A section of Chapter 2 - Weak Signal Operation
The EME-2 mode is an enhanced Earth-Moon-Earth echo system utilizing Long-Term Integration to improve the apparent signal-to-noise ratio. The transmitter sends out a 2 second pulse, after which the receiver is enabled. After a delay of 2.6 seconds from the start of the pulse (to allow for the EME delay) the spectral analysis is started. This analysis continues for 2 seconds to process the entire received pulse. The spectral data is sent from the DSP to the PC for further processing.
The 2 second pulses are repeated every 5 seconds for as long as one wishes. The spectral data consists of the average power received over the 2-second period for each spectral (FFT) "bin." These bins are narrow-band filters spread across the receiver pass-band and are the source of the standard display that is used in the DSP-10. The only difference from, say, USB is that the EME-2 measurement is triggered by the software. In USB it is free running and reoccurs as rapidly as possible.
A "Long-Term trace" is produced on the DSP-10 screen by simply averaging the total of the spectral signal powers that have been received. With the default DSP-10 colors, this produces a Yellow trace, and this is the common nickname for the upper display in this mode. The yellow trace does not occupy the full width of the screen since the only portion of the display of interest are near the Doppler-shifted operating frequency. This is marked by a tall red-line (when using the default colors). Detailed data is taken for 10 bins above the center frequency and 10 bins below. In addition, numerical data is available in a "data box" on the returns received for the 21 bins of special interest (activated by Alt-A).
Doppler correction is automatically applied to the receiver frequency so that the return will always line up with the center marker.
It takes anywhere from 1 to 10,000 or more pulses to receive an unambiguous echo, depending on the station capability. The yellow trace will start as noise and, in time, this will average towards a smooth line. At the center frequency there is a sum of signal and noise power and the trace will show a spike over the noise curve. This continues to be more obvious as time progresses. Because of variation in the receiver response across the frequency band, the noise trace will not be straight as one might wish. A flattener function fits the noise curve with a fourth-order polynomial which can be subtracted from the yellow and white traces to make them nominally straight and horizontal.
EME-2 is capabable of major enhancements in sensitivity. Early experiments have shown the ability to work with signals 25 or more dB below the audible levels. For instance, W7LHL receives reliable returns on 1296 MHz using 50 Watts and a 44 element loop yagi. W7SZ has had similar success using 5 Watts and a 13 foot dish. At 2-meters, this author (W7PUA) is receiving well defined echoes at 5 Watts with 4x12 element Yagis.
The following material describes the controls and display for the EME-2 mode. Following this is a discussion of the steps that are ordinarily followed to obtain EME echoes in this mode, and then some Q&A on this mode.
ALT-B BOX FOR EME-2 - There are not many options for the EME-2 mode. Most of these come from the Alt-B modal dialog box with a heading EME2 Echo Mode. This dialog is opened by the Alt-B (or Alt-b) command when in the EME-2 mode (Alt-M or m). The rules for all the dialog boxes are the same: The up/down cursors move between items which become highlighted. Radio buttons or check-boxes are changed by highlighting the item and hitting the space bar. Text entry is at the position of the cursor in the text box. Backspace over existing text and type in new items. Close the dialog box by hitting Enter.
Item-by-item in the Alt-B Box:
Random Frequency Spread, Hz - The maximum total range that the transmitter and receiver frequencies are shifted to minimize problems due to "birdies" that can corrupt the EME data. Too little spread allows birdies to cluster near the center and to be a source of interference. Too much spread makes it difficult to find an operating frequency that does not include birdies. The selection of this parameter depends on the operationg enviornment. A value of 200 Hz is often adequate.
Record Data to Disk - Provides for a disk file that saves the important spectral data for later analysis. This feature is not normally used, but it gives a way to reanalize the received data after Moon exhoes have been received. The data format for the resulting files is in the appendix.
LTerm Trace max dB/ - This series of radio buttons prevent the hiding of details in the yellow trace by a strong, off-frequency signal. The yellow trace is self scaling and sometimes it is desireable to allow portions of the trace to run off the screen. Normally this is not a problem and the max can be left at the first button, 10 dB/.
Ident String - Up to 9 characters can be entered in the text box. Every few minutes this string is sent in CW to identify the station sending the 2 second pulses. Either capital or lower-case letters will work along with any of the CW character set.
Noise Blank, dB - Wide band noise can disrupt the EME-2 results by adding a large amount of power to all bins. This tends to cause the signal to sink into the noise. The average of the 18 noise-measurement bins is compared with a running average of these same bins. If the current average exceeds the running average by more than the "Noise Blank, dB," all the data from that 5 second period is discarded. A value of 0 dB will exclude roughly half the data. A value of 99 dB will never exclude any data. Values aroud 1 dB are good starting points. When the data is discarded, the yellow trace is not shown and a note "*NB*" appear on the left side of the upper spectral box.
TRANSCEIVER SETTINGS FOR EME-2 - Many of the settings for EME-2 are automatic. The data is always 16 bit and the spectral display is always the lower half of the spectrum analyzer width. The SpecAve value (the effective number of FFT's being averaged per screen update) is always the value corresponding to 2 seconds worth of data, i.e., it is 4.5, 9 or 18 depending on whether the SPecAnl (Spectrum Analysis) is set to 1200, 2400 or 4800 Hz width.
In EME-2 the audio portions of the receiver are fully operational, but have no interaction with the display. Controls such as Filter, LMS and AF Gain can be changed at will. The same is true of the settings that affect the upper and lower spectral displays, such as Contrast, Brightness, Trace Normalization, AutoDisplay, db/div and dB Offset. The value of the SpecAve, which is the number of power spectrums averaged before a display update takes place, is set automatically in EME-2 to update every 2 seconds. The value of SpecAve still changes with SpecAnl width since the amount of time for each FFT changes with this setting.
The transmit base frequency is displayed in the Transmit Freq Box and should be chosen to find a "birdie" free frequency.
TRANSMIT POWER - this mode has a 40% duty-cycle and the transmit power should be kept consistent with the cooling capabilitiees of any following amplifiers.
CLOCK SET - No software Clock Setting is required for EME-2.
The windowing function (Alt-W, Alt-w or Scrl-W with kbd_alt2=1 in CFG file) is programmable for None, Tukey-25 dB, Hamming or blackman-harris 92 dB. These windowing functions allow trading off the selectivity of the FFT relative to the off-frequency rejection. No windowing function gives the best sensitivity and can often be used. If there are problems with strong signals or birdies, one can use a windowing function such as Tukey-25 dB, or possibly Hamming.
The width of the spectral display is set with Alt-J. All FFT's are 1024 point, but the sample rate is 2400, 4800 or 9600 Hz. This produces bin resolution bandwidths, without windowing functions, of about 2.3, 4.6 and 9.2 Hz. The display in the left-hand column is "SpecAnl Window Width" where Window is the windowing functions and Width is the 1200, 2400 or 9600 Hz. Experience to date shows that the 1200 Width can be used at frequencies up to 1296 MHz. It gives better sensitivity because of the reduced noise bandwidth. As an aside, when one cuts the bin bandwidth in half, the noise bandwidth is half and the S/N of a single FFT measurement improves by 3 dB. But, each FFT measurement takes twice as long and so in any given time there is only half as much non-coherent integration. This decreases the advantage of the narrower bin bandwidth to 1.5 dB.
The center FSK frequency varies with the SpecAnl width in order to center the display. For a 1200 value, the center frequency is 323 Hz, for 2400 it is 600 Hz and for 4800 width it is 900 Hz. This is shown by the 5 division tall line. The horizontal line below the tall center line shows the region for the 21 bins discussed above that get careful study.
To allow the Doppler correction, it is necessary to have a "Moon:" line at the bottom of the screen (Scrl-F3). This designates the latitude and longitude of the station or stations being used. The coordinates are set in the .CFG file as is explained there, and in the previous software notes. No command is needed to activate the Doppler correction, nor can it be turned off. The receive frequency is shifted for Doppler correction. The receive frequency is shown just below the Transmit-Frequency Box and reflects the affects of both randomization and EME Doppler. To indicate that these corrections are being made, there is an "R' on the left and an "E" on the right.
The transmit frequency is the "base frequency" and the FSK modulation that modifies the transmit frequency is shown just above the Transmit-Frequency Box. This system allows one to tune the frequency being used without ambiguity.
ALT-A BOX for EME-2 - A non-modal information box is available for the EME-2 display. This occupies the right edge of the spectral waterfall. As the display scrolls the data up, it gets covered by this box, but no data is actually lost. The 21 bin area is far enough to the left to never be covered by the Alt-A box. This box is both opened and closed by the Alt-A keyboard command.
The box is updated every 5 seconds. The top line shows the number of data points that have been averaged together (there are no limits) and a note "Rcd=N" or "Rcd=Y" to indicate the status of data recording. The equivalent amount of effective Signal-to-Noise improvement is shown in the second line as "NonCohInt=xx.xdB." The third and fourth lines indicate the amount of signal found at the center frequency. The display shows "fffHz S+N/N=y.yyydB" and the next line is "Cntr Sig= -zzz dBm." The signal + noise/noise values are determined from the running average of the 18 noise bins. Calculations for each of the 21 center bins is made to determine the presence of a signal. The S+N/N value will be both + and - across the set of bins. Negative values are noise generated errors and correspond to signals that have negative power. Positive values allow the calculation of a signal level as shown in the "Cntr Sig=" line. An assumption needs to be made about the noise-power density and this comes form the eme2_te entry in the .CFG file. The default value for this noise temperature is 290 degrees K and corresponds to -174 dBm/Hz. One can fill in a different value for the effective noise temperature, if it is known.
Finally at the bottom of the Alt-A box are the Randomization value in Hz and in the bottom line the Running Average and the Current Noise power values, in relative dB. This last line allows one to observe the operation of the Noise Blanker. The difference between the values is compared with the "Noise Blank, dB" setting form the Alt-B Box to see if noise blanking will occur.
OPERATION OF EME-2 - This mode is capable of considerable sensitivity when operated for sufficient periods of time. It produces an estimate of the actual returned signal strength and can be used as a system test device to insure that all elements of an EME system are operating properly. In addition, it allows stations that cannot hear their own echoes to see how far they are below audability and to evaluate how well the station is working.
Several requirements are placed on the station hardware. The transmitter is on for about 40 percent of the time and cooling of any power amplifiers must support this level of activity. The frequency reference being used by the station need not be precise, since the return frequencies are all relative to the transmitted frequency. However, the frequency reference must not change between transmit and receive. This can be a problem when the station uses a transverter to translate microwave frequencies. The oscillator in the transverter may shift enough to prevent the returns from hitting in the 21 center bins. The use of RIT to work around this problem is described below.
Sequencing for the Transmit/Receive changeover is available in the DSP-10 software. These delays are adjusted in the .CFG file and include a special delay for EME-2 that is called dly_emet2r. This delay needs to be adjusted to center up the timing of the EME-2 return. It depends on the other delays and the formula is :
dly_emet2r= 575 - dly_ant2amp - dly_amp2xmit - dly_xmit2amp - dly_amp2ant
where all times are in milliseconds. Nominal values are:
dly_ant2amp = 50
dly_amp2xmit = 75
dly_xmit2amp = 25
dly_amp2ant = 25
dly_emet2r = 400
To use the EME-2 mode, first select the EME-2 mode (Alt-M) and open the Alt-B Box. Select parameters as described above. Close this box (Enter) and open the Alt-A box to show the result data. After the antenna is positioned on the Moon, using the Az-El data at the bottom of the screen, activate the transmissions by the Home key. If transmissions are at the proper power level and you are ready for the EME-2 measurement, clear the long-term data with a Ctrl-W. The yellow trace will first look like the white trace and probably start at 5 dB per division. After a few transmissions the yellow trace will begin to smooth into a better defined curve. As it does this, the scale of dB/division will reduce automatically and the noise will be reducing in magnitude. The signal will appear as a narrow spike at the tall red line. The S+N/N numbers and the center estimated signal levels will be displayed in the Alt-A box.
Let the process run until the signal becomes well defined from the noise. The amount of time required for this varies greatly depending on the station capabilities. If the station is capable of hearing their own echoes, this requires 5 to 30 seconds for an excellent display. Stations with 25 dB less capability can receive their echoes and see a high confidence display by waiting for about 30 minutes to an hour.
Ctrl W clears the display and all data for a fresh start.
Ctrl E straightens the displays by "normalizing" across the entire 450 to 1350 Hz region. This is a 2 step process. First a 4th order polynomial is fit to all the power data. This defines a smooth curve through the measured datsa. Then any signals are "ignored" by calculating a new 4th order polynomial, this time weighting the data points that are far from the curve by 1/abs(power) of the data point. Data points far from the curve get little recognition. Ctrl E can be applied any time you think the yellow curve is bending at the ends, or whatever. The estimate of the noise power is applied to both the white and yelow curves.
The yellow curve has its center fixed 3 divisions below the top. It is scaled, automatically, to keep the curve on the screen, at least in the center region, over the red line. The scale factor (up to a max of 1 dB/div) is shown in yellow. The yellow curve is not changed by the commands that change the white one. It gets its data before the offsets and scaling is done.
QUESTIONS AND ANSWERS ABOUT EME-2
Q - What should I do if the display is not centered on the tall red line?
A -Several things can cause this. The Doppler calculation has errors up to about 15 Hz per GHz. Until the causes of this error are identified and removed this will require a correction. Shifts in transmit/receive frequency can also cause this error. The correction is applied with RIT. To determine the amount of RIT that should be applied, some calculation is needed (this is planned to be done automatically in a future version of the program). First the bin spacing is needed. This is based on SpecAnl value:
SpecAnl=1200 Bin Spacing B=2.34 Hz
SpecAnl=2400 Bin Spacing B=4.69 Hz
SpecAnl=4800 Bin Spacing B=9.37 Hz
The RIT shift needed is roughly that to move the largest N-value to the center. N-values on the left are referred to as negative N-values as they correspond to frequencies below the expected center. Suppose, for example, that the maximum response was at N2. The RIT value needed is thus 2xB. If B was 2.34, the needed RIT would be 2 x 2.34 = 4.68 Hz and the closest available value is 5 Hz. This should be applied. If the maximum N-value was a negative N-value, the RIT would set to a negative value. In all cases, after the RIT has been changed, one should clear all the long-term data with the Ctrl-W command.
Next it is possible to apply fine tuning of the RIT to not only make the center frequency return the largest value(this is the one shown at the top of the Alt-A Box as "xxxHz S+N/N+y.ydB"). The RIT can experimentally be moved a small amount to make the -N1 and +N1 values about the same. Depending on the signal strength this may require some patience.
Q - How do I make the yellow noise trace straight?
A -The Ctrl-E line straightner can be applied at any time. It processes the long-term averaged data (the entire yellow trace) to fit a fourth-order polynomial to the noise data. This is used as a normalization curve for both the yellow and white traces. To prevent the bending of the curve towards a strong signal, there is a weighting function applied. This decreases the importance of points that are far from the smooth curve.
Q - Can I interrupt the transmission?
A -Yes. All the long-term data is saved and when you hit the Home key again, it will pick up where you left off. There are two ways to clear the long-term data: the Ctrl-W key and changing modes.
Q - How do I select the best SpecAnl width?
A -This is somewhat complicated! To a point, the narrower widths have better signal-to-noise ratios since the bandwidth of the FFT bins is narrower. This does not apply if the signal being received is broader than the bin bandwidth. This can be caused by the libration of the Moon, especially for frequencies above 432 MHz. In addition, W7LHL and W7SZ have been able to observe dispersion due to rain scatter at 1296 MHz that is wider than the 2.34 Hz bin bandwidth of the narrowest width. In general, the phase noise of the DSP-10 does not seem to be a limiting factor for the available bin bandwidths. So, the answer is that one should use the narrowest width for which most of the energy is concentrated in a single bin. As the operating frequency is increased this can require going to SpecAnl widths of 2400 or 4800 Hz.
Q - Do the contrast and brightness controls affect the EME-2 measurement?
A -No, these are for the waterfall
only. The same is
true with all controls that affect the audio signal. It is easy to get
a good yellow trace when no trace is observable on the waterfall.