# Operating several base stations on one radio frequency

In an RTK survey you can reduce the effects of radio interference from other base stations on the same frequency by operating your base station with a different transmission delay.

When you use multiple base stations, you set the transmission delay for each base when you start the base survey. Each base must broadcast with a different transmission delay and station index number. The delays allow the rover to receive corrections from all of the base stations at once on one frequency. The station index numbers let you select which base station to use at the rover.

- You can set the base radio transmission delay only when using Trimble GNSS receivers or the Spectra Geospatial SP100 GNSS receiver.

- When you carry out surveys using different base stations in one job, make sure that the coordinates of the base stations are in the same coordinate system and are in terms of each other.

Hardware and firmware requirements

To operate several base stations on one frequency, you must use receivers that support the CMR+ or CMRx correction format.

All other base and rover receivers must be Trimble GNSS receivers.

Do not use transmission delays if you intend to use radio repeaters.

To start the base with a transmission delay

Before you start the base receiver, do the following:

1. Select the CMR+ or CMRx correction format. Select this in the survey style for both the base and the rover.

2. Set the over air baud rate in the radio to at least 4800 baud.

   If you use a 4800 over air baud rate you can only use two base stations on one frequency. Increase the over air baud rate if you want to increase the number of base stations on one frequency.

When you start the base survey, do the following:

1. In the Station index field, enter a value within the range 0–31. This number is broadcast in the correction message.

   You can configure the default station index number in the survey style. See Base options.

2. If the receiver you are using supports transmission delays, the Transmission delay field appears. Choose a value in ms, depending on how many base stations you want to use:

   | Base 1 | Base 2 | Base 3 | Base 4
   --- | --- | --- | --- | ---
   One base station | 0 | – | – | –
   Two base stations | 0 | 500 | – | –
   Three base stations | 0 | 350 | 700 | –
   Four base stations | 0 | 250 | 500 | 750

To swap bases during a real‑time rover survey

If you are using multiple bases on the same frequency, you can swap bases during the rover survey.

To swap bases, from the Measure menu, select Swap base receiver.

The Select base station screen appears. It shows all the base stations operating on the frequency you are using. The list shows the station index numbers of each base and the reliability of each. Tap the base you want to use.

When you change to a different base, your OTF receiver automatically starts the initialization.