Topcon GR-3 unit is rugged GPS receiver uses 36 satellites to give you the geographical coordinates down to the last millimeter. It will also allow you to locate reference points with a high degree of precision.The GPS receiver is easy to hold due to the dual hot-swappable batteries. For balancing, the 2 batteries are oriented vertically to keep the center of gravity close to the range pole axis. The device is shock- proofed, so it can "easily" handle frontal cliff impacts (but don't try it on purpose, though). The main housing is is an I-beam shaped rugged magnesium, engineered to resist a fall from a range pole. It's weather proof too: the micro-tuned antenna, the switches and the connectors are housed under a plastic and rubber dome which overlaps the base and battery handle. The GR-3 series following features: 915 MhHz Spread Spectrum internal Tx/Rx radio Advanced Rugged System Design 72 Universal Tracking Channles Bluetooth Wireless Technology G3 Satellite Tracking (GPS, Glonass, Galileo) Optional Internal GSM/GPRS Cellular CommunicationThis Sale Includes: 1x GR3 Base Station 1x GR3 Rover 4x Batteries 2x Antennas 1x Tribrach 2x Battery Chargers 1x Additional charging Station 1x Original carrying case