 and fitted (green) data is observed for a beam profile with 4 degree FWHM,  small ($\pm 0.5$ deg) departures from nominal zenith pointing, and 20kHz constant frequency offset. }
      \end{figure}
      
\subsection{GNSS transits}
GNSS satellite transmissions in the L5 (1176.43 MHz) and L2 (1227.6 MHz) bands are observed clearly (Fig. \ref{fig:GNSS_fits}, left) and can be used to measure beam pointing and beam widths. For each dish and polarization, we used the following method:

\begin{itemize}
    \item For each day of observing, identify GNSS satellites that pass near the BMX zenith. We use the two-line element orbit parameters on that day for each satellite to determine local altitude and azimuth as a function of MJD, then sort by maximum altitude and select the 16 satellite transits closest to the BMX nominal (zenith) pointing.
    \item Convert (altitude, azimuth) to a coordinate system centered at zenith with (x,y) axes oriented along (E-W, N-S) directions
    \item Identify windows of length 65  minutes centered around the MJD of transit for each satellite.
    \item Extract the auto-spectrum signal for each window where a satellite is predicted to pass. Integrate over the band of frequencies 1105 - 1369 MHz containing the GNSS transmissions.
    \item Assemble a data cube by stitching together the predicted locations and BMX signals for each of the 16 time windows
 
        \item Perform a joint fit to all 16 transits using a simple Gaussian beam model of the form \[B(x,y) = Ae^{-((x-x_0)^2/{2\sigma_x^2} + (y-y_0)^2)/2{\sigma_y^2})} + C\]
    Note that the amplitude and offset parameters $A$ and $C$ are treated as invariant and included in the joint fit, even though they may be expected to vary from transit to transit due to satellite transmit power differences and gain drifts. The fit for the shape parameters $x_0, y_0, \sigma_x, \sigma_y$ was found to be insensitive to these variations. Also, frequency dependence is disregarded and the fitted shape parameters may be taken as an average over the 269 MHz GNSS L2+L5 band.
\end{itemize}
An example of the resulting fit for 16 transits occurring on 25 Jan 2020 is shown in the center upper panel of Fig. \ref{fig:GNSS_fits}, along with the predicted tracks for each (center lower panel).  Satellites from GPS, GALILEO, GLONASS, and BEIDOU constellations were observed. Note, not all GPS satellites transmit in the L5 band. Fits were poor for the GLONASS constellation satellites, posibly due to ephemeris errors, but generally good for GPS, GALILEO, and BEIDOU. The extracted shape parameters for the West dish, Y polarization channel over the 5-day period from 21 to 25 January 2020 were found to be stable to better than 0.2$^{\circ}$ (Fig. \ref{fig:GNSS_fits}, right panel).
 

  \begin{figure} [H]
   \begin{center}
   \begin{tabular}{c}
   \includegraphics[width=\textwidth]{GNSS_wfall_fits_pmts.png}
   \end{tabular}
   \end{center}
   \caption[] 
   { \label{fig:GNSS_fits} 
Left, waterfall data showing signals from GPS, GALILEO, and BEIDOU satellites on 3 Jan 2020. Center top, observed BMX signals (integrated autospectra, blue) for one-hour periods around the predicted transit of 16 satellites on 25 Jan, with fits to simple Gaussian beam model (green). Center bottom, predicted satellite tracks on zenith-centered coordinates with fitted beam center and widths. Right top, statistics of fitted beam parameters over 21-25 Jan 2020, mean-subtracted. Right bottom, resulting parameters (in degrees). Data for West dish, Y polarization.}
   \end{figure} 
   
\subsection{Cygnus-A transits}
Of the 24 cross-correlation products that a 4-dish, dual-polarization configuration like BMX can produce, we compute only the 12 like-polarization visibilities. Fig.\ref{fig:CygA_waterfall} shows an example waterfall plot of the real part of the East-West, Y-polarization visibilities over a 3.8-hour period near a transit of the radio source Cygnus-A. Dashed horizontal lines are labeled with frequencies for which visibilities were analyzed. Transmissions from four GNSS satellites are seen in the figure in addition to CygA; their signals are confined to the L2 and L5 bands between 1130-1230 MHz while CygA exhibits a falling power-law spectrum. The observed fringes from CygA can be used to estimate beam parameters using fitting methods similar to those used for UAV, GNSS satellite, and galactic HI calibrations, with the advantage that CygA is effectively a point source, its horizon coordinates can be known with high accuracy, and its fringe rate is compatible with BMX's baselines, beam width, frequency bandpass, and integration time.

  
  \begin{figure} [H]
   \begin{center}
   \begin{tabular}{c}
   \includegraphics[width=0.65\textwidth]{CygA_Waterfall_POC.png}
   \end{tabular}
   \end{center}
   \caption[] 
   { \label{fig:CygA_waterfall} 
Waterfall chart showing cross-correlation amplitude for the East-West dish pair $\sim$3.8 hours of observation near CygA transit. Four GNSS satellites also transited the BMX beam during this period. Dashed horizontal lines mark the frequencies for which CygA fringing was analyzed.}
   \end{figure} 
   
\noindent
Fig. \ref{fig:CygA_fringes_2freqs} shows visibilities for a CygA transit on 17 April 2019, at 1105.5 MHz and 1450 MHz. Each sub-panel shows the 6 like-polarization visibilities for the (N-E), (N-S), (N-W), (E-S), (E-W), and (S-W) dish pairs, along with model fits and fitted parameters. The N-S fringes (second row in Fig.\ref{fig:CygA_fringes_2freqs}) are slow since that baseline is nearly orthogonal to the direction of CygA motion. Otherwise, the visibilities have $SNR >$ 200 and the simple Gaussian model fits well except for those transits where nearby GNSS satellites or RFI spikes perturbed the fit.

\noindent
BMX was upgraded from a single-dish spectrometer to a 4-dish interferometer in early 2019, and after commissioning the instrument regular observations began in early 2020. Between 14 January and 17 April we logged 81 days of observations; 16 non-observing days involved weather events, hardware and software maintenance, and coronavirus-related site restrictions starting in late March. Stability of the system is illustrated by the histograms of baseline length and RA offset fitted to CygA transits in Fig.\ref{fig:CygA_fitHistograms}. Fig.\ref{fig:CygA_baseline_stats} gives the statistics for the fitted baselines over this period. After rejecting outliers, pointing and beamwidth fits for a given visibility were generally stable to  0.1 -- 0.2 degrees and baseline fits to within 6 -- 10 cm during this period.
   
  
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.45\textwidth]{fringes_190417_1105.pdf}
\includegraphics[width=0.45\textwidth]{fringes_190417_1450.pdf}
\end{center}
\caption[]{Visibilities (real part) for a CygA transit on 17 April 2019. Left: at 1105.5 MHz. Right: 1450 MHz. Arrangement of each 6 x 2 panel has like-polarization visibilites in columns and like dish pairs in rows, in the order (N-E), (N-S), (N-W), (E-S), (E-W), (S-W) dish pairs. X-axis is time in MJD. Measured data is in blue; fit in red; envelope of visibility in green. The dashed vertical line is the predicted time of CygA transit over BMX. Inset boxes have the fitted beam parameters: A (amplitude in ADU**2), th0 (RA offset in degrees), b (E-W projection of baseline length in meters), sigma (beam width in degrees), tau (time offset in seconds), and pcq (goodness-of-fit measure). N-S baselines (second row) display very slow fringes, indicative of a small misalignment in the array layout. Overall features