2m FSK beacon

Following up on the success of the low-power CW beacon built by Steve G4AQB, Ross G6GVI has built a second 2m beacon transmitter, this time using an ESP32 microcontroller and incorporating a GNSS satellite receiver to determine time and position, plus an atmospheric sensor to measure temperature, pressure and humidity.
Rather than Morse On-Off Keying, this uses two modes of Frequency-Shift Keying to transmit the data at a much faster rate.

This prototype was assembled from left-over bits from other projects and as the radio board was designed for use at 433MHz, a custom low-pass filter is needed to reduce spurious harmonic emissions when operating at 144MHz (see below). The reference for the transmit frequency is just a crystal, so it may drift over a few hundred Hertz as the temperature changes.

This beacon operates around 144.021MHz, transmitting just 10 milliwatts every 30 seconds: first with simple 2FSK "RTTY" and then with the more complex 4FSK "Horus Binary v2" modulation, both at a rate of 100 baud. It will be interesting to compare the performance of these modes under weak signal conditions.
Both of these FSK formats can be decoded uisng the HorusGUI software, although the RTTY (7-bit ASCII with No parity and 2 stop-bits) can also be decoded with many other digimode programs. The freqency shifts are 427Hz for the 2FSK and 244Hz for the 4FSK modulation (note that these are different from the defaults).

Each telemetry frame consists of the following elements:
Callsign, frame_number, time, latitude, longitude, altitude, satellites, supply_voltage, temperature, pressure, humidity, fieldstring, checksum
The HorusGUI program will automatically calculate the distance, bearing (and elevation) from your receiver and when operating in 4FSK mode display the sensor readings too:

Note that in Horus Binary, the battery voltage is in "modulo-5" format, so voltages in the 5-7V range will be shown as 0-2V.

If the "Upload to Sondehub" box is ticked, it will automatically send the received packet to the Sondehub Amateur site, along with your received Signal-to-Noise Ratio and frequency (if you've entered your Radio Dial Freq, in the panel). Please don't enable this feature whilst we're testing on the ground (instead you could Enable Logging in the "Other" tab and save a local .CSV file).

The Horus Binary 4FSK mode has been popular on 70cm for High-Altitude Balloon flights in Australia and around continental Europe over recent years, but we believe that this is the first time it's ever been deployed on 2m.

Output spectrum of 433MHz unit operating on 2m

Output spectrum with low-pass filter fitted

The beacon will be run on test from time to time from G6GVI's QTH in central Bolton.
One test used a lightweight "halo" (omnidirectional horizontal) antenna at about 5m above ground, but different antennas may be used at other times.

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