This answer applies to a previous, broader version of the question and remains in case it is of more general interest.
A distance of 2500 km rules out a line-of-sight link due to the curvature of the earth; you would need a tower a hundred times taller than the tallest manmade structure. Therefore, in order to achieve radio communication, you must reflect your signal off something, or have at least one intermediate station which receives and retransmits it.
In any case, the latency is essentially the length of the path divided by the speed of light.
Some options for pure radio communications, roughly in order of increasing latency but not necessarily practical, are:
HF radio. Here, your signals are reflected/refracted by the ionosphere, so that they can proceed around the curvature of the earth entirely on their own.
The catch is that the ionosphere is not always in a condition to serve this function — primarily dependent on the time of day, but in general as variable as the weather. HF propagation is a quite complex topic.
Also, a good HF station requires physically large antennas. They can be as simple as wire strung between two trees, though.
Relay through a satellite. This increases latency because the signal is taking a longer path (two straight lines meeting at the satellite rather than a “great circle” path).
The time taken will depend on the satellite's orbit, which also determines whether and when you can use it; notably, a geosynchronous satellite is apparently fixed in the sky so you can use it continuously, but is so far out that you will always have a quarter-second of latency.
You need a satellite to use. All (nearly all? I forget) amateur radio satellites are in low earth orbit (so moving rapidly over the ground and not covering a very large area), and generally heavily used the easier they are to contact. Commercial communications satellites are available but would require some contract to make use of.
You need a ground station, that is, a radio station that is capable of making itself heard by the satellite. This means directional, steerable antennas, for low orbit satellites; geosynchronous ones still need a directional antenna but it can be installed in a fixed direction.
Relay stations on the ground along your path.
Here, your problem is finding land you can put up towers on, or using a network someone else has already built. Also, the relays will add latency — more or less depending on whether they are full-duplex and how efficient they are.
The amateur APRS network is in theory capable of supporting this, but in practice rarely gets a packet a long distance on purely RF relays and uses the Internet as a fallback.
Reflect your signal off the Moon!
This is much like using a low-orbit satellite, in that you need high-gain (large) steerable antennas, and it adds 2.5 seconds of latency because the Moon is even farther away.
There's a good chance that your best option is to take none of the above and instead use the Internet, which will do a really good job of getting your data where you need it fast — if you can find Internet access at both ends. Or you could use a radio link over a short distance at one or both ends and transfer the data over the Internet the rest of the way.
