Whether you are buying your first Trimble GPS / GNSS system or adding on to an existing fleet, the question of which radio option is best is one we often get. The answer is not always straight forward because it can depend on your application, existing GPS and radio equipment, environment and location. The information below is focused on Trimble RTK technology utilized in land surveying, construction, agriculture and mining applications.
900 MHz vs 450 MHz - Which is Better for GPS?
The 900 MHz band is highly regarded for its reliability and effectiveness in ensuring uninterrupted communication - which can be crucial on projects where multiple 3D machine control systems are operating and any downtime cuts into the bottom line. The downside of 900 MHz is that it's a low power radio which is limited to shorter distances - typically under 3 miles.
900 MHz (902-928 MHz):
900 MHz radios do not require a license and can provide excellent coverage on subdivision type projects where the change in topography is mild. Because they do not require a license, the user can change what channel they transmit on anytime - without the assistance from their dealer. Additionally, the 900 Mhz spectrum offers more bandwidth which can be used to transmit as-built data from machines to the office - while simultaneously receiving and transmitting base station corrections. 900 MHz radios also receive far less interference compared to 450 MHz when working in urban areas.
UHF (403-473 MHz):
Land surveyors tend to choose UHF radios because they are capable of transmitting longer distances - as far as 20 miles away from the base station. UHF signals are better in hilly terrain - though selecting the right antenna is also very important. UHF interference is less of an impact on a single surveyor vs a construction project where multiple GPS machine control systems require uninterrupted corrections for most of the day.
UHF Requires a FCC License:
In the United States, transmitting corrections on UHF radios requires a license from the FCC. This license is not difficult to get, but typically costs around $500 and can take over a month to receive approval. Additionally, you cannot add the assigned frequencies to your radio on your own, that requires sending your FCC license to your Trimble dealer who will then program your assigned transmit frequencies into your hardware which can cause downtime while you are waiting.
Additionally, before transmitting on UHF users are required to check their assigned frequencies to ensure the channel is clear. If the channel is occupied, you'll change frequencies and check again until you find a clear channel.
Why do you need a radio for GPS systems (RTK)?
If you need GPS accuracy, you need RTK technology. Real-Time Kinematic (RTK) is the technology used by GPS / GNSS receivers that enable real-time, centimeter-level accuracy. RTK corrections are transmitted from a base station to your GPS rover over the air by either radio, cellular, internet or satellite (RTX).
Without RTK corrections, your GPS receiver’s accuracy is 3-feet / 1-meter at best. That’s right, even the most advanced “survey-grade” GNSS receivers like the Trimble R12i, SPS986 and R980 can only provide 3-feet of accuracy when RTK corrections are not available.
How RTK works (the basics):
RTK requires two GPS / GNSS receivers and a communication link (radio) between the two receivers. Both GPS receivers take continuous measurements from satellites as they fly overhead. The communication link sends position corrections from one receiver to the other.
As mentioned before, a single GPS receiver's accuracy is between 1 to 3 meters. This accuracy error is caused by different atmospheric layers which disrupt GPS signals as they travel to the Earth's surface.
The genius of RTK technology is by placing one of the receivers at a stationary location - with fixed coordinates - it is possible to track the accuracy errors caused by the atmosphere. This stationary receiver (base station) then broadcasts it's corrected position through the communication link. The GPS rovers receive the broadcast and apply the corrections to their position. These calculations happen nearly instantaneously, providing centimeter-level accuracy in real time.
Bottom line:
While 900 MHz is the preferred choice for most machine control applications because of it's simplicity and uninterrupted performance. A 450 MHz radio will perform better over longer distances and hilly terrain, but is susceptible to interference when working in areas where UHF radio are prevalent e.g., hospitals, hotels, airports, military bases and construction sites.
Looking for Trimble radios & accessories?
One solution is the Trimble EM940 dual-band radio which we offer at Positioning Solutions. The EM940 adds the ability to receive both 900 MHz and UHF corrections to almost any Trimble RTK enabled GPS / GNSS receiver.
Read our overview of the Trimble EM940 >>