Unfortunately, whether two or more people can communicate in simplex mode is highly dependent on the capabilities and locations of the radios involved. The quality of the antenna and the available power output determine the capability. However, even a powerful radio cannot always overcome some of the obstacles, such as an intervening mountain. The most common type of radio uses the 2-meter frequency band, which depends on relative line-of-sight communication. If a person is transmitting from a deep canyon it is very unlikely that anyone outside the canyon will receive the signal.
Because of the constraints of relatively low power and line-of-sight communication, simplex operation presents a large challenge for covering a large area such as San Diego County. In this case it is desirable to have in place a network of operators who can relay messages, and who already know the challenges they face.
This is the goal of the MARA simplex mapping project: to build a database describing "who can hear whom from where" in the event of a serious emergency, and to make this information available to both those having leadership responsibilities and to the general pool of amateur radio operators who may be called upon to pass emergency traffic.
The first item of business is to determine who is taking part, so a roll call is held. Operators are asked to respond with their call sign and location, and these are noted for later use. Instructions are then given on how the mapping will be conducted.
Each participant is called upon one at a time to act as a transmitter. The individual's call sign is given, along with a short description of the transmit power and the type of antenna being used. These details are noted by (at least) the appointed record keeper. The transmitter then does a short count, usually to ten. During this "ten count" all other operators momentarily turn their receive frequency to the input frequency of the repeater. This means that they are at that time listening directly to the same frequency on which the designated person is transmitting. If they hear anything it is because they are receiving a signal directly from the source, i.e., a simplex signal. (Remember that the repeater transmits on a different frequency than the one it receives, so it is not producing a signal on the same frequency as the designated source.) Each listener records what they hear (or don't hear) as a relative signal strength and quality (clear, scratchy, etc.).
After each person has had a chance to act as the transmit source, the exercise is concluded. The participants are asked to submit their records to a central location or person for processing. As of this writing the data is maintained in a Microsoft Access database, and special-purpose software has been written to extract it in various ways for dissemination.