Chapter 11 357
GPS Modulation
Real Time GPS
Setting Up the Real Time GPS Signal
If the signal generator is in the factory-defined preset mode, (Utility > Power On/Preset > Preset Normal User to
Normal) a basic GPS signal is automatically set up when you press the
Preset key. Perform step 2 and steps 6
through 8 to generate a signal at the RF OUTPUT connector. To set up a signal using additional features of
the GPS personality, complete this procedure starting with step 1.
1. Press
Preset.
2. Press
Mode > More (1 of 2) > GPS > Real Time GPS.
3. Press
Satellite ID > 8 > Enter.
4. Press
Doppler Shift > 2.5 > kHz.
5. Press Data Mode Enc TLM to Enc.
6. Press
Data > PN Sequence > PN15.
To create a user file, refer to “Creating a User File” on page 195.
7. Press
Real-time GPS Off On to On.
8. Press the
Frequency hardkey. Using the numeric keypad, set the signal generator RF output frequency to
1.57542 GHz (L1 carrier frequency).
9. Press the
Amplitude hardkey. Using the numeric keypad, set the signal generator RF output amplitude to
−120 dBm.
10. Turn the RF output on by pressing the
RF On/Off hardkey on the signal generator front panel.
The real-time GPS signal is now available at the signal generator RF OUTPUT connector. Figure 11-6 on
page 358 shows what the signal generator display should like after all steps have been completed. Notice the
GPS, I/Q, and RF ON annunciators are on and the parameter settings for the signal are displayed in the
status area of the signal generator display.
This procedure sets up a GPS signal that incorporates a subframe structure using 10 words, each with 24 raw
data bits and 6 parity bits computed from the data source. The signal generator created this subframe
structure when you selected Enc as the data mode and a PN15 sequence as the data type. The PN15 sequence
was encoded with the C/A code at 50 bps. Using factory preset settings, the C/A chip rate is 1.023 Mcps
with a clock reference of 10.23 Mcps. A 2.5 kHz doppler shift simulates the relative motion of the satellite
with respect to the receiver. For more information on the GPS option, refer to “Real Time GPS” on page
351.
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