Master / Slave Operation

RPG’s profiler software is capable of handling multiple instruments and treating them virtually as a single radiometer. The internal instrument design limits the total number of receiver packages to two per radiometer, e.g. a humidity and temperature profiler (RPG-HATPRO) or a humidity profiler and 90 GHz channel (RPG-LWP-U90). Therefore e.g. a dual profiler plus 90 GHz channel cannot be combined in a single instrument. Instead, a RPG-LWP-U90 plus RPG-TEMPRO could be used in a Master / Slave configuration to realize the dual profiler + 90 GHz frequency set. The two radiometers are connected independently to the host PC which handles all the synchronization tasks of calibrations, TB acquisition, elevation/azimuth scanning etc. between the instruments.
The software combines the Master and Slave radiometers to a virtual single instrument which means that all available frequency sets from both units are stored together in brightness temperature data files (or level 0 channel voltages files), the applied online retrievals can use the total combined frequency set and scanning in elevation and azimuth is synchronized as well as all calibration procedures like skydips or other automatic calibrations. The BL mode stores ALL TBs together with the elevation angle information into BLB files. 

The RPG radiometer models that can be combined in a Master / Slave configuration are the following:

•	RPG-HATPRO: Humidity and Temperature Profiler (7 channels in the 22.2-31.4 GHz range, 7 channels in the 51-59 GHz range)
•	RPG-HUMPRO: Humidity Profiler (7 channels in the 22.2-31.4 GHz range) for mid-latitude to tropical humidity levels >2 kg/m^2 (2mm)
•	RPG-TEMPRO: Temperature Profiler (7 channels in the 51-59 GHz range)
•	RPG-LHATPRO: Humidity and Temperature Profiler (7 channels in the 22.2-31.4 GHz range, 6 channels in the 183-198 GHz range), humidity profiler for ultra low humidity levels < 2 kg/m^2 (2mm)
•	RPG-LWP: Two channel (23.8 / 31.4 GHz) radiometer for LWP / IWV retrieval
•	RPG-LWP-U90: Humidity Profiler (7 channels in the 22.2-31.4 GHz range) plus 90 GHz channel for improved LWP accuracy
•	RPG-150-90: Two channel (90 / 150 GHz) radiometer for the detection of low LWP levels
•	RPG-DP150-90: Three channel (90 / dual pol. 150 GHz) radiometer for the detection of low LWP levels and ice particles
•	RPG-36-90: Two channel (36 / 90 GHz) radiometer for atmospheric attenuation measurements. Intended as a Slave instrument for other models
•	RPG-TEMPRO-90: Temperature Profiler (7 channels in the 51-59 GHz range) plus 90 GHz channel. Ideal combination with RPG-HUMPRO
•	RPG-TEMPRO-150: Temperature Profiler (7 channels in the 51-59 GHz range) plus 150 GHz channel. Ideal combination with RPG-HUMPRO
•	RPG-15-90: Two channel (15.3 / 90 GHz) radiometer for atmospheric attenuation measurements. Intended as a Slave instrument for other models

The best frequency coverage is given for the combination RPG-LWP-U90 / RPG-LHATPRO which is a triple profiler for medium/low humidity, ultra low humidity and temperature plus a 90 GHz channel for improved LWP retrieval. 

Both radiometers are connected to different serial ports of the host PC. These ports have to be defined in the Define Serial Communication Ports menu. The Activate checkbox in the Slave Port window must be checked in order to support the second radiometer. As before, an additional azimuth positioner can be connected. The host PC has to provide 3 serial ports for this configuration. 
The Define Serial Communication Ports menu provides the checkbox Try again, if not successful . If this is checked, the host automatically retries a connection after a connection failure one more time. 
If a Master and Slave unit have been successfully detected, the host PC’s Status and Config. tag displays an additional radiometer status box for the Slave instrument. In addition, there is a second status bar displayed at the bottom of the screen to indicate the Slave connection status and interface settings.
Several other menus are showing a Master / Slave toggle button like the diagnostics menus, the file transfer menu or the absolute calibration menu. Before the user is issuing a command from one of these menus, he/she specifies the receiver of the command by pressing the toggle accordingly. Both radiometer IDs are displayed on the headlines of menus like the Send Measurement Configuration or the Absolute Calibration menu. Where frequency lists are displayed, the Master channels are preceded by ‘M:’ while the Slave channels are preceded by ‘S:’. When enabling file backup, all data files are stored on the Master’s flash memory; no data is stored to the Slave’s flash.
When the HKD (housekeeping data) product is selected and a Slave radiometer is present, a second HKD monitoring window is created during measurements for the display of the Slave housekeeping data. The HKD data file names for the Slave instrument have the extension *_S.HKD instead of  *.HKD (Master radiometer).  

The selectable COM-ports are enabled in the upper button list. The user can only select one of the available ports for interconnection with the radiometer(s). 
The baud rate parameter defines the communication speed. For copper cable lengths up to 50 m and for fiber optic cables, the highest baud rate should be used (115200). This is particularly important when files that have been backed up on the radiometer DOM (disk on module) need to be transferred to the host. Some of these data files might be several MBytes in size so that an optimally fast transfer rate is desirable. At 115200 baud the transfer speed is about 6 kByte/sec.
If Auto Connect is checked, the host software automatically attempts to connect to the radiometer during the starting phase (if a data cable is detected). This feature enables an auto-startup function after a power failure of the host PC. The radiometer embedded PC will automatically continue a measurement after a power failure when the power returns. To start the host software automatically after reboot of the operating system, the R2CH.EXE should be entered into the Auto Start directory of the operating system or an appropriate task should be defined in the scheduler.
When using an older version of the optional azimuth positioner (see section 8.2) to allow for full sky scanning, the azimuth controller is connected to the host PC through a second RS-232 interface. This interface is also defined in the Define Serial Interface menu. The baudrate is auto-adjusted to the fixed azimuth controller baudrate. The automatic controller activation at software start is enabled by checking the Activate box. 
If a first attempt to connect to the radiometer fails, the program can retry a second time if the Try again, if not successful is checked. If the user wants to adjust the host PC time by synchronizing it to the radiometer GPS clock he can check Synchronize to Radiometer GPS Clock.
A second radiometer (Slave) can be run by the same host program to combine the channels of two systems to a virtually single instrument (Master / Slave operation). In this case the Slave radiometer needs a second serial interface and must be separately activated. 
Fiber optics cables are available in two versions:
•	6 line fiber optic cable: This cable provides double hardware handshaking for maximum transfer speed. A special RPG made RS-232 to fiber converter has to be implemented with this cable type. A disadvantage is that commonly available RS-232 to USB converters cannot be used on the host, because such devices only offer poor speed grades for handshaking lines. Instead, if the host motherboard does not provide a serial port, an extension card must be installed (e.g. PCMCIA or Express Card on laptops) that connect to the 6 line RS-232 to fiber converter. Maximum cable length is 2000 m.
•	2 line fiber optic cable: No hardware handshaking is implemented for these cables, only TX / RX in combination with an optimized software handshaking. The communication with the radiometer is only slightly slower compared to the 6 line version which can be neglected. An advantage of the 2 line cable is that it can be interfaced with any type of commonly available serial converters (e.g. USB, PCMCIA, Express Card) and commercially available serial to fiber converters (most of them only support TX / RX comminucation).
All copper cables (old version) are 6 line cables with double hardware handshaking. These cables can be run with maximum speed for cable lengths up to 50 m.
The connection to a Master or Slave radiometer is realized by one of these cable types. RPG delivers cables that allow the host and radiometer PC to auto-detect the type of cable attached to it. If the radio button Auto Detect is checked, the host will use this cable feature. But if the user wants to enforce another cable type one of the other buttons 6 line Com. or 2 line Com. are checked. E.g. if a 6 line cable is installed but a 2 line communication is desired in combination with a USB to serial adapter on a laptop, the user checks the 2 line Com. radio button to enforce the host to implement no hardware handshaking in the communication with the radiometer. In this case, only 2 of the 6 lines is used.