Perhaps you should try some spelling variations:
"ground penetrating radars" » "ground penetrating bars", "ground penetrating beams", "ground penetrating bodies"
"rod penetration radar" » "rod penetration raman", "rod penetration rate", "rod penetration der", "ion penetration radar", "cloud penetration radar", "cone penetration radar"
"rod penetrating radar" » "from penetrating radar", "non penetrating radar", "bow penetrating radar"

Try widening your search to All Fields.

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (((("rod penetration radar") OR ("rod penetrating radar"))) OR ("ground penetrating radars")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"rod penetration radar\") OR (\"rod penetrating radar\"))) or (\"ground penetrating radars\"))".
• Removing quotes may allow a broader search: (((( rod penetration radar ) OR ( rod penetrating radar ))) or ( ground penetrating radars )).
• Adding a wildcard symbol may retrieve word variants: (((("rod penetration radar") OR ("rod penetrating radar"))) or ("ground penetrating radars"))*.