Differential corrections on OxTS units- Serial Interface

There is a glossary at the end for some technical terms.

The priority is to ensure that the radios are in the same frequency and that this frequency is viable. On the back of the radio should be a sticker describing the possible frequency range, which should be checked with the number that periodically flashes on the radio display.

This frequency must be a multiple of 25 kHz (i.e. 0.025 MHz). It is possible that other radio users at OxTS may be on the same, or similar frequencies so you may want to ask around in your building to check for interference.

Step 2 - Baud Rate

There is a chain of four items which must all have the same baud rate - the base station, the sending radio, the receiving radio and the mobile station.

OxTS devices (RT3003 etc.)	9600 bps by default
NovAtel devices (PwrPak7 etc.)	19200 bps by default

 

The next step is to check that each radio is in the correct mode - RX for the receiver and TX for the transmitter. For the Satel radio this can be found in:

When correctly set up, the RD or TD LED lights should be flashing on the front of the device, as well as the CD light. 

Depending on your radio, receiver and supplied cables, you may be using a type of serial converter or gender changer:

Unit	Likely use case	Check for
PwrPak7 direct connection to RT3003G	14C0097A cable in between	Power provision and data pins 2 and 3
PwrPak7 connection to Satel radio	Gender changer to fix the female end connector	Mirroring the pins (2 and 3) rather than being carried through

Base CD	Base TX	Base CTS	Mobile CD	Mobile TX	Mobile CTS	Descriptions
Red	Red	Red	Red	Off	Red flash	Power no link
Green	Slow red flash	Slow red flash	Green	Slow red flash	Slow red flash	Linked
Green	Green	Green	Green	Green	Green	Setup mode

 

Glossary

Carrier Detect Line

The "Carrier Detect" (CD) LED refers to a signal from the radio modem to the DTE (Data Terminal Equipment). It indicates whether there is activity on the radio channel - it is active whenever a signal exceeding the level required for reception exists on the radio channel, or when the radio modem itself is transmitting.

It does not make any difference if the signal is an actual data transmission, a signal of a radio transmitter not belonging to the system, or even an interference signal.

RSSI Level

RSSI, or “Received Signal Strength Indicator,” is a measurement of how well your device can hear a signal from an access point or router.

Because an RSSI value is pulled from the client device’s WiFi card (hence “received” signal strength), it is not the same as transmit power from a router or AP.

Common issues with using NTRIP for differential corrections using the serial interface

When using NTRIP to receive differential corrections, there are a number of common issues that may arise. This article provides some possible causes for problems receiving NTRIP corrections, although it is by no means an exhaustive list.

Note: this article does not cover troubleshooting the Internal NTRIP Client feature.

Configuration

Configuration issues are a common cause for problems with receiving NTRIP corrections. You should check your configuration to ensure all settings are correct.

There are two major aspects to configuring for NTRIP corrections: the configuration of the INS and the configuration of the interface device connected to the unit to provide corrections (e.g. a Bluetooth RS232 Serial Adaptor or NTRIP modem).

For the majority of our products, configuring the unit to use NTRIP corrections is a matter of configuring the relevant settings in the GNSS Differential Corrections tab in NAVconfig. You will need to ensure that:

1. The Enable NTRIP checkbox is ticked
2. The correction format is the same as that used by the NTRIP provider
3. The serial port settings match those of your interface device

If you are using an xNAV250 or xNAV550, the steps for configuration are slightly different. Instead of configuring these settings in the GNSS Differential Corrections tab, they must now be configured in the Serial Input and Serial 1 Output tabs (under Interfaces).

First, in the Serial Input tab, the Input Type must be set to GNSS Differential Corrections. As above, the correction format should be set to that used by your NTRIP provider, and the serial port settings should match those of your interface device.

Note: for these types of units, the Enable NTRIP box will remain grey and cannot be ticked. This is expected behaviour and is not a cause for concern.

Then, in the Serial 1 Output tab, you should configure the Packet as NMEA, and set the GPGGA message to a periodic output of 1Hz. The Baud rate should be set to the same value as in the Serial Input tab.

When configuring your interface device, you should ensure that the settings match what you have set in the INS configuration. Other settings may be dependent on the type of interface device you are using.

Hardware Setup

The hardware being used and the connections between different pieces of equipment in your setup can make a difference to whether or not using NTRIP corrections will be successful.

One of the most common errors in setup is to connect the interface device to the J2 connector on the user cable. It should instead be connected to the J3 connector via a 15-to-9-way serial adapter cable.

Two-way communications are required, as the unit must provide the GGA message to the NTRIP provider; you must therefore make sure the serial adapter cable you are using has both Tx and Rx connected correctly. The pin assignments are shown in the diagram below.

Using a xNAV and AV200 with a 15 pin radio

If using a xNAV or AV200 the cable below is needed as the serial port on these units does not provide power. This requires a 9-15 pin cable to connect the 15 pin radio to the 9 pin RS-232 port on the user cable.

It is also possible to use the 9 pin serial port on the RT3000 and RT1003 to connect to the radio. Please refer to the follow guide.

Using the J2 Serial connector as access point for Differential corrections

Network and NTRIP Software

If you have checked the configuration and hardware setup, but are still having issues receiving NTRIP corrections, it may be caused by network or signal issues, or problems in the NTRIP client software you are using.

It is worth checking:

• Have you correctly paired your Bluetooth RS232 serial adapter with your smartphone/laptop?
• Do you have a strong enough network signal? You may be able to view the corrections in your NTRIP client software as they are received; you can use this to check whether the problem is with the network or just the connection between your device and your OxTS unit
• Have you selected the correct stream in the NTRIP client software?
• Do you have a valid subscription for the NTRIP service you are using? Are you using a free service?
• Is the phone network you are using blocking the port used by the NTRIP client software?