Recording/Playback of GPS Signals

Doc: 9101
Recording/Playback of
GPS Signals
TECH Note
WBT-Your All In One RF
Test Solution
TM
12 March 2015
Rev: 001
TECH Note: Doc: 9101
Recording/Playback of GPS Signals
12 March 2015
Table of Contents
1.0
OBJECTIVE ......................................................................................................................... 1
2.0
SETUP .................................................................................................................................. 1
3.0
RECORDING ........................................................................................................................ 2
4.0
PLAYBACK .......................................................................................................................... 4
5.0
POTENTIAL ISSUES ........................................................................................................... 6
5.1
5.2
5.3
5.4
Q R C
RECEIVE LEVEL ........................................................................................................... 6
GPS RECEIVER COLD START ....................................................................................... 6
LIVE GPS SIGNAL INTERFERENCE ............................................................................... 6
WARNING ABOUT INDOOR GPS REPEATERS ............................................................... 7
T E C H N O L O G I E S
i
TECH Note: Doc: 9101
Recording/Playback of GPS Signals
12 March 2015
1.0 Objective
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.
2.0 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
meets these requirements.

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).
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.
Q R C
T E C H N O L O G I E S
1
TECH Note: Doc: 9101
Recording/Playback of GPS Signals
12 March 2015


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.
3.0 Recording
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.
2. From the Settings ->System Tab set;




Enable - Receive via Specials Use Connector [1]
Enable - Enable 3V Special Use Connector [2]
Set - RX Attenuation (dB) to 25. [3]
Tx Gain isn’t used, and can be any value [4]
1
2
4
3
3. Tap the Home GUI icon.
Q R C
T E C H N O L O G I E S
2
TECH Note: Doc: 9101
Recording/Playback of GPS Signals
12 March 2015
4. Attach the GPS puck antenna to the Special Use Connector.
5. Optionally attach the second GPS puck antenna to the GPS Connector.
6. 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.
7. Record at least 5 minutes of data to allow the receiver to “warm up” later when playing
back.
Q R C
T E C H N O L O G I E S
3
TECH Note: Doc: 9101
Recording/Playback of GPS Signals
12 March 2015
4.0 Playback
1. From the Settings ->System Tab set;



Disable - Receive via Special Use Connector. [1]
Set – TX Gain (dB) to 10. [2]
The RX Attenuation setting, Receive, 3V Power settings do not impact playback, and
can be any setting. [3]
1
3
2
2. Connect the TX/RX port of the tuner used to collect the data, to the GPS receiver with an
RF cable.
Q R C
T E C H N O L O G I E S
4
TECH Note: Doc: 9101
Recording/Playback of GPS Signals
12 March 2015
3. Be sure the GPS receiver is OFF (cold).
4. Start playing back the GPS data file.
5. Turn on the GPS receiver from a cold start (the almanac and clock must be cleared).
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.
Q R C
T E C H N O L O G I E S
5
TECH Note: Doc: 9101
Recording/Playback of GPS Signals
12 March 2015
5.0 Potential Issues
5.1
Receive Level
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.
5.2
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.
5.3
Live GPS Signal Interference
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).
Q R C
T E C H N O L O G I E S
6
TECH Note: Doc: 9101
Recording/Playback of GPS Signals
12 March 2015
5.4
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.
Q R C
T E C H N O L O G I E S
7
Systems
Integrator for the
Cellular
Community
Since
1987
WBT-Your All In One RF
Test SolutionTM