Timing errors in two-way satellite time and frequency transfer using spread spectrum modulation

The delay instabilities of a spread spectrum two-way satellite time and frequency transfer system (TWSTFT) are caused by errors in the measurement of group delay. A non-coherent code-tracking loop is used to make the group delay measurements. The largest errors are caused by delayed coherent interference, distortion of the spread-spectrum signal, and non-coherent code interference. Delayed coherent interference causes the most significant errors that are on the order of σ_x (τ = 1 day) ∼ 2 ns–10 ns. Delayed coherent interference is caused by signal reflections along the propagation path that result in a delayed replica of the direct signal arriving at the detector. In the TWSTFT system these reflections arise from impedance mismatch along transmission lines and electronic components such as filters and amplifiers. Phase distortion is caused by non-linearities such as gain compression in the electronic components of an earth station. The measurement errors caused by signal phase distortion due to gain compression in the earth station electronics are on the order of 100 ps/dB–1 ns/dB. Non-coherent code interference is caused by the presence of multiple codes on the same frequency (code division multiple access). These signal jamming effects are on the order of σ _x (τ = 1 day) ∼ 50 ps–200 ps.; An earth station delay calibration system cannot be used to correct the group delay measurement errors because the delay calibration system is also sensitive to the same type of group delay measurement errors that affect the two-way time and frequency transfer system. The improvements in the TWSTFT system depend on the reduction of the level of delayed coherent interference as well as the reduction of the sensitivity to interference of the code tracking loop employed in the spread spectrum modems. The sensitivity of a correlator to interference can be reduced by increasing the chipping rate and by using a correlator with smaller early/late correlator chip spacing. Maintaining linear operation through all earth components reduces signal phase distortion and introducing code dependent frequency offsets of order 10 to 100 kHz will reduce signal-jamming effects to an undetectable level.