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 σ_x(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

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
Tx mon	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
1 pps in	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
1 pps out	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)

Figure 3. 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
Tx	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 Eltvor 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.

Figure 4. Installation of Ku-band terminal and experimental campaign with ELT-TT-12 modem

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.