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Experimental Results

Experimental Results. Static Testing. Objective Characterize performance improvement obtained by increasing the Gold code length on the ATS 6502 (Time Transfer Modem) Description Compare TWTT measurements of the same event using two independent systems

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Experimental Results

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  1. Experimental Results

  2. Static Testing • Objective • Characterize performance improvement obtained by increasing the Gold code length on the ATS 6502 (Time Transfer Modem) • Description • Compare TWTT measurements of the same event using two independent systems • Event: Clock offset between U.S. Naval Observatory and Symmetricom (Boulder) • System 1: TSC 4402 (Device under test) • System 2: Timetech SATRE TWSTFT Modem (Baseline) • Differences in the measurements represent the combined measurement error of the two systems • Experimental setup (diagram on next slide) • Each site clock used as a common reference for both TWTT systems • Both TWTT systems utilize same type of Ku-band transceiver • Minimize performance differences due to ancillary equipment • Each TWTT system operated over a different satellite • Effects from satellite motion will be different for the two systems

  3. Experimental Setup AMC-1 (129º West) Intelsat 707 (53º West) Size: 1.8 meter BW: 3.0 MHz Data Rate: 0.25 kbps Code: 6 Size: 1.8 meter BW: 3.0 MHz Data Rate: 0.25 kbps Code: 7 Size: 1 meter BW: 2.5 MHz Data Rate: 2.5 kbps Code: 11 Size: 1 meter BW: 2.5 MHz Data Rate: 2.5 kbps Code: 27 Symmetricom (Boulder) U.S. Naval Observatory

  4. Antenna Configuration • SATRE TWTT system • 1.8 meter dish • 4 Watt Ku-band Transceiver • Anasat 4Ku • Symmetricom TWTT system • 1.0 meter dish • 4 Watt Ku-band Transceiver • Anasat 4Ku 1.8 meter 1.0 meter Symmetricom (Boulder)

  5. Performance Comparison • Systems agree well during first half of experiment • Short-term noise is comparable • Long-term agreement is very good • 40 ns cycle slip removed from the ATS 6502 data (shown later) • Last half of experiment showed significant discrepancies • Multiple events of > 3 ns • One event of > 15 ns • Root cause of the events is being investigated (discussed later) ATS 6502 (black) vs SATRE (red) TWTT Comparison σ = 1.7 ns Residual (1 ns / division) Offset (5 ns / division) Time (1.2 hours / division) Time (1 day / division)

  6. Gold Code Comparison • One of the primary noise sources from earlier experiments was due to interference between the signals • Extending the Gold code from 127 bits to 511 bits improved the cross-correlation properties of the signals • Medium term variations (2-3 hours) reduced considerably • Trade-off is that the link operates at 2.5 kbits/second instead of 10 kbits/second 127 bit Gold Code (1 sec) 511 bit Gold Code (1 sec) σ = 0.95 ns σ = 1.3 ns Residual (2 ns / division) Residual (2 ns / division) Time (0.5 days / division) Time (0.5 days / division)

  7. Cycle Slip • ATS 6502 data experienced a 40 ns cycle-shift early in the experiment • 40 ns corresponds to the 25 MHz clock used internal to the system • Currently investigating the source of the shift • Performing simulations within the FPGA to determine if there is a potential bug that causes it to get off by a clock cycle • Trying to replicate the event on multiple systems to determine if it is a hardware failure that is isolated to a specific unit 127 bit Gold Code (1 sec) Residual (2 ns / division) Time (0.5 days / division)

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