MET Light Detection and Ranging (LIDAR)
The Phoenix mission to Mars [SMITHETAL2008] will advance our knowledge of the climate on Mars by combining lidar remote sensing of atmospheric dust and clouds with measurements of solar radiation [LEMMON2008] and in situ sampling of temperature, pressure, wind [TAYLOR2008], and water vapour. The day to day variation of the local weather on Mars is controlled primarily by the amount of solar radiation reaching the surface and this depends on the optical thickness of the dust suspended in the atmosphere. This affects local and global meteorological patterns which in turn determine the lifting of dust from the surface and long range transport [LEOVY2001; NEWMAN2002]. There is also a climate interaction with the distribution of water ice above and below the surface of Mars [BOYNTON2002, PATHAK2008]. This involves transport of water through the atmosphere and previous lidar measurements from orbit have indicated that clouds could have a substantial role [NEUMANN2003].
The lidar on the Phoenix mission will record the height profile of laser backscatter from the dust and cloud layers that drift past the landing site with a view that is continuous in time. The attenuation of the lidar signal can be used to derive the optical extinction coefficient and this is related to atmospheric properties. For example, the extinction of solar radiation due to dust has a first order effect on heating of the surface of Mars. The measured optical extinction can also be related to the amount of scattering material. For example, for a thin cloud the extinction coefficient measured by a lidar can be used to derive the ice water content within the cloud [HEYMSFIELD2005]. Such a measurement will be of vital importance to the mission goal to investigate water on Mars. The combination of the lidar and passive remote sensing with in situ sampling will provide a view of the interacting processes that determine the local weather at the surface and also the role of the atmosphere in the water cycle on Mars.
A basic atmospheric lidar (light detection and ranging) emits pulses of laser light into the atmosphere then detects and records the backscattered light as a function of time [MEASURES1984]. The time resolved signal is converted to distance using the speed of light and a factor of two to account for the round-trip path length.
The lidar transmitter is based on a diode pumped, passively Q-switched, neodymium-doped yttrium garnet (Nd:YAG) laser. This configuration was chosen for its robustness and technological maturity as much as for its suitable lidar performance. The pump diode array emits at a wavelength of 808 nm and is used to provide the energy to the Nd:YAG crystal within the laser cavity. Lasing is inhibited within the cavity through the use of a saturable absorber. The photon density within the oscillator cavity builds to a level where the saturable absorber bleaches and the energy within the cavity is dumped as a laser pulse over a very short time (a passive Q-switch). For this laser the output light pulse has a length of approximately 10 ns and an energy of 0.7 mJ.
After the laser cavity emits its light pulse, part of the optical energy is converted from a wavelength of 1064 nm to 532 nm by second harmonic generation in a Potassium Titanyl Phosphate (KTP) crystal. The laser output is then expanded by a factor of 10 in order to reduce the divergence to 0.5 mrad. A small fraction of the outgoing laser pulse is 'picked off' and separated into the two wavelength components in order to measure the relative amplitude of each wavelength with photodiodes. The signal from the photodiode detecting the 1064 nm pulse is used to trigger the data acquisition electronics, providing a zero-time reference, and also to shut off the pumping diodes.
The atmospheric backscatter signals are collected by an afocal reflective telescope and split into the two relevant wavelengths using dichroic mirrors. The 1064 nm backscatter is detected by an Avalanche Photo-Diode (APD), and recorded using 14-bit analog to digital conversion (ADC). The 532 nm backscatter is detected by a Photomultiplier (PMT) and the signal is recorded using both ADC and photon counting. Photon counting is required to record the weak signals from heights above 5 km and up to 20 km.
The main objective of the Lidar is to measure the vertical distribution of scattering particles (both dust and clouds) in the Martian atmosphere. These measurements are expected to provide estimates of:
Martian ice cloud base heights and cloud thickness (for optical thick but spatially thin clouds)
Diurnal variation of surface ice fogs
Diurnal variation of the boundary layer height
Variation of vertical dust structure owing to local weather conditions.
The Phoenix Mars Lander arrived at 68.2184N, 234.2487E on May 25, 2008.
Operational Modes and Measured Parameters
The signal as a function of height is integrated over a set number of laser pulses and height bins. For example the standard operating mode is to use 50m vertical resolution (ten 5-meter bins averaged) and 20.48 second temporal resolution (1000 laser pulses averaged) for the Photon Counting Channel, and 10m, 20.48 sec resolution, respectively, for the two Analog Channels. These profiles are then stored sequentially within a data file. Typical operational times are 15min, with the expectation that this will be increased whenever power and data volume permit. The observable range is 0-20km for the photon counting, and 0-10km for the analog data, though it is possible to slide the observation window up (e.g. 1-21km) using a timing offset parameter. The temporal and spatial averaging are parameters that are part of the commands transmitted from the ground station during the mission. Owing to the flexibility in these parameters, there are no set operational 'modes' other than the 'default' outlined here.
Each vertical/temporal bin represents the total number of measured photons (i.e. counts) for the Photon Counting Channel, and the average voltage for the 532nm and 1064nm Analog Channels. The voltages are sampled at an effective range of 13 bits, with the sum for each spatial/temporal bin being stored at 32 bits before transmission to Ground. Subsequent processing converts this sum into an average in Volts: that is the average Voltage per shot and per 2.5m vertical bin (the highest possible resolution) for the purposes of inter-comparison. The cumulative Photon Counts are left unprocessed.
Laser Power data are also provided as a relative measure of the transmitted laser intensity, and comparison with zenith pointing SSI images may provide an estimate of the receiver efficiency.
Verification and Calibration
Verification that the Phoenix lidar will function on Mars was achieved by operating it within a thermal vacuum (T-vac) chamber at the pressures and temperatures expected on the surface of Mars (e.g. 8 Torr, -70C). A window was installed in the T-vac chamber so that the output laser pulses and the receiver field of view could be directed across the clean room and through another window into the atmosphere in the zenith direction. Atmospheric observations could then be acquired and compared with simultaneous measurements with another lidar system.
The lidar used for side-by-side comparison in testing was supplied by York University (referred to as the York Lidar). This has the same essential characteristics as the Phoenix Lidar. For example, the transmitted wavelengths are the same (1064 nm and 532 nm), and the same detectors are applied in the receiver. The data acquisition electronics provide the same function as in the Phoenix lidar: analog to digital conversion (ADC) at 1064 nm; both ADC and photon counting at 532 nm. The main difference is that the York Lidar has a more powerful laser than the Phoenix lidar.
An important part of the testing was to ensure that the receiver and transmitter remain aligned over the full range of temperatures that are expected during the mission. The temperature within the lidar is controlled by heaters and the coldest it will get during the mission is -40C. The heat dissipated by the laser while operating will increase the temperature of the instrument by about +3C over a 15 minute interval. The baseline plan is to have the lidar operate for four separate 15 minute intervals at mid-day, evening, midnight and morning. In this scenario the temperature of the lidar chassis in not expected to rise above -20C. If the lidar is run for longer periods and more often, then the lidar chassis will get warmer. The alignment of the transmitter was optimized so that it was pointing within the receiver telescope FOV over a temperature range of -40C to -10C on the lidar chassis.
The direction of the transmitted laser light relative to the receiver telescope field of view (FOV) was determined by deflecting the transmitted light with a variable optical wedge. The direction of the transmitted light was deflected until the atmospheric backscatter signal at a height of 1.5 km started to decrease and this was done in the four cardinal directions. The outline of the FOV could then be determined relative to the transmitter output. This was done at various temperatures between -40C and 0C. It was determined that the relative angle between the transmitted light pulse and the receiver telescope FOV axis changed by 0.9 mrad over the temperature range from -40C to 0C, moving inward toward the telescope axis as the temperature increased. The transmitted light pulse has a divergence of 0.5 mrad, so there is 1 mrad of tolerance within the 1.5 mrad FOV to account for temperature variations. Thus it was possible to optimize the alignment of the system so that the transmitter is pointing within the FOV over the range of temperature from -40C to 0C. This was done so that at -40C the transmitter output is pointing 0.4 mrad away from the central axis of the receiver telescope FOV. It is still within the full 1.5 mrad of the FOV, with a 0.1 mrad tolerance between the outside edge of the transmitted laser pulse and the edge of the FOV. At a temperature of 0C the transmitter was pointing 0.5 mrad toward the receiver telescope.
The alignment of the Phoenix lidar in Mars conditions was verified by direct comparison to the York Lidar with simultaneous atmospheric measurements. An example of a comparison between the York lidar and Phoenix lidar with photon counting signal acquisition is shown in Fig. 4. The Phoenix lidar was in the T-vac chamber with 8 Torr of CO2 at a temperature of -70C, and the lidar chassis temperature was -36C. The ratio of the signals from the two lidars is constant with height up to above 15 km and this is an indication that the Phoenix lidar transmitter is aligned within the receiver field of view. Another indication that the system is properly aligned is in the comparison with the expected signal from molecular backscatter. There is an enhancement in the signal between heights of 8.5 km and 10.5 km due to a cirrus cloud layer. There is also a thin cloud at a height of 1.5 km. It is expected that such thin ice clouds will be detected in the atmosphere of Mars [PATHAK2007]. Below a height of 2 km there is a reduction in the signals recorded with both lidars in comparison with the expected molecular backscatter signal. This is a well known effect: the photon counting saturates due to electrical pulse overlap at count rates greater than 10 MHz. This well known nonlinear effect in photon counting has been characterized and can be corrected when there is not substantial variability within the averaging period. However, there is no saturation in the signal recorded by analog to digital conversion.
The ratio of the Phoenix Lidar to the York Lidar signals was found to be constant with height above about 400m. A difference below 400 m is owing to incomplete overlap between the outgoing laser pulse and the receiver FOV. This is consistent with the geometry of the system: the 12 cm separation between the telescope axis and the transmitter output, the 1.5 mrad FOV, the 0.5 mrad divergence, and the 0.1 mrad tilt of the transmitter output away from the telescope axis that occurs at a chassis temperature of -30C. There is complete overlap at this temperature above 400 m. Plots of this can be found in [WHITEWAY2008]."