SimTX API/Transmitter session

A transmitter session puts a signal on the air. The client binds the transmitter to one of its persistent stations, waits for a transmitter slot, and then streams audio or IQ samples in real time while the server modulates them, models the power amplifier and antenna, and propagates the result to every receiver. Transmitter sessions use ROLE_TX_AUDIO or ROLE_TX_GENERATED and need an account with the game.transmit permission.

Frames

Direction	#	Message	When
C2S	12	TransmitterStart	Once, right after the hello.
S2C	10	SessionStatus	BOUND or REJECTED after the start; later UPDATED and ENDED.
S2C	13	TxQueue	While waiting for a slot.
S2C	14	TxAccept	When the transmitter goes on the air.
C2S	13	SampleBlock	Continuously after TxAccept (ROLE_TX_AUDIO).
C2S	15	TxModeInfo	Optional, after TxAccept.
C2S	14	TxUpdate	Optional, to change settings on air.
S2C	18	TxUpdateAck	In reply to each TxUpdate.
C2S	16	AntennaPointingUpdate	Optional, to steer the antenna or change tracking.
S2C	17	TxTelemetry	About every 50 ms while on air.
C2S / S2C	2	Ping / Pong	Any time.
S2C	3	Error	When a pointing request fails, or on a protocol violation.

Sequence

 client                                                    server
   | POST /api/v1/game-tickets {role: "tx-audio", stationId}   (HTTPS)
   | -- preface, [1] hello (ROLE_TX_AUDIO, ticket) --------> |
   | <----------------------------- [1] hello_result         |
   | -- [12] transmitter_start (site.station) -------------> |
   | <----------------------------- [10] session_status      |   BOUND, station snapshot
   | <----------------------------- [13] tx_queue            |   (only if all slots are busy;
   | <----------------------------- [13] tx_queue            |    repeats about once a second)
   | <----------------------------- [14] tx_accept           |
   | -- [13] samples --------------------------------------> |
   | <----------------------------- [17] tx_telemetry        |
   | -- [13] samples --------------------------------------> |
   | -- [14] tx_update ------------------------------------> |
   | <----------------------------- [18] tx_update_ack       |
   |                      ...                                |
   | -- close ---------------------------------------------> |   off the air

Placing the transmitter

A transmitter normally runs on one of the account's own persistent stations:

1. Choose a station, an antenna and optionally a rig from GET /api/v1/stations. See SimTX API/Station endpoints.
2. Mint a tx-audio or tx-generated game ticket with the same stationId, antennaId and rigId.
3. Send the hello with the ticket, then a TransmitterStart whose site.station is a StationBinding with the same ids. Empty antenna and rig ids accept what the ticket bound.

The station dictates the callsign, placement, antenna and pointing defaults; the bound rig dictates the PA cap and most imperfection knobs. The session chooses the modulation, frequency, power up to the cap, source, sample rate, gain, bandwidth, low cut, tone and duty cycle, plus drive, compression, frequency offset and AM index. See What the station dictates. Only the owner may transmit through a station.

On a satellite, the transmitter is a downlink transmitter at the satellite's position, and moves with it.

Ad-hoc transmitters

A start whose site is an AdhocStation is accepted only for principals that hold tx.adhoc (service integrations) or when the server's ad-hoc policy allows it. By default servers refuse it with ERROR_CODE_STATION_REQUIRED. Third-party clients should always bind a station.

Admission

After the start, the server answers with SessionStatus:

• REJECTED: the start was refused. code and reason say why, and the connection closes. See REJECTED.
• BOUND: the start is accepted. For a station-bound session, station describes the station, antenna and rig and the effective PA cap. Then:
  • TxQueue if every transmitter slot on the server is busy. Further TxQueue frames report the position as it changes, at most about once a second. Keep reading and do not send samples yet.
  • TxAccept when the transmitter is on the air. Start streaming.

Sources

source_type decides what the client streams.

Source type	Role	Client sends	Server does
AUDIO_STREAM, MICROPHONE, PIPEWIRE	ROLE_TX_AUDIO	Mono audio, one float per sample at sample_rate, nominal range ±1, in SampleBlock.audio	Applies audio_gain, the audio low cut and compression, then modulates with the requested modulation.
GNU_RADIO_IQ	ROLE_TX_AUDIO	Complex baseband, I and Q interleaved, at sample_rate, centred on the carrier frequency, in SampleBlock.iq or SampleBlock.compact_iq	Applies audio_gain, normalises average power to at most 1.0 (equal to power_watts), and transmits it as it is. Use modulation IQ.
WHITE_NOISE	ROLE_TX_GENERATED	Nothing	Generates the signal itself.

With an IQ source, the client is the modulator. Anything that can be expressed as complex baseband, including digital modes the server has never heard of, can be transmitted this way. TxModeInfo gives such a signal a name.

Sample rates

The server processes each transmitter at a rate equal to the declared sample_rate, raised to at least 240,000 for WFM and limited to 10,000,000. Audio is resampled to that rate by a factor of at most 128; a lower audio rate is refused. Raw IQ above 10,000,000 samples per second is refused.

Streaming

Samples are sent in SampleBlock frames of up to 65,536 values. IQ sources may use the compact 16-bit form, which halves the uplink.

Pacing

The client must send samples in real time, at the rate it declared.

• The server keeps about 0.5 s of samples as a playout buffer. When its ring buffer is 90 % full, it stops reading from the socket, and TCP flow control slows the client down. Sending slightly ahead is therefore safe.
• Input arriving faster than 1.05 times the declared rate, beyond a burst allowance of about two seconds of samples, is discarded block by block without notice.
• buffer_seconds in TxTelemetry reports the server's buffer. It is the best pacing signal available: keep it above zero and below about two seconds.

Keying

There is no push-to-talk frame. A transmitter is on the air from TxAccept until the connection closes. To go quiet without disconnecting, send silence, or set the power to 0 with TxUpdate.

An audio source that delivers no samples for 15 seconds is disconnected. An IQ source is not disconnected for being idle, but it is still on the air, transmitting nothing, and still occupies a transmitter slot.

To leave cleanly, shut down the sending side of the socket (half-close) and then close it.

Live updates

TxUpdate changes frequency, power, bandwidth, audio low cut, drive, headroom, compression and audio gain while on air. Every field is optional; send only those that change. The server answers each update with a TxUpdateAck that reports, for every field set, whether it was applied and the value now in effect. At most one update is applied per 250 ms, and the frequency at most once per second; updates sent faster are refused in the ack, not queued.

Other settings, such as the modulation, sample rate, source or station, cannot be changed on air. Close the session and open a new one. Antenna pointing and tracking change with AntennaPointingUpdate.

Tracking and Doppler

A transmitter can follow a target with its antenna and pre-correct its carrier for the target's Doppler shift, so that the signal arrives on frequency at a satellite: set TransmitterStart.tracking, or send an AntennaPointingUpdate later. See SimTX API/Tracking and pointing.

Telemetry

While on air the server sends TxTelemetry about every 50 ms. It reports forward, delivered and reflected power, SWR, amplifier compression and temperature, thermal foldback, and the playout buffer level, the same values the official transmitter shows on its meters. Telemetry is coalesced: a slow reader gets the newest values, not a backlog.

The client must keep reading. A client that stops reading for 5 seconds is disconnected.

Taken off the air

The server ends a running session with SessionStatus ENDED, a code and a reason, when a moderator or the operator removes the transmitter, the address is banned, the station is deleted or loses the bound antenna or rig, or a satellite reenters. The connection closes right after. When the account lease expires the connection is closed. See SimTX API/Session status.

Limits

• The power is capped by the PA cap: the bound rig's paMaxWatts and the account's tx.pa_limit.N capability, whichever is lower (BoundStation.pa_cap_watts). Requests above it are reduced to the cap; TxTelemetry shows the actual power.
• The number of simultaneous transmitters per account and per station is limited. See SimTX API/Limits and errors.

Example

Building a station-bound TransmitterStart and an audio SampleBlock with the helpers of Quick start:

import uuid

# Ids from the game ticket response ("station": {"stationId", "antennaId", "rigId"})
station_id = uuid.UUID("3f2504e0-4f89-41d3-9a0c-0305e82c3301").bytes
antenna_id = uuid.UUID("9b2c1e47-2f0a-4c55-8d1e-6a7b8c9d0e1f").bytes

binding = f_bytes(1, station_id) + f_bytes(2, antenna_id)   # rig_id empty: the rig the ticket bound
start = (
    f_bytes(1, f_bytes(1, binding))      # site { station { ... } }
    + f_bytes(3, "USB")                  # modulation
    + f_double(4, 14_230_000.0)          # frequency_hz
    + f_float(5, 100.0)                  # power_watts
    + f_bytes(6, "AUDIO_STREAM")         # source_type
    + f_varint(7, 48_000)                # sample_rate
    + f_float(8, 1.0)                    # audio_gain
    + f_float(9, 2_700.0)                # bandwidth_hz
    + f_float(10, 300.0)                 # audio_low_cut_hz
    + f_float(12, 1.0)                   # duty_cycle; tone_hz (11) stays 0
)
send_frame(sock, 12, start)              # transmitter_start


def sample_block(samples):               # SampleBlock { audio: PackedFloats { values } }
    packed = struct.pack("<%df" % len(samples), *samples)
    return f_bytes(1, f_bytes(1, packed))


# after TxAccept, every 20 ms:
send_frame(sock, 13, sample_block(block))    # samples

Checklist

1. Obtain a bearer credential through device login (scope game.transmit) or a personal credential.
2. GET /api/v1/stations; pick an ACTIVE station (or a DEORBITING satellite before its reentry), an antenna and optionally a rig.
3. POST /api/v1/game-tickets with {"serverId": ..., "role": "tx-audio", "stationId": ..., "antennaId": ..., "rigId": ...}.
4. Connect with TLS. Send the preface and a hello with role = ROLE_TX_AUDIO, auth = AUTH_KIND_GAME_TICKET and the ticket. Require HELLO_STATUS_ACCEPTED.
5. Send transmitter_start with site.station and the signal parameters.
6. Read session_status (BOUND: show the station; REJECTED: show the code and reason), then tx_queue frames until tx_accept.
7. Stream samples in real time. Read telemetry on another thread.
8. Use tx_update for live changes. Close the socket to go off the air. On ENDED, show the reason.