Docs update

Author: nseidle

docs/configure_base.md

@@ -2,6 +2,8 @@
 
 Surveyor: ![Feature Supported](https://raw.githubusercontent.com/sparkfun/SparkFun_RTK_Firmware/main/docs/img/GreenDot.png) / Express: ![Feature Supported](https://raw.githubusercontent.com/sparkfun/SparkFun_RTK_Firmware/main/docs/img/GreenDot.png) / Express Plus: ![Feature Not Supported](https://raw.githubusercontent.com/sparkfun/SparkFun_RTK_Firmware/main/docs/img/RedDot.png) / Facet: ![Feature Supported](https://raw.githubusercontent.com/sparkfun/SparkFun_RTK_Firmware/main/docs/img/GreenDot.png)
 
+**Note:** The RTK Express Plus does not support Base mode. The Express Press contains an internal IMU and additional algorithms to support high precision location fixes using dead reckoning. 
+
 In addition to providing accurate local location fixes, the SparkFun RTK devices can also serve as a correction source, also called a *Base*. The Base doesn't move and 'knows' where it is so it can calculate the discrepancies between the signals it is receiving and what it should be receiving. Said differently, the 'Base' is told where it is, and that it's not moving. If the GPS signals say otherwise, the Base knows there was a disturbance in the ~~Force~~ ionosphere. These differences are the correction values passed to the Rover so that the Rover can have millimeter level accuracy.
 
 There are two types of bases: *Surveyed* and *Fixed*. A surveyed base is often a temporary base setup in the field. Called a 'Survey-In', this is less accurate but requires only 60 seconds to complete. The 'Fixed' base is much more accurate but the precise location at which the antenna is located must be known. A fixed base is often a structure with an antenna bolted to the side. Raw satellite signals are gathered for a few hours then processed using Precision Point Position. We have a variety of tutorials that go into depth on these subjects but all you need to know is that the RTK Facet supports both Survey-In and Fixed Base techniques.

docs/configure_ports.md

@@ -19,7 +19,7 @@ By default the **Radio** port is set to 57600bps to match the [Serial Telemetry
 The **Data** port on the RTK Facet, Express, and Express Plus is very flexible. Internally the **Data** connector is connected to a digital mux allowing one of four software selectable setups. By default the Data port will be connected to the UART1 of the ZED-F9P and output any messages via serial.
 
 * **NMEA** - The TX pin outputs any enabled messages (NMEA, UBX, and RTCM) at a default of 460,800bps (configurable 9600 to 921600bps). The RX pin can receive RTCM for RTK and can also receive UBX configuration commands if desired.
-* **PPS/Trigger** - The TX pin outputs the pulse-per-second signal that is accurate to 30ns RMS. The RX pin is connected to the EXTINT pin on the ZED-F9P allowing for events to be measured with incredibly accurate nano-second resolution. Useful for things like audio triangulation. See the Timemark section of the [ZED-F9P Integration Manual](https://cdn.sparkfun.com/assets/learn_tutorials/1/8/5/7/ZED-F9P_IntegrationManual__UBX-18010802_.pdf) for more information.
+* **PPS/Trigger** - The TX pin outputs the pulse-per-second signal that is accurate to 30ns RMS. This pin can be configured as an extremely accurate time base. The pulse length and time between pulses are configurable down to 1us. The RX pin is connected to the EXTINT pin on the ZED-F9P allowing for events to be measured with incredibly accurate nano-second resolution. Useful for things like audio triangulation. See the Timemark section of the [ZED-F9P Integration Manual](https://cdn.sparkfun.com/assets/learn_tutorials/1/8/5/7/ZED-F9P_IntegrationManual__UBX-18010802_.pdf) for more information.
 * **I2C** - The TX pin operates as SCL, RX pin as SDA on the I2C bus. This allows additional sensors to be connected to the I2C bus.
 * **GPIO** - The TX pin operates as a DAC capable GPIO on the ESP32. The RX pin operates as a ADC capable input on the ESP32. This is useful for custom applications.