Over at Turn Island Systems (TIS) and the HamSCI group, we are studying ways to measure Time Of Flight over various propagation paths and frequencies. We can currently do this crudely with WSPR and FST4W transmissions but the Time Of Transmit is only roughly known and the modulation characteristics only allow measurement accuracy in the millisecond region.
WWV and other frequency and time standard transmitters include precise timing information in their signals, but we need more transmitter locations and frequencies for a more complete data-set.
Work on the receive end is being done with the RX-888 SDR and ka9q-radio, and I have designed some units that provide a local in-band time reference signal for these receivers — testing is underway.
For the transmitter, some work has been done using GPS-synchronized 1-second BPSK. I am expanding this here to use not only the 1-second modulation, but also a faster PRBS data pattern that still complies with the new occupied bandwidth regulations. This is definitely a work in progress, and my transmitter will be turned on and off while we figure out the details. But for now, here is the signal format:
Carrier freq: 7.090 MHz
- Minute 0: transmit morse ID message
- Minute 1-2: transmit PPS BPSK
- Minute 3-4: transmit PRBS DBPSK
- Repeat
The PRBS pattern is the 1023-bit GPS CA code #1, 170.5 Hz chip rate, 10-second sequence rate. The modulation is DBPSK (NRZI/BPSK).
The transmitter location is Occidental, California, and the antenna is an off-center-feed 40 meter dipole. The photo shows my stack of little boxes I am using to generate these signals. From top to bottom they are:
- TIS-1W One-Watt amplifier – logic-level in, square wave out, 1-50 MHz, powered via 5V USB-C. The input is provided by:
- BeaconBox – a lightly-modified early TIS test board, designed during the early stages of the TIS WSPRSONDE. This is connected to a GPS antenna for time and frequency synchronization. I am also giving this board a reference clock from an external 10 MHz OCXO.
- A TIS Filter-Combiner – this takes the output from the 1W amplifier, turns it into a clean low-harmonics sinewave, and sends it to the antenna.
Note that the PRBS modulation is technically *not* Spread Spectrum (that may come later, pending rules changes). We have chosen this particular PRBS pattern because it is well-documented, and has good characteristics permitting potential multiple transmit sources to share a common frequency.
Acronyms, etc.:
- PPS : Pulse Per Second
- BPSK: Binary Phase Shift Keying
- DBPSK: Differential BPSK
- PRBS: Pseudo-Random Bit Sequence
- GPS CA Code: Here’s a good link that describes this:
https://natronics.github.io/blag/2014/gps-prn/
Please contact me if you have any questions or comments!
paul (at) wb6cxc.com