Proximity Sensors and Range Finders

[Builder1]

Hello All,
I am trying to write an algorithm for my robot's navigation program. The biggest problem is I don't know whether I should go with proximity sensors or range finders, which are some of the key pieces needed in order to come up with an algorithm. I am trying to build a robot that patrols a piece of property which means it needs to be able to operate outside, traverse most any terrain, and stay within the boundary of the property. I've got the "traverse most any terrain" part covered, but I am having problems with the other two. Proximity sensors and Range-Finders are sensitive electronic devices which need to be sheltered from the elements, the act of encasing them in a protective shell would probably render them useless. I've thought of making the robot stay within the boundary of a property using something like an electronic dog fence, but I realized that it's not economical if the property is very large. What do you suggest I use as my robot's sensors if I intend to have it patrol a considerably large property? Thank you in advance for your response.

[mbateman]

GPS?

[ArcMan]

Just a note that ultrasonic rangers can be fairly well protected from the elements.  I have found that the rangers work fine through low-density foam (e.g. - that foam that electrostatically sensitive components are pushed into for shipment and storage).  Also, speaker grill covers work.
So you could encase the sensor on all sides (with aluminum, plastic, etc.) except the front, where you would use the foam.

[Builder1]

Thank you all for your response. I have looked into GPS units and found that the really precise ones that have precisions up to a few centimeters cost several hundred dollars, and that is the kind of precision I need in order to make this work. The idea of using a low density foam to protect an ultrasonic ranger is intriguing, I don't know whether or not it can be used to shelter the ranger from rain but I will look into it. Thanks again for your input.