What Is My Base Station Actually Sending Me?
Most GNSS users who rely on a base station, whether their own or someone else's, have never actually looked inside the stream it produces. Understanding it by purpose, rather than by message number, is worth doing before building anything, since every decision ahead in this companion depends on it.
An RTCM stream is not one thing. It carries several distinct categories of information: where the base claims to be, what it actually observed, satellite-specific bias information, details about its own antenna, and the rate and bandwidth all of that arrives at.
Where the Base Claims to Be
One category of message simply states the base's own coordinate, the position everything else in the stream gets compared against. This is the single most consequential piece of information in the entire stream, since every rover using it inherits whatever error this one message carries, uniformly and invisibly.
A base position message is the one piece of the stream that cannot be double-checked by anything downstream of it. Every other message describes an observation. This one describes an assumption.
What the Base Actually Observed
A second, much larger category, commonly referred to by its message set name, MSM, carries the base's actual satellite observations, code and carrier-phase measurements across whatever constellations and frequencies the base is tracking. This is the raw material a rover's differencing actually uses, and its completeness, which constellations, which frequencies, how often it updates, directly limits what a rover connected to it can achieve.
MSM observation messages are not a correction in the sense of a ready-made adjustment. They are the base's own raw measurements, made useful only once a rover differences its own observations against them.
Bias, Antenna Information, and the Practical Details
Additional message types carry satellite-specific bias information, corrections for known systematic effects in certain signals that would otherwise degrade multi-constellation processing, and details describing the base's own antenna, which a careful rover setup can use to account for that antenna's specific characteristics. Underneath all of this sits the practical layer that determines whether any of it is even usable: how often each message type is sent, how much bandwidth the whole stream requires, and how much delay exists between an observation being made and it reaching the rover.
A rover connects successfully to a base's stream and never loses connection, yet performs worse than expected. Before suspecting the rover, what part of the stream itself is worth checking? Whether the message set actually includes the constellations and frequencies the rover needs, and how much latency exists between observation and delivery, since a technically successful connection does not guarantee complete or timely content.
An RTCM stream carries the base's claimed position, its raw satellite observations, bias corrections, antenna details, and the rate and latency all of it arrives at. Understanding a stream by what each piece actually does, rather than by message number, is what makes every decision later in this companion an informed one.
Knowing what the base sends is only half the picture. The next chapter follows that stream to the rover, and explains exactly what happens to it once it arrives.