An INS will suffer from errors due to accelerometer drift. Using a Kalman filter along with better and more accurate Giros will help this situation greatly. Basically doing the maths means better accuracy giros can compensate for not so great accelerometers. You can always add some extra sensors such as magentometer and this will give you another input into your system, once again it will help reduce drift errors.
Wheel encoders are great, provided you do not suffer from slippage. As soon as a wheel slips you will get innaccurate readings.
Another method (as used on mars pathfinders!) is to use optical systems. Two cameras, one processor, and some algorithms and you will be able to accurately and reliably calculate distance travelled! This is easier to implement if you have lots of easily identified static items around the room for the algorithm to pickup on.
You could also instrument your room! this means you could place RF, magnetic, or white lines around the room and use an appropriate sensor and use them for navigation!
Another method is to use simple ultrasonic sensors (reverse parking sensors anyone?) these will aide collision avoidance! fuse them with INS sensors and magnetometer and you will far more accurate position tracking system. Or use them on their own and avoid hitting stuff!!!!
I don't think that using the signal of an LDR can do much because the circuit already
has some kind of ambient light suppression: it's the high pass filter at capacitor C8.
I agree with MikeJ-UK that the signal probably is saturated by ambient light.
If you just want to get the proximity sensor working with more ambient light
I'd suggest to put an IR filter in front of the detector.
If this is too easy (or you also have a lot of ambient IR light, e.g. because the sun is shining
at the detector):
You have to solve the problem of the signal being totally jammed by the ambient light.
Lets suppose the photocurrent caused by the signal is some micro amps or less and the ambient light gives you
already some 0.1mA there is only a very very small signal voltage at the input voltage divider (D1/R10).
The more current (caused by ambient light) is flowing in the voltage divider, the smaller your singal will be.
Just increasing the amplification doesn't help, because the noise will be amplified too and
I think you come into regions where signal-to-noise ratio is what you have to take care about.
So instead of having a voltage divider at the detector a better approach would be to utilize a transimpedance amplifier:
Its output voltage is linear to the photo current.
So this will give you at least constant signal level, no matter how much ambient light you have
(see also this article about this problem by Bob Pease).
Of course this is only true within limits: if your amplifier is jammed, you can't do much.
So the amplification before bandpass filtering must not be too large.
But if you make your bandpass filter narrow enough you
can do huge amplification afterwards (like in radio receivers).
Best Answer
Consider these receiver "phototransistors". They automatically decode a 38 kHz signal, which is what most remote controls use. Because it is pulsing it is virtually immune to ambient IR light including sunlight, although there is a limit as to how much ambient light they can reject. There are plenty of code examples and projects on the net which send out 38 kHz signals to control TVs, DVD players, etc. but the idea is applicable to IR beacons; just choose a code/sequence which won't interfere with other products.