There are a lot of publications about this subject. A lot of informations you will find in "Photovoltaik Engineering - Handbuch für Planung, Entwicklung und Anwendung", written by Andreas Wagner, Springer Verlag, ISBN 978-3-540-30732-7. Very good is also the book "Simulation der Abschattungsverluste bei solarelektrischen Systemen" by Prof. Dr. Volker Quaschning which is also available online. More information about this book you will find here.
The PVPM measure the voltage and the current of the pv generator. When there is a current on the line, a voltage will drop due to the resistance of the cable itself. The voltage at the end of the cable is different to that on the beginning. This would be a measurement fault. To avoid this problem we use a different pair of wires for the voltage measurement (where no current will ocurr) and so have a four-wire-cable - the measurement of current and voltage is as precise as we need it.
We have important reasons to keep the size of the PVPM. The critical value is the measurement speed: the smaller the device, the higher is the speed of the measurement. The higher the speed of the measurement, the higher is the risk of measurement faults because of the increasing portion of capacitive and inductive parts from the pv generator and the cabling. That is why we try to keep the measurement speed relativly low and at last that is what defines the size of the PVPM.
The connection of the reference sensor by radio transmission can in fact be very comfortable. But there is one serious disadvantage: the transmission by radio needs some time. We actually know that the irradiance differs within milliseconds. The exactness of the peak power measurement depends on the correctness of the dependency of irradiance and module current. By radio transmission we only get the "historical" irradiance. That is why we prefer the cable to the radio transmission.
You can connect to the PVPM every irradiance sensor that produces a voltage according to the short circuit current of the reference cell. This method is standard. At an irradiance of 1000W/m2 the sensor should deliver a voltage between 10 and 100 mV. The connector to the PVPM is a common part, please request the article number and pin assignment. It is important to use an irradiance sensor that matches the spectral attributes of the measured module (according to IEC 60904).
No, the PVPM can be operated stand-alone. The PVPM has an graphical display, an internal computer, a battery supply and so is independent from other devices. But If you want you can connect an external PC or Laptop and save the stored data or make reports or other evaluation.
The internal serial resistance Rs is a measure of the quality of the cabling in the pv generator. Problems like corrosion in connectors, cable damage or a too small dimension of the cables will increase the Rs. You can calculate the expected Rs for a Module with the Software PVPM:disp. If you find out that the measured Rs is higher than the calculated the cabling should be checked for faults.