GPS Playing Note
You cannot loop playback GPS files, as soon as the end of the file is reached you need to reset the GPS Receiver and clear it’s almanac before repeating your test.

Potential GPS Record and GPS Playback Issues


GPS Receive Level too Low

When recording, the GPS “ noise floor ” should be no higher than -80 dBm to avoid strong out-ofband signals saturating the front end. If necessary, add more attenuation if saturation is observed.

GPS Receiver Cold Start

GPS receivers do not like jumping around in time. They will not restart correctly if time suddenly jumps backwards. When playing back a GPS file, the GPS receiver should be purged of all time/almanac information first. The receiver must be fully cold started to pick up the played back GPS time and satellites, or it will latch on to the live current time and get confused when seeing new signals which from its point of view are back in time.

Our in-house test receiver used for testing (a Linx Technologies RXM-GPS-FM development board) has a “ Cold Start” option in a command string that lets us do this quickly. Other units may need to be powered down and have their batteries removed. Please consult the manual for the GPS you wish to play back into to determine how to clear its almanac. As a note, from our experience, we know that hand-held Garmin devices behave very erratically when playback starts, and recommend the device be power cycled after the playback has begun so it restarts with the signal being played to it already present.

Making sure the GPS sees only the signal being played back and no other GPS signals (including the on the air one). The playback may appear to partially and intermittently work if more than one signal is present. However, receivers typically repeatedly drop tracking and never latch on to the recording, because it is convinced the current time and almanac info are valid, when they are not.

Live GPS Signal Interference on GPS Playback

Many receivers are so sensitive that they can pick up live GPS signals without an antenna, or even thru the external RF cable. In this situation, the played back file must be transmitted with enough power to fully overwhelm these weak signals, or the receiver will become very confused about which is the correct signal.

Most modern GPS receivers are very sensitive (acquiring down to -143 dBm, tracking at -160 dBm). Even with no antenna attached, our test GPS receiver continues to track satellites. This is particularly true if indoor repeaters are being used, as the GPS amplifier in the building is stronger than the real signals outside. When playing back the recorded GPS signal, it is unknown how the live GPS signal is interacting with the played back signal. Our suggested approach is to transmit into the GPS device with more power to swamp out weaker over-the-air signals (which is why we recommend a transmit gain of 10 dB or more).

Warning About Indoor GPS Repeaters

Indoor GPS repeaters may put out a stronger signal than the normal over-air signal, making use of playback files even more difficult. Do not playback into a GPS receiver directly under an indoor GPS repeater antenna. Increasing gain of the playback will help, but moving away from the indoor repeater is preferable.

Differences with GNSS Record and GNSS Playback from GPS

GNSS refers to multiple Global Position Systems solutions, including GPS, GLONASS, Galileo, And Beidou. From the WBT's point of view, the differences are largely a matter of setting the correct center frequency and bandwidth to get the desired signal. With this exception, generally the rest of the Tech Note applies, with minor adjustments in antenna or filters only. The following image shows all of the frequencies and bandwidths that would be used to record these systems.

Common GNSS Frequencies and Bandwdith

As can be seen, the WBT is well equipments with 80MHz of bandwidth between 50MHz and 6 GHz to perform well for all your GNSS Record and GNSS Playback needs. Even better, with two seperate tuners, the WBT can record GPS L1 and GPS L2 (for example) at the same time if needed, a common requirement in Military system testing that most other systems are simply incapable of.