{"id":267,"date":"2018-09-28T07:27:29","date_gmt":"2018-09-28T07:27:29","guid":{"rendered":"https:\/\/ejmilton.uk\/?page_id=267"},"modified":"2020-10-24T11:08:09","modified_gmt":"2020-10-24T11:08:09","slug":"mmr","status":"publish","type":"page","link":"https:\/\/ejmilton.uk\/index.php\/mmr\/","title":{"rendered":"MMR"},"content":{"rendered":"\n

The Milton Multiband Radiometer<\/h2>\n\n\n\n

Although I had encountered satellite remote sensing as part of my first degree in Geography (1972-75), I was not aware of the potential of satellite sensors to make quantitative measurements until I started my PhD at the University of Reading (1977). As a naive but enthusiastic researcher I seriously underestimated the challenges involved in making accurate measurements of spectral reflectance in the natural environment. This led to a lifelong interest in the principles and practice of ‘field spectroscopy’, perhaps best captured by these publications that book-ended my academic career:<\/p>\n\n\n\n

Milton, E.J., Schaepman, M., Anderson, K., Kneub\u00fchler, M., & Fox, N. (2009). Progress in Field Spectroscopy<\/a>. Remote Sensing of Environment<\/em>, 113, S92-S109.<\/p>\n\n\n\n

Milton, E.J. (1987). Principles of field spectroscopy<\/a>. International Journal of Remote Sensing<\/em>, 8, 1807-1827.<\/p>\n\n\n\n

One of the first practical things I did was to design and build a simple instrument to measure reflected light in similar wavebands to those sensed by the Landsat Multispectral Scanner (MSS). The resulting instrument was tested in the field in Morocco by my colleague Peter Hancock, and feedback from Pete led to a much-improved second version which we both used in our PhD researches in southern Italy (1978-1982). This instrument became known as the ‘Milton Multiband Radiometer’ (MMR), a title I was not entirely happy with, but one which undoubtably raised my academic profile. Over the next 20 years the MMR was developed further, first by me, and then in partnership with Bill Damon, an outstanding Electronics Technician employed by the University of Southampton.<\/p>\n\n\n\n

Challenges in designing the MMR<\/strong><\/p>\n\n\n\n

Although it was simple enough to design a spectral radiometer, to make an instrument that was optimised for the particular task in hand was quite challenging. First, the wavebands sensed had to be matched to those sensed by the Landsat MSS, and later the Landsat Thematic Mapper (TM). This was achieved by selecting a silicon photodiode which, when combined with gelatin filters from the Kodak Wratten range, had a comparable spectral sensitivity.<\/p>\n\n\n\n

Identifying the optimum combination of filter and sensor involved a lot of trial-and-error and extensive measurements in the Physics Lab at the University of Lancaster. Two silicon photodiodes were eventually selected from the manufacturer Ferranti Ltd., both of which were optimised for sensing the visible light region. In particular their peak sensitivity was shifted significantly to shorter wavelengths compared with standard silicon photodiodes. In addition, the MS601 had an integral blue-green glass filter designed to match the response of the human eye. When supplies of the MS601 dried up an alternative was found comprising a BPW21 photodiode and a Schott glass filter, type BG18. Choosing a silicon photodiode for the near infra-red region was easier, with the Centronics SD172 (and later the OSD5-5 being the preferred choice). Many different Wratten filters were tested before eventually settling on numbers KW45, KW99, KW29 and KW87 for ‘blue’, ‘green’, ‘red’ and ‘near infra-red’ wavelengths respectively (Landsat TM bands 1-4). A match was found for Landsat MSS band 6 (near infra-red), but this band proved to be of little practical use. The short-wave infra-red bands (TM 5 & 7) proved more difficult to match, so the MMR is best regarded as an instrument for the visible and near infra-red region.<\/p>\n\n\n\n

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Figure 1. Relative spectral response of the MS600 & MS601 silicon photodiodes
Source: Ferranti MS600\/MS601 data sheet, Issue 3, Sept 1974<\/em> <\/figcaption><\/figure>\n\n\n\n

<\/p>\n\n\n\n

Summary of MMR versions<\/strong><\/p>\n\n\n\n

1. Prototype (1977)<\/strong>
The prototype MMR comprised a single hand-held box and had just two spectral bands: Red and Near Infra-red. I used a CdS light dependent resistor (LDR) for the visible wavelengths and a general purpose silicon photodiode for the near Infra-red. The LDR was not ideal because these devices have a logarithmic response to light intensity. This makes them very suitable for use in camera light meters as this is similar to the way the human eye responds to light, but a linear response was needed for the MMR.<\/p>\n\n\n\n

The prototype MMR was useful in three respects: first, it revealed that the range of light levels in nature is very large, meaning the MMR would either have to have variable gain settings or a very large dynamic range. Second, it confirmed that low-cost Wratten gelatin filters were highly suitable for such a simple instrument, and third, it showed that measuring spectral reflectance by using a calibrated grey reference panel was a viable method, provided that some means of capturing the signal from the surface being measured and that from the reference panel with minimal delay couild be achieved.<\/p>\n\n\n\n

2. Series 1 (1978-1980)<\/strong><\/p>\n\n\n\n

The next version of the MMR separated the sensor head from the meter unit to allow the former to be mounted on a lightweight aluminium mast up to 4 metres high. Silicon photodiodes were used for all four bands, the gain of each could be adjusted by DIP switches within the sensor head. The reference panel was mounted on the mast below the sensor head and could be rotated (2m mast) or lowered (4m mast) into position. Reflectance was measured using the sequential method. Inevitably, light levels sometimes changed slightly between the surface measurement and that of the reference panel, so this source of error was addressed in the next version.<\/p>\n\n\n\n

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Figure 2. A Series 1 MMR in use from the 4m mast<\/figcaption><\/figure>\n\n\n\n

3. Series 2 (1980-1985)<\/strong>
This series retained the basic mechanical design of the first Series but incorporated two identical sensor heads and eight analogue sample\/hold circuits, four for the radiance data from the surface and four for the radiance data from the reference panel. In operation, the user set a switch to store the radiance data from both the surface and the reference panel. This ‘simultaneous’ mode greatly reduced the errors inherent in the sequential method, but came at the additional cost of needing a second inter-calibrated sensor head.<\/p>\n\n\n\n

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Figure 3. Series 2 MMR (meter unit)<\/figcaption><\/figure>\n\n\n\n
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    Figure 4. MMR sensor unit (Series 2-4)<\/figcaption><\/figure>\n\n\n\n
    <\/div>\n\n\n\n

    <\/p>\n\n\n\n

    \"\"
    Figure 5. Schematic diagram of the MMR Sensor Unit, one of four channels (see below for typical component values)<\/figcaption><\/figure>\n\n\n\n

    <\/p>\n\n\n\n

    The external appearance of the MMR sensor unit hardly changed over time, but the internal electronics were improved and the mechnical design was simplified, making it easier to make and to service. There were two parts to the sensor unit, the optical ‘field stop’ responsible for defining the field-of-view of the instrument in conjunction with the silicon photodiodes, and the preamplifier board responsible for amplifying and conditioning the signal before sending it to the meter unit. High performance operational amplifiers (e.g. Linear Technology LTC1053) were used in the sensor unit, configured as four independent transimpedance amplifiers with adjustable gain. The meter unit was basically a high impedance voltmeter with eight analogue sample\/hold circuits which retained the signals from the Sensor Head until the data could be read off the meter or sent to a data logger.<\/p>\n\n\n\n

    4. Series 3<\/strong> (1985-1995)<\/strong>
    The third series introduced a new method of collecting spectral data in the field, termed the ‘near simultaneous’ mode. This model had eight analogue sample\/hold circuits like the Series 2 but had the option to load these short-term stores with the data from two sensor heads or with the data from a single sensor head. With practice, the delay between sampling the data from the surface and those from the reference panel could be kept to a couple of seconds, greatly reducing the errors due to changing sub-visual cloud conditions. The convenience of near-simultaneous mode meant that it soon became the standard method of using the MMR.<\/p>\n\n\n\n

    The Series 3 continued to improve the electronics design as new components became available, and also replaced the military style ‘Amphenol’ multiway connectors with alternatives that were easier to repair in the field.<\/p>\n\n\n\n

    The third series is the definitive analogue MMR and was a very popular instrument for both teaching and research, both in the UK and internationally.<\/p>\n\n\n\n

    \"\"
    Figure 6. Series 3 MMR (meter unit)<\/figcaption><\/figure>\n\n\n\n

    5. Series 4 (1995-2000)<\/strong>
    The fourth Series combined the proven sensor unit and choice of measurement modes from the third Series with a digital display. The eight analogue sample\/hold circuits were retained as they were very effective for the short-term storage of data, even with a digital display . This version of the MMR was popular with students because it was easier to read the digital display than the rather complex scale of the analogue meter.<\/p>\n\n\n\n

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    Figure 7. Series 4 MMR ( meter unit)<\/figcaption><\/figure>\n\n\n\n

    6. Series 5 (1999-2002)<\/strong>
    The fifth and final Series of the MMR incorporated a PIC microcontroller. There were three distinct versions. The first was based on a design by Alberto Ricci Bitti (Circuit Cellar, October 1998) and used a PIC to interface the MMR to a Casio graphic calculator (fx-9750G). The second was based on an 8-channel data logger designed by John Becker (EPE\/ETI August 1999). The third was an in-house design which used a PIC to control the MMR, either as part of an all-digital design or as an add-on to an analogue sensor unit. Several production versions of the Casio version were built whereas the all-PIC designs were only used in a handful of bespoke instruments.<\/p>\n\n\n\n

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      Figure 8. MMR based on a data logger design by Alberto Ricci Bitti<\/figcaption><\/figure>\n\n\n\n

      <\/p>\n\n\n\n

      7. Specials<\/strong>
      Throughout the 20 years that the MMR was in production there were numerous ‘one-offs’, designed for specific purposes or to test new ideas. For example, ‘The Squealer’ was a version of the Series 4 MMR that had an audio output that indicated the rate of change of solar illumination. This gave an indication of unsuitable cloud conditions and was particularly useful as an adjunct to the digital display.<\/p>\n\n\n\n

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        Figure 9. Examples of Non-standard MMR<\/figcaption><\/figure>\n\n\n\n
        <\/div>\n\n\n\n

        Further information<\/strong><\/p>\n\n\n\n

        <\/p>\n\n\n\n

        <\/td>Ch1 (Blue)<\/td>CH2 (Green)<\/td>Ch3 (Red)<\/td>CH4 (NIR)<\/td><\/tr>
        Filter(s)<\/td>KW45+BG18<\/td>KW99+BG18<\/td>KW29+BG18<\/td>KW87<\/td><\/tr>
        Detector<\/td>MS601\/BPW21<\/td>MS601\/BPW21<\/td>MS601\/BPW21<\/td>OSD5-5T<\/td><\/tr>
        IC1 (Quad)<\/td>LT1053CN<\/td>LT1053CN<\/td>LT1053CN<\/td>LT1053CN<\/td><\/tr>
        R1<\/td>470 K<\/td>470 K<\/td>1 M<\/td>15 K<\/td><\/tr>
        R2<\/td>470 K<\/td>470 K<\/td>1 M<\/td>15 K<\/td><\/tr>
        R3<\/td>16 K<\/td>16 K<\/td>16 K<\/td>16 K<\/td><\/tr>
        R4<\/td>560 R<\/td>560 R<\/td>560 R<\/td>560 R<\/td><\/tr>
        VR1<\/td>1K0<\/td>1K0<\/td>1K0<\/td>1K0<\/td><\/tr>
        C1<\/td>100 nF<\/td>100 nF<\/td>100 nF<\/td>100 nF<\/td><\/tr><\/tbody><\/table>
        Table 1. Typical component values for MMR Sensor Head (see Figure 5)<\/figcaption><\/figure>\n\n\n\n

        <\/p>\n","protected":false},"excerpt":{"rendered":"

        The Milton Multiband Radiometer Although I had encountered satellite remote sensing as part of my first degree in Geography (1972-75), […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"_links":{"self":[{"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/pages\/267"}],"collection":[{"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/comments?post=267"}],"version-history":[{"count":51,"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/pages\/267\/revisions"}],"predecessor-version":[{"id":699,"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/pages\/267\/revisions\/699"}],"wp:attachment":[{"href":"https:\/\/ejmilton.uk\/index.php\/wp-json\/wp\/v2\/media?parent=267"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}