Aha! Late with the proverbial homework are we? Tsk Tsk
Have you:-
a) Searched this, or other, micro Forums to see if the modules have been used?
b) (if so) have you got people's comments on performance and how to squeeze out the best.
c) Looked on an any Forums anywhere to see if these units have been used reliably?
d) Contacted the vendor or manufacturer for ideas on range claim reliability and optimising?
If you haven't then it's worth a look. If there is no info around, then, well, you can be the first....
If the project is crucial and with a looming deadline then, if you haven't time to play, then you will have to dig your wallet out and buy modules which are KNOWN to work over those ranges.
I'm sure you'll get some replies saying I've used a Yingtong plus the wife's underwired bra at over 1km on a sunny day, but unless you have time (and budget) to experiment then it ain't much use.
And, as you know, your physical arrangement (ground planes, antennas) will play a huge role.
Of course there are devices around that will EASILY achieve what you want - but I guess you have a tight budget. Most PICAXE Forumites do.
Again, if your sites are fixed, then a
yagi antenna (or a.n.other gain design) can make a huge difference. (Direction, gain, s/n etc.)
Yagi's are usually only allowed on the Rx end .
It could solve your problem.
I've used
RF Solutions T7G and R7G reliably at 400m with hedges, trees and fat people in the way. I used simple whips and the data was reliable. They claim 1000m range on their Data Sheets. Maybe with greater care in matching and using a fancy gain antenna and PCB design I could have achieved that.
The nice thing about RF Solutions (and other pukka Manufs) is that they have brains, speak English and don't bullship.
An email to them could extract some useful info.
Go to the Horse's Mouth (regardless of manufacturer).
Good luck (or spend the money!)