# Confused about time systems in ephemeris computation (from GPS subframes)

I am using the gnss-sdr library to compute ephemeris from the GPS message, and to try to make sense of things, I am reading the well-known IS-GPS-200E specification. To compute ephemeris, the time from ephemeris reference ephoch is defined as (page 101, table 20-IV)

$$t_k=t-t_{oe}.$$

I am unsure about how $t$ is defined, and I find the specification unclear on that. Rummaging in the source code of the aforementioned library, I found out that $t$ seems to be computed as follows:

$t=t_{x}-b$

where $b$ is the satellite clock bias, and

$t_x=R_x- (\text{pseudo-range})/c$

where $c$ is the speed of light. However, I have been unable, so far, to find out what $R_x$ exactly refers to. It seems to correspond to "time of week at current symbol", but there is no documentation/precison on that. I suppose that an expert could very simply deduce what $R_x$ is just from the formulas, though.

So my question is: what is $R_x$? What time system is it expressed in (satellite time? gps time? receiver time?). And if someone could explain to me what those formula are doing or give me pointers, I'd be extremely grateful.

My understanding is that $R_x$ is the receiver time when the signal is received. So that would make $t_x$ the transmission time. And you subtract the bias to get a more accurate transmission time.