Agenda

• GPS technology concepts

• GPS and GNSS overview

• Assisted GPS (A-GPS)

• Basic tests required for GPS receiver verification

• Test solutions

• Signal creation for GPS receiver test

• A-GPS test systems

© 2010 Agilent Technologies

Typical Tests for GPS Receiver Verification

Time To First Fix (TTFF)

Sensitivity

Cold, warm, hot start conditions

Acquisition sensitivity

Location Accuracy

Acquisition sensitivity

Tracking sensitivity

Absolute and relative accuracy

Moving GPS receiver accuracy

Satellite tracking accuracy

© 2010 Agilent Technologies

Time to First Fix (TTFF)

• Time between start of GPS signal and the acquisition of a location fix by the GPS receiver

• May be tested under different conditions or states of the GPS receiver:

Time To First Fix (TTFF)

For cold, warm, hot start conditions

• May be tested under different conditions or states of the GPS receiver:

– Cold start: Receiver does not have time or position information, no valid ephemeris (or almanac) data (typical TTFF 30-50 sec, may be up to several minutes)

– Warm start: Last position and approximate time known, valid almanac, no valid ephemeris data (typical TTFF 30-40 sec)

– Hot start: Time and last position known, valid almanac and ephemeris (typical TTFF 1-5 sec)

• Need to specify satellite power level when testing TTFF

� Test requires a multi-satellite GPS signal with valid navigational messages

© 2010 Agilent Technologies

Sensitivity

Sensitivityminimum = -174dBm/Hz + C/Nominimum + NFreceiver

Sensitivity

Acquisition sensitivity

Tracking sensitivity

• Minimum level of signal that allows GPS receiver to acquire or track the GPS signal (may also be specified in terms of C/No)

– Acquisition sensitivity: minimum level to successfully perform TTFF under cold start (typically around -140 to -150 dBm)

– Tracking sensitivity: minimum level to maintain location fix once it has been attained (typically -150 to -160 dBm)

� Test requires multi-satellite GPS signal with valid navigational messages for TTFF, and real-time satellite power control to reduce power levels to test sensitivity

© 2010 Agilent Technologies

Location Accuracy

• Absolute location accuracy: Closeness of the receiver’s calculated

Location Accuracy

Absolute and relative accuracy

Moving GPS receiver accuracy

Satellite tracking accuracy

• Absolute location accuracy: Closeness of the receiver’s calculated location fix to the ideal (simulated) location

• Relative location accuracy: Compares location fixes between tests

• May be tested for both stationary and moving GPS receivers

• Satellite tracking accuracy: Track location of single satellite

� Tests require multi-satellite GPS signal for location fix, repeatable test scenarios, moving GPS receiver scenarios, and ability to vary power and other satellite parameters to test tracking

© 2010 Agilent Technologies

Secondary GPS Verification Tests

Calculate reacquisition time

• Time required to resume location fix following loss of

signal

RF interference measurements

• Measures the ability of the GPS receiver to operate in

the presence of interfering (jamming) signals (second RF

source required)

© 2010 Agilent Technologies

Why You Need GPS Simulation

GPS simulation offers the following advantages over “live” satellite

signals:

• Repeatability

– Ensure exact replication of the signal conditions for each test

• Flexibility

– Use different scenarios to simulate different locations and times, with different – Use different scenarios to simulate different locations and times, with different

satellites and orbits

• Control

– Use standard test scenarios throughout your process

• Stress Testing

– Turn individual satellites on/off and change satellite power

– Reduce satellite visibility

– Introduce specific, known impairments

© 2010 Agilent Technologies

Requirements for GPS Test Signals

GPS Signal Simulation

Feature Test Challenges Met

Multiple channels to simulate multiple

satellites (4 to 15)

Minimum of 4 for TTFF. 15 allows you to

reproduce all satellites visible from any

location.

Stationary and moving GPS receiver Test more realistic case of moving GPS

TTFF AccuracySensitivity

Stationary and moving GPS receiver

scenarios

Test more realistic case of moving GPS

receiver as well as simple stationary case

Real-time satellite power adjustments,

satellite visibility on/off

Test sensitivity, ability to reacquire satellite

signals

Custom scenario generation capability Flexibility to create scenarios for any date,

time, or location

Requirements for GPS Test Signals

Impairments

Feature Test Challenges Met

Multipath signals (requires more

channels)

Simulate real-world situation with reflected

signals from obstructions

Reduced satellite visibility: partial or

complete, elevation mask

Test loss of visibility due to tunnels,

mountains, or other obstructions

TTFF AccuracySensitivity

complete, elevation mask mountains, or other obstructions

Ionospheric and tropospheric modeling Account for atmospheric effects on GPS

signals (refraction, delay)

Add calibrated AWGN Add broadband noise to control C/No of

GPS signal

© 2010 Agilent Technologies