ELT-TT-21

  • Ground-station Two-Way Satellite Time and Frequency Transfer modem

  • 1 Tx and 8 Rx CDMA channels

  • Low systematic errors by design, fully digital implementation

  • Fully compatible with current BIMP/UTC TWSTFT modulation and codes

  • Manual as well as remote configuration/commanding

  • Modem local loopback performance σx(τ=1s) ≈ 1 psRMS

Introduction

The Two-Way Satellite Time and Frequency Transfer (TWSTFT) modem is designed to allow highly accurate transfer and/or comparison of time among two or more ground stations via a satellite bent-pipe transponder. The general principle complies with [ITU]. Two or more ground stations are connected to shared satellite channel. All these stations do have some local realisation of the time to be compared to the others. Two-way ranging measurements between any of the two stations are disseminated in a standardised data file format.

Each modem generates a PN-spread ranging signal, which is being transmitted to the satellite. At the same time, the modem receives from the satellite the transponded and delayed echo of own signal, plus superimposed signals from all other stations. In real time, the receiver section processes the received signal and estimates the code delay and carrier phase which are the outputs of the modem.

Figure 1. Concept of operation

Concept of operation


Interfaces

Electrical specification

The target electrical specification for the interfaces are summarised in Table 1.

Table 1.  Electrical interface description

I/F Parameter Specification Condition / note
Tx frequency range 0…100 MHz nominally 70±20 MHz
output level −50…+3 dBm
impedance 50 Ω
reflection coefficient −15 dB
Tx mon output level other −60…−7 dBm fixed Tx −10 dB
other same as Tx
Rx1, Rx2 frequency range 0…100 MHz nominally 70±20 MHz
input level −50…+3 dBm
impedance 50 Ω
reflection coefficient −15 dB
Clk in frequency 10 MHz
input level −10…+10 dBm
impedance 50 Ω
reflection coefficient −15 dB
1 pps in input level 0…5 V
impedance 10kΩ
Clk out frequency 10 MHz
output level 10 dBm
impedance 50 Ω
reflection coefficient −15 dB
1 pps out output level 0/4 V
impedance 50 Ω

Front and rear panel

The modem is housed in a 2U 19 inch rack chassis. The external interfaces of the modem for both front and rear panels are listed in tables below. The modem baseline is 70 MHz intermediate frequency only.

Table 2.  Modem rear panel interface description

Interface Physical Description
Rear panel
Tx N Tx Intermediate frequency (70 MHz)
Tx monitor N Tx, −10 dB
Rx1, Rx2 N Rx Intermediate frequency (70 MHz)
Clock in N 10 MHz reference clock input
1 pps in BNC 1-second frame signal complementing Clock in
Clock out N 10 MHz synthesized clock output
1 pps out BNC 1-second frame signal complementing Clock out
Ethernet 1000Base-Tx (RJ45) NTP, measurement and control
2x Monitor BNC, LVCMOS Generic digital monitoring/debug signals
Keyboard USB-A User external keyboard input
External display HDMI Output for external display for extended information about modem operational status
SSD storage 2.5" SATA Removable drawer for SATA SSD (bit-grabber, high-speed logging)
2x digital data interface Display port Digital data interface for future use, e.g., for transfer of sampled data between cascaded modems
Power supply IEC C14 100V–240V power supply input

Table 3.  Modem front panel interface description

Interface Physical Description
Front panel
Display VFD User interface
Keypad built-in pushbuttons User interface (subset of essential functions)

Figure 2. Modem front panel (not fully assembled)

Modem front panel (not fully assembled)


Figure 3. Modem rear panel

Modem rear panel


Communication interfaces

Besides the user entry, the modem offers full commanding and telemetry via remote interface. The interface implemented is an ASCII based, SCPI-like protocol running over TCP/IP.

Table 4.  Modem user-configurable parameters

Tx output power on/off
output signal level [dBm] range see Electrical specification
carrier phase w.r.t. 1pps [s] 0…1 s, for test purposes
code phase w.r.t. 1pps [s]
Rx start/stop acquisition
input signal attenuation [dBm] 0…60 dB
max. carrier frequency Doppler deviation [Hz]
Tx and Rx carrier frequency [Hz] any 0…100 MHz
code chip rate [Hz] 1 to 20 Mch/s, pre-defined set
modulation scheme | legacy TWSTFT | BPSK
PN code | code number / polynomial MLS polynomial or table upload
PN code length | number of chips up to 65535, pre-defined set
common subsystems modem mode | network / master / slave
TCP/IP settings
SFTP file access settings
NTP settings
station settings (lat/lon/alt)
satellite settings (TLE)
date/time settings, including UTC−TAI offset
ionospheric correction (TEC)

Table 5.  Measurement quantities

Tx read-back of the set parameters
read-back of code and carrier phases (phi, dphi, ddphi)
Rx (per channel) readout of code and carrier phases (phi, dphi, ddphi)
standard deviations
block averages over extended periods
code lock status
signal level [dBm]
C/No [dBHz]

Extension and configurability

The FPGA firmware as well as SW is flexible enough to support various types of modulations and modulation parameters. Notably: independent PN-code on I/Q carriers (“QPSK mode”), data-less pilot signal, SinBOC/CosBOS subcarriers. E.g. a broad range of existing GNSS signals may be received and/or transmitted.

Nevertheless Eltor sees a great opportunity in such a configurability: apart from optimising the TWSTFT performance, it may be useful to receive e.g. GNSS or other DSSS signals using the same modem as TWSTFT signals in order to support various inter-system calibrations.

References

[ITU] Recommendation ITU-R TF.1153-4 (The operational use of two-way satellite time and fre- quency transfer employing pseudorandom noise codes). International Telecommunication Union. 2015.